PROOF 1 2 3 4 5 6The Getty Conservation Institute [612067]

PROOF 1 2 3 4 5 6The Getty Conservation Institute
Los Angeles
Stone Conservation
An Overview of Current
Research
Second Edition
Eric Doehne and Clifford A. Price
2010
research in conservation

PROOF 1 2 3 4 5 6The Getty Conservation Institute
Timothy P. Whalen, Director
Jeanne Marie Teutonico, Associate Director, Programs
The Getty Conservation Institute works internationally to advance conservation
practice in the visual arts—broadly interpreted to include objects, collections, architecture, and sites. The Institute serves the conservation community through scientific research, education and training, model field projects, and the dissemina –
tion of the results of both its own work and the work of others in the field. In all its endeavors, the GCI focuses on the creation and delivery of knowledge that will benefit the professionals and organizations responsible for the conservation of the world’s cultural heritage.
Research in Conservation
The Research in Conservation reference series presents the finding of research conducted by the Getty Conservation Institute and its individual and institutional research partners, as well as state-of-the-art reviews of conservation literature. Each volume covers a topic of current interest to conservator and conservation scientist. Other volumes in the Research in Conservation series include Alkoxysilanes and the Consolidation of Stone (Wheeler 2005), Analysis of
Modern Paints (Learner 2004), Effects of Light on Materials in Collections
(Schaeffer 2001), Inert Gases in the Control of Museum Insect Pests (Selwitz and
Maekawa 1998), Oxygen-Free Museum Cases (Maekawa 1998), Accelerated
Aging: Photochemical and Thermal Aspects (Feller 1994), and Statistical Analysis
in Art Conservation Research (Reedy and Reedy 1988).
© 2010 J. Paul Getty Trust
Published by the Getty Conservation Institute
Getty Publications1200 Getty Center Drive, Suite 500Los Angeles, California 90049-1682www.gettypublications.org
Gregory M. Britton, Publisher
Ann Lucke, Managing Editor
Beatrice Hohenegger, Project Editor
Cynthia Newman Bohn, Manuscript Editor
Pamela Heath, Production Coordinator
Hespenheide Design, Designer
Printed in Canada
Library of Congress Cataloging-in-Publication Data
Doehne, Eric Ferguson.
Stone conservation : an overview of current research / Eric Doehne and
Clifford A. Price. —2nd ed.
p. cm.—(Research in conservation)
Includes bibliographical references and index. ISBN 978-1-60606-046-9 (pbk.) 1. Building stones—Deterioration. 2. Stone buildings—Conservation and
restoration. I. Price, C. A. II. Getty Conservation Institute. III. Title.
TA426.P75 2010 691'.2—dc22 2010019971

PROOF 1 2 3 4 5 6 vi Foreword to the Second Edition, 2010
viii Preface to the Second Edition, 2010
x Foreword to the First Edition, 1996
xii Preface to the First Edition, 1996
xiii Dedication
xiv Intr oduction
Chapter 1 1 Ston e Decay
1 CHARACTERIZING THE STONE
2 DESCRIBING DECAY
3 HOW SERIOUS IS IT? MEASURING THE EXTENT AND SEVERITY OF DECAY
5 Surf ace Techniques
6 Look ing Beneath the Surface
8 All the Information We Need?
9 CAUSES OF DECAY
9 Air Pollution
14 Salt s
20 Biod eterioration
24 Diff erential Stress
25 Intr insic Problems
Chapter 2 27 Putt ing It Right: Preventive and Remedial Treatments
27 PREVENTIVE CONSERVATION
29 ACTIVE CONSERVATION: CLEANING
31 Lase r Cleaning
32 Late x Poultice Method
33 Biol ogical Cleaning
33 Targ eting the Dirt
33 ACTIVE CONSERVATION: DESALINATION
35 ACTIVE CONSERVATION: CONSOLIDATION
36 Lime and Related Treatments
38 Bari um Hydroxide
38 Orga nic Polymers
40 Alko xysilanes
41 Epox ies
42 Acry lics
43 Othe r Materials
43 Emul sionsContents

PROOF 1 2 3 4 5 6iv Cont ents
44 SURFACE COATINGS
44 Wate r Repellents
45 Anti -Graffiti Coatings
46 Emul sions
46 Crys tal Growth Inhibitors
46 Oxal ate Formation
47 Lime and Biocalcification
47 Coll oidal Silica
47 Bioc ides
48 Biol ogical Attack on Treatments
Chapter 3 49 Do T hey Work? Assessing the Effectiveness of Treatments
50 CHARACTERIZING THE TREATED STONE
50 Prop erties That Change with Decay
50 Meet ing Objectives
51 Stan dard Test Methods
51 LONG-TERM PERFORMANCE
53 Docu mentation of Field Trials
Chapter 4 54 Putt ing It into Practice: Conservation Policy
55 RESPONSIBLE USE OF SURFACE COATINGS AND CONSOLIDANTS
55 RETREATMENT
56 RECORDING
Chapter 5 58 Heri tage in Stone: Rock Art, Quarries, and Replacement Stone
58 ROCK ART
59 Rock Art Conservation
61 Rock Art Treatment
62 Rock Art Documentation
63 HISTORIC QUARRIES
64 REPLACEMENT STONE
Chapter 6 66 Doin g Better: Increasing the Effectiveness of Research
66 WHAT IS WRONG?
66 Publ ications
67 Conf erences
67 Stan dards
67 Cond uct and Quality of Research
70 Gett ing the Message Across
70 PUTTING IT RIGHT
70 Qual ity, Not Quantity
71 Conf erences and Other Models for Advancing the Field
71 Conf erence Papers
71 Sele ction of Conference Papers
72 Refe reeing
72 Coll aborative Programs
73 Trai ning
73 Revi ews

PROOF 1 2 3 4 5 6Chapter 7 75 What Has Changed? Some Thoughts on the Past Fifteen Years
80 CONC LUSION
81 Refe rences
140 Appe ndix: Resources for Stone Conservation
152 Inde x
159 Abou t the Authors Cont ents v

Chapter #
Chapter Title
Authors’ names
PROOF 1 2 3 4 5 6Petra, Angkor, Copán, Venice, Lascaux, Easter Island . Stone conservation
research may not be the first thing that comes to mind when reading
these words, but it is because these places of irreplaceable cultural heri –
tage, and many other stone monuments, are eroding at a noticeable rate that the subject of this volume is of such crucial importance. In the sum –
mer of 1994, the Getty Conservation Institute (GCI) invited Professor Clifford A. Price to provide an overview of research on the conservation of stone monuments, sculpture, and archaeological sites. The purpose of the review, which was subsequently published in the 1996 book Stone
Conservation: An Overview of Current Research , was to inform GCI
research policy in this field and to highlight areas into which Getty resources might usefully be channeled. Today, a Google search for “stone conservation” raises this book in the first link—a testament to its endur –
ing usefulness to the wider conservation community.
Stone Conservation remains one of the most cited and down –
loaded of the GCI’s books some fifteen years after it was written. A refreshingly opinionated work, its call to reform the focus and process of research was subsequently echoed and reinforced by other authors and institutions. By raising challenging issues, the book influenced a genera –
tion of conservators and scientists who have worked to advance the field of stone conservation. Indeed, progress on several key issues can be tied directly back to Professor Price’s frank observations and prescriptions. The need for both a conservation journal that reviewed scholarly articles and for rigorous peer review of conservation publications contributed to the subsequent formation of venues such as the IIC Journal, Reviews in
Conservation, and the expansion of the GCI’s Research in Conservation
series. His call to improve the quality and timing of conferences and the accessibility of proceedings was one of the stimuli for the recent develop –
ment of the Torun Guidelines adopted at the ICOMOS International Stone Committee meeting in 2008.
The fact that the stone conservation field has evolved significantly
since 1994 prompted requests to update this popular volume, and in May Foreword to the Second Edition, 2010

PROOF 1 2 3 4 5 6 Fore word to the Second Edition, 2010 vii
2007 GCI scientist Eric Doehne, with the advice and collaboration of
Clifford Price, embarked on a new survey of the field of stone conser –
vation research. The goal was to retain key characteristics of the first
edition (notably its brevity, informal character, and pointed suggestions),
while covering the recent explosion of new research, enlarging the discus –
sion of preventive conservation, and adding new sections on rock art and
other subjects. This required a parallel compilation of a new bibliography, which included a review of more than six thousand abstracts and more than three thousand PDF files of material published between 1995 and 2009. Topics ranged from nano-scale measurements of salt damage by materials scientists to conservators’ documenting the unintended conse –
quences of waterproofing agents. The selected bibliography drawn from this research effort is included in this new edition as an appendix and will be a useful starting point for many researchers.
With increasing reliance on the Internet and the rapid develop –
ment of interdisciplinary research and teaching, we live in a time when all knowledge is being connected to all other knowledge. Building and main –
taining a coherent infrastructure for the conservation field, arguably one of the most interdisciplinary of endeavors, is a particular challenge. To advance the field of stone conservation and manage the growing variety and volume of information, practicing conservators and scientists need a framework for building a coherent base of useful knowledge. The second edition of Stone Conservation: An Overview of Current Research pro-
vides this framework in the form of a strategic overview of the past
fifteen years in stone conservation research and an updated critique
of the field’s strengths and weaknesses, with recommendations for
future research.
Timothy P. Whalen, Director
The Getty Conservation Institute

Chapter #
Chapter Title
Authors’ names
PROOF 1 2 3 4 5 6Being a conservation scientist often means acting as a bridge—between
researchers and conservation practitioners, and between the many differ –
ent fields of research related to the preservation and conservation of carved and worked stone, from Stonehenge to the Sphinx, and from ana –
lytical chemistry to X-ray tomography. Like the first edition, this volume is not a literature review. It is an overview that maps the landscape of stone conservation, cites interesting and representative research, and is intended to serve as a useful point of entry to the field.
I began the research for the second edition in May 2007 as an
effort to update and highlight the significant changes that had taken place in the field since the first edition and to encompass a much larger range of publications. The text was largely written in 2008, with revisions and editing completed in 2009. Such an endeavor unavoidably results in a
particular pers pective. This tendency has been ameliorated by consulting
with an experienced, as well as linguistically diverse, group of conserva –
tion practitioners, researchers, and colleagues who have been very gener –
ous with their time.
In particular, I would like to acknowledge the help and advice
given by John Ashurst, Api Charola, Jose Delgado Rodrigues, Vasco Fassina, John Fidler, William Ginell, John Griswold, Chris Hall, Seamus Hanna, Adrian Heritage, Ioanna Kakouli, Lorenzo Lazzarini, Susan Macdonald, Bill Martin, David Odgers, Leo Pel, Sarah Pinchin, Francesca Preface to the Second Edition, 2010

PROOF 1 2 3 4 5 6 Pref ace to the Second Edition, 2010 ix
Pique, Jerry Podany, Thomas Roby, Carlos Rodríguez-Navarro, Eduardo
Sanchez, Alison Sawdy, George Scherer, Stefan Simon, Michael Steiger, Marisa Laurenzi Tabasso, Giorgio Torraca, Véronique Vergès-Belmin, Heather Viles, Norman Weiss, George Wheeler, Chris Wood, Konrad Zehnder, and Fulvio Zezza. I would also like to acknowledge former
GCI interns and postdoctoral researchers Enrica Balboni, Ann Bourgés, Tiziana Lombardo, Paula Lopez Arce, and Claire Moreau, who helped teach me more about stone through our joint research. The students of the International Course on Stone Conservation have also been a source of inspiration. The GCI’s Beril Bicer-Simsir, David Carson, Giacomo Chiari, Mara Schiro, and Jeanne Marie Teutonico provided important support. I am grateful to Valerie Greathouse and Tina Segler for their help in tracking down references and to Cynthia Godlewski and Cynthia Newman Bohn for their excellent coordination and editorial assistance. Two anonymous peer reviews of earlier drafts of the book were thorough and thoughtful. Finally, my coauthor has been brilliant in skillfully aiding my efforts to bind together the old and the new in this volume, and I extend my kind thanks to him for his enthusiasm for this project.
Eric Doehne
Pasadena, California, June 2009

x Chap ter 2
PROOF 1 2 3 4 5 6The “sympathetic conservation” of historic or culturally significant stone
is a relatively recently recognized practice. In the past, the repair of dam –
aged sculptured stone objects was frequently accomplished using more intrusive means, such as iron dowels, staples, or clamps that often marred the appearance of the object and could lead to further damage. For the patching and filling of defects, lime mortar, cement, plaster of Paris, sodium silicate, and various gums and resins were used—materials no longer considered acceptable. Stone-cleaning processes involved harsh acidic treatments followed, at times, by neutralization, which resulted
in the production of soluble salts that penetrated the stone and increased the potential for future salt-crystallization damage. Damaged architec –
tural stone was either replaced or repaired with little regard to the mate –
rials’ compatibility with the stone, appearance matching, or the durability of the treatment.
The unsuitability of many of these treatments encouraged
research efforts to develop new materials and procedures for the preser –
vation of stone. Over the past twenty years or so, these studies have resulted in the publication of a vast number of reports and papers, most of which were concerned with case studies and how specific stone sub –
strates were treated. Few were accompanied by details of the research that supported the selection of the treatment method or materials used. Fewer yet were those concerned with stone-damage mechanisms or with scientific research on stone conservation processes, materials behavior, and environmental effects.
Although research has proliferated, there has not been a recent,
concerted effort to evaluate the direction in which research has been Foreword to the First Edition, 1996

PROOF 1 2 3 4 5 6 Fore word to the First Edition, 1996 xi
progressing and whether or not the current direction is proving fruitful.
Should the emphasis on stone conservation research be placed on devel –
opment of new materials and new application procedures? Has there
been significant work on the evaluation of the post-treatment stone property improvements? Are the methods for evaluating stone properties universally accepted? Do we need to conduct research on methods for carrying out and assessing the long-term durability of treatments? Are there problems in the process of conducting stone conservation research that bear on our ability to do the research effectively? Can these prob –
lems be defined; and, if so, what can be done to further the effectiveness of stone research? These are some of the many questions that Clifford A. Price has considered in this review of the current status of stone conser –
vation research.
We asked Dr. Price to give us his subjective viewpoint on what is
being done right, what areas of current research should be continued or accelerated, and what new directions should be addressed that would promote an increase in the effectiveness of stone conservation. In the course of preparing this review, Dr. Price has had extensive discussions with a number of active participants in the stone conservation community and what has emerged is an engaging account on whither we seem to be going and in which ways, if any, our paths should be altered.
William Ginell
Emeritus Head, Architecture and Monuments The Getty Conservation Institute, 1996

Chapter #
Chapter Title
Authors’ names
PROOF 1 2 3 4 5 6This volume was written over a short period during the summer of 1994,
following a systematic study of the major publications of the last five years. Inevitably, the volume reflects my own experience, expertise, and linguistic abilities. An international working party could, no doubt, have produced a more objective and comprehensive report—albeit over a
longer span of time. In order that my own prejudices might not shine
through too strongly, I have consulted with other conservation scientists and stone conservators, and I am very grateful for the help and advice that they have given me.
In particular, I would like to acknowledge the help given by John
Ashurst, Norbert Baer, Guido Biscontin, Sue Bradley, Api Charola, Vasco Fassina, John Fidler, William Ginell, Lorenzo Lazzarini, Bill Martin, Antonia Moropoulou, Marisa Laurenzi Tabasso, Jeanne Marie Teutonico, Giorgio Torraca, and George Wheeler. I am also grateful to Sasha Barnes for the help that she has given in rooting out references and to Julie Paranics for help in the final production of the volume.
The emphasis of this publication is on stone as a material. There
is little reference to mortars, and no consideration of the structural per –
formance of stone masonry. This volume is not a detailed, state-of-the-art review, and many of the references I have given are intended as illustra –
tive rather than definitive. It is intended to give a strategic overview of the whole field and to identify areas of strength and weakness where
further research should be focused.
Clifford PriceLondon, 1996Preface to the First Edition, 1996

Chapter #
Chapter Title
Authors’ names
PROOF 1 2 3 4 5 6We dedicate this book to the memory of John Ashurst, 1937–2008,
in recognition of his unparalleled contribution to stone conservation
through research, practice, and training.Dedication

PROOF 1 2 3 4 5 6After presenting his work at a recent stone conservation conference, a
thoughtful researcher was responding to questions from conservators.
A pained expression was evident on his face as he said to the audience,
“I feel as though I am explaining in great detail why I cannot help you.”
This encapsulates the frustration felt by many who are involved
in stone conservation at present. While great strides have been made in understanding why stone decays, the perception is that much less prog –
ress has been made in helping conservators cope with a number of long-standing conservation problems. The researcher’s comment also highlights a central paradox in conservation: while progress is necessarily incremen –
tal, time and the elements steadily take their toll on cultural heritage, and the window for action to ensure that history is preserved for future gen –
erations is limited.
This volume takes a broad and sometimes critical look at the
present state of stone conservation and of the way in which research is conducted. It looks first at the deterioration of stone and ways in which deterioration may be prevented or remedied. Then, it discusses some of the factors that limit the effectiveness of research and makes recommen –
dations as to how research might be made more effective. It concludes with some reflections on changes that have taken place over the past fif –
teen years.Introduction

PROOF 1 2 3 4 5 6The deterioration of stone is all too familiar to anyone who has looked
closely at a historic stone building or monument. While there are a few stones that seem little affected by centuries of exposure to the weather, the majority of stones are undergoing gradual and episodic deterioration. This may not matter much if the stone is an undecorated part of a mas –
sive wall. However, it does not take much deterioration of a carved piece of stone for the sculptor’s original intention to be lost altogether. A high proportion of the world’s cultural heritage is built of stone, and it is slowly but inexorably disappearing.
If we are to do anything to reduce or prevent this loss of our her –
itage, we must first be able to characterize the many stones involved. We need to be able to describe the decay and to measure its extent, severity, and rate. We then need to understand the causes and mechanisms of decay. Only then can we hope to understand the behavior of any particu –
lar stone in a given environment.
CHARACTERIZING THE STONE
The literature is full of papers concerned with stone characterization. Pick
up any set of conference proceedings and you will find numerous papers that first describe the situation and history of some particular monument and then lay out the physical properties of the stones involved. There will be petrological descriptions, followed by measurements of surface hard –
ness, porosity, capillarity, hygric and thermal expansion, pore size
distribution, mechanical strength, velocity of sound, resistance to salt
crystallization, and so forth. There will invariably be photographs taken
on a scanning electron microscope and, for good measure, probably some energy-dispersive X-ray analyses. To what end? The information will no doubt be of value to those who are concerned with the care of that partic –
ular monument, but it is of questionable relevance to a wider audience unless the properties of the stone can be linked to its performance. At this point the field needs to move beyond basic characterization to a better understanding of material behavior (Torraca 2009) and the maintenance
necessary to sustain long-term performance (Brand 1995).
Most of the techniques for characterization are well established.
Many are summarized by Robertson (1982) as well as Borelli and Urland Chapter 1
Stone Decay

2 Chapter 1
PROOF 1 2 3 4 5 6(1999) and Svahn (2006) . Adams and MacKenzie (1998) provide a useful
atlas of petrographic sections, while a more recent petrographic atlas and
applications of polarized light microscopy to building materials conserva –
tion are presented by Bläuer and Kueng (2007) and Reedy (2008).
In the process of characterizing stone, it is important to recognize
that while some stones have a similar composition, their behaviors may
have few things in common . For example, Istrian stone, Lecce limestone,
and Carrara marble are all carbonate materials , but their contrasting
modes of deterioration depend more on their porosity, pore shapes, pore size distribution, and grain size than their chemical composition. One division of stone types is based on the percentage and relative ratio of pore-shaped and fissure-shaped voids (Croci and Delgado Rodrigues
2002)
. A second division can be made on the basis of the degree of hygric
swelling of the stone (Delgado Rodrigues 2001 ; Duffus, Wangler, and
Scherer 2008 ), and a third division on the strength (Winkler 1985;
Bourgès 2006) . Subsequent divisions based on composition, texture, and
homogeneity enable further distinctions to be made, but they may be less important in rating overall performance than the first three. Those stones with high porosity, high rates of swelling, and low strength tend to be rel –
atively poor building materials (e.g., Jackson et al. 2005).
A review of the relationship between pore structure and other
stone characteristics is given by Bourgès et al. (2008). Gauri and Bandyopadhyay (1999) review the interpretation of mercury porosimetry data and cite a number of the seminal papers on pore structure determi –
nation. Analysis of the positive correlation between the fractal dimension, stone pore surface, and the degree of natural weathering has shown that increases in the surface fractal dimension are a more accurate descriptor of the degree of weathering than pore size distribution (Yerrapragada, Tambe, and Gauri 1993; Pérez Bernal and Bello López 2000).
DESCRIBING DECAY
Stone decay takes many different forms. Stone may weather away gradu –
ally, leaving a sound surface behind; at times large scales of stone may
drop away in one episode. Sometimes the surface erupts into blisters;
sometimes the stone loses all integrity and simply crumbles away. Some –
times the stone may look perfectly sound to the naked eye, while below the surface it has lost its cohesion.
One of the problems inherent in discussing stone decay is finding
a common language. Even in English, there are a bewildering number of terms that may mean different things to different people. And even if
we can agree on terms to describe the types of decay that we observe, it can be difficult to determine the severity or rate of decay. A significant advance in this area is the recent publication of a stone decay glossary by the ICOMOS Stone Committee under the editorship of Véronique Vergès-Belmin (2008). Another effort to produce a glossary of decay terms is that of the Italian Commissione NORMAL (UNI 2006). Earlier work in this area came from the building stone industry in an effort to standard –

Stone Dec ay 3
PROOF 1 2 3 4 5 6ize terminology (Stone Federation of Great Britain 1991) , governmental
organizations (Grimmer 1984) , and research groups (Fitzner, Heinrichs,
and Kownatzki 1997) .
The ICOMOS- ISCS Illustrated Glossary on Stone Deterioration
Patterns (Vergès-Belmin 2008) helps define and clarify usage across lan –
guages and within the stone community, providing useful definitions of
terms such as scaling, spalling, and flaking. Weathering is generally defined as the result of natural atmospheric phenomena, while decay is “any chemical or physical modification of the intrinsic stone properties
leading to a loss of value or to the impairment of use,” degradation is “decline in condition, quality, or functional capacity,” and deterioration
is the “process of making or becoming worse or lower in quality, value, character, etc.” Some interesting details of the history of stone glossaries can be found in the introduction to the glossary.
A more guided approach than a glossary can be found in work on
expert systems from the late 1990s, with Van Balen (1996; 1999) produc –
ing an atlas of damage to historic brick structures as part of an expert system for elucidating environmental effects on brick. The atlas evolved into a broader program known as the MDDS (Masonry Damage Diagnostic System) (Van Hees, Naldini, and Sanders 2006; Van Hees, Naldini, and Lubelli 2009). Expert systems have gone in and out of fash –
ion over the past fifteen years, but the need to capture expert experience and judgment has become ever more urgent, given the large number of conservation professionals nearing retirement age.
Fitzner has produced an important classification of weathering
forms as a basis for mapping the deterioration across a building facade (Fitzner, Heinrichs, and Kownatzki 1997). This system has also been pre –
sented in case studies (Fitzner, Heinrichs, and La Bouchardiere 2004). Such complex systems have been criticized because of the number of parameters to be measured (Moraes Rodigues and Emery 2008) as well
as “cost concerns and the extensive training they require” (Dorn et al. 2008). Fitzner’s classification recognizes nineteen different weathering forms and goes some way toward recording the severity of each, based
on visual inspection (Fitzner 2004). Similar, but simpler systems have been described by Massa, Naldini, and Rorro (1991) and by Vergès-Belmin (1992). Zezza (1990; 1994; 2002) has used digital image process –
ing to map different forms of surface decay. Starting with photographs and other nondestructive information, such as ultrasonic measurements, false color images are produced that identify particular forms of decay.
HOW SERIOUS IS IT?
MEASURING THE EXTENT AND SEVERITY OF DECAY
In order to make real progress, we need to quantify decay. In other
words, in addition to describing the type of decay, it is essential that we are able to measure its extent, or the area it covers; its severity, or how advanced the decay is; and the rate of decay over time. First, we need to do so in order to unravel its various causes. For example, how can we say

4 Chapte r 1
PROOF 1 2 3 4 5 6that pollution is causing decay unless we have some way of correlating
pollution levels with decay? Second, we need to have some objective means of assessing the extent and the rate of decay in order to decide whether remedial action is necessary and, if so, how urgent the need is. Third, we cannot establish whether our remedial actions are having any effect unless we can monitor the condition of the stone afterward.
If one accepts these eminently reasonable preconditions, then we
are left with a situation where extremely few monuments today (or even paintings) meet these basic conditions. Conservation decisions most often rest upon a framework of experience and general guidelines for treatment compatibility, instead of data on the actual behavior or rate of loss of the monument. Conservation documentation for the majority of our cultural heritage appears to consist of a few uncalibrated photographs taken under different lighting conditions over a few decades. Helping to fill this void with more quantitative and reproducible approaches has been the objective of many of the research projects cited in this volume: turning “weathering” or “decay” into numbers.
No single technique is sufficient to measure stone deterioration,
since decay takes many different forms. Some techniques, such as 3D laser scanning and fluorescence LIDAR (light detection and ranging), look only at the surface, and they are well suited to decay that consists of a gradual loss of surface, leaving sound stone behind. Other techniques, such as ultrasonic measurements, thermography, or magnetic resonance imaging (MRI) are designed to probe below the surface, and these are useful where decay consists of a loss of cohesion within the stone, or the development of detached layers, blisters, or internal voids.
Before using more complex methods, simple visual examination
plays an important role in quantifying decay. A single examination can convey the state of the stone at a particular moment, but it does not cap –
ture the rate of decay. For this, a series of inspections is required, usually over a period of several years. Photographs are of immense value here, but their objectivity can be abused. Winkler (1975, p. 87), for example, constructs an alarming graph of exponentially increasing decay on the basis of just two photographs. Even within a series of photographs, a fundamental difficulty is that often they have been shot under differing lighting conditions, making the interpretation of surface loss challenging (GCI and IHAH 2006; Thornbush and Viles 2008).
Two improvements in traditional photographic documentation
show promise. One is the use of time-lapse methods to provide more
frequent images in order to correlate surface loss with environmental
changes ( Sawdy and Heritage 2007 ; Doehne and Pinchin 2008 ; Zehnder
and Schoch 2009 ). The other is a new method known as polynomial
transform mapping (PTM), a subset of RTI (Reflectance Transform Imaging), that is, the use of multiple photographs from different angles to document more comprehensively the texture of stone surfaces. This gives the viewer the ability to control the angle of the light source in a given image using Java-based software ( Malzbender, Gelb, and Wolters 2001 ;
Padfield, Saunders, and Malzbender 2005 ). See examples at : http://www

Stone D ecay 5
PROOF 1 2 3 4 5 6.hpl.hp.com/news/2004/jan-mar/ptm.html and http://www.southampton
.ac.uk/archaeology/acrg/acrg_research_PTM_amazon.html .
Surface Techniques
Surface techniques for quantifying rates of stone loss include the use of
a microerosion meter, profilometry, close-range photogrammetry, laser
scanning, and laser interferometry. The microerosion meter is a simple
micrometer device that measures surface height at a number of predeter –
mined points relative to datum studs set into the stone. It was used, for example, to monitor the rate of stone decay at St. Paul’s Cathedral, London, over a twenty-year period, during which atmospheric sulfur dioxide levels in the region fell by 50 percent (Trudgill et al. 1992; Trudgill et al. 2001). Erosion rates on horizontal sites were found to have decreased from 0.045 mm/year in the period 1980–90 to 0.025 mm/year in 1990–2000.
Optical profilometry is a contact-free technique that consists of the
projection of a grid of lines onto the surface at an angle of 45°. Any irregu –
larities in the surface are immediately evident. Aires-Barros, Maurício, and Figueiredo (1994) have demonstrated its use, coupled with image analysis, to construct a weatherability index. Similar optical methods include laser triangulation, confocal microscopy, and digital holography.
A technique for monitoring surface roughness known as contact
profilometry was utilized by Jaynes and Cooke (1987) to monitor the decay of limestone when exposed to a range of different pollution envi –
ronments. It measures irregularities by means of a stylus that is drawn across the surface; movement of the stylus produces an electrical signal in a transducer.
Grissom has compared stylus profilometry, reflected-light image
analysis, and visual/tactile evaluation to assess the roughness of abrasive-cleaned stone. The results found tactile evaluation to be the “more practical and cost-effective technique” (Grissom, Charola, and Wachowiak 2000).
Close-range photogrammetry was described by Coe and others
(1992), who demonstrated that the technique was sufficiently sensitive
to detect surface loss of 0.1 mm per year over a four-year period. More recent work has pointed out the importance of human interpretation in close-range photogrammetry (Inkpen, Collier, and Fontana 2000) and has shown how to combine laser scans with close-range photogrammetry (Ressl 2007).
Asmus and co-workers (1973) were among the first to propose
the use of laser interferometry to monitor surface loss in stone. The tech –
nique has now been developed to the point where deformations as small as 0.5 microns can be detected. Laser profilometry has also been used to quantify changes in surface roughness due to laser cleaning (Colombo et al. 2007). Meinlschmidt and others (1992; 1998) have demonstrated the
use of a portable system based on electronic speckle pattern interferome –
try (ESPI) or video holography. They were able, for example, to monitor deformations that took place during the hardening of a mortar or the growth of efflorescence over a period of just a few days. Recent advances

6 Chapter 1
PROOF 1 2 3 4 5 6have made such systems less expensive and more practical in field condi –
tions (Keene and Chiang 2009) .
Many stone decay processes can be evaluated by focusing on
solution chemistry and mineral reactions. Microcatchment studies are a
useful way to evaluate the chemical dissolution of stone surfaces, where the ions in rain runoff are measured to evaluate reaction rates (Halsey
2000) . Finally, atomic force microscopy (AFM) and vertical scanning
interferometry (VSI) have been used to monitor mineral reactions and the effects of biodeterioration (Davis and Lüttge 2005; Perry, McNamara, and Mitchell 2005; Herrera, Le Borgne, and Videla 2009).
Looking Beneath the Surface
Outward appearances may be sufficient in some instances, but they can be deceptive. It is not unusual to find a stone surface that looks perfectly sound but which sounds hollow when tapped. Sooner or later, we need a way of measuring what is going on beneath the surface.
Many techniques are available and some of the more important
are reviewed by Facaoaru and Lugnani (1993). These are typically divided into in situ field methods and laboratory-based methods. Lab tests are performed on collected samples or on samples subjected to accelerated or artificial weathering.
In Situ Field Methods
Preeminent among field methods is the use of ultrasonics to
detect the presence of cracks, voids, and other inhomogeneities in stone
(Mamillan 1991; Bläuer Böhm 2004). This may take a variety of forms, such as using the longitudinal wave or the transverse component run –
ning parallel to the surface. Galán and co-workers (1992) provide an early case study demonstrating the reliability and cost-effectiveness of the technique.
The transmission of ultrasonic waves in stone depends on many
factors, and interpretation of the data is not necessarily straightforward. Valdeón, King, and De Freitas (1992) used digital analysis of the surface wave to demonstrate that wave attenuation can provide a sensitive mea –
sure of stone decay. The velocity of the longitudinal wave was a less sen –
sitive measure. Montoto, Valdeón, and Esbert (1996) have used ultrasonic tomography to investigate the internal deterioration of megaliths in northwestern Spain. The technique was useful for determining the posi –
tion of internal fissures but was less reliable at assessing the condition of stone immediately below the surface. Simon and co-workers (1994) have used formal concept analysis to optimize the interpretation of ultrasonic velocity measurements, while Mosch and Siegesmund (2007) correlated a large data set of physical stone properties with ultrasonic measurements. Weiss, Rasolofosaon, and Siegesmund (2002) found ultrasonic measure –
ments a useful method to measure degradation of marble from thermal cycling. However, because the presence of moisture can produce mislead –
ing results, it is critical that the marble be dry before ultrasonic measure –
ments are taken to ensure consistent results (Siegesmund, Weiss, and Rüdrich 2004). Ultrasonic testing has also been found useful for address –

Stone Dec ay 7
PROOF 1 2 3 4 5 6ing the difficult challenge of long-term evaluation of stone treatments
(Simon and Lind 1999 ; Favaro et al. 2006 ; Favaro et al. 2007 ).
The development and application of the drilling resistance mea –
surement system (DRMS), also known as the drilling force measurement system (DFMS), has provided an extremely useful and minimally destruc –
tive method for evaluating the condition of stone and the performance
of treatments for stone ( Lotzmann and Sasse 1999 ; Leroux et al. 2000 ;
Delgado Rodrigues, Ferreira Pinto, and Rodrigues da Costa 2002 ;
Pamplona et al. 2008 ). The system uses a portable drill and ceramic drill
bit with a sensor to measure the force needed to advance the drill bit a given distance. In principle, the DFMS can determine depth of deteriora –
tion and the penetration depth of consolidants, in situ, with a minimum of destruction (a 3 mm hole).
Ground-penetrating radar is increasingly used in archaeological
prospecting, and it is natural that its use should be extended to historic buildings (Finzi, Massa, and Morero 1992). It has seen wider application recently by a number of researchers (Binda et al. 2003; Binda, Lualdi, and Saisi 2007; Huneau et al. 2008; Palieraki et al. 2008). The method is use –
ful in detecting flaws, voids, moisture, metal straps, and the thickness of stone masonry.
Infrared thermography has been used by a wide range of
researchers (Moropoulou, Avdelidis, and Theoulakis 2003; Grinzato et al. 2004; Tavukçuoglu et al. 2005) to study moisture in stone. To provide useful results, a thermal contrast, such as solar heating or deliberate heat –
ing by infrared lamps, is often needed to identify the different surface temperatures related to differences in moisture content. This method is known as photothermal radiometry and has been developed to detect delaminations and voids (Madrid, Coffman, and Ginell 1993; Candoré et al. 2008). Most building materials have significant thermal inertia, and practitioners using thermography on a casual basis will not necessarily find useful temperature contrasts.
One interesting way to look into a stone’s subsurface for small
surface detachments is to use the sensitivity of laser holography interfer –
ometry in combination with varying sound vibrations from a loud –
speaker to map detached segments of wall paintings or stone surfaces (Castellini et al. 2003; Gulker, Hinsch, and El Jarad 2004; Keene and Chiang 2009).
A range of field evaluation methods has proven useful for quanti –
fying water uptake and surface coherence, including the sponge and Scotch tape tests (Urzï and De Leo 2001; Vergès-Belmin and Laboure 2007; Vandevoorde et al. 2009).
Laboratory-Based Methods
So far, we have considered minimally destructive techniques. It is,
of course, possible in some instances to remove samples for analysis in
the laboratory. These will often consist of core samples, which are sliced parallel to the original surface. The slices may be examined using the nor –
mal techniques for characterizing stone, such as polarized light micros –
copy, scanning electron microscopy combined with energy- dispersiv e

8 Chapter 1
PROOF 1 2 3 4 5 6spectroscopy, hygric tests, and biaxial flexural strength measurements
(Mamillan 1991 ; Snethlage, Wendler, and Sattler 1991 ; Bläuer Böhm
2004 ). Surface hardness measurements may be useful, and the salt con –
tent of the slices may also be determined (Bläuer Böhm 2005) .
The European Commission (EC) projects COMPASS and
DESALINATION have developed a simple test for salt content based
on the hygroscopic moisture content (HMC) (Gonçalves and Delgado Rodrigues 2006; Gonçalves, Delgado Rodrigues, and Abreu 2006; Nasraoui, Nowik, and Lubelli 2009). Kaminski (2008) has proposed an alternative gravimetric system and makes some constructive criticisms
of common aspects of the diagnosis and analysis of moisture and salts, including misleading readings from moisture meters based on electrical resistance or dialetric properties, dry drill powders showing lower than expected results, and salt solution–conditioned chambers not providing consistent conditions for HMC measurements.
Jacobs, Sevens, and Kunnen (1995) proposed the use of comput –
erized X-ray tomography (CT) to gain further insight into the internal structure of stone and the changes that occur during the deterioration of building materials. Procedures were developed to bring the resolution down to grain-size level (about 100 microns or less). Mossotti and Castanier (1990) used CT scanning to show that for Salem limestone, capillary water reached the surface except under windy conditions, when the air/water interface moved into the stone. In the past decade, resolu –
tion of the CT method has advanced significantly (Bugani et al. 2008; Cnudde et al. 2009; Ruiz de Argandoña et al. 2009); however, it is still difficult to see treatments and salts inside pores owing to the lack of
contrast and the small amount of material scanned. A promising way to overcome the limitations of x-ray CT is the use of high-speed neutron tomography (synchrotron radiation) for in situ dynamic analysis of wet –
ting/drying, moisture transport, salt development, or the curing and eval –
uation of protective coatings and consolidants within a porous stone (Vlassenbroeck et al. 2007; Cnudde et al. 2008).
The linear variable differential transformer (LVDT; also known
as a linear velocity displacement transducer) has also proven to be an important lab tool in the quantitative evaluation of the thermal and
hygric response of building materials to wetting and humidity cycles,
measuring expansion and contraction behavior on a micron scale (Martin, Röller, and Stöckhert 1999; Lombardo, Doehne, and Simon 2004; Poupeleer et al. 2006).
Nuclear magnetic resonance (NMR) imaging (also known as
MRI) of building materials has advanced rapidly in the last fifteen years. This method allows the measurement of hydrogen and sodium ions in solutions present inside porous materials and has provided an important new dimension for understanding the behavior of moisture in building materials, especially at the millimeter to centimeter scale (Pel, Kopinga, and Brocken 1996; Pel, Huinink, and Kopinga 2002; Rijniers et al. 2004; Huinink et al. 2006; Gonçalves, Pel, and Delgado Rodrigues 2009). For example, when a stone has finer pores than an overlying plaster, NMR

Stone Decay 9
PROOF 1 2 3 4 5 6has shown that if both layers are fully saturated with water at the start of
a drying experiment, the stone will dry after the plaster and soluble salts in the stone will tend to be retained.
All the Information We Need?
With such sophisticated forms of investigation being pursued, one might be forgiven for thinking that no problems remain in the measurement of stone decay. There is, however, a long way to go. Stone decay is a com –
plex phenomenon, and no single technique can disentangle and quantify its causes and effects. Advances in experimental work, field measure –
ments, and theory—each building on the other—are still needed. The techniques that we have looked at thus far are certainly useful, but
the methodical measurement of decay and our understanding of decay processes over time have not yet met the goal set forth earlier of conser –
vation decisions being based on measurements instead of assumptions.
CAUSES OF DECAY
Before we can take any action to prevent or to remedy the deteriora-
tion of stone, we must understand what is causing that deterioration.
Sometimes the cause is obvious; sometimes there may be several differ –
ent causes acting at once. In an attempt to clarify the relative impor –
tance and interdependency of individual causes, Verdel and Chambon
(1994) have introduced the principles of system dynamics.
1 Stone decay
mechanisms and rates are reviewed in the proceedings of two Dahlem meetings ( Doehne and Drever 1994 ; Viles 1997 ), and both reports point
out areas where additional research is needed, essentially providing use –
ful road maps for research. An interesting example of quantifying the relative importance of a range of factors—in this case for absorption and desorption of moisture—is the careful research by Sawdy ( 1995 ;
2002 ). She found, for example, that for environmental control of salt
decay in wall paintings, relative humidity (RH), airflow, substrate type, and temperature are important factors, while earlier research had emphasized only RH.
Some of the causes of stone decay are sudden and rapid in their
effect. Those toward the latter part of the following list are slow and more insidious: earthquake, fire, flood, terrorism, vandalism, neglect, tourism, previous treatments, wind, rain, frost, temperature fluctuations, chemical attack, salt growth, pollution, biodeterioration, intrinsic factors, and so on.
The literature includes many papers dealing with the causes of
decay and some reviews are available, e.g., Ashurst and Dimes 1998 ;
Honeyborne 1998 ; Grassegger 1999 ; Feilden 2003 ; Smith, Gómez-Heras,
and McCabe 2008 . Goudie and Viles (2008) trace the remarkable history
of the study of physical, chemical, and biological weathering. Recent lit –
erature is dominated by three topics: air pollution, salts, and biodeterio –
ration. These are considered in the following sections.

10 Chapt er 1
PROOF 1 2 3 4 5 6Air Pollution
Air pollution is, to the minds of many, the prime culprit in stone decay. Everyone has heard of acid rain, and it is easy to conjure up an image
of old buildings slowly dissolving in the rain. Needless to say, the true
situation is a good deal more complex, as reviews of the role of air pollu –
tion and soiling in stone decay have found ( Charola and Ware 2002 ;
Mitchell and Searle 2004 ; Brimblecombe and Grossi 2007 ; Siegesmund,
Snethlage, and Ruedrich 2008 ). The emphasis of these studies has largely
been on limestone, marble, lime mortars, and carbonate-cemented
sandstones, as these are the most vulnerable to acidic pollution. However,
soiling from atmospheric particulates is a universal problem for all types of stone.
Until recently, all the attention was given to the direct effects of
air pollutants on stone, and research focused on the “traditional” pollut –
ants: sulfur oxides, nitrogen oxides, and carbon dioxide. All are naturally occurring, although human activity has greatly increased the amounts that are to be found in urban areas, as well as significantly increasing background levels of pollution in rural areas. All are capable of dissolv –
ing in water to create an acidic solution and so are capable of reacting with calcareous materials.
The direct effects of air pollution on stone received enormous
attention from the mid-1970s to the early 1990s. This is due, at least in part, to concerns about the effects of pollution on health, agriculture, and the environment. Stone research in Western Europe and the United States was able to ride on the back of these concerns and to benefit from the funding of large research programs.
2
Since the early 1990s, priorities have shifted as progress has
been made in reducing sulfur dioxide (SO2) levels in major metropolitan
areas in Western Europe and the United States. Consequently, funds for research on air pollution on stone have steadily decreased and a number of larger programs have been discontinued altogether. Infrastructure and research groups, originally dependent on these large programs for the development of laboratories and funding for students, must now try to survive where there is no longer any state-supported program of research. Germany, for example, has had no federal support for stone conservation research since 1998.
In spite of the funding decrease, several conferences over the past
decade have addressed important open questions regarding the impact
of air pollution on rates of stone soiling and decay. One grew out of a EC-funded project
3 (Saiz-Jimenez 2004) . Another set of conferences
grew out of SWAPNET (Stone Weathering and Atmospheric Pollution Network), a group of researchers focused on the topic of stone decay
in polluted environments that started meeting at University College London in the late 1980s. Since 1993 SWAPNET has held twelve meet –
ings, mostly in the UK ( Jones and Wakefield 1999 ; Mitchell and Searle
2004 ; Smith and Warke 1996 ). The most recent SWAPNET meeting was
in Malta in 2007 (Gómez-Heras 2007) , which reported progress in under –
standing the rapid decay of certain stones affected by air pollution.

Stone Decay 11
PROOF 1 2 3 4 5 6Damage to stone by air pollution is still an important problem in
parts of central Europe, China, India, Russia, and other industrialized
regions (Larssen et al. 2006) . For example, while scrubbers were installed
to reduce SO2 near the Taj Mahal, a lack of water, power outages, and the
corresponding use of diesel generators were found to reduce the effective –
ness of the scrubbers and decrease air quality near the site. This outlines the importance of infrastructure development to monument health (Hangal and Harwit 1997) . In the past decade the rapid development of
India and China’s economies has in some measure been built on burning coal. This raises concerns for human health and corresponding concerns for well-known monuments, through both the direct and indirect effects of pollutants ( Xingang Liu et al. 2008 ; Thakre, Aggorwal, and Khanna 1997 ;
Zhao et al. 2007 ).
There is a general perception that air pollution is a modern prob –
lem, but Brimblecombe ( 1992 ; 2001 ) has shown that it is a problem that
dates from antiquity. By examining the effects of pollution on individual historic buildings over periods of several hundred years, he has also attempted to correlate pollution levels with observed damage. This links in with another widespread perception: that decay rates are accelerating rapidly, despite falling levels of several major pollutants. There are insuffi –
cient data to prove conclusively whether this is indeed the case. It is pos –
sible that the perception is due largely to an increasing public awareness of the problem and to the fact that stone loss through pollution is cumu –
lative. Also, the reaction products of air pollution, such as soluble salts, often remain on sheltered stone surfaces and result in ongoing damage.
The direct effects of acidic pollutants on calcareous stones depend
very much on the immediate environment of the stone. If the stone is in an exposed position where it is regularly washed by rain, the reaction products are washed away and the surface of the stone gradually recedes. If, however, the stone is in a relatively sheltered position, the reaction products accumulate and may form dense black crusts on stone surfaces.
A great deal of research, particularly in Italy, has been concerned
with the nature and the origins of the black crust ( Camuffo et al. 1982 ;
Del Monte 1992 ; Fassina 1992 ; Ausset et al. 1992 ). These studies have
shown that carbonaceous particulate pollution resulting from the com –
bustion of fossil fuels in electrical power generation is responsible for the blackness of the crust. More important, however, is the discovery that
the particles are not passive prisoners in the crust: they contain metal oxides that catalyze the oxidation of sulfur dioxide and hence promote formation of the crust in the first place (McAlister, Smith, and Török
2008) . While greater attention has now been paid to treating or removing
these crusts, current research on black crusts has largely confirmed the earlier studies. Isotopic analysis has been useful in isolating the generally anthropogenic sources of black crust (Pr ˇikryl et al. 2004; Vallet et al. 2006; Siegesmund et al. 2007).
In an attempt to clarify the growth mechanism, Schiavon (1992)
has studied the “stratigraphy” of black crusts. He concludes that growth occurs in two directions: inward and outward with respect to the original

12 Chapte r 1
PROOF 1 2 3 4 5 6stone surface, but with inward growth predominating. Vergès-Belmin
(1994) proposed a three-step process to explain the inward and outward
formation of the black crust, making a distinction between a clear gyp –
sum layer, growing inward through pseudomorphic replacement, and a
dark one, which is a deposit, thus developing on the surface of the stone
and growing outward. Work by Toniolo, Zerbi, and Bugini (2009) divides
black crusts into three types, with marble substrates exhibiting differen –
tial preservation beneath each type.
While the vast majority of research on black crusts has focused
on carbonate stone, interesting research on silicate stones has found high sulfation rates associated with diesel soot and accelerated rates of granite kaolinization associated with black crusts ( Simão, Ruiz-Agudo, and
Rodríguez-Navarro 2006 ; Schiavon 2007 ). In certain cases, black crusts
forming on granites appear to be geochemically unrelated to the substrate and are thus accumulated from atmospheric (Silva et al. 2009) and bio –
genic sources (Aira et al. 2007) .
Diakumaku and others have observed that some black fungi pro –
duce small spherical particles that might, under some circumstances, be confused with fly ash (1995) . Microflora are also capable of producing
sulfates. In the opinion of these authors, the formation of some black crusts in unpolluted environments may be attributable to biological fac –
tors. In addition, Ortega-Calvo and co-workers (1995) have demonstrated
that sulfate crusts may provide an ideal habitat for some cyanobacteria through the gradual dissolution of the sulfate. Work by Mansch and Bock
(1998) found greater concentrations of nitrifying bacteria and greater
stone decay rates associated with air pollution and black crusts. Gonzales-
del Valle and others (2003, 219) have found that “building stones host an active microflora that degrades fossil fuel derivatives.” Schiavon, Chiavari, and Fabbri (2004) found organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), which appear to represent markers for present-
day vehicular pollution, on the limestone walls of Emmanuel College, Cambridge, UK. It seems likely that the accumulation of PAHs (Hermosin, Gaviño, and Saiz-Jimenez 2004) provides a food source for microbial communities (Saiz-Jimenez 1995; also see biodeterioration section). Despite the extensive research already carried out on black crusts, our understanding is not yet complete.
Another important area of research has been centered on the rate
of decay attributable to pollutants and on the likely effect of reductions in pollution levels—a crucial issue for policy makers. For example, what would be the overall savings in building maintenance costs if sulfur
dioxide levels were reduced by 10 or 20 percent? Several authors have
addressed this issue through the use of damage functions, which are mathematical expressions that attempt to express the rate of stone decay as a function of several different variables. Although the damage func –
tions differ in detail, a fairly consistent overall picture emerges ( Lipfert
1989 ; Reddy 1990 ; Benarie 1991 ; Butlin et al. 1992 ; Livingston 1992 ;
Webb et al. 1992 ; Meierding 1993 ; Livingston 1997 ; Viles et al. 1997 ;
Schreiber and Meierding 1999 ). The rate of decay depends largely on
three factors: pollution levels, rainfall acidity, and amount of rainfall.

Stone Decay 13
PROOF 1 2 3 4 5 6Some intriguing findings have led to a better understanding of the inter –
play between material, environment, and weathering rates: for example,
tropical weathering has been found to be less detrimental to marble tombstones than an acidic, polluted atmosphere (Meierding 1993 , 2000 ).
Some authors argue that sulfur dioxide levels in certain cities
have decreased to the point where sulfur dioxide is no longer a major contributor to decay. In other words, there may be a “safe level” of around 30 μg/m
3, below which sulfur dioxide is not a significant contrib –
utor to decay (Sharma and Gupta 1993).4 This view is not universally
upheld, with some experts finding that for many pollutants there is no safe threshold and that resulfation of cleaned monuments is proceeding apace in some places.
5
One area where consensus is emerging is in the relative impor –
tance of wet and dry deposition. Where sulfur dioxide levels are high (urban areas), dry deposition appears to predominate on vertical surfaces. On horizontal surfaces and in rural areas, wet and dry deposition may
be of comparable importance (BERG 1989; Butlin 1991; Furlan 1992; Cooke and Gibbs 1993; Charola and Ware 2002). According to Grossi and Murray (1999), stones with a high specific surface area and/or a deli –
quescent salt content were found to promote more nitrogen oxides (NO
x)
dry deposition.
Some recent findings concerning the effects of air pollution have
been unexpected, such as the observation of a decrease in dissolution from stone surfaces blackened with diesel soot as measured in micro –
catchment studies, apparently due to a higher mean surface temperature resulting in faster drying (Searle and Mitchell 2006). This counterintuitive result suggests the importance of “time of wetness” in the damage to stone, as confirmed by earlier research (Charola and Ware 2002). Recent decay of marble in New York was evaluated using mass balance methods sensitive enough to detect a 2 nm surface loss (Livingston 2008). Dissolution was found to be mostly due to gypsum dissolution originat –
ing from dry deposition with less contribution from karstic processes due to carbonic acid or from neutralization of acid rain. This result is in con –
trast to earlier US National Acid Precipitation Assessment Program (NAPAP) studies, which found carbonic acid responsible for approxi –
mately 70 percent of carbonate dissolution (Baedecker and Reddy 1993).
Despite significant cleaning campaigns in many European capi –
tals, the soiling rates of historic structures remain high, apparently due to a substantial increase in diesel emissions (Grossi et al. 2003; Searle and Mitchell 2008). Recommendations for human health and monument health include increasing the distance between diesel emissions and important sites such as schools and monuments (Nord and Holenyi 1999; Sagai, Furuyama, and Ichinose 1996; Qinghua Sun et al. 2005).
What are the issues that have still to be addressed? They include
the following:
• What is the role of high nitrog en oxide levels on stone decay?
The substantial increase in vehicular emissions of nitrogen oxides (NO
x) has not resulted in acid attack on the same scale

14 Chapter 1
PROOF 1 2 3 4 5 6as SO2. Yet despite several studies, the situation is still unclear.
Some authors have found synergistic effects for NOx on SO2
reactions, while others have not (Kirkitsos and Sikiotis 1995;
Sikiotis and Kirkitsos 1995; Massey 1999; Searle and Mitchell 2006; Allen 2007; Metaxa et al. 2009). It seems part of the issue is that a catalytic effect for NO
x on SO2 is present in dry
conditions, but not in wet (Bai, Thompson, and Martinez-Ramirez 2006). In a larger context, research is showing that the geochemical cycle of nitrogen is being altered in ways, including the impact on historic stone, that are still poorly understood (Gruber and Galloway 2008).
• What is the mechanis m by which sulfur dioxide is oxidized to
produce sulfuric acid? Does oxidation take place before the pollution reaches the stone, or is it catalyzed by other pollut –
ants on the surface of the stone? Is the oxidation catalyzed by other air pollutants, such as ozone, nitrogen oxides, or die –
sel soot (see, for example, Rodríguez-Navarro and Sebastian 1996)? Do bacteria in the stone play a part?
• To what extent are today’s decay rates influence d by pol-
lution levels of the past (the memory effect)? For example, sulfate and nitrate salts that are already present in the stone will continue to cause damage even if further pollution were eliminated altogether. The “memory effect” story is not yet complete (Vleugels, Dewolfs, and Van Grieken 1993; Pr ˇikryl and Smith 2007).
• Recent research has examined the role of formates , acetates ,
and airborne microbes ( Kumar et al. 1993 ; Grossi et al. 2003 ;
Gibson et al. 2005 ; Maruthamuthu et al. 2008 ). Are other
important pollutants being overlooked?
• What are the relative roles of carbonic acid versus sulfuric ,
nitric, or other acidic species? This is an issue that still remains controversial (Baedecker and Reddy 1993; Charola and Ware 2002).
More recently, the focus has shifted away from the direct effects
of pollutants to their indirect effects. Carbon dioxide, generally regarded as a minor culprit where direct effects are concerned, now takes center stage. It is regarded as the primary cause of climate change, and the impact of climate change on the built heritage may far exceed the direct effects of pollutants—severe though they may be.
International concern over climate change and global warming
continues to grow. Because the impact on people is the primary concern, it is easy to think that stone monuments will be immune to global warm –
ing of just a few degrees. This is not the case, however, and recent studies have started to demonstrate the widespread impacts of climate fluctua –
tions such as floods, droughts, and humidity cycles ( http://noahsark.isac
.cnr.it/ ) (Cassar 2005; Sabbioni et al. 2006). For example, concern has
been expressed that an increase in biodeterioration of stone in Scotland can be expected due to higher temperatures and rainfall (Duthie et al. 2008). And in central Europe, the yearly number of humidity fluctuations

Stone D ecay 15
PROOF 1 2 3 4 5 6crossing the deliquescence point of sodium chloride (~75 percent RH) are
projected to increase two- to four-fold by the end of the century due to drier summers, which is likely to increase damage from salt crystallization (Brimblecombe and Grossi 2007 ; Grossi et al. 2008 ). Climate change is a
very real threat to our monuments and cannot be ignored.
Salts
Along with air pollution, soluble salts represent one of the most impor –
tant causes of stone decay. Salts cause damage to stone in several ways. The most important is the growth of salt crystals within the pores of
a stone, which can generate stresses that are sufficient to overcome the stone’s tensile strength and turn the stone to a powder. The deterioration of many of the world’s greatest monuments can be attributed to salts, from Angkor Wat (Siedel, von Plehwe-Leisen, and Leisen 2008) to Venice
(Lazzarini et al. 2008) , and from Petra (Simon, Shaer, and Kaiser 2006) to
the Great Sphinx of Giza (Reed 2002) .
There are many ways in which stonework can become contami –
nated with salts. Air pollution is a major source of sulfates and nitrates. Other sources include the soil, from which salts may be carried into masonry by rising damp; salts blown by the wind from the sea or the des –
ert; deicing salt; unsuitable cleaning materials; incompatible building mate –
rials; garden fertilizers; and, in the case of some medieval buildings, the storage of salts for meat preservation or even for gunpowder.
The growth of damaging salt crystals is usually attributable to
crystallization, caused by the evaporation or cooling of salt solutions within the stone. In the past, there was much reference to “hydration damage,” building on the fact that some salts can exist in more than one hydration state. The prime example is sodium sulfate, one of the most damaging of soluble salts, which can exist as the anhydrous salt thenardite (Na
2SO4) or the decahydrate mirabilite (Na2SO4·10H2O)
(Doehne 2003; Espinosa Marzal and Scherer 2008a). Thenardite increases in volume by more than three times on conversion to mirabilite, and it has been argued that this growth in volume was the cause of so-called hydration damage. However, it is now recognized that a crystal cannot magically transform from one form to the other without first dissolv –
ing and then recrystallizing in the new form. Hydration damage thus becomes a special case of crystallization damage (Doehne 1994; Flatt and Scherer 2002; Flatt 2006). Having said that, it is now becoming recog –
nized that the sodium sulfate system presents yet further challenges, with researchers demonstrating the crystallization of the metastable heptahy –
drate (Na
2SO4·7H2O) in preference to mirabilite in some circumstances
(Hamilton, Hall, and Pel 2008; Saidov and Pel 2008).
Salt damage does not occur only in an outdoor environment,
where the stone is subjected to cycles of rainfall and subsequent drying. It can also take place indoors, through the hygroscopic action of the salts. Severe damage to stonework held in uncontrolled museum environments is not uncommon (Hanna 1984; Rodríguez-Navarro et al. 1998).
On first sight, it appears surprising that salt damage should occur
at all. Crystallization, for example, results in the formation of crystals that occupy a smaller volume than the solution from which they have

16 Chapte r 1
PROOF 1 2 3 4 5 6been deposited. Is there not ample room for the crystals to develop in
the pores, without the necessity of forcing the pore walls apart? However,
this simplistic view overlooks the dynamic aspects of stone decay (Yu and
Oguchi 2009) . A stone may be fed constantly with salt-bearing moisture
from the soil, for example, so that salts are constantly accumulating at the point of evaporation. Detailed analyses of this situation are given by Lewin (1982) and by Hall and Hoff (2007) and in a useful new book
by the Italian engineer Edgardo Pinto Guerra , Risanamento di murature
umide e degradate (Restoration of Damp and Deteriorated Masonry
Walls) (2008) . Work in Rhodes shows that the amount of salt is cor –
related to the severity of damage to the stone (Stefanis, Theoulakis, and
Pilinis 2009) .
Several tables of salt levels that are considered potentially
hazardous for porous materials have been published in Germany (Wissenschaftlich-Technische-Arbeitsgemeinschaft für Bauwerkserhal-tung und Denkmalpflege e.V. 1999) , Austria (Österreichisches
Normungsinstitut [ON] 2006) , and France (Ministère de la culture et de
la communication 2003) . Simply measuring the concentration of salt in
stone captures only part of the issue, since substrate characteristics (resis –
tance to salt weathering) as well as the severity and frequency of envi –
ronmental fluctuations are important in determining rates of salt damage (Doehne 2002). Any proposed international norm for salt levels in porous materials would have to take these factors into account, in addition to addressing the issue of identifying a method for measuring salt levels in building materials that is less costly than ion chromatography.
Modeling by Hall, Hoff, and Hamilton (2008) shows that in the
UK rising damp can typically transport several hundred liters of moisture per year, per linear meter of stone, which can easily result in the accumu –
lation of salts even from dilute groundwater solutions. The accumulation of salts and whether they crystallize on the surface or as a subflorescence has been related to the interfacial properties (wetting) and to the trans –
port properties of the liquid. For example, De Witte (2001) and Miquel and others (2002) have clearly shown that subflorescence can develop at the interface between treated and untreated stone, and subsequent con –
tour scaling can be due to the presence of water repellents. In lab experi –
ments, Shahidzadeh and others (2008) have confirmed that interfacial properties are of key importance for where and how the crystals form. Pel, Sawdy, and Voroninaa (2010) have described the Peclet number
6 as a
useful parameter that relates the rate of advection of a flow to its rate of diffusion in building materials . When advection dominates, salts will tend
to accumulate at the surface of a stone. When diffusion dominates, ions will be more widely distributed.
There have been major advances in our understanding of salt
weathering over the past fifteen years (Rodríguez-Navarro and Doehne 1999), although research into ways to prevent, mitigate, and treat the problems has lagged somewhat behind. The first big advance is related to the behavior of solutions containing more than one salt—the situation that is almost invariably found in practice. It is a straightforward process to predict the environmental conditions under which a single salt will

Stone Dec ay 17
PROOF 1 2 3 4 5 6pick up moisture from the air and subsequently lose it (causing damage
by crystallization). However, the conditions under which salt mixtures will cause damage are much more difficult to predict and entails thermo –
dynamic modeling. This work has advanced in several steps ( Steiger and
Zeunert 1996 ; Price 2000 ; Steiger 2006 ; Sawdy and Heritage 2007 ; De
Clercq 2008 ; Franzen and Mirwald 2009 ). In an example that highlights
the importance of understanding material behavior, recent work has shown that the type of salt is critical in determining if damage may occur. A pillar at Angkor Wat with severe erosion at its base was found to contain the same amount of salt in damaged and undamaged areas, leading to questions about whether salts were or were not the cause of the damage. Thermodynamic calculations subsequently showed that there were differences in the salt type present that explained the damage pattern, with highly hygroscopic salts that did not crystallize often being present in the undamaged zone and salts that crystallized frequently being present in the damaged zone (M. Steiger, personal communication). Computer programs can now predict the “safe” ranges of temperature and relative humidity in which crystallization damage may be minimized (Sawdy and Price 2005; Simon and Doehne 2006b; Price 2007; Steiger, Kiekbusch, and Nicolai 2008). Inevitably, there are limitations, the most important being that the programs can predict only what will happen under equilibrium conditions; they say nothing about the rate at which
it will happen (Prokos and Bala’awi 2008).
The second important area of research is concerned with the mech –
anism by which damage occurs. Some of the papers are quite daunting, but the ideas are essentially quite simple (Hamilton and Hall 2004; Espinosa Marzal and Scherer 2009). Consider a crystal bridging a pore and exerting a pressure on the pore walls. If it is to grow any further, and thereby do damage, it is necessary for the surrounding solution to be able to get in between the crystal and the pore walls. If the pressure gets so high that this solution is squeezed out, no further growth can occur and there will be no damage. There is therefore a tug of war (or perhaps “push of war” would be more appropriate) between various opposing forces related to the sur –
face energies of the respective stone/solution/crystal interfaces. As the
surfaces of the salt crystal and the pore wall get to within 10 nm or so,
repulsive forces arise due to the mismatched surface energy of the mineral surfaces and this keeps them apart, much like what happens when an attempt is made to push two magnets together. This mismatched surface energy can be thought of as the degree of lattice compatibility or incompat –
ibility between two minerals. NMR, AFM, thermo-mechanical analysis (TMA), and environmental scanning electron microscopy (ESEM) have provided direct evidence of the existence of salt crystallization or disjoining pressure (Rijniers et al. 2005; Hamilton, Koutsos, and Hall forthcoming; Balboni et al. forthcoming) and the process has been modeled as well (Espinosa, Franke, and Deckelmann 2008). Future work on the calculation and measurement of the actual supersaturation that occurs in a porous medium at the exact moment of salt crystallization will help in understand –
ing the widespread variability in resistance of various building materials to salt weathering. Research is continuing using synchrotron X-rays, NMR

18 Chapter 1
PROOF 1 2 3 4 5 6(Hamilton, Hall, and Pel 2008) , and differential scanning calorimetry
(DSC) (Espinosa Marzal and Scherer forthcoming) . Some additional work
on direct measurement of the disjoining pressure using AFM is also needed
to help clarify some of these issues.
The size of the substrate pores is important in salt weathering,
as shown by measurements and models developed over the past decade (Scherer 1999 , 2000 ; Steiger 2005a , 2005b ). Under equilibrium condi –
tions, a crystallization pressure can only occur in the smallest pores (less than 30 nm) (Rijniers et al. 2005) . Since most types of stone have few
pores in this range, it is predicted that most salt weathering damage takes place during nonequilibrium conditions, such as rapid drying. Another way that damage increases is when the stone pores fill with salt, which modifies the pore size distribution, essentially creating small pores where crystallization pressure can occur, even under equilibrium conditions. This may help explain the sudden onset of some salt weathering problems, since damage may not start until the pores are full of salt.
Other researchers have looked at the initial nucleation and
growth stages of crystals in pores, with a view to inhibiting nucleation
or modifying the shape and size of the crystals that form by using trace amounts of the chemicals commonly used to inhibit mineral scaling on pipes in industrial applications (Selwitz and Doehne 2002; Rodríguez-Navarro, Hernandez, and Sebastian 2006; Cassar et al. 2008; Ruiz-Agudo, Putnis, and Rodríguez-Navarro 2008). In some ways it is a risky strategy, for it has long been known that crystallization pressure is related to the degree of supersaturation of the solution from which the crystals grow. If nucleation is inhibited or postponed, this will lead to even higher levels of supersaturation, so that damage (if and when it does occur) may be more severe than it might have been. Modifiers may also behave differ –
ently when in solution than when absorbed to stone surfaces.
A further aspect of recent research concerns the role of wind in
salt weathering or alveolar weathering.
7 The formation of alveolar or
honeycomb weathering patterns is apparently due to the preferential accumulation of salt in sheltered hollows, where it was not washed away by rain as it would be in the ridges surrounding the hollows, and the
protective effects of endolithic microbes ( Laue et al. 2005 ; Siedel 2008 ;
Mustoe 2010 ). The hollows are the last place to dry, and thus the place
where salts tend to accumulate. Also, the ridges would generally be dry, while fluctuations in moisture in the depth of the cavity would result in cycles of salt crystallization and further erosion at the deepest point of the cavity. The main source of salts is thought to be wind-borne dust from nearby playas or sea-salt aerosol (Kirchner 1996).
Early laboratory work on wind’s effect on stone showed that
the boundary between air-filled pores and solution-filled pores in a
stone could be moved into the sample by placing a fan facing the stone (Mossotti and Castanier 1990). Thus, the location of salts (efflorescence or subflorescence) is due in part to the rate of air exchange at the surface of the material, and windy conditions can result in the crystallization of salts as a more damaging subflorescence, rather than as a surface efflores –
cence. More recent modeling indicates that the development of a uniform erosion pattern or a honeycomb pattern of weathering may be explained

Stone D ecay 19
PROOF 1 2 3 4 5 6by differences in the duration of drying periods (Huinink, Pel, and
Kopinga 2004) . The researchers found that short drying periods tend to
result in the accumulation of salts on the surface (resulting in more uni –
form erosion). For longer drying periods (slow evaporation rates), salts
accumulate in sheltered areas with lower evaporation rates (tending to expand pits and resulting in honeycomb patterns). Experimental labora –
tory work has also shown that wind (Selwitz and Doehne 2002) and
related rapid drying (Lombardo, Doehne, and Simon 2004) increases
damage rates due to increases in salt supersaturation, and that variable weathering rates related to wind can result in honeycomb patterns (Rodríguez-Navarro, Doehne, and Sebastian 1999) . Recent modeling of
the effect of wind on the Sphinx found that areas of rapid erosion corre –
lated with areas of high wind friction and enhanced drying (left shoulder and the top of the haunches) (Hawass 1998; Hussein and El-Shishiny 2009). Lab experiments and work at sites such as Petra in Jordan show that wind speed strongly influences the rate of damage and pattern of salt distribution (Bala’awi 2008). Pore blocking by salts also appears to be
an important factor in controlling the pattern of salt weathering damage (Espinosa Marzal and Scherer 2008b; McCabe, McKinley, and Smith 2008; Espinosa Marzal and Scherer forthcoming) and may result in greater crystallization of salts as subflorescence.
Is crystallization the only way in which salts can cause damage?
It seems not. It appears that they can also cause damage through stress from differential thermal expansion, since sodium chloride, for example, expands at about five times the rate of calcite at surface temperatures (Nocita 1987; Holmer 1998; Smith et al. 2005). Schaffer (1932) attrib –
uted this idea to Scott Russell. Salts also have a role to play in the weath –
ering of stones that contain clay minerals (Snethlage and Wendler 1997; Rodríguez-Navarro et al. 1998; Scherer 2006; Scherer and Jiménez-González 2008), in some cases enhancing the swelling potential of these stones. While most of the damage from salts is physical, work shows that salt solutions enhance the dissolution of calcite (Ruiz-Agudo, Martín-Ramos, and Rodríguez-Navarro 2007) and the alteration of biotite
(Silva and Simão 2009), quartz (Young 1987), and feldspars (Bernabe, Bromblet, and Robert 1995). And while one might expect salt weathering to have little in common with biodeterioration, recent work has found that halophilic bacteria are often present and may enhance physical dam –
age mechanisms (Laiz et al. 2000; Papida, Murphy, and May 2000).
There are several recent reviews that give further details of
research in this area. They include a thoughtful overview of the role
of salts in the deterioration of porous materials by Charola (2000) and an excellent discussion of salts and crusts by Steiger (2003). Doehne (2002) reviews the scope and interdisciplinary nature of salt weathering in a paper that brings in perspectives from conservators, geomorpholo –
gists, and cement specialists. Simon and Doehne (2006a; 2006b) summa –
rize a series of discussions and expert papers on salt weathering and masonry desalination. A special issue of the journal Environmental
Geology was devoted to salt decay with three groups of papers devoted
to salt weathering tests, salt behavior, and field studies (Steiger and Siegesmund 2007). A detailed summary of the fundamental basis of salt

20 Chapte r 1
PROOF 1 2 3 4 5 6decay mechanisms can be found in Scherer (2004) . “Salt Weathering on
Buildings and Stone Sculpture” conferences were held in Ghent in May
2007 and in Copenhagen in October 2008 (Albertsen 2008) ; the next
conference will take place in Cyprus in October 2011. Finally, a recent review of salt weathering calls for new field research on building material behavior and soluble salts (Gulotta et al. 2008) .
Closely related to the issue of salt damage is the issue of damage
from frost. The topic has been reviewed by Scherer and Valenza (2005)
and Matsuoka and Murton (2008) . In France, the standard on frost resis –
tance of natural stone (Norm XP B 10-601, see LERM 2006 ) gathers all
the tests to be performed and gives the appropriate thresholds, according to the destination of the stone in the building and according to local
climate. Created in 1984, the standard is regularly revised to fit with field
observations and climate change.
Inevitably, further questions remain. Why are certain types of
stone much more vulnerable than other types to salt damage? Why are certain salts much more damaging than others? Is damage caused mostly by relatively rare environmental events (rapid cooling, drying, or conden –
sation) or cumulative everyday stresses (humidity cycling)? What are the long-term effects of various conservation treatments, such as desalination or consolidation, on salt damage? How can desalination and preventive conservation efforts be enhanced? Can general agreement be achieved regarding the fundamental mechanisms of salt weathering? Can the salt damage process and weathering forms such as tafoni be accurately mod –
eled using existing knowledge? How does the hydration of salts progress, and how are crystallization pressures sustained in situ? And, above all,
how can the great fundamental strides of recent years be converted to practicable applications?
Biodeterioration
In 1932, in his classic report The Weathering of Natural Building Stones ,
Schaffer wrote:
Living organisms also contribute to the decay of stone
and similar materials and, although their action is, generally, of
somewhat less importance than certain of the other deleterious agencies which have been considered, their study presents numerous features of interest. The effect of certain organisms, such as bacteria, is still a matter of controversy, but the effect of others, such as the growth of ivy, is generally considered to be detrimental.
In two of these areas, he is still remarkably up to date. There is contro –
versy over the role of bacteria, and we still need to weigh the importance
of biodeterioration against the importance of other causes of decay. However, recent work on ivy suggests that the shade and thermal stability provided by ivy on stone walls may be beneficial in certain situations (H. Viles, personal communication; see also: http://www.srs.ac.uk/
scienceandheritage/presentations/Ivy_presentation2.pdf ).

Stone Decay 21
PROOF 1 2 3 4 5 6Biological growths on stone are both a blessing and a blight.
Colorful lichens and creepers, such as ivy, can contribute an air of age
and romance to a monument, and their removal can leave the stone look –
ing stark and denuded. Nevertheless, many organisms contribute to the deterioration of stone, and it is necessary to find the right balance between appearance and longevity. The discussion surrounding this topic has become more complex and nuanced, as evidence has accumulated that complex biofilms in some situations may help to stabilize fragile stone surfaces and in other cases may strongly accelerate decay ( Uchida et
al. 2000 ; Chiari and Cossio 2004 ; Caneva et al. 2005 ; De Muynck, De
Belie, and Verstraete 2010 ). For example, in laboratory experiments, bio –
films have been shown to result in a 40–70 percent decline in dissolution rates of calcite (Davis and Lüttge 2005) . In more recent work, the contri –
bution of bacteria to dissolution or protection has been related to the amount and type of “food for microbes” present, such as nitrate versus ammonium ions and organic carbon species (Jacobson and Wu 2009) .
Research on the action of biofilms on silicate stones (granite and basalt) has shown they may enhance dissolution rates in some situations ( Wu et
al. 2007 ; Wu, Jacobson, and Hausner 2008 ). While additional work is
needed, research in this area suggests that some surface patinas may be an effective natural protection for carbonate stones, while other biofilms, particularly in polluted environments, may be deleterious.
Bioremediation and biocides are related topics of recent research
that are discussed later in the section on surface treatments in chapter 2.
The biological degradation of rocks is well known and has been
studied for a long time: it is one of the weathering mechanisms responsi –
ble for the formation of soil. The deterioration of stone in buildings and monuments through the action of biological organisms has also been acknowledged since the mid-1960s, but the topic has received increasing attention over the past decade. Some of the literature is concerned pri –
marily with the influence of organisms on the appearance of stone sur –
faces, while other research deals primarily with the deterioration of the stone itself. In the past, microbiologists studying this topic have tended to focus more on characterizing the species and ecosystems found on stone and less on the nature of the effects of biological agents on stone decay. This is changing with new research on how biofilms change the thermal, hygric, and mechanical behavior of stone, thus enhancing decay by increasing the duration of surface wetting and providing a source of organic acids and complexing agents.
Excellent reviews of the topic are provided by Warscheid and
Braams (2000); Caneva, Gasperini, and Salvadori (2008); Warscheid (2008); and Scheerer, Ortega-Morales, and Gaylarde (2009). Other useful overviews are given by Wakefield and Jones (1998); Ciferri, Tiano, and Mastromei (2000); and Crispim and Gaylarde (2005). A burst of earlier reviews can be found in Gómez-Alarcon and de la Torre (1994); Jain, Mishra, and Singh (1994); May et al. (1993); and Tiano (1994). Krumbein and Urzï (1992) have set out a comprehensive terminology for describing aspects of biodeterioration on stone.

22 Chapte r 1
PROOF 1 2 3 4 5 6Much of the recent research has been centered on algae, lichens,
and bacteria. Adamo and Violante (2000) ; Jie Chen, Blume, and Beyer
(2000) ; Schiavon (2002) ; Wilson (2004) ; St. Clair and Seaward (2004) ;
and Piervittori, Salvadori, and Isocrono (2004) have reviewed the action
of lichens, confirming that their effects are both physical and chemical.
Mechanical damage is caused by penetration of the hyphae into the stone and by the expansion and contraction of the thallus (the vegetative part of the fungus) under changes of humidity. Chemical damage, how –
ever, is more important and may arise in three ways: by the secretion of oxalic acid, by the generation of carbonic acid, and by the generation
of other acids capable of chelating ions such as calcium. Field examples of damage from lichens to stone monuments have recently been described in contexts ranging from Persepolis to the Alhambra palace and the Jeronimos Monastery ( Mohammadi and Krumbein 2008 ; Sarró et al.
2006 ; Ascaso et al. 2002 ).
The secretion of oxalic acid, which reacts with a calcareous stone
to produce calcium oxalate, is of particular interest. A number of authors have noted the presence of calcium oxalate on the surface of stone monu –
ments, where it can form part of a coherent and seemingly protective layer known as scialbatura . Del Monte and Sabbioni (1987), for example,
have argued that scialbatura is caused solely by lichen activity, whereas
Lazzarini and Salvadori (1989) have enumerated other possible causes, including the deliberate application of a protective coating. Correlating the environmental limits for lichen growth with the distribution of
oxalate on Trajan’s Column, Caneva (1993) found that the oxalate dis –
tribution pattern was the opposite of that expected for lichens and thus lichens were perhaps not the best explanation for the column’s patina. Analysis of rock outcrops suggests that some oxalate patinas may be rel –
ics of past paleo-environments that were more suitable for lichen growth during an interval of greater surface moisture (Moore et al. 2000). Subsequent experimental work has shown that the alteration of an organic coating can result in the formation of calcium oxalate (Cariati et al. 2000). Analysis of oxalate patinas in the field has also provided sup –
port to the idea that microbial alteration of organic material contributes to oxalate formation (Casoli and Palla 2002). In addition, Monte (2003) has performed experiments showing oxalate can be produced by the action of fungi alone on marble. More on biomineralization can be found in chapter 2.
Researchers have found a range of microbes that are endolithic—
colonizing the interior of porous stone (Walker and Pace 2007). There they take advantage of the light, moisture, and shelter found inside the stone (Caneva, Gasperini, and Salvadori 2008) and may also modify their surroundings (McNamara et al. 2006). It seems clear that the presence of endolithic communities should be assumed in many environments and stone types. Further study is needed of their role in stone alteration, their modification of moisture transport, and their interaction with conserva –
tion treatments such as consolidants and biocides.
The heavyweight controversy is saved for bacteria. They have
long been implicated in stone decay, but acceptance of their role has sometimes been hindered by the emotive stance of some researchers, who

Stone Decay 23
PROOF 1 2 3 4 5 6have appeared determined to see bacteria and nothing else. A troubling
number of authors have noted high numbers of bacteria in decaying stone, in comparison to low numbers in sound stone, and have concluded that the bacteria cause the decay. However, an alternative explanation could be that decayed stone presents a preferred habitat for the bacteria.
Bacteria that attack stone chemically fall into two groups: auto –
trophic bacteria derive their carbon from carbon dioxide (CO
2), and may
derive their energy from light (photolithotrophs) or from chemical redox reactions (chemolithotrophs). Heterotrophic bacteria, by contrast, utilize organic compounds on the stone to derive their carbon. Autotrophic bac –
teria include those that are capable of oxidizing sulfur and nitrogen com –
pounds to produce sulfuric acid and nitric acid, respectively. They are one more means, therefore, by which air pollutants such as sulfur dioxide and nitrogen oxide are turned into sulfates and nitrates. This underlines the difficulty of separating out the individual causes of stone decay; sev –
eral different factors may play integral roles in the overall decay process.
The question remains of whether bacteria or catalyzing metal com –
pounds, for example, are the main routes of sulfate production. How-ever, if oxidation by both bacteria and metal compounds is rapid by comparison with the rate at which sulfur dioxide arrives at the stone surface, then the arrival rate will be the rate-determining step, not the route taken. The synergistic effects of air pollution and biofilm forma –
tion have been researched, with the finding that there is strong evidence that biofilms enhance the absorption of air pollutants ( Young 1996 ;
Mansch and Bock 1998 ).
Heterotrophic bacteria produce chelating agents and organic
acids that are weaker than the inorganic acids produced by the sulfur-oxidizing and nitrifying bacteria. They have received comparatively little attention, but their role in deterioration is well established nonetheless (Lewis, May, and Bravery 1988 ; Saarela et al. 2004 ; Gorbushina 2007 ;
Maruthamuthu et al. 2008 ).
Recent work on bacteria has helped to quantify the effect they
have on the dissolution of limestone, with one example showing a two-fold increase in the laboratory dissolution rate compared to control lime –
stone samples (McNamara et al. 2005). Bacteria typically produce slime or extracellular polymeric substances (EPS) as part of a complex biofilm made up of polysaccharides, water, and proteins that has been shown to change the dissolution rate and dissolution pit morphology on samples of limestone (Perry et al. 2004). Damage from bacteria in the field has been described by McNamara and others (2006) and Mansch and Bock (1998).
Biodeterioration studies of important cave painting sites such as
Altamira have resulted in recent advances in understanding. Researchers have found that cyanobacteria and algae (phototrophs) and networks of heterotrophic bacteria increase stone deterioration through their meta –
bolic products, biomediated dissolution, and mechanical alteration, such as scaling (Cañaveras et al. 2001). As expected, the control of moisture, food, and light levels appears to be the most effective prevention method (Dornieden, Gorbushina, and Krumbein 2000; Zammit et al. 2008).
Environmental control of cave environments has proven to be
complex and controversial, as in the cave at Lascaux, where efforts to

24 Chapte r 1
PROOF 1 2 3 4 5 6limit the introduction of fungal strains through the use of a formalde –
hyde foot-wash treatment for visitors resulted in the growth of a
formaldehyde-resistant strain of white Fusarium solani fungus ( Bastian
et al. 2007 ; Dupont et al. 2007 ; Jurado et al. 2009 ; Bastian and
Alabouvette 2009 ; Bastian et al. 2009 ; Bastian, Alabouvette, and Saiz-
Jimenez 2009 ). This condition may have been exacerbated by the installa –
tion of a new ventilation system ( Brunet, Malaurent, and Lastennet 2006 ;
Lacanette et al. 2009 ). Computer modeling of the airflow at the Lascaux
cave suggests that reducing the airflow may help avoid future damage
(Malaurent et al. 2007) .
The role of halophilic microbes (mostly archaea, with some bacte –
ria) is important in stone decay (Laiz et al. 2000; Saiz-Jimenez and Laiz 2000). A significant and open question is if hydroscopic salts may raise moisture levels to the point where halophilic microbes increase in abun –
dance, setting the stage for further microbial development of adjacent areas of stone.
Differential Stress
While air pollution, salts, and biodeterioration capture the lion’s share
of attention, there are advances in our understanding of other, often related decay mechanisms that are worth some consideration. Reviewing the recent literature on stone conservation, it is clear that there is an important trend in decay mechanism research that is focusing on what
is called here (for want of a better term) “differential stress.” This decay mechanism includes the effects of wet/dry cycling, clay swelling, differen –
tial hygric stress, differential thermal stress, and stress from differential
expan sion rates of material in pores (such as salts or organic material)
versus in the stone. The general idea is that treatments, salts, water films, or biofilms—anything that causes the stone surface to react differently than the interior—can result in a shear stress, crack propagation, and,
eventually, surface parallel detachment (e.g., flaking). For example, sig –
nificant shear stress is generated when, during a brief afternoon rain, the surface of a clay-containing stone swells, while the interior of the stone remains dry (Doehne et al. 2005). This would be considered an example of differential hygric stress and is typically found on the corners of stones such as Sydney sandstone and Portland brownstone. As mentioned earlier, sodium chloride expands at approximately five times the rate of calcite at surface temperatures, so decay in limestone from this mechanism would be an example of stress induced by differential thermal expansion (Nocita 1987; Holmer 1998; Smith et al. 2005). Note that salts naturally tend to accumulate near the stone surface, setting up differences in how the two parts of the stone (surface and interior) react to environmental changes. Modeling has shown that there may be a particular depth beneath the stone surface where moisture is present and salts may accumulate (Snethlage and Wendler 1997). This depth is often the same as the thick –
ness of stone flakes or scales. Differential thermal expansion stresses may also be induced at the interface between minerals having different colors, for instance in granites exposed to direct sunlight (Casta 1988). Field measurements of stone surfaces show that rapid thermal variations are more common than previously thought (Molaro and McKay 2010).

Stone Decay 25
PROOF 1 2 3 4 5 6Organic material in pores, whether it is a polymeric consolidant
or originating from biological sources, may expand significantly faster
than the stone on wetting ( Laurenzi Tabasso 1995 ; GCI and IHAH 2006 ).
Work by Yang, Scherer, and Wheeler (1998) highlighted the importance of
making sure consolidants have thermal expansion properties similar to the substrate being treated. Some recent work on thermal damage to stone reveals that it is an important decay factor, and stress may result from differential heating, such as when areas of stone undergo short-term cooling events from shade ( Weiss et al. 2004 ; Gómez-Heras, Smith, and
Fort 2006 ; Hall and André 2006 ; Sumner, Hedding, and Meiklejohn
2007 ; Gómez-Heras, Smith, and Fort 2008 ). This effect may be more pro –
nounced at high-altitude sites such as Tiwanaku in Bolivia (Maekawa,
Lambert, and Meyer 1995) , where the drop in temperature when a cloud
blocks the sun is substantial.
Work by Warke and Smith (1998) found that climate chamber
simulation studies do not take into account the important effects of radi –
ant heating and thus are not representative of field conditions. The bal –
ance between thermal and hygric damage is addressed in work at Petra by Paradise ( 2002 ; 2005 ). Research on clay swelling has advanced signifi –
cantly based on the research at Princeton University ( Wangler and Scherer
2008 ; Duffus, Wangler, and Scherer 2008 ; Jiménez-González, Rodríguez-
Navarro, and Scherer 2008 ), where it was found that shear forces can
cause buckling of wetted stone surfaces and that intracrystalline swelling of clay is the primary mode of swelling for Portland brownstone, despite the proportion of swellable clay being only 1 percent of the stone. The clay is present as a cement at sand grain boundaries, permitting the clay sufficient leverage in brownstone. Osmotic swelling (salt-activated
clay swelling) was found to be important in sepiolite-rich Egyptian lime –
stone (Rodríguez-Navarro et al. 1998) . Understanding the relative role
and dynamics of differential stress as it relates to air pollution, biodeteri –
oration, salt weathering, and conservation treatments remains an area for future research.
Intrinsic Problems
“Intrinsic problems” (or “inherent vice”) is an expression that places the blame for stone decay squarely on the material, rather than the particular environment. Every region seems to have a stone that ought to have remained in the ground, rather than being used to create sculpture, monu –
ments, and buildings. Some examples include Reigate stone in the UK (which contains unstable silica, glauconite, and smectite), Lausanne molasse in Switzerland (containing swelling clays), and the Lecce lime –
stone of Italy (with a porosity of 50 percent). These “difficult stones” appear to account for a disproportionate share of stone conservators’ attention. Recent research into these problematic stones includes several studies of Lecce and similar highly porous limestones (Fratini et al. 1990; Calia et al. 2004; Atzeni, Sanna, and Spanu 2006).
As far as Reigate stone is concerned, decay of this material is not
a new phenomenon. Reporting in 1713 on the condition of Westminster Abbey, Sir Christopher Wren wrote, “the Ashlar of the whole fabric . . . is disfigured in the highest degree . . . and the stone is decayed 4 inches deep

26 Chapte r 1
PROOF 1 2 3 4 5 6and falls off perpetually in great scales.” He comments wryly that Reigate
“is a stone that would saw and work like wood, but not durable, as is manifest” (as quoted in Prudon 1975 ). An alternative point of view notes
that the decay of Reigate stone is mainly confined to surface layers and has not been responsible for structural failure (Lockwood 1994) .
Work on Swiss molasse and similar stones has included the use
of grouts for the extensive detached areas often found on buildings and a new treatment for reducing the swelling of clays (discussed in more detail in chapter 3) (Jiménez-González and Scherer 2004; Rousset et al. 2005).
One intrinsic issue that researchers have puzzled over for several
decades is the bowing of thin marble slabs on emblematic modern
buildings, such as the Amoco building in Chicago, the Grande Arch de
la Defénse in Paris, and Alvar Aalto’s Finlandia city hall in Helsinki. Substantial recent research has found that differential expansion of
calcite enhanced by moisture, microstructure, and differential residual
strains in the marble is the main cause of this problematic and still some –
what mysterious phenomena (Siegesmund, Koch, and Ruedrich 2007; Grelk et al. 2007; Siegesmund, Ruedrich, and Koch 2008; Malaga, Schouenborg, and Grelk 2008; Marini and Bellopede 2009).
Notes
1 System dynamics deals with understanding the behavior of complex systems over
time. It is an approach that uses internal feedback loops and time delays to characterize the entire system and nonlinear behaviors.
2 Major programs were coordinated by the European Union through its STEP and Environment initiatives, the NATO Committee for the Challenges of Modern Society, the United Nations Economic Commission for Europe (UNECE), the US National Acid Precipitation Assessment Program (NAPAP), the UK National Materials Exposure Programme, and the German Bundesministerium für Forschung und Technologie (BMFT).
3 EC project “Carbon content and origin of damage layers in European monuments–CARAMEL” (EVK4-CT-2000-00029). Related EC projects on air pollution and climate change include: MULTI-ASSESS (2002–5), CULTSTRAT (2004–7), and Noah’s Ark (2004–7).
4 Sulfur dioxide is detectable to the human nose at concentrations of about 0.5–0.8 parts per million (1400–2240 µg/m
3).
5 SO2 limits: WHO (2008): 20 μg/m3; US EPA (1997): 80 μg/m3; EU (2008): 20 μg/m3
6 The Peclet number is the ratio of the rate of solute transport by advection to the rate of transport by molecular diffusion. For Pe « 1, diffusion dominates and ion transport proceeds according to the concentration gradient. If Pe » 1, advection dominates and ion transport takes place due to capillary water flow. The Peclet number is defined at the macroscopic scale of the bulk porous material.
7 “Alveolization is a kind of differential weathering possibly due to inhomogeneities
in physical or chemical properties of the stone. Alveolization may occur with other degradation patterns such as granular disintegration and/or scaling. In arid climates large alveoles of meter size are frequently formed (e.g., Petra Jordan)” (Vergès-Belmin 2008, 28).

PROOF 1 2 3 4 5 6Chapter 2
Putting It Right: Preventive and Remedial Treatments
When confronted with decaying stonework, one’s immediate instinct is to
“do something about it.” Traditionally, this has meant doing something
to the stone: perhaps patching it up with mortar, applying some kind of protective coating, or cutting out decayed stone and replacing it with new stone. Regular maintenance is vitally important, wherever practicable; William Morris (1877) wrote of the need to “stave off decay by daily
care,” and in a textbook for conservators encouragingly titled Preventive
Conservation of Stone Historical Objects, Domaslowski (2003) persua –
sively argues that routine maintenance is an often-underappreciated aspect of preventive conservation. Now, however, there is an increasing emphasis on doing something not only to the stone itself but also to the environment in which the stone is found. This reflects a growing aware –
ness of the importance of preventive conservation, of the principle of minimum intervention, and of the need to limit the use of materials that might prove harmful to either the stone or to the environment. Also, now that there is a better understanding of decay mechanisms, a conservation strategy can be designed to reduce the rate of damage by focusing on points of leverage that can mitigate some decay processes. An interesting example is the use of multispectral satellite images of a historic city to provide an automated assessment of the condition of the roofs, where building degradation often begins (Gonçalves et al. 2009) .
PREVENTIVE CONSERVATION
Doing something to the stone’s environment is not simply a matter of
temperature and relative humidity. Preventing damage can embrace a very wide range of topics: legislation to protect individual buildings and mon –
uments, pollution control, traffic control, control of groundwater, visitor management, and disaster planning ( Baer 1991 ; Baer and Snethlage 1997 ;
Baer and Snickars 2001 ). Such topics may seem remote from the prob –
lems of an individual block of stone, but they are nonetheless of great importance. Other areas of preventive research on immovable stone heri –
tage have included shelters, wind fences, and reburial ( Demas 2004 ;
Teutonico 2004 ), as well as modeling of interior environments to help
determine needed interventions (Albero et al. 2004) .

28 Chapt er 1
PROOF 1 2 3 4 5 628 Chapt er 2
PROOF 1 2 3 4 5 6Preventive conservation measures of more immediate effect are
usually concerned with keeping water out of the stone and with control –
ling the relative humidity and temperature of the air around the stone.
This is relatively easy for stone artifacts within a museum, and it may also be feasible for stone masonry that is exposed on the interior of a building ( Price and Brimblecombe 1994 ; Price 2007 ). It is less easy for
stonework on the outside of a building, although a dramatic example of this approach is provided by the glass envelope constructed over the ruins of Hamar Cathedral in Norway.
More modest protective shelters are frequently used on the out –
side of a building to protect those features that are particularly impor –
tant. They may be part of the original design (for example, a canopy protecting a statue in a niche), or they may be a later addition. As an extreme measure, they may enclose the feature altogether. Their purpose is to reduce the amount of rain that reaches the stone and, insofar as is practicable, to stabilize the temperature and moisture content of the stone. If the shelter is a later addition, it is likely to be visually intrusive—
unless it is so small as to serve little purpose.
Few studies have been undertaken of the design requirements of
such shelters, and it is possible that their benefits are more psychologi –
cal than actual. This has been evaluated in practice in only a few cases (Agnew et al. 1996 ; Aslan 2007 ). One case study is at the site of the
Hieroglyphic Stairway, in Copán, Honduras, where a simple canvas
shelter has prevented lichen growth and the swelling of the clay-
containing stone due to frequent rainstorms ( Doehne et al. 2005 ; GCI
and IHAH 2006 ). A second case study, which calculated protective
indices for several styles of shelter at the archaeological site of Joya de
Ceren in El Salvador, found that evaporation was reduced and thermal and relative humidity stability improved in several cases (Maekawa
2006) . A further useful study was undertaken for a pavilion at
Chartwell, Sir Winston Churchill’s country house in Kent, England (Lithgow, Curteis, and Bullock 2007) . The pavilion was open on two
sides, and interior decoration suffered from condensation events, which were mitigated by roof repairs and a temporary wall to buffer the microenvironment during winter.
The main purpose of relative humidity control or buffering is
to reduce damage from salt and moisture cycles. The humidity regime required to prevent damage in a stone or a wall painting that is contami –
nated with a single salt is well established. However, stone is more com –
monly contaminated with a mixture of salts. As discussed in the section on salts in chapter 1, the behavior of salt mixtures is complex ( Steiger
and Zeunert 1996 ; Price 2000 ; Steiger 2005b ; Sawdy and Heritage 2007 ;
De Clercq 2008 ; Franzen and Mirwald 2009 ), and there are now method –
ologies to help with selecting appropriate humidity ranges, even for com –
plex mixtures (Bionda 2004) . Arnold has proposed a methodology for
reducing salt damage to wall paintings by monitoring the relative humid –
ity and temperature, and observing salt efflorescence over the course of one year ( Arnold and Zehnder 1991 ; Arnold 1996 ). Then, the periods
where salts appear can be correlated with the environmental parameters

Stone Decay 29
PROOF 1 2 3 4 5 6 Puttin g It Right: Preventive and Remedial Treatments 29
PROOF 1 2 3 4 5 6and the environmental conditions modified to reduce the incidence of salt
crystallization events (Laue, Bläuer Böhm , and Jeannette 1996) . There is
increasing evidence that drying rates are important and that even a small reduction in drying rate can result in salts crystallizing on the surface as relatively harmless efflorescence (Selwitz and Doehne 2002) . This was the
logic behind the suggestion that a row of trees be planted to help protect salt-laden structures at the site of Port Arthur in Australia (Thorn and
Piper 1996) .
The remainder of this chapter is devoted to research related to
active conservation: doing something directly to the stone itself. In keep –
ing with the title of this volume, this chapter is not a handbook of repair techniques. Information on the routine practice of stone conservation is available elsewhere ( Ashurst and Ashurst 1988 ; Ashurst and Dimes 1998 ;
Ashurst 2007 ; Snethlage 2008 ).
ACTIVE CONSERVATION: CLEANING
Cleaning is often one of the first steps to be undertaken after a condition
survey has been completed. As expected, carbonate materials are the most reactive to acidic pollution and thus have received the lion’s share of attention in studies of stone cleaning. By removing the dirt, one can bet –
ter see the condition of the underlying stone and thus judge what further conservation may be necessary. Cleaning may also serve in some circum –
stances to remove harmful materials from the surface. However, the pri –
mary reason for cleaning will often be the dramatic change in appearance that can be achieved. A dirty building or monument does not look well cared for, and the dirt may well obscure both fine detail and major archi –
tectural features. Nonetheless, there are those who would argue that cleaning contravenes one of the fundamental principles of conservation—reversibility—and that by removing the dirt one is removing both the sense and the evidence of history.
From a morphological point of view, the original stone surface
may be present under a layer of soot or black crust. However, the stone cannot be considered original from the chemical point of view, having undergone a series of changes as the surface equilibrates with its
varying environment ( Vergès-Belmin 1994 ; Smith, Gómez-Heras, and
McCabe 2008 ). Different types of gypsum crust morphology have been
used as criteria for determining the appropriate degree of gypsum removal, and in some cases it has been deemed no longer a desirable goal to eliminate all gypsum from stone surfaces ( Bromblet and Vergès-
Belmin 1996; Siegesmund et al. 2007). After removal of black crusts, the persistence of a gypsum layer bearing no airborne particles may indicate that the original surface has been preserved. This type of layer is approximately 30–500 µm thick. It cannot be recognized with the naked eye; however, it is often detected in cross sections using optical microscopy, ESEM, or EDS (energy dispersive X-ray spectrometry) (Vergès-Belmin 1994). In other cases, a clear gypsum layer occurs
underneath the fragile, hardened stone surface and therefore, when

30 Chapter 1
PROOF 1 2 3 4 5 630 Chapter 2
PROOF 1 2 3 4 5 6reached, it means that the original surface is completely gone (José
Delgado Rodrigues, personal communication).
A wide range of techniques is available for cleaning stone, rang –
ing from those that are intended for use on large facades to those that
are intended for meticulous use on finely carved and delicate sculpture.
Techniques are reviewed by a range of researchers and practitioners: Fassina 1994 ; Andrew, Young, and Tonge 1994 ; Ashurst 1994 ; Cooper,
Emmony, and Larson 1995 ; BSI 2000 ; Vergès-Belmin and Bromblet 2000 ;
Rodríguez-Navarro et al. 2003 ; Normandin et al. 2005 ; Worth 2007 . This
is an area where much progress has been made in the past twenty years, although only a portion is reported directly in the literature. The basic techniques have remained largely the same, although they have become more refined. This reflects an increasing awareness of the damage (and consequent litigation) that may be caused by inappropriate or overenthu –
siastic cleaning and also of the environmental issues posed by the use of certain chemicals or excessive quantities of water (Maxwell 1996; Young, Urquhart, and Laing 2003). With some exceptions, such as latex cleaning films, developments have largely come about through care and attention on-site rather than in the laboratory. These lessons from the field have been consolidated into guidelines (BSI 2000; Young et al. 2003).
It should be noted that any cleaning method requires judgment
and an agreed-upon definition of the target cleaning level before the work
begins. For example, in the present urban environment, uncleaned lime –
stone surfaces may range in color from white (where water runoff has
taken place) to dark br own and black, depending on the amount of accu- dark brown and black, depending on the amount of accu –
mulated dirt. All of these surfaces differ substantially from the “original” freshly cut surfaces, and establishing a target level of cleaning is not an easy task when a single building may contain a wide range of surfaces.
A number of authors have emphasized the damage that can be
caused by cleaning: loss of surface, staining, deposition of soluble salts, or making the stone more vulnerable to pollutants or biological growths. They include Maxwell (1992); MacDonald, Thomson, and Tonge (1992); Young and Urquhart (1992); Andrew, Young, and Tonge (1994); Maxwell (2007); and Delegou and others (2008). It is undoubtedly the case that very severe damage can arise, but a degree of skepticism would perhaps be justified over “damage” that is observable only through a scanning electron microscope.
In most cleaning methods no attempt is made to collect the dirt
and detritus, which is instead allowed to run down the stone and pass into the drains. Some attention is now being given to techniques that
collect the detritus and, for example, permit recycling of the abrasive
(Hoffmann and Heuser 1993) . A commercial system has been developed
that uses fine powders and an air extraction system to capture the debris. This and similar methods have seen wide application ( Vergès-Belmin and
Bromblet 2000 ; Iglesias, Prada, and Guasch 2008 ).
The effectiveness of a cleaning technique is usually assessed sub –
jectively, although objective procedures have been described by many authors ( Werner 1991 ; Young 1993 ; Andrew, Young, and Tonge 1994 ;
D’Urbano et al. 1994 ; Vergès-Belmin 1996a ; Kapsalas et al. 2007 ; Hauff,

Stone Decay 31
PROOF 1 2 3 4 5 6 Puttin g It Right: Preventive and Remedial Treatments 31
PROOF 1 2 3 4 5 6Kozub, and D’ham 2008 ). Vergès-Belmin (1996b) gives a particularly use –
ful overview of methods for evaluating cleaning treatments for stone.
Recent work has shown that quantitative measurements of color change after stone cleaning vary considerably, mainly due to the action of hygro –
scopic salts (Vergès-Belmin, Rolland, and Leroux 2008) . Precautions
should be taken to account for the influence of salts when making such measurements. When discussing color change due to cleaning, it should be made clear that once aged, the stone surface can never be returned to
the freshly cut color. Color can be used as criteria for cleaning only when a “reference surface” is defined and taken as a target for the cleaning level to be reached in the intervention. Color changes related to laser
cleaning are dealt with in the next section.
Laser Cleaning
Using lasers to clean stone is now routine, and large-scale commercial application of laser cleaning has become more common over the past fifteen years (Dajnowski, Jenkins, and Lins 2009) . Its great attraction
is that it does not entail any physical contact with the stone and so lends itself to the cleaning of very delicate surfaces. There are no sol –
vents or water to redistribute potentially harmful salts. The technique is selective and sensitive in terms of the degree and control of removal. The principle is essentially simple: a laser beam impacts the surface, and the energy of the infrared beam is dissipated by the sudden heat –
ing and expansion of light-absorbing material on the surface, such as particles rich in carbon, and the nearly instantaneous vaporization of moisture in the surface layer, which acts to remove surface dirt. Spraying the surface with water just before laser cleaning can enhance the effectiveness of the treatment (Siedel, Neumeister, and Sobott
2003) . For light-colored stones with dark surface deposits, the infrared
beam continues to be absorbed while the stone remains soiled and cleaning proceeds. Once the dirt has been removed, however, the light is reflected by the clean surface, and no more material is removed. This
is not the case for biotite-bearing granites and painted stones, where
laser cleaning may not be appropriate . The technique is described in
detail by a number of authors, including Cooper, Emmony, and Larson
(1993) ; Cooper (1998) ; Maravelaki-Kalaitzaki, Zafiropulos, and
Fotakis (1999) ; and Orial and others ( Orial and Riboulet 1993 ; Orial,
Vieweger, and Loubiere 2003 ).
With early systems, the speed of cleaning was comparable to that
achieved with a pencil-sized air-abrasive gun. The use of optic fibers to
transmit the laser beam was a significant advance (EC project: LAMA—
LAser MAnuportable pour le nettoyage des façades courantes et des monu- pour le nettoyage des façades courantes et des monu –
ments historiques; BRITE/EURAM BRE CT93-560 ). Now entire facades
have been laser cleaned (Pini, Siano, and Salimbeni 2000), including the town hall in Rotterdam (Nijland and Wijffels 2003), and many monuments in Poland (Koss and Marczak 2008). The technique is seeing additional test –
ing and application in the United States as well (Normandin et al. 2007).
Current research is aimed at selecting the optimal wavelength
and pulse energy; at examining the effects on the stone, both physical and

32 Chapter 1
PROOF 1 2 3 4 5 632 Chapter 2
PROOF 1 2 3 4 5 6chemical; at comparing the performance of lasers with other cleaning
techniques; and at identifying possible hazards to the operato r (Vergès-
Belmin et al. 2003 ; Bromblet, Labouré, and Orial 2003 ; Rodríguez-
Navarro et al. 2003 ). The use of a laser requires special caution when
cleaning surfaces with traces of polychromy (Fassina, Gaudini, and
Cavaletti 2008) . A set of conferences devoted to the use of lasers in art
conservation (Lasers in the Conservation of Artworks, or LACONA) has
taken place every two years since 1995: for example, Liverpool in 1997 and Madrid in 2007. A European Cooperation in Science and Technology project on the topic of artwork conservation by laser, funded by the European Science Foundation, ran from 2000 to 2006 and resulted in
a handbook available for download ( http://www.cost.esf.org/library/
publications/05-40-Cleaning-Safely-with-a-Laser-in-Artwork-Conservation ).
Further development of equipment has taken place, identifying,
for example, the appropriate means and timing of delivering the laser pulse to the surface of the stone ( Margheri et al. 2000 ; Mazzinghi and
Margheri 2003 ; Dogariu et al. 2005 ; Siano et al. 2008 ). An important
issue with laser cleaning is the color of the cleaned surface. In some cases, a yellow surface layer is revealed, which in some examples is related to previous restoration treatments ( Vergès-Belmin and Dignard 2003 ;
Zafiropulos et al. 2003 ; Gaviño et al. 2004 ; Gaviño et al. 2005 ; Vergès-
Belmin and Laboure 2007 ; Andreotti et al. 2009 ). Color changes after
laser cleaning may happen due to modifications in the substrate (pink feldspars, for instance), to modifications in any covering colors, or to changes in deposited dirt particles. The last situation may indicate that the target cleaning level has not been reached.
Latex Poultice Method
An important challenge for stone conservation has been the cleaning
of large, public interiors, such as cathedrals, while allowing them to remain open during the process. This stricture generally rules out the use of toxic chemicals and abrasives. One innovative response to this chal –
lenge has been the development over the past fifteen years of the latex poultice method; it is known commercially as Arte Mundit. Originally developed as an improvement to the Mora poultice (Woolfitt and Abrey 2000) by Eddy De Witte (De Witte and Dupas 1992) as a spray-on film containing EDTA (ethylene diamine tetra acetic acid) and other additives, it has seen adaptation and application to a wide range of sites, including St. Paul’s Cathedral in London (Miget 2000; Odgers 2003; Jacobs 2004; Stancliffe, De Witte, and De Witte 2005; Odgers 2006; Allanbrook and Normandin 2007). The method is best used on sound interior surfaces.
If the soiling has been trapped in an encrustation such as a gypsum crust, the latex poultices no longer work. Recent research on latex poultices
has raised the issue of residues left on stone surfaces by the method, which deserves further study (Morasset 2008; Morasset et al. 2009),
and there may also be concern over unintentional mechanical damage to friable surfaces during removal. There are interesting parallels between the residue issue and the use of gels for the cleaning of paintings (Stulik et al. 2004).

Stone Decay 33
PROOF 1 2 3 4 5 6 Puttin g It Right: Preventive and Remedial Treatments 33
PROOF 1 2 3 4 5 6Biological Cleaning
Hempel (1976) was one of the first to raise the possibility of biological
cleaning. He had been surprised by the effectiveness of a clay poultice containing urea and glycerol and proposed that microorganisms were at least partially responsible. Kouzeli (1992) has reported favorably on the
technique in comparison with pastes based on EDTA or ammonium bicarbonate.
Biological cleaning, in general, has been little researched ( Ranalli
et al. 1996 ; Ranalli et al. 2000 ). Gauri has demonstrated the use of the
anaerobic sulfur-reducing bacterium Desulfovibriode sulfuricans in remov –
ing the black crust on marble (Gauri et al. 1992) . He has argued, moreover,
that the bacterium was converting calcium sulfate back into the calcium carbonate from which it was originally formed ( Atlas, Chowdhury, and
Gauri 1988 ; Gauri and Chowdhury 1988 ). Konkol has demonstrated that
using an enzymatic cleaner derived from the fungus Trametes versicolor
may reverse biological staining of marble (Konkol et al. 2009) . Efforts to
remove lichen from concrete through the use of Thiobacillus bacteria
have been evaluated by De Muynck, De Belie, and Verstraete (2010) .
Comparison of sulfate-reducing bacteria treatment versus conventional chemical cleaning procedures on a marble element of the Milan Cathedral is reported by Toniolo et al. (2008) and Cappitelli et al. (2007a) .
Targeting the Dirt Gauri’s work is interesting because it takes account of the nature of the dirt. It is true that this may be implicit in other cleaning techniques (e.g., the use of complexing agents to increase the solubility of calcium sulfate
or the use of hydrofluoric acid to dissolve silica), but it is disappointing that only a few developments in cleaning techniques have flowed out of
the extensive studies on black crusts. One example is the work of Vergès-Belmin, Pichot, and Orial (1994) determining the point at which to stop the removal process. Livingston (1992) has studied the solubilities of cal –
cium carbonate and calcium sulfate; Schiavon (1992) has commented on the distribution of calcium sulfate within the pores of stone and on that distribution’s implications for water washing; and Skoulikidis and Beloyannis (1984) have attempted to convert calcium sulfate back into cal –
cium carbonate by the use of potassium carbonate, blissfully ignoring the potentially harmful effects of the resulting potassium sulfate. Few other researchers, however, have focused directly on the nature of the dirt depos –
its in an attempt to develop more effective cleaning techniques. Partially, this has been due to the fact that it is only recently that the complex amal –
gam of organic fractions contained in patinas and the role microbes play in this ecology have become better known (see the Biodeterioration section
in chapter 1 and the Rock Art section in chapter 5).
ACTIVE CONSERVATION: DESALINATION
In situations where soluble salts are a major contributor to decay, it
makes sense to try to remove the salts. The word try is used deliberately.

34 Chapter 1
PROOF 1 2 3 4 5 634 Chapter 2
PROOF 1 2 3 4 5 6The removal of water-soluble salts sounds tantalizingly easy, but it can
prove difficult in practice. Salt reduction may be a more appropriate
term ( Redman 1999 ; Sawdy, Heritage, and Pel 2008 ; Pel, Sawdy, and
Voroninaa 2010 ).
Salt reduction is relatively straightforward in the case of small
artifacts, which can, for example, be immersed in water or enclosed completely in a poultice, though even here problems can arise through the frailty of the surface or the presence of pigments ( Beaubien et al.
1999 ; Paterakis 1999 ; Muros and Hirx 2004 ; Franzen et al. 2008 ).
The real problems start when one attempts to remove salts from the masonry of a building or monument. In an early desalination study, Bowley (1975) demonstrated that it was possible to extract a worth –
while quantity of salt from masonry through the repeated use of clay poultices, although little would be gained in the long run unless one could eliminate the source of further salt. An excellent review (Vergès-
Belmin and Siedel 2005) makes it clear that larger-scale masonry desali –
nation needs further study.
Desalination of masonry is usually attempted through the use of
poultices, which may consist of a range of materials (e.g., clay, sand, and paper pulp) (Auras 2008) . In those instances where calcium sulfate
is to be removed, additional materials may be added in order to increase its solubility. Clearly there are overlaps here with cleaning, especially in
the removal of black crusts. The additives may include EDTA and its sodium salts, sodium bicarbonate, ammonium bicarbonate, and ammo –
nium carbonate ( Maravelaki et al. 1992 ; De Witte and Dupas 1992 ;
Alessandrini et al. 1993 ; Leitner 2005 ; Henry 2006 , p. 153). A word of
warning may be appropriate: If a limestone is heavily sulfated, the cal –
cium sulfate may be all that is holding it together, and total removal could be disastrous.
An EC project, Assessment of Desalination Mortars and Poultices
for Historic Masonry (DESALINATION) 2006–9, has worked to provide a scientific foundation and guidelines for the efficient application of desalination poultices (Bourguignon et al. 2008; Doehne et al. 2008; TU Delft 2009). Principles involve matching the poultice to the pore charac –
teristics of the substrate (kaolin helps with finer stones), preventing rapid drying of the poultice, using less water, and thinner poultices. While counterintuitive, using less water helps remove salts that are near the stone surface and helps avoid pushing the salts deeper into the stone. Some improvements, using finer poultices and both sides of the wall, have also been proposed by other researchers to improve the efficiency of the desalination process (Friese and Protz 1997; Friese, Protz, and Peschl 1997). More recent work has shown that poultice shrinkage and detach –
ment are further important parameters in improving poultice efficiency (Bourgès and Vergès-Belmin 2008a; Bourgès and Vergès-Belmin 2008b; Sawdy, Heritage, and Pel 2008; Heritage et al. 2008).
Desalination efforts often need to be coupled with efforts to
reduce the supply of salts, such as the maintenance or installation of a damp-proof course (DPC) at the base of the building foundation (Pinto Guerra 2008; Young and Ellsmore 2008). Installing new DPCs to deal

Stone D ecay 35
PROOF 1 2 3 4 5 6 Putting It Right: Preventive and Remedial Treatments 35
PROOF 1 2 3 4 5 6with the accumulation of salts and damp has a mixed record in some
church monuments ( Henry 2006 , p. 277).
Finally, the use of bacteria in desalination may merit further
attention. Gauri’s use of sulfur-reducing bacteria to eliminate the black crust has already been mentioned, and Gabrielli (1991) gives an anec –
dotal account of using the reducing atmosphere created by cow dung to convert nitrate salts into elemental nitrogen gas. One wonders, however, if other salts are added at the same time. Removal of salts by microor –
ganisms has also been proposed by Webster and others (Webster, Vicente, and May 2004; Webster and May 2006) as a central part of the EC BIOBRUSH project (BIOremediation for Building Restoration of the Urban Stone Heritage; May et al. 2008). However, these studies found that any effects of the bacteria were masked in many cases by the effect of the material used to apply them and that there were practical prob –
lems in supporting the weight of the application material on large areas. One is left with the feeling that additional development is needed before practical biological cleaning can be readily applied. In contrast, biocalci –
fication appears to be at a much higher level of development (see Lime and Biocalcification section below).
ACTIVE CONSERVATION: CONSOLIDATION
Where stone is severely weakened by decay, some form of consolidation
may be necessary to restore some strength. Ideally, one might hope to make the stone at least as strong as it was originally (Snethlage 2008; Scherer and Wheeler 2009), so it might resist further decay, but even the strength to resist the battering of the wind or the wing of a bird may be enough to prolong survival.
It all sounds so easy. One just has to find something that will
penetrate the decayed stone, binding it together and securing it onto the
sound stone beneath (Ginell, Wessel, and Searle 2001) . And why stop
there? Why not find something that will also protect the stone from fur –
ther decay? Perhaps it could prevent damage from cycles of salt crystalli –
zation. Or perhaps it could make the surface of the stone water-repellent or able to resist hygric swelling. Of course, the treatment will need to
be reasonably cheap, easy to apply, and safe to handle. VOC (volatile organic compound) regulations mean that any treatment needs to be for –
mulated to be environmentally friendly. It will need to remain effective for decades at a time, in order to last from one maintenance cycle to the next (often dictated by the cost of scaffolding). The treated stone will need to have much the same moisture expansion, thermal expansion, and elastic modulus as the untreated stone in order to avoid internal stresses and assure compatibility. Ideally, the treatment should work equally well
on any type of stone, regardless of the cause of decay. And let’s not forget
that it must be completely invisible.
Put like this, it sounds absurd to attempt the task. It is like trying
to find one pill that will cure all the diseases known to humankind. But this has not hindered the search for an all-singing, all-dancing stone

36 Chapt er 1
PROOF 1 2 3 4 5 636 Chapt er 2
PROOF 1 2 3 4 5 6 consolidant-cum-preservative. It is a wonder we have made as much
progress as we have. An enormous variety of materials have been tried
since time immemorial ( Barff 1860 ; Egleston 1886 ), each with its own
advocates (Palmer 2002) .
One has to start somewhere, and one of the properties that a
consolidant must have is the ability to penetrate the stone. This, in turn,
requires a low viscosity and a low contact angle. Next, the consolidant needs to stiffen or set once it is in place in order to strengthen the stone. These requirements can be met in three ways: first, one could think of applying a substance that is liquid at high temperature and stiffens as it cools down—wax for instance. In practice, it is hard to get a low enough viscosity without excessive heat, and wax tends to be sticky and to pick up dirt. The consolidation might become risky in areas having significant exposure to the sun. The second approach is to use a consolidant dis –
solved in a solvent. One cannot assume, however, that the consolidant necessarily penetrates as far as the solvent, and there is always a danger of the consolidant being drawn back to the surface as the solvent evapo –
rates. Third, one can use a low-viscosity system that undergoes a chemical reaction in situ to give a solid product.
Consolidants are usually applied to the surface of the stone by
brush, spray, pipette, or by immersion and are drawn into the stone
by capillary action. Domaslowski (1969) experimented with a “pocket
system” that was intended to hold the consolidant against the stone, and Mirkowski (1988) has described a system employing bottles to maintain
a steady supply of the consolidant at a large number of points. At St. Trophime (Arles, France), consolidant was fed using “intravenous” tubes, allowing a slow drop-by-drop application to the stone surface (Mérindol
1994) . Schoonbrood (1993) has developed a low-pressure application
technique that maximizes capillary absorption. Vacuum systems may also be used to facilitate penetration into movable objects and ashlars (see, e.g., Hempel 1976 ; Török 2008 ). The vacuum system developed by
Balfour Beatty Limited (Balvac) for use on monuments (see , e.g.,
Antonelli 1979 ) did not find extensive application in practice. Various
vacuum systems for sculpture are in use (Pummer 2008) , and damage to
fragile stone surfaces can be reduced by wrapping them with cotton.
The majority of materials that have been tried as stone consoli –
dants have been organic polymers, but several inorganic materials deserve a particular mention, as their mode of operation is rather different: cal –
cium hydroxide (slaked lime) and barium hydroxide.
Lime and Related Treatments
Nothing could be more natural than putting lime into limestone. The emotive appeal of lime must account for at least some of its popularity. There is, however, a sound rational basis for its use. If a saturated solu –
tion of calcium hydroxide is allowed to penetrate into limestone, subse –
quent evaporation of the solution will lead to the deposition of calcium hydroxide within the stone. This, in turn, will react with carbon dioxide in the air to produce calcium carbonate. This could serve to consolidate

Stone De cay 37
PROOF 1 2 3 4 5 6 Putting It Right: Preventive and Remedial Treatments 37
PROOF 1 2 3 4 5 6the stone, in much the same way as carbonation of calcium hydroxide
leads to the hardening of a lime mortar.
This basic chemistry forms the basis of the “lime technique”
(Ashurst 1998), which has been used extensively in England and to a lesser extent elsewhere. The technique, in its entirety, can quite transform the appearance of decayed limestone. However, Price, Ross, and White (1988) demonstrated that the lime was deposited largely in the outer
couple of millimeters of the stone and that no deep consolidation of the
stone could be attributed to the calcium hydroxide. However, it is con –
ceivable that some consolidation could be attributed to the redeposition of calcium sulfate within the stone, a suggestion supported by the appar –
ent effectiveness of distilled water under some circumstances (Clarke and Ashurst 1972). The conclusion of Price, Ross, and White was that the success of the technique was largely attributable to the subsequent use of well-designed mortars, which filled surface fissures and other defects. An alternative suggestion, put forward by R. White (personal communica –
tion) and by Anagnostidis et al. (1992), is that the lime is serving to kill bacteria and other organisms and so reduces decay. Krumbein and others (1993) suggest that the observed sterility of marble treated with lime may be due not to biocidal action but to pore closure, which prevents colonization.
Despite the hope that the lime treatment would lead to the depo –
sition of interlocking calcium carbonate crystals, in the manner of lime mortars, the available evidence suggests that it is deposited in an amor –
phous form that can have little consolidating effect. Tiano and others, however, have proposed a pretreatment based on glycoproteins derived from marine organisms and biomineralization (Tiano, Addadi, and Weiner 1992; Tiano 1995; Tiano 2004). The pretreatment is reported to induce the nucleation of calcite, leading to well-formed crystals that adhere strongly to the underlying stone. More recent work undertaken by Jiménez-Lopez and colleagues (Jiménez-Lopez et al. 2007; Jiménez-Lopez et al. 2008) tested the consolidating effect of soil microbes precipitating calcite in porous limestones.
The lime technique is still in use (Fidler 1995; Brajer and
Kalsbeek 1999; Fidler 2002; Woolfitt and Durnan 2002; Oudbashi et al. 2008). However, new nano-lime technology is now available after some years of development (Giorgi, Dei, and Baglioni 2000; Ambrosi et al. 2001; Dei and Salvadori 2006; Adolfs 2007; Ziegenbalg 2008). This tech –
nology, which suspends nano-scale calcium hydroxide particles in alcohol, permits deep penetration into stone surfaces. The use of alcohol instead of water limits carbonation by CO
2 before the particles are deposited in
the porous stone and facilitates much higher loadings of lime than is pos –
sible with aqueous solutions. The method is commercially available and has been used in some specific cases (Howe 2007; Daniele and Taglieri 2010). Future work should include the long-term testing of nano-lime materials, and an EC project on the topic is in progress: STONECORE (Stone Conservation for the Refurbishment of Buildings, http:// www
.stonecore-europe.eu/ ; Drdácký, Silzkova, and Ziegenbalg 2009).

38 Chapt er 1
PROOF 1 2 3 4 5 638 Chapt er 2
PROOF 1 2 3 4 5 6Barium Hydroxide
Barium hydroxide is another material with a long pedigree. Chemically, bar-
ium compounds and calcium compounds share many of the same charac –
teristics, the one notable difference being the insolubility of barium sulfate as compared with the sparing solubility of calcium sulfate. Barium hydro-xide treatments thus have a number of possible objectives, which are not always clearly spelled out. They may serve to convert calcium sulfate to barium sulfate and thereby reduce damage due to the solution and recrys –
tallization of calcium sulfate; they may serve, after carbonation, to deposit a coating of barium carbonate, which will be more resistant than calcium carbonate to acid rain; and they may serve to consolidate the stone through the formation of solid solutions of barium calcium carbonate (Lewin and Baer 1974) . The advantages and disadvantages of barium
treatments are reviewed by Hansen and others (2003) .
A number of techniques have been proposed for introducing the
barium hydroxide into the stone. Simple application of barium hydroxide solution appears to be ineffective and led Schaffer (1932, p. 84) to dis –
miss the process in just seven words: “In practice the method proved a failure.” Lewin and Baer (1974) , by contrast, described a technique that
ensured the slow growth of well-formed barium carbonate crystals within the stone, a technique Lewin was still advocating fifteen years later (Lewin 1988) . Schnabel (1992) has cast doubt on the effectiveness of the
process when applied by capillarity in situ. More recent work on barium includes “not satisfying” results from Toniolo et al. (2001) , good results
on Gioia marble (Bracci et al. 2008) , and its use as an additive in lime
mortars (Karatasios et al. 2007) . An EC project evaluating a range of
consolidant treatments, including barium hydroxide, found improvements in drilling resistance to a depth of 2 cm in porous limestones (Bracci
et al. 2008) .
The widest application of barium hydroxide has come in the field
of wall paintings, where Matteini (1991) proposed that barium hydroxide
treatment should be preceded by the use of ammonium carbonate to
dissolve the calcium sulfate (Ambrosi et al. 2000) . Barium oxalates and
aluminates have also been tested on a range of materials (Matteini
and Zannini 2004) .
Organic Polymers From naturally occurring compounds, such as linseed oil and cactus juice, to the synthetic polymers of the twentieth century, somebody somewhere will have tried it as a stone consolidant. Generally speaking, such trials have been on a rather hit-or-miss basis. Materials have been selected more on the grounds of availability than of any predetermined qualities. Provided they will penetrate the stone and then set, they have been worth a try. In a number of cases, the use of incompatible materials on stone has led to a series of difficult and unintended consequences, even with ostensibly removable materials (Nimmrichter and Linke 2008) .
1
While it is easy to sound contemptuous about such an empirical
approach, it is hard to see how things could have been any different. Because our knowledge of decay processes is still incomplete, our

Stone Decay 39
PROOF 1 2 3 4 5 6 Putti ng It Right: Preventive and Remedial Treatments 39
PROOF 1 2 3 4 5 6 knowledge of how to combat them is incomplete, as well. Of necessity,
we are learning by experience.
The vast majority of researchers believe that stone needs to
“breathe.” In other words, stone should remain permeable to water
vapor, in order to avoid any buildup of moisture and soluble salts (and consequent shear stresses) at the interface between the treated zone
and the untreated stone below. Rapid drying of stone surfaces reduces the potential for biological growth and decreases the time of wetness—
a parameter associated with damage to stone from air pollution.
Little attention has been given to the distribution of consolidants
within stone at the microscopic level, despite numerous photomicro –
graphs taken with the scanning electron microscope. Many authors have been content simply to state that a treatment “lines the pores.” Sasse and
Honsinger (1991) have described a “supporting corset” model, consisting
of an impermeable layer that coats and protects the internal surfaces of the stone, while imparting mechanical strength. Hammecker and others (Hammecker, Esbert Alemany, and Jeannette 1992 ; Hammecker 1993 )
describe the use of mercury porosimetry to monitor changes in pore structure due to treatment, but such studies may be hindered by the change in contact angle following treatment.
Little is known about the bonding, if any, that takes place
between a consolidant and the substrate, and much is left to chemi –
cal intuition. It is widely argued, for example, that alkoxysilanes will form primary chemical bonds to the Si-OH groups on the surface of sandstones, but that they will not be able to form primary bonds to
limestones. Lack of bonding need not necessarily mean failure, however,
for an unbonded network of consolidant could still provide strength. The stability of polymers used for protective purposes has been evaluated with increasingly sophisticated methods ( Gembinski et al. 2000 ; Chiantore and
Lazzari 2001 ; Favaro et al. 2005 ), both in the lab and the field, detailing
their alteration and loss of efficiency over time.
More needs to be known, not just about stability but also about
the molecular structure of the polymer that is deposited within the stone. We speak glibly, for example, about the network polymer that is formed by the hydrolysis and subsequent condensation of tri-alkoxysilanes and tetra-alkoxysilanes. But how many siloxane bonds are formed, on aver –
age, by any one silicon atom? What is the structure of the polymer? How is it influenced by the presence of water, of solvents, of salts, or of partic –
ular minerals? How does it affect the strength of the polymer? Our
present knowledge of consolidants may be likened to folk remedies in
medicine. We have gained a lot of experience of what is, and what is not, effective, but we have little understanding of how polymer consolidants work. Once we have a deeper understanding of the properties that are required of a consolidant, we shall be in a better position to synthesize compounds that incorporate those properties.
Alkoxysilanes
The alkoxysilanes and alkyl alkoxysilanes, or “silanes” for short, have undoubtedly been the most widely used stone consolidants over the past

40 Chapt er 1
PROOF 1 2 3 4 5 640 Chapt er 2
PROOF 1 2 3 4 5 6twenty years ( Snethlage and Wendler 2000 ; Wheeler and Goins 2005 ;
Price 2006 ; Wheeler 2008 ; Scherer and Wheeler 2009 ). Two compounds,
in particular, have been dominant: methyltrimethoxysilane (MTMOS) and
tetra-ethoxysilane (TEOS). The silanes are hydrolyzed by water to form silanols, which then polymerize in a condensation reaction to give a sili –
cone polymer. The water may come from the atmosphere or from the stone itself, or it may be added as a deliberate ingredient. In the latter case, a solvent may be required in order to make the mixture miscible. A catalyst may also be added, usually in the form of an organo tin or lead compound. The condensation reaction, and often the hydrolysis reaction also, takes place after the treatment has been absorbed by the stone, and the resulting polymer imparts the required strength to the stone.
The popularity of MTMOS and TEOS is no doubt due in part to
their commercial availability, and a number of proprietary products are available that are based on these two compounds. A number of other silanes have also been tried, usually involving substitution of the methyl group for larger alkyl or aryl groups.
A thoughtful review by Wheeler (2008) of the use of alkoxysi –
lanes for stone consolidation deals with three important issues: the use of alkoxysilanes on clay-rich stone, alkoxysilanes used on limestone versus quartz sandstones, and the use of alkoxysilanes on marble. Results for clays are mixed: two important studies found that ethyl silicate treatment of clay-rich stone initially resulted in a strength increase, but that this improvement was lost after three to ten wet/dry cycles ( Félix 1996 ;
Scherer and Jiménez-González 2008 ). This suggests that for clay-rich
stone, the focus should be on reducing clay swelling, not on increasing strength (see the Differential Stress section in chapter 1 for more on anti-swelling treatments).
The difficulty of bonding a silicate material to calcite has long been
considered an important problem, resulting in some new research on cou –
pling agents and alternative consolidants ( Wheeler, Mendez-Vivar, and
Fleming 2003 ; Correia and Matero 2008 ; Ferreira Pinto et al. 2008 ;
Ferreira Pinto and Delgado Rodrigues 2008 ). Wheeler (2008) points out
that while the percent strength increase for limestone after ethyl silicate treatment is not as great as for sandstone, comparing the absolute level of the modulus of rupture (generally higher for limestone) provides a more realistic perspective and helps explain the widespread use of this material on limestone. The use of alkoxysilanes on marble is explained as filling narrow voids between calcite grains, which can help lock in particles expe –
riencing granular disintegration (Ruedrich, Weiss, and Siegesmund 2002) .
Recent work on nano particle–modified silanes show they reduce
the cracking seen in conventional treatments and result in improved con –
solidation ( Escalante, Valenza, and Scherer 2000 ; Miliani, Velo-Simpson,
and Scherer 2007 ; Kim et al. 2008 ). Elastified silanes have also been
developed to help create a less brittle film ( Boos et al. 1996 ; Kim et al.
2008 ; Maravelaki-Kalaitzaki et al. 2008 ). A commercial elastified version
is available (E. Wendler; Remmers KSE 500 E). Surfactants have also
been tested and result in a less brittle silane treatment—a hybrid nano-
composite ( Mosquera and de los Santos 2008 ; Simionescu et al. 2009 ).

Stone Decay 41
PROOF 1 2 3 4 5 6 Puttin g It Right: Preventive and Remedial Treatments 41
PROOF 1 2 3 4 5 6Important research on application procedures has shown that the
timing and number of applications can result in important differences in the
pore-blocking effect and general hardness of TEOS (De Clercq, De Zanche,
and Biscontin 2007) . The development of microporosity during the curing
of Funcosil stone strengthener was noted by Barajas and others (2009) .
Although the literature contains many papers describing the use
of silanes on stone, there are few that attempt to come to grips with the underlying chemistry or the associated sol-gel technology. Some excep –
tions are studies by Wheeler ( Wheeler, Mendez-Vivar, and Fleming 2003 ;
Wheeler and Goins 2005 ; Scherer and Wheeler 2009 ), Scherer ( Scherer,
Flatt, and Wheeler 2001 ; Miliani, Velo-Simpson, and Scherer 2007 ),
and Snethlage ( Snethlage 2002 ; Meinhardt-Degen and Snethlage 2007 ;
Snethlage 2008 ). Other recent work includes efforts to evaluate and con –
trol the relationship of pore evolution and solvent (Salazar-Hernández et al. 2009). Research continues on extending sol-gel treatments beyond stone to other diverse heritage materials, including bronze, pyrite, and unstable historic glass (Bescher and Mackenzie 2003; Khummalai
and Boonamnuayvitaya 2005; Dal Bianco and Bertoncello 2008). Kumar (Kumar and Price 1994) has reported on the influence that soluble salts may have on the hydrolysis and condensation of MTMOS. Sodium sul –
fate, for example, markedly decreased the rate of both hydrolysis and condensation, whereas sodium chloride increased the rate of condensa –
tion. Silica-sol treatments at Petra were found to perform poorly in the presence of salts, resulting in the need to poultice areas to be treated prior to application (Simon, Shaer, and Kaiser 2006). Consolidation of stone does not encapsulate salts that may be present, and research shows that salts can be removed by poulticing after treatment if salt concentra –
tions are low to moderate. However, some of the consolidation effect is lost after wetting the samples, depending on the salt tested (Costa and Delgado Rodrigues 2008a).
Epoxies
Epoxy resins have had some bad press as far as consolidation is con –
cerned. Many conservators see them as viscous, brittle, yellowing materi –
als that may make admirable adhesives in some circumstances, but which are certainly not to be considered as consolidants.
It is true that there have been some notable failures, but it would
be foolish to dismiss epoxy resins entirely on these grounds. Selwitz, in three reviews (1991, 1992a, 1992b), summarized the use of epoxies as consolidants, charting the successes and failures. He highlights the pio –
neering work of Domaslowski (Domaslowski and Strzelczyk 1986; Domaslowski and Sobkowiak 1991) and Gauri (1974; Gauri and Appa Rao 1978) and emphasizes the two different paths they have adopted in order to treat relatively small objects and large facades, respectively. The choice of solvent, the means of application, and postapplication proce –
dures are vitally important to a successful outcome (Pinto and Delgado Rodrigues 2008b).
Cycloaliphatic epoxy resins (Eurostac EP2101) have been success –
fully used in some important field consolidation cases in Italy, such as

42 Chapte r 1
PROOF 1 2 3 4 5 642 Chapte r 2
PROOF 1 2 3 4 5 6the deep consolidation under vacuum of large, fissured granite columns
(Cavalletti et al. 1985) . More recent work has focused on application
methods that minimize color change with aging (Ginell and Coffman
1998) , the use of waterborne epoxy emulsions ( Kozub 2004 ; Luan
Xiaoxia et al. 2008 ), and complex hybrids, such as epoxy-silica materials
(Cardiano et al. 2003 ; Cardiano et al. 2005 ). Further work is needed to
evaluate these newer materials.
Acrylics
Although in situ polymerization of methyl-methacrylate (and other acrylic monomers) has its advocates, the high rigidity and glass transition tem –
perature of polymethyl-methacrylate are generally considered to make it unsuitable as a stone consolidant. Far more attention has been given to the use of acrylic resins dissolved in solvents, and the ubiquitous Paraloid B72 (Acryloid B72) inevitably makes its appearance.
Many conservators have experimented with B72 dissolved in an
alkoxysilane such as MTMOS, the reasoning being that the B72 brings adhesive properties that the alkoxysilane lacks. The idea seems to have been that B72 is capable of securing pigment or loose flakes, for example, while the alkoxysilane provides deep consolidation. This treatment was used by Nonfarmale and Rossi-Manaresi in San Petronio Cathedral in
Bologna, from where the term “Bologna cocktail” was coined ( Gnudi,
Rossi-Manaresi, and Nonfarmale 1979 ). In San Petronio, the limestone is
very compact and virtually nonporous, and the decay progresses mainly with the formation and detachment of scales and other fragments. The cocktail was used in this case for gluing the scales, and because it was properly done by a very experienced conservator, the result was satisfac –
tory and those surfaces are apparently still in good condition (Laurenzi Tabasso 1995). The problem arises when the Bologna cocktail is trans –
posed to very porous limestones with pore-shaped voids (J. Delgado Rodrigues, personal communication). Under these circumstances, B72 has a very low impregnation capacity, forming indurated crusts and leading to severe detachments some time after application. In such cases it may con –
stitute a disaster. In short, Paraloid B72 is an excellent adhesive, but it is not necessarily a good consolidant outdoors. The Bologna cocktail is a useful example of the need to match the treatment to the problem and the need for critical thinking when navigating the conservation literature .
More recent research on the aging of Bologna cocktail mixtures (Paraloid B72 and Dri Film 104) has been undertaken by Favaro and others (2006; 2007).
Wheeler and co-workers (Wheeler et al. 1991; Wheeler, Wolkow,
and Gafney 1992) have shown that the resulting composite gel is weaker than the polymers derived either from neat MTMOS or from a solution
of B72 in a nonreactive solvent. Research has continued on acrylic/
siloxane composites ( Zielecka, Bujnowska, and Bajdor 2007 ; Sadat-Shojai
and Ershad-Langroudi 2009 ) with some promising results. Other work on
B72 has focused on characterizing its long-term stability and field perfor –
mance ( Roby 1996 ; Bracci and Melo 2003 ).

Stone Dec ay 43
PROOF 1 2 3 4 5 6 Putting I t Right: Preventive and Remedial Treatments 43
PROOF 1 2 3 4 5 6Other Materials
Innovative approaches to consolidation have come from several research –
ers, such as the use of calcium alkoxides (Favaro et al. 2008) , the trans –
formation of gypsum or calcite into calcium phosphate based on historic patinas ( Martín-Gil et al. 2005 ; Xiangmin Zhang and Spiers 2005 ;
Vazquez-Calvo, Alvarez de Buergo, and Fort 2007 ; Snethlage et al. 2008 ),
and frontal (in situ) polymerization ( Proietti et al. 2006 ; Mariani,
Capelletti, and Brunetti 2008 ).
Research on the use of tartrates has led to a patented product
that creates a conversion coating on calcite that can also act as a cou –
pling agent for ethyl silicate–based treatments (Slavid and Weiss 2001). Known commercially as HCT (Prosoco, Inc.), the product has been on
the market for some time (Correia 2005; Correia and Matero 2008; Pinto and Delgado Rodrigues 2008a), and results from longer-term trials are expected in due course.
Previous accounts of isocyanates, polyurethanes, and polyureas
may be found in Hansen and Agnew (1990); Coffman, Agnew, and Selwitz (1991); Zádor (1992); Littmann et al. (1993); Auras (1993); and Riecken and Sasse (1997). The use of cyclododecane, largely as a temporary, reversible consolidant that sublimes over time, has been explored over the past decade as a useful new component of the conservator’s toolbox (Stein et al. 2000; Maish and Risser 2002; Muros and Hirx 2004; Anselmi, Doherty, and Presciutti 2008). Some health and safety issues regarding cyclododecane remain to be resolved (Rowe and Rozeik 2008). Advanced research in self-cleaning surfaces, such as titanium-coated glass, has led
to interest in biomimetic surfaces that may have potential application for developing compatible coatings for the conservation of stone (Solga et al. 2007; Qiang Liu et al. 2006; Kun Hong and Yuzhong Zhan 2008).
Emulsions
Organic consolidants frequently rely on the loss of volatile reaction prod –
ucts or solvents during the curing process. This can make application impracticable in hot climates, and it can pose a hazard both to the conser –
vator and to the wider environment. Attention has been given to the devel –
opment of aqueous emulsions for use as consolidants and as surface coatings (see the following section). Snethlage and Wendler (1991) discuss the possible use of an aminoalkyl silane to stabilize a silane emulsion, and Piacenti, Camaiti, Brocchi, and others (1993) report on the development of emulsions based on a hexafluoropropene-vinylidene fluoride elastomer. More recent work illustrates the diverse application of emulsions contain –
ing acrylic, fluorinated acrylic, methacrylate/alkoxysilane, or epoxy resin as conservation treatments (Castelvetro et al. 2004; Luan Xiaoxia et al. 2008; Theoulakis et al. 2008). Further work in this area seems probable.
SURFACE COATINGS
Surface coatings is a bit of a catchall category that includes a range of
materials applied to stone—protective water repellents, emulsions,

44 Chapte r 1
PROOF 1 2 3 4 5 644 Chapte r 2
PROOF 1 2 3 4 5 6anti graffiti coatings, salt inhibitors, protective oxalate layers, sacrificial
lime coatings, colloidal silica, biocides, and bioremediation treatments.
A  substantial research effort in the 1970s and 1980s was aimed at find –
ing a single treatment that would both consolidate and protect stone.
However, the naïveté of this approach has become increasingly apparent, and many conservators now accept the need for two treatments: one to consolidate and one to protect. The soundness of the latter approach
has been borne out by Félix and Furlan (1994) and Alonso and others
(1994) , who reported damage to certain stones following treatment with
tetra- ethoxysilane (TEOS) unless the stones were also given a water-
repellent coating.
Protective treatments need to be maintained, and this means
retreatability needs to be taken into consideration when designing a treat –
ment system. Surface coatings can be renewed at regular intervals, but the initial consolidation will, it is hoped, last much longer.
Some researchers have suggested doing away with the consolidant
and relying solely on the water repellent (Sramek 1993) . However, the
long and disappointing history of water-repellent coatings on the more porous limestones and sandstones should not be dismissed too readily (Honeyborne et al. 1990) .
Water Repellents The property that has been most sought in surface coatings is water repellency. The logic behind the approach is simple: Since water is involved in most forms of stone decay, a treatment that prevents the ingress of water should help to reduce decay. Reviews of the use of water –
proofing agents on stone can be found in Charola (1995), Bromblet and Martinet (2002), as well as Vallet and others (2000). The influence of the substrate and the temperature of application for water repellents have been investigated by De Clercq and De Witte (2001). A series of confer –
ences on water repellents have been held, with the most recent in Brussels, Belgium in 2008 (International Conference on Water Repellent Treatment of Building Materials: Hydrophobe V 2008), (De Clercq and Charola 2008).
Water repellency has been provided largely by alkoxysilanes,
silicones, and fluoropolymers. The development of the fluoropolymers
provides an interesting, and regrettably rare, instance of “tailor-made” products. The polymers are close relatives of polytetrafluoroethene (PTFE, or Teflon), renowned for its nonstick properties. The early fluoropolymer coatings worked well, except for a rather poor ability to stick to the stone! Subsequent development has entailed the synthesis of compounds containing functional groups that can adhere to the stone surface, thereby providing more persistent protection (Piacenti, Camaiti, Manganelli del Fa, et al. 1993). It has been argued that such water repellents should help to prevent resoiling, although this claim has not been adequately substan –
tiated. The rapid loss of water-repellent properties after accelerated ( arti-arti-
ficial) and field (natural) weathering has been noted by several researchers
and deserves further study.

Stone D ecay 45
PROOF 1 2 3 4 5 6 Putting It Right: Preventive and Remedial Treatments 45
PROOF 1 2 3 4 5 6Another example of “tailor-making” is provided by Fassina and
co-workers ( Aglietto et al. 1993 ; Fassina et al. 1994 ), who have synthe –
sized a range of fluorinated acrylic polymers. The intention, which was
partially achieved, was to improve water repellency and resistance to photooxidation, by comparison with nonfluorinated analogues such as Paraloid B72. In a different approach, research on the use of polyure –
thanes on stone, known as the “Aachen concept,” has been reviewed by Snethlage and Wendler (2002).
More recent work on several types of water repellent (an acrylic
dispersion, an oligomeric alkylpolysiloxane, a solution of silicone resin and an alkylalkoxysiloxane in aqueous microemulsion) applied to seven types of limestone found that: “Due to the diverse petro-physical nature and properties of each stone, the results indicate that no universally com –
patible protective treatment exists” (Boutin 2001, 233) . Accelerated or
artificial tests of hydrophobic coatings as a method for reducing the
effects of air pollutants on porous, calcareous stone have had mixed results, with the protective effect decreasing rapidly with time in bulk samples (Camaiti et al. 2007) , while X-ray photoelectron spectroscopy
(XPS) microanalysis showed adequate performance after aging 240 hours (Torrisi 2008) .
In addition to surface layers on stone, the water-repellent proper –
ties of silanes have also been used to create chemical damp-proof courses (DPC) along the base of foundations of buildings that lack this common feature of modern masonry buildings (Pinto Guerra 2008; Young and Ellsmore 2008). While an ancient idea (see Vitruvius 7.4), DPCs began to be standardized in new construction only starting in the mid-nineteenth century (Schmidt 1999). Chemical DPC application methods have included gravity feed and pressure injection of silanes into regularly spaced holes drilled into the foundation. Current methods include a cream containing silane injected into holes drilled along a mortar joint. The silane appar –
ently diffuses out of the cream and some distance into the mortar to form a chemical DPC. The long-term performance of various DPC measures suggests that some may experience a rapid loss of effectiveness over time (Alfano et al. 2006; Lopez-Arce et al. 2009; Henry 2006, p. 277).
Recent work shows that sodium chloride preferentially crystal –
lizes on hydrophobic surfaces (Shahidzadeh et al. 2008), suggesting that water repellents are not compatible where salts may accumulate (Lubelli et al. 2007). An EC-funded project, SCOST (Salt Compatibility of Surface Treatments), addresses this issue in detail (De Witte 2001; Miquel et al. 2001).
Anti-Graffiti Coatings
The problem of graffiti has spread across diverse urban environments over the past fifteen years and is affecting not just modern buildings but historic monuments as well. A new EC project on the topic, comparing five graffiti protectives in six countries (Gardei et al. 2008), has found that four commercial anti-graffiti agents strongly reduce water and
vapor transport and thus are not compatible with most historic building

46 Chapter 1
PROOF 1 2 3 4 5 646 Chapter 2
PROOF 1 2 3 4 5 6materials. However, a new product developed specifically for historic mat-
erials was found to have acceptable performance and is undergoing field
testing. Recent work on an anti-graffiti coating containing perfluoropoly –
ether and epoxysilanes in aqueous microemulsion with an epoxide curing agent found good resistance to repeated cleaning cycles (Licchelli and
Marzolla 2008) . Earlier work by Mertz, Grunenwald, and Ternay (2003)
found that some reduction in water vapor permeability was necessary to get efficient protection and that the preventive anti-graffiti treatments do not perform the same on substrates with high and low capillary absorp –
tion coefficients.
Emulsions
Complex emulsions as stone protectives have been studied by a number of researchers. The emulsions have included acrylics ( Kumar and Ginell 1995 ;
Theoulakis et al. 2008 ; Karatasios et al. 2009 ), silicones ( Snethlage and
Wendler 1991 ; Ren and Kagi 1995 ; Mao and Kagi 1995 ; Van Hees and
Koek 1995 ; Ciabach 1996 ; Boutin 2001 ), silanes ( Biscontin et al. 1993 ;
Licchelli and Marzolla 2008 ; Wittmann et al. 2008 ), and fluorina ted poly –
urethanes (Guidetti, Chiavarini, and Parrini 1992; Croveri and Chiavarini 2000). Performance varies from stone to stone but is generally promising.
Crystal Growth Inhibitors
Another possibility is to treat the stone surface with compounds that inhibit the growth of salt crystals, as was mentioned briefly in the section on salts. Relevant technology already exists in such diverse fields as anti-caking agents for road salt and in oil extraction, where phosphonates are used to prevent the precipitation of barium sulfate and calcium sulfate (Black et al. 1991). Applications in the field of conservation have been proposed from time to time (e.g., Puehringer and Engström 1985), and recently this area has received some further research (Selwitz and
Doehne 2002), including an EC project, SALTCONTROL, on the topic (Rodríguez-Navarro, Hernandez, and Sebastian 2006; Cassar et al. 2008). Inhibitors used to treat stone surfaces, such as phosphonates and carbox –
ylates were found to be a mixed blessing. In some instances they decrease damage by letting salts reach the surface as less harmful efflorescence. However, in other situations they enhance solution supersaturation ratios and absorb to surfaces, resulting in increased rates of damage.
Oxalate Formation
Building on the protective properties of scialbatura
(see the Biodeteriora –
tion section in chapter 1), Matteini, Moles, and Giovannoni (1994) tested
the use of ammonium oxalate to produce a shallow film of calcium oxalate on calcareous surfaces such as wall paintings. Both calcium car –
bonate and calcium sulfate react with a poultice containing a solution of ammonium oxalate to produce a cohesive, hydrophilic film that reduces rates of acid attack ( Hansen et al. 2003 ; Doherty et al. 2007 ; Sikka et al.
2008 ). The method has been used to help protect objects and surfaces
that cannot be removed to a more protective environment ( Ambrosi et al.
2000 ; Mairani, Matteini, and Rizzi 2000 ).

Stone De cay 47
PROOF 1 2 3 4 5 6 Putting It Right: Preventive and Remedial Treatments 47
PROOF 1 2 3 4 5 6Lime and Biocalcification
The final stage of the lime treatment consists of the application of a very thin coating of lime and fine aggregates rubbed firmly into the surface of the stone (see above under the Lime and Related Treatments section). The
coating is intended to protect the stone, and it is reapplied as necessary. An alternative approach, which started first in France, utilizes microbes to produce a sacrificial surface layer of calcite ( Orial et al. 1996 ; Le
Métayer-Levrel et al. 1999 ; Castanier et al. 2000 ; Orial, Vieweger, and
Loubiere 2003 ). Results from an EC project on bioremediation
(BIOBRUSH) have been presented by Webster and others ( Webster,
Vicente, and May 2004 ; Webster and May 2006 ; May et al. 2008 ).
Biocalcification in the context of conservation treatments is reviewed by Tiano (2008) , and promising test data is presented by Zamarreño,
Inkpen, and May (2009) .
Colloidal SilicaKozlowski, Tokarz, and Persson (1992) have adopted a rather different approach for forming a protective coating on calcareous stones. They have used sols of colloidal silica that deposit silica particles within the outer pores of the stone. The resulting surface is hydrophilic, but the pas –
sage of water through the pores is impeded by the presence of the parti –
cles. The material has been used at several sites to help protect vulnerable calcareous materials from acidic pollution (Stepien, Kozlowski, and Tokarz 1993; Mangio, Simpson, and Tokarz 1996). The method has been further developed by the conservator Egon Kaiser for use as a void filling and repair mortar at Petra and other sites (Kühlenthal, Kaiser, and Fischer 2000; Simon, Shaer, and Kaiser 2006).
Biocides
There is a long history of research into surface treatments that will kill biological growths and, if possible, inhibit regrowth. Such treatments must meet a large number of criteria, and this can prove difficult in the outdoor environment, where there is a continual supply of moisture to promote regrowth. They must not only kill the growth in the first place but also be resistant to new strains. They must not have any harmful effect on the stone itself, nor must they change its appearance. They must not be
washed out by rainfall before taking effect or destroyed by ultra violet
light, and they must be safe both to the person applying them and to the wider environment. The last requirement has been applied evermore strin –
gently over the past few years, with the result that a number of proven biocides have been banned by law. It follows that there is still a need for research in this area. The related area of biological stain removal has seen some development and success in removing some stubborn materials (Delgado Rodrigues and Valero 2003; Konkol et al. 2009).
Most of the existing research on biocides has been concerned with
algae, lichens, and higher plants like weeds, mosses, and ivy. Some of the research has been on cultures in the laboratory, while most of it has been based on site trials. Examples of such research are provided by Agarossi, Ferrari, and Monte (1990); Monte et al. (2000); and Anagnostidis and

48 Chapte r 1
PROOF 1 2 3 4 5 648 Chapte r 2
PROOF 1 2 3 4 5 6 others (1992) . The last also emphasize the need for regular observation and
retreatment, and they suggest early warning systems to indicate the
moment for retreatment. A promising new approach to biocontrol using anti-biofouling agents is presented by Cuzman, Tiano, and Ventura (2008) .
An interesting example of a complex fungal treatment is outlined by Orial
and Brunet (2004) , while a recently proposed treatment for lichens is
removal by a low-pressure abrasive technique using dry ice (Rosato 2008) .
Laser treatment for lichens has been investigated by DeCruz and others
(2009) . A book reviewing the topic of biocides for natural and artificial
stone is in preparation (Daniela Pinna, personal communication).
Caneva, Nugari, and Salvadori ( 1991 ; 2008 ) provide a valuable
account of the many available biocides, which are normally applied to
the surface of the stone by brush or spray. Portable objects may also be treated by fumigation: Elmer and others (1993) , for example, report the
use of ethylene oxide. Bassier (1989) reports the use of ultraviolet radia –
tion to sterilize mineral surfaces. Caneva, Nugari, and Salvadori ( 1991 ,
p. 119; 2008 ) mention the possibility of preventive conservation by the
deliberate introduction of suitable vegetation in the vicinity. Some water-repellent treatments act to prevent biological growth by limiting available water. Low tech is still a useful approach, as shown by work using
hot water vapor to kill lichens and algae (Orial and Bousta 2005) .
Sorlini, Falappi, and Sardi (1991) report the inhibition of fungal
growth by a methylphenyl silicone resin, but other workers ( Petushkova
and Grishkova 1990 ; Santoro and Koestler 1991 ; Krumbein et al. 1993 )
have reported the opposite effect: the biodegradation of silicones.
Relatively little research has been conducted on antibacterial
treatments for stone. This is surprising, perhaps, in view of the extensive work on the role of bacteria in decay, but it may reflect the difficulty of finding antibacterial treatments with sufficient persistence (Gorbushina
et al. 2003) . Nonetheless, Orial and Brunet (1992) present a satisfying
account of the use of streptomycin and kanamycin to substantially reduce bacteria in stonework at Elne Cathedral for a period of more than seven years, with a resulting cessation of decay.
Biological Attack on Treatments
In some cases polymeric treatments of stone become food for microbes, leading to the production of organic acids and other biological activity related to the consumption of surface treatments (Cappitelli et al. 2007b; Cappitelli and Sorlini 2008). However, this biological affinity for certain otherwise insoluble, cross-linked organic material has also been used as a bioremediation treatment to remove the hardened glue from the surface of a fresco fragment in storage for twenty years (Antonioli et al. 2005).
Note
1 This was also discussed in a paper presented by Simon Warrack at the Stone
Consolidation in Cultural Heritage: Research and Practice Symposium, held in Lisbon, May 6–7, 2008.

PROOF 1 2 3 4 5 6One might suppose that the most practical approach to stopping or
reducing stone decay would be simply to apply a treatment and see if
it works. But how can we tell if it is working? What do we really mean by “working”? How long does a treatment need to be left in place? Can things be speeded up a bit? Will it keep on working indefinitely? Will it work on other stones in other environments? What about other treat –
ments that come along while a lengthy evaluation of one is being
carried out?
Price (1982) reviewed strategic approaches to the evaluation of
treatments, an issue that lies at the crux of the conflict between “doing something” and not causing harm. It is a subject that is of vital impor –
tance. We need answers straightaway in order to devise responsible pro –
grams for the conservation of monuments that are decaying before our eyes. But if we act too quickly and apply the wrong treatment, we may make matters even worse.
Many researchers have devised their own procedures for evaluat –
ing treatments, using a range of tests to build up an overall picture ( Sasse
and Snethlage 1996 ; Van Hees 1998 ; Moropoulou et al. 2000 ; Haake,
Simon, and Favaro 2004 ; Laurenzi Tabasso and Simon 2006 ; Bracci et al.
2008 ; Costa and Delgado Rodrigues 2008b ). This is both inevitable and
understandable, since individual researchers are constrained by the range of techniques that are available to them. Having a range of techniques also has the advantage that the procedure can be tailored to suit a partic –
ular stone and environment (Galán and Carretero 1994) . It is unrealistic
to think that any single procedure could fit all circumstances. However, it can be very difficult to compare the findings of one researcher with those of another, and there is a need for standardized procedures. This was the underlying objective of the RILEM (Réunion Internationale des Laboratoires et Experts des Matériaux, systèmes de construction et ouvrages) Commission 25-PEM (Protection et érosion des monuments) Working Group, albeit not fully achieved ( RILEM 1980 ; Price 1982 ).
The definition of individual test methods has had more success (see the following), although the “not invented here” syndrome frequently hin –
ders their widespread adoption. Some useful advances in standardized evaluation procedures have been made by CEN (Comité Européen de Normalisation) Technical Committee 346 (Fassina 2008) . Chapter 3
Do They Work? Assessing the Effectiveness of Treatments

50 Chapte r 1
PROOF 1 2 3 4 5 650 Chapte r 3
PROOF 1 2 3 4 5 6It is convenient to divide evaluation procedures into two catego –
ries: those that characterize the stone shortly after treatment has taken
place, and those that are concerned primarily with monitoring long-term performance. Questions that should always be asked before proceeding are: “What criteria apply, and is enough information available to sustain a recommendation to use this or that stone treatment?”
CHARACTERIZING THE TREATED STONE
There are some properties that are helpful in building up an overall pic –
ture of the treated stone, even though they may not give any direct indi –
cation of the treatment’s effectiveness. These include the porosity and
pore size distribution of the treated stone, its appearance, and the depth of the treatment’s penetration. The majority of tests, however, are con –
cerned with measuring properties that are known to change as the stone decays or with assessing the extent to which the treatment has met cer –
tain clear objectives.
Properties That Change with Decay
We have already looked at a number of tests that are intended to measure the extent of stone decay. These tests can obviously be applied to stone that has been treated with a consolidant or surface coating in order to determine whether there has been an improvement in performance. Such tests might include water uptake, Scotch tape tests, surface hardness, drilling resistance profile, and ultrasonic pulse velocity ( Giorgi, Dei, and
Baglioni 2000 ; Vergès-Belmin and Laboure 2007 ). The paper by Villegas,
Vale, and Bello (1994) illustrates the difficulties of interpretation that
may arise. If it is difficult to characterize decayed stone, then characteriz –
ing treated stone is doubly difficult. And if one treatment is difficult enough to characterize, work on stone treated with both consolidant and waterproofing agents shows that together they have a larger effect on pore structure than any single treatment (Iñigo et al. 2001) . Then con –
sider the important issue of retreatment of treated stone.
Meeting Objectives
Some tests are designed to assess the extent to which the treatment is meet –
ing certain objectives. If the treatment is intended to impart water repellency, for example, then measurements of contact angle and water absorption are appropriate. If it is intended to provide protection against acid rain, then measurements of weight loss or of salt formation may be necessary. The logi –
cal outcome of this approach is the definition of a set of performance criteria against which a treatment may be judged; Sleater (1977) provides a good example. However, Sleater was unable to find any treatment that met all his criteria, and this may account in part for the lack of attention subsequently given to establishing overall criteria.
Caution must be exercised when using tests intended primarily
for untreated stone. For example, a crystallization test that relies on the absorption of a sodium sulfate solution is frequently used to determine a

Stone D ecay 51
PROOF 1 2 3 4 5 6 Do They Work? Assessing the Effectiveness of Treatments 51
PROOF 1 2 3 4 5 6stone’s resistance to salt weathering (Doehne 2002) . The test should not
be applied unthinkingly, however, to a stone that has been coated with a
water repellent. Excellent performance in the test (e.g., Villegas and Vale
1992 , p. 1259) would not necessarily indicate increased resistance to salt
growth; it could simply indicate that the water repellent had prevented the ingress of salt in the first place.
Standard Test Methods
Standard test methods are essential if one is to compare the results of dif –
ferent laboratories in any meaningful way. Even for a seemingly straight –
forward property such as water repellency, differing procedures yield differing results (Henriques 1992) . Subsequent interlaboratory testing of
some hygric European Norm (EN) standards has found that while many work well, vapor transmission tests were found to have large variations when done according to EN norms (Roels et al. 2004) .
The standardization of test methods has been the objective of both
national and international committees. Notable among them are the rec –
ommendations of the RILEM 25-PEM and 59-TPM (Traitement des mon –
uments en pierre) Working Groups ( RILEM 1978 ; Pien 1991 ) and the
standards published by the Italian Commissione NORMAL (Alessandrini
and Pasetti 2004) . It is regrettable that details of these test methods have
not been more readily available and more widely translated. However, European standards for stone conservation are currently being integrated under the EN norms, with CEN Technical Committee 346 being led
by Fassina (2008) . The work incorporates the Italian NORMAL, German
DIN, RILEM, and other national standards groups ( Koestler and Salvadori
1996 ; Alessandrini and Laurenzi Tabasso 1999 ; Fontaine, Thomson, and
Suter 1999 ). Other work in building materials standards can be found in
various ASTM (American Society for Testing and Materials) and RILEM committees ( http://www.astm.org ; www.rilem.net ).
The trend of research on standards has been to find and define
quantitative parameters to characterize materials and help guide treat –
ments as a way to ensure compatibility between interventions and exist –
ing materials (Sasse and Snethlage 1997; Bromblet et al. 2002; Laurenzi Tabasso and Simon 2006; Laurenzi Tabasso 2008). However, the exten –
sive list of parameters that researchers suggest should be measured to evaluate treatments has been shown to be unrealistic in the field (Moraes Rodrigues and Emery 2008). As a practical matter, most treatment evalu –
ations have focused on changes in water uptake, color, ultrasonic mea –
surements, or drilling resistance profiles (Ferreira Pinto and Delgado Rodrigues 2008).
LONG-TERM PERFORMANCE
It is one thing to find a treatment that performs well in the short run; it is
another thing altogether to be sure that it will keep on performing year
after year when exposed to the weather. When a water repellent progres- When a water repellent progres –
sively loses it hydrophobic properties, we may say that its effectiveness is

52 Chapt er 1
PROOF 1 2 3 4 5 652 Chapt er 3
PROOF 1 2 3 4 5 6decreasing. However, when the application of a stone consolidant leads
(with time) to differential stress and the eventual detachment of the indu –
rated scales, we may say that it is showing a “delayed harmfulness.” Both are examples of long-term performance, yet they represent two distinct phenomena.
Natural exposure trials provide the only true test. They may be
carried out in situ on monuments or on small blocks of stone that can
be brought into the laboratory at intervals for evaluation. Either way, the trials can provide information only on a limited number of stones, treat –
ments, and environments, and it may be many years before reliable infor –
mation is obtained. A new range of treatments will inevitably have emerged in the meantime. Additionally, one is still confronted with the difficulty of evaluating the effectiveness of the treatment. The surface may look sound on the outside, but what is going on underneath? In situ mon –
itoring relies heavily on the techniques described in chapter 1.
Nevertheless, there are a number of important questions that
have been asked and answered by long-term natural exposure trials.
For example, Moreau and others (2008) asked the question “Do water-
repellent treatments reduce soiling in protected parts of monuments, and do they allow for easier cleaning?” They found after a ten-year study that silicone water-repellent treatments did not decrease the limestone soiling rate, while a fluorinated acrylic resin decreased it significantly. This result is encouraging and suggests that new-generation fluorinated acrylic resins could be used to protect stone against soiling. Not all fluorinated acrylic resins are suitable for every type of stone, however, since some are film forming and may peel away. Sulfation rates were not decreased by water-repellent treatments. After measuring soiling and sulfation, the test slabs were cleaned by micro sandblasting and laser to determine if the coatings had changed the cleaning efficiency. The results show that treated samples typically were not easier to clean by micro sandblasting but instead became lighter in color than untreated samples. After laser-cleaning treated and untreated samples, the typical yellowing observed after laser cleaning was less noticeable on samples treated with silicon-based water repellents. The study also showed that the darker the samples were after exposure, the yellower they were once laser-cleaned (Moreau 2008) .
Another interesting example of long-term analysis of treatment
performance is research by Favaro and others ( 2005 ; 2006 ; 2007 ), where
they analyzed the effectiveness (1979–2005) of consolidants and water repellents on marble in the field in Venice and in the laboratory. One of the findings was that Paraloid B72 undergoes irreversible modifications over time and becomes impossible to remove completely. Thus, while a treat –
ment may be technically removable at the time of application and in com –
pliance with the dictum of reversibility, over time this may not be the case.
Accelerated weathering chambers are extensively used to simulate
decay (see, for example, Sasse and Riecken 1993 ), but they also introduce
another layer of uncertainty (Warke and Smith 1998) . Do they accurately
reflect long-term behavior? By what factor do they increase the rate of weathering and decay? When consolidated stones are subject to artificial
aging, it is common to call these trials “durability” tests. However, what

Stone Decay 53
PROOF 1 2 3 4 5 6 Do The y Work? Assessing the Effectiveness of Treatments 53
PROOF 1 2 3 4 5 6these trials essentially assess is not the durability of the consolidation
properties but the delayed harmfulness introduced when a consolidated layer is present. A stone consolidant may keep its strengthening proper –
ties intact and show poor performance—delayed harmfulness—in an
aging test.
It is noteworthy that most of the literature on long-term perfor –
mance is concerned with the behavior of the treated stone per se. There have been surprisingly few in-depth studies of the breakdown of the treatment itself. Even in the field of alkoxysilane consolidants, few authors have made systematic studies of the long-term weathering of the resulting silicone polymer (Favaro et al. 2006) . Some authors, however,
have highlighted the fact that treatments may serve as an energy source for microorganisms ( Koestler and Santoro 1988 ; Petushkova and
Grishkova 1990 ; Krumbein et al. 1993 ; Cappitelli et al. 2002 ; Cappitelli
and Sorlini 2008 ). This important aspect had been largely overlooked
hitherto.
Documentation of Field Trials
There is a regrettable tendency for researchers to set up field trials, to monitor them for a few years, and then to forget about them as further treatments become available. There is a need for systematic, long-term monitoring of trials. This is often hindered, however, by woefully inade –
quate or missing records.
The availability of sophisticated databases offers the possibility of
creating good, centralized records, and this possibility has been seized by a number of workers. Fitz (1991; 1996) described the MONUFAKT data –
base adopted by the German federal environmental agency, and Rosvall and Lagerqvist (1993) developed the EUROCARE database. More recent work on databases has focused on regional issues (Klamma et al. 2006; Hyslop et al. 2009) and specific projects (Inkpen et al. 2004; May et al. 2004; Cassar 2004). The difficulty lies in persuading researchers to put reliable, comprehensive information into the system and in persuading others to use it in future years. This is on top of the difficulty of curating digital records over long time periods, during which hardware, software, and even institutions may rapidly become unsustainable. There is now a movement to encourage researchers to put machine-readable data directly onto the Internet (Grossenbacher 2009; Rosling 2009). Thus, while the technology is available and the potential benefits of data sharing are evi –
dent, this approach to databases has seen little implementation and sus –
tained effort, aside from project- or region-specific efforts (GCI 2009).

PROOF 1 2 3 4 5 6Conservation is not immune to the vagaries of fashion—fashion that
varies with both time and place. In England, for example, it was
fashionable one hundred years ago to replace decayed sculpture with
“copies”— conte mporary interpretations of what the originals might
once have looked like. By contrast, the current normal philosophy is
to “conserve as found”—to keep original material and prevent further
deterioration as far as is practicable. A further approach is common in the Far East, where the emphasis is more on preserving the function
of a monument than on preserving the materials from which it is constructed.
Numerous attempts have been made to codify conservation prin –
ciples and to introduce international uniformity. Notable among them are the 1964 International Charter for the Conservation and Restoration of Monuments and Sites (the Venice Charter), the Burra Charter (Australia, ICOMOS 1999), and Principles for the Conservation of Heritage Sites in
China (China, ICOMOS 2000). It is beyond the scope of this publication
to discuss conservation principles in detail, but it is relevant to note that many parties play a role in shaping conservation policy: the architect, the art historian, the scientist, the archaeologist, the conservator, the owner, and ultimately, the general public. The scientist may be convinced of the validity and importance of his or her results, but there are others to be convinced before the results can impact on conservation policy. One example of an interesting policy discussion of how research in heritage science is organized and carried out on a national level took place in 2005–6 in the UK (House of Lords, Science and Technology Committee 2006; House of Lords, Science and Technology Committee 2007) (see also: http://www.heritagescience.ac.uk ).
This chapter focuses on just three aspects of conservation policy:
the responsible use of surface coatings, adhesives, and consolidants; the problems posed by multiple treatments; and recording. These three issues have been chosen because they have a common thread, which is the fact that no treatment can be expected to last forever. However much we may be lured into thinking that a treatment will last indefinitely (or, perhaps, until we are no longer accountable for it?), we must accept that all treat –
ments have a finite life. This has direct implications for conservation pol –
icy in the three areas indicated. Chapter 4
Putting It into Practice: Conservation Policy

Stone Decay 55
PROOF 1 2 3 4 5 6 Putti ng It into Practice: Conservation Policy 55
PROOF 1 2 3 4 5 6RESPONSIBLE USE OF SURFACE COATINGS AND CONSOLIDANTS
If a treatment is not going to last forever, should we use it in the first
place? As we have seen above, we cannot be absolutely sure that the treatment will not lead to some unforeseen problem in the future. At what point should we take the risk of applying a treatment to an impor –
tant stone object/monument?
Conservators paid homage for a long time to the principle of
reversibility: no treatment should be used unless it can be removed at some future date, should that prove necessary. In the context of stone conservation, however, reversibility is more idealistic than realistic. It can be extremely difficult, in practice, to remove even the most soluble of treatments. It is wiser, therefore, to assume that a treatment, once applied, cannot ever be totally removed. Succeeding generations are going to have to live with the consequences of our actions.
What should we do? Treatment is irreversible, in practice, but
decay through neglect is irreversible too. The dilemma highlights the importance of preventive conservation, but there are instances where
preventive conservation is not enough. Ultimately it will be necessary to
reach a carefully balanced decision, taking into account all aspects of each individual case. Sometimes we will conclude that treatment is justi –
fied; at other times, we may conclude that we can safely defer treatment for the time being.
This polemic is all very well, but sadly it is often irrelevant. In
many of the cases with which we are confronted, the stonework has already been treated by previous generations who were perhaps less cau –
tious or more optimistic than we may be. Often we do not know with any certainty the identity of the treatment, and often there may have been more than one treatment. This leads us to the problems of retreatment.
RETREATMENT
Virtually all research on stone treatment is based on the assumption that
the treatment is to be applied to stone that has never been treated before. It is astonishing that so little work has been done on the effects that one treatment might have on another. While we hear much about reversibility, we hear little about retreatability, even though the latter is a far more important concept in practice ( Teutonico et al. 1997 ; Van Balen, Ercan,
and Patricio 1999 ; Hansen et al. 2003 ).
Any consolidant that blocks the pores of the stone and prevents
the subsequent application of another consolidant must clearly be regarded with some caution. The topic that demands research, however, is the physical and/or chemical interaction of one consolidant with another. The swelling of polymers under the influence of solvents is a well-known phenomenon, but little attention seems to have been paid to the swelling of a consolidant when a second consolidant is applied. It is possible that such swelling might cause damage to the stone—which can safely be

56 Chapt er 1
PROOF 1 2 3 4 5 656 Chapt er 4
PROOF 1 2 3 4 5 6assumed to be fragile, or the consolidant would not have been applied in
the first place. Moreover, one can imagine that the second consolidant will not be deposited as a coherent layer on top of the previous treatment but will form an intermingled mixture. It is not an appealing prospect, and it certainly deserves more attention. In one example, surface over-strengthening following reconsolidation treatment has been examined by Meinhardt-Degen and Snethlage (2007) using biaxial flexural strength
and modulus of elasticity as the criteria.
There is equally a need to ensure that there are no unforeseen
consequences of multiple applications of maintenance coatings. For exam –
ple, recent work by Moreau and others (2008) has shown that the effec –
tiveness of a water-repellent coating is reduced if it is applied either on top of, or beneath, a quaternary ammonium biocide.
RECORDING
If we cannot preserve it forever, it is imperative that we make the best pos –
sible record of stone as it exists. Indeed, one could argue that recording
should have a higher priority than preserving the stone itself— provi ded, of
course, that we are confident that the records can, in turn, be properly
maintained and curated indefinitely. This is a very big proviso, and it is one that also relates to the recording of field trials. All too often, careful records may be made of treatment applications, and they are duly lodged in filing cabinets, archives, or computer disks. But ten or twenty years later, when individuals have moved on or retired, these records can disap –
pear without a trace; much of the benefit of the trial is lost, and later con –
servators have no record of what was applied in the past.
Drawing and photography still have a place in recording, but
attention is turning increasingly to techniques of three-dimensional recording. Molding and casting is the traditional technique, but it is not always practicable on very delicate or undercut surfaces. It is accordingly being replaced by techniques that do not entail any physical contact with the stone surface.
Raking light photography, including PTM imaging, can provide a
useful documentation of the surface texture of stone, which can be later draped over a digital elevation model (DEM). Stereophotography has been known for a long time but provides only an illusion of depth from a single viewpoint. Photogrammetry, a related technique, has been widely used for producing contoured images, but it still suffers from the drawback of a single viewpoint. Holography overcomes this problem, and its use for recording sculpture was first proposed by Asmus and others (1973) .
Nonetheless, the role of holography has been limited largely to the pro –
duction of images that, while visually striking, do not really provide a quantitative record. In this respect, laser scanning has taken the lead.
The operation of a laser scanner to record stone weathering is
described by several authors ( Kleiner and Wehr 1994 ; Ball, Young, and
Laing 2000 ; Warrack 2000 ; Bates et al. 2008 ; Tiano and Pardini 2008 ).
The time it takes to scan an object depends upon its size, the desired res –

Stone De cay 57
PROOF 1 2 3 4 5 6 Putting It into Practice: Conservation Policy 57
PROOF 1 2 3 4 5 6olution, and the surface texture (shiny surfaces, such as polished or wet
stone, are challenging). The primary output of the scanner is a digital record of the three-dimensional form of the object, and this can be used in a number of ways. It may be used purely as an archival record (e.g., http: //archive.cyark.org ); it may be used (in conjunction with subsequent
scans) to monitor the deterioration of an object (Smith et al. 2008); and it may be used to drive a milling machine in order to produce a replica (Ahmon 2004).
One of the great advantages of the laser scanner is that it does
not entail any physical contact with the object and is hence suitable for even the most delicate surfaces. It has also been developed to the point where it is now capable of providing stereoscopic color images with a resolution of about 0.025 mm, which may be viewed, sectioned, and mea –
sured. Data processing and data management are significant challenges, and data clouds of 3D points are challenging to work with, in part because the 3D laser-scanning software equivalent of Adobe Photoshop does not yet exist and thus far, 3D software tends to be complex and expensive. Creating a DEM or 3D surface out of a cloud of 3D points acquired by the laser scanner requires significant processing and manipu –
lation. In order to monitor change over time, subsequent DEMs need to be registered in three dimensions using a number of control points, whose position is known not to have changed.
The ability to produce highly accurate replicas of decayed stone –
work is an attractive proposition that has been seized upon by some con –
servators (Larson 1992; Ahmon 2004). Original sculpture can be taken indoors to the safety of a museum, while an exact copy can be put in its place. Nonetheless, there are those who argue that it may be inappropri –
ate to install an exact copy that will be missing features already lost from the original and that it may be preferable to re-create those features in as sympathetic a manner as possible. In any event, the costs of implementing such a process remain significant, and it has not yet become a common tool in the conservator’s toolbox.
As recording technologies rapidly advance, notions about the pos –
sibility of digital preservation of monuments regularly crops up, but it behooves the conservation profession to act to prevent further damage to cultural heritage as its first priority.

PROOF 1 2 3 4 5 6Issues of stone conservation extend beyond conventional notions of
masonry buildings, carved gargoyles, and beautiful stone entryways.
The effort to preserve our heritage in stone includes research in the
conservation of rock art, the preservation of historic quarries (both for
replacement stone and as historic industrial technology), the specialized conservation needs of some ornamental stones (from colored marble to mosaic stone), and the related arts of stone decoration (stone carving, polychromy, and wall painting). Finding replacement stone is an impor –
tant problem in maintaining historic structures. Often the lack of timely availability of suitable stone may result in one aspect of performance being emphasized over another (e.g., initial color match, match after weathering, durability, or compatibility with existing stone). Researchers have addressed this problem in different ways in different regions.
Reviewing research related to heritage in stone is important to
enhancing cross-fertilization between these specialized areas, which often have substantial interests in common. In this chapter we will focus on the first two issues—conservation of rock art and preservation of historic quarries—since the second two—ornamental stones and stone decoration
—are well covered by existing reviews ( Stieber 1995 ; Viles 2003 ; Henry
2006 ; Pepi 2008 ).
ROCK ART
A simple definition of rock imagery, as rock art should be called, encom –
passes engravings (petroglyphs) and paintings (pictographs) on rock
surfaces ( Ward and Ward 1996 ; Whitley 2001 ; Whitley 2005 ). While
Lascaux in France and Altamira in northern Spain are among the best-
known sites, important rock art collections are to be found around the world, notably in the southwestern United States, Australia, South Africa, India, Scandinavia, and the Sahara.
With respect to the state of the field of rock art conservation,
“there has always been a large public interest in ancient pictures painted or carved on stone, but the archaeological study of rock art is in its
infancy.”
1 The same may be said for rock art preservation. One specialist
has commented that, in Australia, “reactive management is in vogue and Chapter 5
Heritage in Stone: Rock Art, Quarries, and
Replacement Stone

Stone Decay 59
PROOF 1 2 3 4 5 6 Herit age in Stone: Rock Art, Quarries, and Replacement Stone 59
PROOF 1 2 3 4 5 6not proactive research, conservation and planning” ( Watchman 2005 , 17).
There are, however, signs of progress. Norway implemented an extensive
program for rock art preservation, undertaking an unprecedented eight-million-dollar, ten-year project between 1998 and 2008 for the research and conservation of three hundred important sites ( Bjelland and Thorseth
2002 ; Bakkevig 2004 ; Bjelland et al. 2005 ; Hygen 2006 ; Gran 2008 ).
MacLeod (2000) provides a useful review of the complex issues sur –
rounding rock art preservation.
Interest in the conservation of cave paintings and rock art has
increased significantly since the first edition of this book in 1994. The close relationship between the conservation of building stone, wall
paintings, and sculpture and the conservation of rock art panels is self
evident, with research opportunities for cross-fertilization between these disciplines.
Of paramount interest is the long-term preservation of fine sur –
face details in carved stone and paint layers on stone. Traditional building stone conservation has something to learn from rock art conservation, not least as a laboratory of long-exposed art in a range of stone types and environments. Now that more rock art can be reliably dated, ques –
tions of mutual interest such as the protective and destructive aspects of lichens on longer time scales can be considered.
Approaches to the conservation of buildings and of rock art dif –
fer in interesting ways, with a number of rock art programs making effec –
tive use of volunteers for documentation and evaluating change over time, whereas in the building conservation field, such tasks have tradi –
tionally been the province of conservation professionals. A common prob –
lem in both fields is the need for useful monitoring, with Neville Agnew suggesting, “It is incumbent on us to find ways to slow rates of deteriora –
tion, which can vary enormously. One of the things not adequately stud –
ied is the rate of deterioration of rock art” ( Dean et al. 2006 , 11).
Rock Art ConservationA key problem in cultural resource management is the identification
of those archaeological remains in need of immediate conservation. Traditional management of rock art has focused on keeping site locations confidential, providing visitor control at public sites, and performing doc –
umentation. Natural weathering processes and conservation interventions have often not been included in conservation management planning for rock art sites. Professional conservators specializing in rock art are few, while scientific research on rock art has tended to focus on characteriza –
tion and dating issues rather than conservation.
The issue of rock art’s long-term sustainability and the fact that
decisions must be made when allocating scarce resources suggest the need for a method of evaluating rock art stability. In some cases, rock panels have been documented to be stable over a fifty-year period (Hoerlé 2005) ,
while other panels have been recorded as suffering rapid decay over a period of a few years (Meiklejohn 1995 , 1997 ). The establishment of a
Rock Art Stability Index has recently been proposed, involving regular

60 Chapte r 1
PROOF 1 2 3 4 5 660 Chapte r 5
PROOF 1 2 3 4 5 6recording of surface loss ( Cerveny 2005 ; Dorn et al. 2008 ) to address the
need for data on rates of change and to build on earlier efforts to docu –
ment rock art ( Turpin et al. 1979 ; Bell et al. 1996 ; Padgett and Barthuli
1997 ; El-Hakim et al. 2004 ; Barnett et al. 2005 ; Chandler, Fryer, and
Kniest 2005 ; Trinks et al. 2005 ; Chandler, Bryan, and Fryer 2007 ). A
range of proxy measurements of variables thought to be related to long-
term stability or damage rate, such as surface temperature, have also begun to be evaluated to better manage risks to rock art (Hoerlé 2006) .
There are important differences in perception, resources, and
scale when comparing traditional approaches to building stone conserva –
tion and rock art conservation. Buildings are large, freestanding, or inde –
pendent objects, while rock art is wholly embedded in natural settings (Dean 2001) . This has raised the question of whether conservation treat –
ments developed for problems affecting some more-durable building stone, such as biocide treatment for lichen control, are appropriate for fragile rock art surfaces (Tratebas, Cerveny, and Dorn 2004) . Methods
for measuring building stone decay typically require extensive training and testing and therefore carry a relatively high cost (Fitzner and
Heinrichs 2002) . In contrast, rock art condition assessment is often per –
formed by volunteers using a necessarily simplified approach (Dorn et al.
2008) , due to lack of resources and the large scale of the problems.
One of the most common deterioration factors for rock art is
salt weathering, often from gypsum ( Charola, Weber, and Bolle 1990 ;
Hernanz et al. 2008 ; Meiklejohn, Hall, and Davis 2009 ). Strontium iso –
tope analysis has shown that road salt from deicing has migrated to a rock art site in Norway (Áberg, Stray, and Dahlin 1999) and has caused
crystallization damage. Analysis of important rock art in Nine Mile Canyon in central Utah shows that the use of magnesium chloride salt as a treatment to reduce dust on dirt roads through the site appears to be having a deleterious effect on adjacent rock art (Kloor 2008) . The dust
raised by truck traffic on the road is also obscuring the visibility of some petroglyphs and paintings. Road dust has also been found to obscure rock art at other sites as well (Watchman 1998) .
The extremely well-preserved condition of cave paintings at
Lascaux, Altamira, and elsewhere led to the realization of the critical role microbes play in the long-term stability of cave paintings. Rapid cave painting decay following disturbance of the microbiological environment has reminded conservators that our knowledge of microbial decay is still inadequate (Dornieden, Gorbushina, and Krumbein 2000). The debate over how to respond to microbial “outbreaks” on cave paintings has spilled
over into the popular press (Allemand and Bahn 2005; Castellani 2005; Pringle 2008; Bahn 2008). Research into microclimate stabilization and shelters for rock art sites has found that such systems can significantly improve environmental stability and that human visitation often negatively affects the stability at cave sites (Dragovich 1981; Wainwright, Sears, and Michalski 1997; Hoyos et al. 1998; Brunet, Vouvé, and Malaurent 2000; MacLeod and Haydock 2002; Sanchez-Moral et al. 2005; Canals i Salomó et al. 2005; Brunet, Malaurent, and Lastennet 2006).

Stone Decay 61
PROOF 1 2 3 4 5 6 Herita ge in Stone: Rock Art, Quarries, and Replacement Stone 61
PROOF 1 2 3 4 5 6Progress in our understanding of how microbial activity can dam –
age rock art has improved over the past decade. For example, MacLeod
and others (1995) have documented an increase in surface acidity related
to increased seasonal moisture on Aboriginal rock art surfaces. Rapid
lichen growth over rock art in Australia was found to be related to the amount of sunlight falling on rock surfaces, resulting in proposals for shelters and other minimally invasive lichen-control methods (Ford and
Officer 2005) . In South Africa, cracks in pigment layers are allowing
water and fungi to penetrate rock paintings (Arocena, Hall, and
Meiklejohn 2008) . In an example of preventive conservation, researchers
caution against removing any vegetation that provides thermal buffering of rock art surfaces (Hall, Meiklejohn, and Arocena 2007) .
In Norway, a series of proposals have been made to reduce the
rates of damage to rock art, including sheltering, reburial, and modifica –
tion of environmental conditions to help neutralize acids and reduce oxi –
dation (Walderhaug 1998) . A review of decay mechanisms at these sites
found frost and tree roots to be of greatest concern, with acid rain and mineral leaching of lesser importance (Walderhaug and Walderhaug
1998) . The use of insulating materials on Scandinavian rock art was
found to significantly reduce the impact of freeze/thaw cycles (Gran 2008). A recent dissertation on lichen damage to rock art offers advice
to heritage managers (Dandridge 2006) .
Fire has long been recognized as a deterioration factor for rock
art. Research shows its effects are more widespread than previously thought, and preventive measures, such as clearing vegetation by hand, are recommended to reduce fire risk (Tratebas, Cerveny, and Dorn 2004) .
Similar problems have been found in different parts of the world.
For example, conservation assessments of rock art sites in Bolivia found damage from graffiti, salts, humidity cycling, and uncontrolled tourism and proposed more integrated site management (Taboada Téllez 2007) .
A case study in Brazil similarly found salt efflorescence, dust, and animal activity (nests and droppings) resulted in fading, flaking, and loss of read –
ability of rock art panels (De Oliveira Castello Branco and Cruz Souza
2002) . A higher level of coordination and information sharing in rock art
conservation research would be beneficial.
Rock Art Treatment
Conservation treatments to date have included moisture control, consoli –
dation of rocks and pigments, removal of mud nests and lichens, graffiti removal, surface cleaning, and repair of scratches or gunshot damage resulting from recreational firearm use (Pearson and Clarke 1978; Andersson 1986; Lambert 1988; Rosenfeld 1988; Brunet, Guillamet, and Plassard 1997; Dean 1997; Dean 2001; Jeyaraj 2004). In a recent treat –
ment example, test areas of schist in the Côa Valley of Portugal were treated to evaluate the long-term effects of drainage and flood protection. Outcrops were covered with “reinforced soil,” and openings between blocks were filled with layers to encourage drainage and normalize the surface (Batarda-Fernandes and Delgado Rodrigues 2008). In another

62 Chapter 1
PROOF 1 2 3 4 5 662 Chapter 5
PROOF 1 2 3 4 5 6example, a range of biocide treatments for algae on marble petroglyphs
were tested, and several were found to be effective ( Laver and Wainwright
1995 ; Young and Wainwright 1995 ).
A small core of specialist conservators has worked in the field
of documentation and conservation of rock art sites (Dean 1998; Whitley 2006). An overview of treatments used on rock art is the sub –
ject of a master’s thesis (Dandridge 2000). The use of organic glue to stabilize fragments and cement mortar to fill cracks in rock art panels has been evaluated and criticized (Bakkevig 2004). Researchers have
tested antifungal and anti bacterio logical treatments to help mitigate
biodeterioration of rock art (Gorbushina et al. 2003) . The EC has spon –
sored research into rock art conservation, including a project titled “Non-destructive technique for the assessment of the deterioration
proc esses of prehistoric rock art in karstic caves: The paleolithic paint –
ings of Altamira” (Zezza 2002).
The final report on Norway’s ten-year rock art preservation pro –
gram concluded with an appeal to restrict the use of Mowilith2 DM
123 S for conserving rock carvings, due to stability problems and the fact that Mowilith swells with the addition of ethanol. In Norway, etha –
nol is used to remove lichens, so Mowilith is now considered an incom –
patible material for which the long-term effects are unknown (Hygen 2006). Over the evolution of the project, the primary investigator became more reserved concerning direct interventions, and the project moved increasingly in the direction of indirect and preventive methods. Norwegian research has also advanced the debate over whether lichens are protective, neutral, or damaging to rock art, finding some lichens are more aggressive than others (Bjelland and Thorseth 2002; Bjelland and Ekman 2005; Bjelland et al. 2005). However, current Norwegian guide –
lines discourage lichen removal through chemical treatment and indicate that “removal of lichens should only be done in the instances where there are binding plans for regular follow-up of the actions” (Hygen 2006, 19).
Approaches to cleaning of rock art sites also vary according to
the specific environmental problems affecting the art. To remove graffiti at the cave of Rouffignac, the ceiling was cleaned with compresses soaked in a diluted ammonia solution, while in areas where the graffiti was more difficult to remove, special erasers of differing density were used (Brunet, Guillamet, and Plassard 1997). In Zimbabwe, paint stripper and toluene were recommended to clean the graffiti from rock art (Taruvinga 2003), because, when tested, laser cleaning was found to remove both the graffiti and the original paint layers.
The overall impression gained from this literature survey is that
the need to protect rock art has led to treatments being applied somewhat ahead of the scientific study of appropriate interventions. Nonetheless, looking beyond the material decay of rock art to the problem of increas –
ing tourism and the need to link rock art conservation efforts to local economic development has received some needed attention in recent years (Walderhaug Saetersdal 2000; Smith 2006; Deacon 2006).

Stone Decay 63
PROOF 1 2 3 4 5 6 Herita ge in Stone: Rock Art, Quarries, and Replacement Stone 63
PROOF 1 2 3 4 5 6Rock Art Documentation
Given the large number of rock art images and sites worldwide, docu –
mentation is seen as an important preservation tool. In a number of countries, documentation of rock art has been largely implemented by volunteers (Chandler, Bryan, and Fryer 2007) . A survey of those involved
in documenting UK rock art sites found a perception of rapid, variable degradation from the impact of humans and animals, superimposed over a slow background level of erosion caused by physical and chemical agents (Barnett and Díaz-Andreu 2005) . Laser scanning of rock art as
a way to monitor decay rate has been researched by several authors (El-Hakim et al. 2004 ; Barnett et al. 2005 ; Trinks et al. 2005 ). One of
the reasons for making rubbings of rock art and gravestones is that fine details not visible to the naked eye can be recorded using this method. An attempt to record an example of fine detail using 3D laser scanning was undertaken (Díaz-Andreu et al. 2006) , but scanning was not able to
detect a spiral feature recorded in a rubbing in 1995.
Good documentation has led to preventive conservation recom –
mendations, such as the employment of mitigation practices to reduce
the abrasive effects of blowing sand (Keyser, Greer, and Greer 2005) .
Knowing that most of the four hundred thousand rock art sites around the world (Clottes 2006) will never receive a conservation intervention,
let alone a conservation assessment, good documentation (often by volun –
teers or students) has been the most common method of capturing a
durable and accessible record of these sites ( Padgett and Barthuli 1997 ;
Swartz and Hale 2000 ; Larkin 2002 ).
HISTORIC QUARRIES
The preservation of historic quarries is of interest to the field of stone
conservation for several reasons (Ashurst 2007, 306). One is that quarries provide important evidence of how stone production technology has evolved. This technology has a profound effect on stone durability, as we saw with the issue of the bowing found in thin marble panels. The thin –
ner panels were made possible by new production technology (Scheffler 2001). Second, historic quarries may need to be reopened to provide replacement stone for important buildings. In Sydney, the local “yellow block” sandstone is being quarried when the foundations of modern sky –
scrapers are dug and stockpiled by the local conservation authorities for later use as replacement stone on nearby historic buildings.
Perhaps the most extensive work on ancient quarries is the
EC-sponsored project known as QuarryScapes (Conservation of Ancient Stone Quarry Landscapes in the Eastern Mediterranean), coordinated by the Geological Survey of Norway (2005–8), which dealt with issues of inventorying, managing, and conserving ancient quarry sites, with case studies in Egypt, Jordan, and Turkey (Abu-Jaber, Al Saad, and Al Qudah 2006; Bloxam 2006; Degryse et al. 2006; Heldal et al. 2006; Caner Saltik 2007; Heldal, Bloxam, and Storemyr 2007). The other main resource

64 Chapter 1
PROOF 1 2 3 4 5 664 Chapter 5
PROOF 1 2 3 4 5 6for information on historic quarries is the research group ASMOSIA
(Association for the Study of Marble and Other Stones in Antiquity; ASMOSIA.org ). A series of conference proceedings contains the publica –
tions of geologists and archaeologists working on discovering ancient sources of stone, as well as stone transport, trade, conservation, and archaeometry (Schvoerer 1999; Lazzarini 2002; Herrmann, Herz, and Newman 2002). Another useful volume on Egyptian quarries is the work by Klemm and Klemm (2008).
REPLACEMENT STONE
Related to the preservation of historic quarries is the issue of obtaining
adequate replacement stone for repairs, which has become a critical
problem for many important sites as historic quarries are closed due to
development and other economic pressures. Standards and resources for replacement stone vary enormously from country to country, and a criti –
cal review article on this topic with a global view is overdue.
When repairs are being planned for a large building, quarrymen
and geologists are often asked: “Which of the available stones will pro –
vide good durability and a compatible match to the existing stone?” (Jefferson et al. 2006). During the renovations of the British Museum,
the “wrong” stone was used (Niesewand 1999). Recent research by Rozenbaum and others (2008, 345) found that for French limestones it was difficult, but not impossible, to “select substitution stones with satis –
factory aesthetic aspect and properties that enable to expect a satisfactory compatibility with the original stone.”
Finding appropriate replacement stone requires tools for stone
selection such as atlases and databases (Dingelstadt et al. 2000; Hyslop
et al. 2009). A useful discussion of aspects of selecting replacement stone based on material properties can be found in two recent works (Pr ˇikryl 2007; Yilmaz 2008).
Clearly, to improve on the current situation, each country with
significant heritage in stone should have a centralized lithological library, and an associated database, that includes not only the petrographic and mineralogical characteristics of its stone but also petrophysical ones, including pore size distribution, porosity, capillary uptake coefficient,
and hydric and hygric dilatation.
To this end, English Heritage is working with the British
Geological Survey and local experts to expand their database of English stone with a new GIS site called EBSPits (England’s Building Stone Pits) and to identify the most important building stones used, representative buildings, and historic quarries (English Stone Forum 2009; English Heritage 2009).
Research by Blanc and others (Blanc and Lorenz 1988; Blanc and
Lorenz 1992; Holmes, Harbottle, and Blanc 1994) has helped to identify many quarries in France, in part with the goal of architects being able to better match replacement stone. The Bureau de Recherche Géologique

Stone Decay 65
PROOF 1 2 3 4 5 6 Herit age in Stone: Rock Art, Quarries, and Replacement Stone 65
PROOF 1 2 3 4 5 6et Minière (BRGM) and the Laboratoire de Recherche des Monuments
Historiques (LRMH) are collaborating on a project to gather into a data –
base all the information related to the stones of monuments, ancient quarries, and modern quarries (V. Vergès-Belmin, personal communication).
Recent work by Hyslop and others ( Hyslop and McMillan 2004 ;
Hyslop 2008 ) discusses the challenges of finding replacement stone for
the important and well-known stone buildings of Glasgow and Edinburgh. In Glasgow, Duthie and others (2008) found significant varia –
tions in the extent of microbial growth on a range of replacement sand –
stone blocks that had been exposed for twelve years, illustrating the importance of selecting appropriate replacement stone.
Related to the issues of stone replacement is the general question
of loss compensation for stone. This topic has been reviewed by Griswold
and Uricheck (1998) , who suggest that this area be prioritized in future
research and evaluation.
Notes
1 Whitley 2001, book jacket.
2 Mowilith is an aqueous emulsion of polyvinylchloride, polyvinylacetate, and
different stabilizing agents.

PROOF 1 2 3 4 5 6WHAT IS WRONG?
The purpose of this chapter is to suggest some ways in which our
research might be made more effective. The views expressed are unasham –
edly personal, and not everyone will agree with them. It is hoped, none –
theless, that they will stimulate some serious thought and discussion in order that limited research resources may be put to the best possible use.
In the last fifteen years, three factors have helped increase the
effectiveness of research: the entry of topflight researchers into the field, increased access to existing research via Internet databases and PDFs, and the increase in research done by universities, particularly those participat –
ing in EC-sponsored programs. Much work in conservation and conser –
vation research is published only as “gray literature,” and the Internet
has vastly increased accessibility to this material (see, for example:
http://repository.upenn.edu/hp_theses ; http:// www.ncptt.nps.gov/product
-catalog/ ). There have been some corresponding changes that have
decreased the effectiveness of research over this period as well: the decrease in funding of research programs at the institutional and national level (BRE, CSIRO, GCI, ICCROM, national research programs, etc.),
1 the
need to test university innovations and transfer them to the field,
and the need for longer-term research programs. These factors are
discussed in more detail below and in the final chapter.
Publications
The number of published papers relating to stone is growing relentlessly. Every four years a large stone conservation meeting is held, and this is reflected in the overall pattern of publication.
On the face of it, this must surely be welcomed. It indicates the
growing concern about stone and the growing numbers of researchers who are working on stone, many of whom bring important new perspec –
tives and discoveries. However, the quality of many of the papers is still disappointing. Why?
The following criticisms are often made:
• The same mater ial is publi shed on more than one occas ion.
While it is acceptable to publish interim reports on a major Chapter 6
Doing Better: Increasing the Effectiveness of Research

Stone Decay 67
PROOF 1 2 3 4 5 6 Doing Better: Increasing the Effectiveness of Research 67
PROOF 1 2 3 4 5 6piece of work, there is no excuse for publishing the same mate –
rial, with only minor variations, time and time again.
• Many papers consi st of the appli cation of well- tried proce dures
to a specific building or monument. The results are of inter –
est only to a limited audience and should be written up as an
internal report of the organization carrying out the research. They do not warrant full publication in journals or conference proceedings.
• Many papers fail to set the resea rch into conte xt. They are
essentially descriptive; they describe the work that was under –
taken but do not say why it was done.
• Many papers negle ct to indic ate the signi ficance of the resul ts.
Having failed to say why the work was done, they provide insufficient discussion of the results and therefore do not explain what, if anything, was achieved. The reader is left won –
dering whether any advance was made and, if so, what it was.
• Few papers ident ify promi sing avenu es for furth er resea rch.
• Underlying the previ ous probl ems is the frequ ent negle ct of
the scientific rigor of hypothesis —experiment —conclusion.
Conferences Conferences provide unparalleled opportunities for meeting fellow researchers: for making new contacts, finding new collaborators, compar –
ing notes, sharing ideas, and keeping up to date. They also provide a much-needed opportunity to stop and think and to see one’s research in
a broader context.
On the negative side, however, conferences often provide an
opportunity for publishing substandard, nonrefereed work. The prolifera –
tion of conferences, however desirable it may be, can all too easily lead
to a proliferation of poor-quality papers. These and related issues have recently been addressed by the Torun Guidelines for stone meetings, which serve as an example of what can be done to improve stone confer –
ences (see sidebar, page 68).
Standards
The lack of internationally agreed-upon standards, be they for nomencla –
ture or for testing procedures, hinders the interpretation, understanding, and evaluation of research. Without standards, there is no common lan –
guage. The situation is slowly improving, with the adoption of English as the current language of science, which provides greater opportunities for communication and collaboration among researchers and research groups, and with collaborative tools and more universal evaluation stan –
dards beginning to be adopted ( Fassina 2008 ; European Committee for
Standardization = Comité Européen de Normalisation), such as drilling
resistance and ultrasonic testing.
Conduct and Quality of Research
Research into stone conservation demands an interdisciplinary approach. Many researchers, however, find themselves working alone or in relatively

68 Chapt er 1
PROOF 1 2 3 4 5 668 Chapt er 6
PROOF 1 2 3 4 5 6The Torun Guidelines for
Conferences in the Field of
Stone Conservation
Introduction
In an era of increasing informa –
tion and changing dissemination
technology it seems an appropri –
ate moment to reflect on ways to improve the quality and accessibility of knowledge in the field of stone conservation.
As knowledge increases rapidly,
teams working on stone conservation have become more specialised and often present their results at special –
ist meetings. This trend may increase the potential for isolated perspectives and the risk that knowledge may not reach its intended goals.
The general congresses on
stone deterioration and conservation, organised every 4 years since 1972 give a useful snapshot of the differ –
ent trends of stone conservation and provide a multidisciplinary forum for discussion, complementing the specialist meetings. However, it can be difficult for them to encompass all the different trends and fields of stone conservation.
In recent decades there have
been a number of calls to improve the quality and impact of knowl –
edge in the conservation field. In response, there have been a num –
ber of improvements, such as more review articles and multi-author textbooks which give new research –
ers some of the background needed. Electronic publication of full text articles from most journals makes the peer-reviewed literature more readily available. Nevertheless, most confer –
ence proceedings still have limited electronic distribution.
With the aim of improving
the quality and the dissemination of knowledge through congresses in the field of stone conservation, the 11th International Congress on Deterioration and Conservation of Stone, and the 13th meeting of the ICOMOS International Stone Committee, which met in Torun on September 15th to 20th 2008, adopted the following text.
The Guidelines
1 Planning
When planning conferences organis –
ers should review other conferences already scheduled in the field, in order to separate their own confer –
ence from others by at least six months. The aim is to increase the potential pool of participants and to increase the likelihood of original research being presented.
2 Selection of papers
The selection of papers for formal conferences should be based on a thorough review by at least two experts. Organisers, assisted by their scientific committees, should check for and refuse ‘doublons,’ i.e. papers that have been, or are about to be, published in proceedings of another conference. Published papers (whether oral or poster) should meet minimum standards, including:
• precisely define d resea rch
methodologies
• appropriate refer ence citat ions
• advancing knowl edge in the field.
3 Communication among
participants
Organisers should encourage formal and informal communication among conference participants. These may include discussion sessions, panel dis –
cussions and workshops.4 Seeking quality and measuring
outcomes
Organisers, assisted by their scientific committees, should ensure good qual –
ity papers. In addition, organisers should measure the outcomes of their conference. Measures adopted may include reviews of the conference and opportunities for user feedback, such as a web page for participant responses, and quality rankings.
5 Dissemination strategy
To facilitate rapid dissemination of the ideas presented at the conference, organisers should plan for electronic dissemination of the proceedings. This should be arranged within a short period of time (e.g. a year) to ensure that the results achieve a wide and long-lasting distribution.
The following persons participated
to the drafting of the Torun
Guidelines: Akos Török, Hungary — Clifford Price, UK — Dagmar Michoïnova, Czech Republic — Daniel Kwiatkowski, Sweden — David Young, Australia — Elsa Bourguignon, France — Eric Doehne, USA — Hilde De Clercq, Belgium — Jadwiga W. Lukaszewicz, Poland — Jean-Marc Vallet, France — Jo-Ann Cassar, Malta — Johannes Weber, Austria — Jose Delgado Rodrigues, Portugal — Milos Drdacky, Czech Republic — Marisa Laurenzi Tabasso, Italy — Myrsini Varti-Matarangas, Greece — Philippe Bromblet, France — Stefan Simon, Germany — Vasco Fassina, Italy — Vasu Poshyanandana, Thailand — Véronique Vergès-Belmin, France.
http://www.iccrom.org/eng/news_
en/2009_en/field_en/01_01Torun Guidelines_en.pdf .

Stone Decay 69
PROOF 1 2 3 4 5 6 Doing Better: Increasing the Effectiveness of Research 69
PROOF 1 2 3 4 5 6small teams. As a result, research can become too narrow, failing to take
into account factors that might seem self-evident to somebody trained in another discipline. For example, an analytical chemist might look primar –
ily at the composition of a stone, while a materials scientist would per –
haps focus on its behavior, a biologist could discover a new species of microbe in the pores, an engineer would core the stone and measure its strength, and a geologist might make a thin section and evaluate its microtexture.
This recalls the famous story from India of the truth being similar
to a group of blind men trying to describe an elephant by touching it, when each has access to just a single, and different, part of the beast (trunk, leg, ear). A corollary of this situation is the phrase: “When the only tool you have is a hammer, it is tempting to treat everything as if it were a nail” (Maslow 2006, 15) . It is useful for researchers to work
closely with conservators and conservation architects to mitigate such tendencies. At worst, researchers can become so introspective that they take little or no account of work being undertaken elsewhere; the researcher whose citations are solely to his or her own work is clearly falling into this trap.
A great deal of research into stone is conducted at a rather super –
ficial level, but this is changing. The first volume of this book complained that much of the work on consolidants, for example, was very empirical, and that a particular material would be evaluated simply because it was available, not because there were sound theoretical reasons for believing that it would be effective. Some work on decay mechanisms was seen as equally superficial. However, the depth of research has increased enor –
mously in the intervening years, and some areas have changed beyond recognition, thanks to the contributions of exceptionally talented individ –
uals. Nonetheless, there is still a danger that research can become so the –
oretical that it loses sight of its main purpose. The researcher needs to
be fully aware of what is desirable and practicable from a conservation standpoint, while conducting research at a level that is deep enough to solve the fundamental problems.
Some of these issues were summarized succinctly by Chamay
(1992) in his closing remarks at a conference:
Je m’inquiète un peu de constater que vos recherches
sont menées sans concertation organisée, chacun travaillant de
son côté, l’échange d’information restant très limité . . . J’ai aussi le sentiment que la tendance générale parmi les chercheurs est
de rester confiné dans sa spécialité . . . Attention à l’arbre qui cache la forêt! Avant d’entrer dans le détail, une appréciation d’ensemble est nécessaire. [I am a bit worried to notice that you
are carrying out your research without organized dialogue, each person working in his or her own corner, the exchange of infor –
mation remaining very limited . . . I also have the feeling that the general tendency among researchers is to remain confined to one’s own specialty . . . Don’t fail to see the wood for the trees! Before going into detail, an assessment of the whole is necessary.]

70 Chapt er 1
PROOF 1 2 3 4 5 670 Chapt er 6
PROOF 1 2 3 4 5 6It is interesting that one of the concerns that led to a recent conference
on the conservation of the cave at Lascaux (AP 2009) was the need to
have specialists working more closely together and synthetically, just as Chamay suggested.
Getting the Message Across
There is no point in doing research unless the outcome can be applied in practice. This does not mean that there is no place for long-term, strategic research, but that any worthwhile research must ultimately contribute to the care and conservation of the heritage.
There are many ways of getting the message across, including lec –
tures, publications, personal contacts, and advice on specific problems. The message needs to reach other researchers, but it must also reach, for example, conservators, architects, archaeologists, and administrators. It does not follow automatically that a good researcher is a good communi –
cator, and all researchers should ask themselves whether their research is achieving the full impact it deserves. The Internet has changed expecta –
tions about the ease of access to high-quality information.
PUTTING IT RIGHT
What can be done to make our research more effective? There are
no simple solutions. While some steps may be taken by individual
researchers, other solutions lie with research administrators, conference organizers, editors, publishers, training institutions, and funding bodies. The following proposals deserve consideration.
Quality, Not Quantity
Any institution that funds research may reasonably expect to see some return for its money. This necessitates some means of measuring research output. How else may the institution be sure that its money is being well spent? The simplest indicator, and one that appeals to many administra –
tors, is the number of papers that result from the research. It is an
objective, quantitative indicator, but it is one that undermines quality.
Individuals find themselves under immense pressure to produce a certain number of publications each year, and it is no wonder that quality suf –
fers. Publishing the same thing several times is an easy way of meeting the target. Other tactics include the publication of a string of interim reports, the publication of material that warrants no more than an inter –
nal report, publishing papers that report on what one proposes to do
in the future, and publishing papers with a long and unjustified string of authors.
Journal impact factors and citation indices, such as Google
Scholar, ISI Web of Knowledge (Science Citation Index), and Scopus (by Elsevier), can provide an indication as to what references and journals are having the most effect. The number of times an article is cited is tracked as a measure of its popularity and potential usefulness. Journals that con –
tain articles that have higher rates of citation have higher impact factors.

Stone Decay 71
PROOF 1 2 3 4 5 6 Doing Better: Increasing the Effectiveness of Research 71
PROOF 1 2 3 4 5 6The use of citation indices is rapidly increasing in biology and medicine
as an important way to filter the wheat from the chaff of research publi –
cations and to provide employers with an independent assessment of quality, such as the H-index ( http://en.wikipedia.org/wiki/H-index ). As
with all rankings, the system is open to abuse. Popularity is not the same as quality, since once an article begins to be cited, its chances of being cited again increases. Assessment of quality in research is not a simple matter of numbers. It entails a high degree of subjective judgment, both by research managers and by other researchers. Funding bodies must be prepared to appoint research managers whose judgment they trust and then be prepared to accept that judgment concerning the quality of research being conducted under those managers’ supervision. They must be seeking value for money, which entails both quality and quantity, rather than quantity alone.
Conferences and Other Models for Advancing the Field
Other types of scientific meetings should be considered as role models, such as workshops, the Gordon Research Conferences (GRC) (2010),
2 the
Dahlem Conferences (Freie Universität, Berlin 2006),3 and other forums
based on new technologies, such as online discussions of presentations at conferences and online proceedings where attendees can post questions and comment on articles. Conference organizers today can take for granted that most participants possess a Wi-Fi–enabled laptop, netbook, or cell phone. Meetings where participants can actually take part in
discussions, interact, and comment in concrete ways are often more pro –
ductive than conventional meetings at which participants are often over –
whelmed by too many presentations, too much information, and too little time for useful discussion.
Conference Papers
It is a common practice for employers not to fund an individual’s atten –
dance at a conference unless he or she is presenting a paper or a poster. It is a practice that makes the research administrator’s life much simpler, but one that again encourages the production of superfluous publications. To solve this problem, one option would be amending the conference attendance policy to include publishing a conference review as a qualify –
ing activity. At a large conference, multiple reviewers, who may be paid small honoraria for their contribution, can cover parallel sessions. A good conference review is often worth more than several case studies.
Selection of Conference Papers
Another important way of preventing the publication of substandard
conference papers lies with the conference’s technical committee. All too
often, papers are selected on the basis of an abstract submitted some eighteen months or more before the conference. At that time, the research will almost certainly not have been completed; indeed, it may not even have begun. The prospective author, therefore, makes a guess as to the likely outcome of the research and writes an abstract that strikes a deli –
cate balance between the specific and the noncommittal. The technical

72 Chapt er 1
PROOF 1 2 3 4 5 672 Chapt er 6
PROOF 1 2 3 4 5 6committee reviews the abstracts and, on this flimsy evidence, decides
which papers to accept. By the time acceptance is gained, the author has twelve months or less in which to complete a paper—regardless of how the research is going. Then, the technical committee and the editors, when they finally receive the paper, have little option but to publish it much as it stands.
Not all conferences operate this way, but many do. It means that
much of the literature of conservation has been subjected to the very min –
imum of refereeing, if any. Quality assurance is all but nonexistent.
If preprints are to be issued at the time of the conference, there
may be insufficient time for full refereeing. Nonetheless, a significant step forward could be made if technical committees were to insist on seeing the full text of a paper before deciding whether to accept it for presenta –
tion and publication. It is true that it takes longer to read a paper than it takes to read an abstract and that technical committees are composed of busy people. However, it does not take long to decide whether a paper consists largely of previously published material or whether it is of local interest only. A lot of substandard papers could be weeded out very quickly. Another problem is that authors may not be prepared to take the time to write a full paper if there is a risk that it may not be accepted. Too bad—if poor papers were weeded out, there would be correspond –
ingly more space for good papers, so the author who has something worthwhile to say need not fear rejection.
Refereeing
Ideally, all published material should be subjected to peer review. It is a process that is open to criticism in that it slows down publication and can fall afoul of an ill-informed or prejudiced referee. It is, however, the fairest way of ensuring that papers are of sufficient quality to merit publi –
cation. Conservation has suffered greatly from the fact that so much of its literature has been in unrefereed publications. As one conservation
scientist observed: “Why try harder, when you can get away with
being sloppy?”
Collaborative Programs
The time has long passed when a well-educated individual might have a working knowledge of the whole of science and the humanities. We are all highly specialized in our individual fields, and we need to collaborate with specialists in other disciplines if we are to solve the very broad prob –
lems posed by stone conservation. Not only do we need to collaborate with other conservation scientists from different disciplines, but we also need to draw in talented researchers who are not involved in conserva –
tion. Such collaboration is not without dangers (Torraca 1999) , but it is
essential nonetheless.
Some funding bodies are in a position to enforce collaboration.
An example is to be found in EC-operated programs. Research projects are not funded unless they entail genuine collaboration between partners in more than one member state, with each partner making a clearly defined contribution based on a particular expertise. In a relatively short

Stone Decay 73
PROOF 1 2 3 4 5 6 Doing Better: Increasing the Effectiveness of Research 73
PROOF 1 2 3 4 5 6time, these programs have brought about a much greater degree of col –
laboration between relevant European research institutions.
Training
Good research requires good researchers. To be a good researcher in the scientific aspects of stone conservation, one needs a thorough grounding in science, training in research, and a sound appreciation of conservation issues. These qualifications are not readily found in any one individual, and a significant proportion of “conservation scientists” do not have suf –
ficient knowledge of science to enable them to undertake research at a fundamental level. They may, for example, have trained primarily as con –
servators; although their training may well have included some science, they are conservators first and scientists second. As a result, a good deal of research is rather superficial.
Much attention has been paid to the training of conservators, and
lists of training courses are readily available (Rockwell 1994) (also see appendix, List of Conservation Related Sites, pages 150–51). Less atten –
tion has been paid to the training of conservation scientists, although there has been some useful discussion of the different approaches to training researchers (Mazzeo and Eshøj 2002; Chiari and Leona 2005; Trentelman 2005; Mazzeo and Eshøj 2008). There are very few training programs for conservation scientists ( http://www.episcon.scienze.unibo.it ),
and a worldwide survey of current training opportunities would be advantageous. A number of possible pathways can be envisaged: doctoral research followed by a fellowship in a major conservation institution or museum, for example, or a master’s degree in a particular aspect of con –
servation science. A first degree in a scientific subject should be a prereq –
uisite, in any event.
Some attention also needs to be given to ways of attracting high-
caliber students to conservation. The subject does not, on the whole, attract the outstandingly capable researcher. Such individuals are more likely to be found in medical research, in nuclear physics, or in military research, where they will benefit from better funding and from the stimu –
lus of working in large, highly focused teams. Ways must be found of bringing conservation to the attention of science students during the course of their first degree and of presenting stimulating and challenging career opportunities. Part of this issue is that to be more successful, the conservation field needs to scale up its ambitions and build support for larger-scale, coordinated projects to compete with “Big Science.” In the United States the Mellon Foundation has been effective in bringing scien –
tists into museums through endowed chairs;
4 however, permanent posi –
tions dedicated to monuments research remain unconscionably rare worldwide—an important gap that could be filled with the requisite
institutional support.
Reviews
The conservation literature is still remarkable for its relative lack of scholarly review articles. In all the mainstream scientific disciplines,
the need for state-of-the-art reviews is well recognized, and the authors

74 Chapt er 1
PROOF 1 2 3 4 5 674 Chapt er 6
PROOF 1 2 3 4 5 6highly acclaimed. The conservation literature, by contrast, is full of
isolated pieces of work, with very little effort being made to pull the
information together.
Review articles enable researchers to put their work in context
and to see where further work would be worthwhile. However, they are
not easy to write. They require a lot of time and a high degree of compe –
tence. They may have to be specifically commissioned and funded. The National Center for Preservation Technology and Training (NCPTT)
has funded some small grants for researchers to write reviews, and
the International Institute for Conservation (IIC) journal Reviews in
Conservation has proven itself to be an extremely useful resource to the
conservation community.
5 Progress is certainly being made, but there is
plenty of scope for more.
Notes
1 BRE = British Research Establishment; CSIRO = Commonwealth Scientific and
Industrial Research Organization, Australia; GCI = Getty Conservation Institute; ICCROM = International Centre for the Study of the Preservation and Restoration of Cultural Property, Rome (UNESCO).
2 Gordon Conferences are organized around a theme, with few presentations, much discussion, and with contributions “off-record” to encourage free exchange, often of unpublished material.
3 “The Dahlem Conference in Berlin is a unique forum for analyzing, in a multidisciplinary way, complex topics. For five days fifty selected participants are cloistered together, divided into four groups. Each group studies background papers prepared by a few selected individuals, which serve as a basis for further discussion and the preparation of a report. The goal of these reports is to define what is not known in the field rather than rehearsing what is known and to
present ideas for further research and their priorities. The four group reports are hammered together under the guidance of a rapporteur ” (Wolff 1992) .
4 Museum science involves research and service work in support of curators and conservators and revolves largely around issues of technical art history as well as art conservation.
5 Regrettably, the recent financial crisis, or “great recession,” has led to the consolidation of the journal Reviews in Conservation into the IIC journal Studies
in Conservation .

Chapter #
Chapter Title
Authors’ names
PROOF 1 2 3 4 5 6The first edition of this book was written at a time when research on
stone seemed to many people to have stagnated. That perception has changed completely in the intervening years, and real progress has been made in many areas. Significant gaps in knowledge have been substan –
tially narrowed, including many of the fundamental aspects of damage to stone from cycles of frost, salt, moisture, and heat. In a number of cases our descriptive nineteenth-century notions of “weathering” have now been deeply probed and quantified, measured in the field, and replicated in laboratory experiments. These insights have in some cases led to inno –
vations in the preventive treatment of stone, with more sophisticated models of damage advancing hand in hand with more quantitative obser –
vations from field measurements and laboratory experiments.
Five important trends can be identified: 1) a perception that our
understanding of fundamental conservation problems is far ahead of solu –
tions to these problems; 2) new solutions to stone conservation problems often need long-term testing, but resources for such testing are lacking; 3) climate change is an important issue in stone conservation; 4) biodete –
rioration should be increasingly understood in an ecological context; and 5) the locus of stone conservation research activity may be beginning to shift to countries such as China, India, Brazil, and South Korea. Heritage conservation in these countries is becoming a national priority due to unacceptable rates of heritage loss and greater economic success.
The traditional neat classification of weathering mechanisms
into physical, chemical, and biological factors is receding as an accepted approach to this field. A new approach emphasizes material behavior
and the important interrelationships between environmental, material, and historical variables. As is the case with most natural systems, a few key parameters often dominate in each weathering process (Goudie 1995) ,
and the result can be nonlinear and even chaotic, in contrast to previous assumptions about linear rates of erosion. This schism is reminiscent of the nineteenth-century debates over catastrophism and Darwinian gradualism (Viles 2005 ; Giavarini et al. 2008 ).
The straightforward concepts of magnitude, frequency, and dose-
response developed for air pollution studies on stone (Charola and Ware
2002) are being modified by ideas of thresholds, feedback loops, and non –
linearities ( Goudie and Viles 1999 ; Norwick and Dexter 2002 ) within the
large framework of conservation risk assessment (Brokerhof et al. 2007) . Chapter 7
What Has Changed? Some Thoughts
on the Past Fifteen Years

76 Chapte r 1
PROOF 1 2 3 4 5 676 Chapte r 7
PROOF 1 2 3 4 5 6One example of this profound shift over the past two decades is the con –
servation of the wall paintings of Queen Nefertari since the late 1980s.
Development of the conservation program went through several steps: 1) assessment, monitoring, and conservation of the wall paintings in the late 1980s; 2) visitor impact (carrying capacity) studies in the 1990s; and, 3) most recently, research that suggested that the greatest risk to the wall paintings appears not to be humidity cycles that might activate salt weathering but instead rare flash floods in the area. Accordingly, the need to prepare for rare, but catastrophic risk has assumed the same impor –
tance as the need to manage more gradual risk factors ( Agnew and
Maekawa 1999 ; Wüst and Schlüchter 2000 ; McLane et al. 2003 ).
In one of the most important trends over the past fifteen years,
universities have embraced many of the compelling multidisciplinary
challenges found in stone conservation, bringing to bear new, topflight
researchers and new tools from materials science, cement chemistry, geo –
technical engineering, geology, physics, geochemistry, microbiology, and geomorphology, and adding these to the historic tradition of chemists at the center of much research in stone conservation. As a consequence, the standard of research has improved beyond recognition in some areas, and many more papers are being published in the peer-reviewed mainstream literature. The publication of an increasing number of reviews is also much to be welcomed.
One discipline that should be added to the mix is that of (for
want of a better term) heritage hydrology. This is the nanometer- to
kilometer-scale study of the effects of water transport on the stability of
historic architecture and monumental complexes. Archaeologists have long realized that hydrology was critically important in sustaining the cultures that built Tiwanaku, Copán, Moenjadaro, Baghdad, Petra, and Angkor, for example. And for architects, one of the key elements of build –
ing design is how a structure sheds water. Increasingly sophisticated mod –
els of moisture transport and material behavior are developing rapidly (such as WUFI , ASTRA, or CESA) (Sedlbauer and Künzel 2000; Holm
and Künzel 2003 ; Franke et al. 2007 ). These areas have also benefited
from advances in the modeling of the structural behavior of building materials (Binda 2007) . Future researchers in monument conservation
should be encouraged to specialize in heritage hydrology.
Multidisciplinary research has been strongly promoted by
EC-funded research projects over the past fifteen years, resulting in an
experienced network of about eighty multi discip linary researchers across
Europe with an interest in the subject. Indeed, it could be argued that the gradual evolution of this informal research network has been of even more value in promoting research than the projects themselves. Monuments
research networks supported by simila r levels of funding are currently
lacking in the Americas and Asia.
It is uncertain whether overall research funding has improved or
declined—there do not appear to have been any attempts to gather the necessary data. There is, however, a general impression that expenditure on stone research has increased somewhat within the university sector and declined substantially elsewhere. For example, expenditure by gov –
ernments and NGOs, such as BRE, CSIRO, EH, GCI, ICCROM, NPS,

Stone Decay 77
PROOF 1 2 3 4 5 6 What H as Changed? Some Thoughts on the Past Fifteen Years 77
PROOF 1 2 3 4 5 6NCPTT, TNO, and the former Swiss Expert Centers has generally
decreased over the past fifteen years.1
In the United States, research funding for stone conservation
remains difficult to obtain, with some work on biodeterioration and building materials receiving limited National Science Foundation (NSF) support and more applied research funding coming in the form of small grants from the National Center for Preservation Technology and Training (NCPTT) and the Kress Foundation. A July 2009 Mellon Foundation–sponsored meeting with the US National Science Foundation on heritage and science suggests that there is growing interest in the field at the national level in the United States. However, one colleague has quipped, “The larger science community ‘rediscovers’ conservation about every ten years. But when they find out that the problems are difficult and that funding is scarce, they lose interest.”
In an encouraging development, funding is coming increasingly
from outside Europe and the United States. Researchers in India and China are beginning to publish conservation research at a greater rate, partly in response to significant challenges from air pollution, tourism, and climate change. Russia, Brazil, and South Korea also have seen grow –
ing interest in research related to conserving heritage in stone as their economies have developed.
Despite the vagaries of funding, the number of research publica –
tions has continued to increase, as researchers in allied disciplines, from geography to materials science, have discovered compelling scientific chal –
lenges in the field.
Stone conservation issues have increasingly been covered in the
popular press, including public policy as regards conservation research (House of Lords, Science and Technology Committee 2006 ; House of
Lords, Science and Technology Committee 2007 ), the biodeterioration of
monuments (Venkataraman 2008) , and the crumbling of cathedrals (Petre
2006). The sites of Easter Island, Petra, and Angkor are compelling for conservation professionals and the public alike, not only for their beauty and history but also because they are inexorably eroding as unresolved conservation and funding issues continue. Petra ( Paradise 2005 ; Simon,
Shaer, and Kaiser 2006 ; Heinrichs 2008 ) and Angkor (Leisen 2002;
Leisen, von Plehwe-Leisen, and Warrack 2004; André et al. 2008; Siedel, von Plehwe-Leisen, and Leisen 2008) are also important examples of new knowledge from conservation research being brought to bear.
The increasing importance of English as the current common lan –
guage at most conferences and for many journals has helped to consoli –
date the field of conservation.
2 Nevertheless, the fact that many important
works of research are published only in the French, German (see appen –
dix), and Italian literature remains a barrier (Cabreroravel 1993; Alessandrini and Pasetti 2004; Snethlage 2005; Pinto Guerra 2008). Some recent translations have been useful (Caneva, Nugari, and Salvadori 2008), but they are relatively rare. Standards committees (ISO, CEN, ASTM, RILEM) have also brought a needed level of integration, especially at the European level, for example, Technical Committee 346 (Fassina 2008).
The Internet has made a huge impact. Enormous amounts of infor –
mation on stone conservation are more readily available and accessible

78 Chap ter 1
PROOF 1 2 3 4 5 678 Chap ter 7
PROOF 1 2 3 4 5 6than ever before. Nonetheless, many of the tools needed to access that
information are still lacking, such as a citation index, more-comprehensive databases of conservation-related research material, and wider electronic distribution of conference proceedings, past and present. Opportunities, such as a Wiki, for more widespread conservation community feedback and contributions to conservation research, would also be valuable. Without these tools, it is a time-consuming challenge to find high-quality research that advances the field.
3 Addressing these gaps would accelerate
the return on our investment in research and would add new tools to the stone conservator’s kit.
So far, so good—there have been many changes for the better.
Nonetheless, many of the issues that dogged research in 1994 (when the first edition of this volume was written) are still with us: the tendency to publish research in conference proceedings that are not refereed and not widely available, the variable quality of research, the multiplicity of con –
ferences, and the ongoing reinvention of the wheel due to the difficulties of accessing previous work in the field. National funding cycles for stone conservation still tend toward large-scale interventions on sites “in crisis,” while funding for routine maintenance remains in short supply and fund –
ing for long-term research is even more difficult to come by.
Long-tem funding is of particular importance, given the need to
evaluate and document treatments over long periods of time—much lon –
ger than the duration of individual research projects. The nonuniversity institutions have an important role here, facilitating long-term applied research. Discrete or isolated measurements and projects are of limited utility. A more nuanced understanding of material behavior and the effects of conservation interventions over time is essential for balanced and effective decision making, and this can be achieved only in the con –
text of long-term research.
What is the relevance of all this for the stone conservator? It
sometimes seems that the tool kit of a stone conservator has not changed much in the past two decades and may even contain fewer options now than then due to regulations, environmental concerns, health concerns, compatibility concerns, and lessons learned from unintended conse –
quences. As conservators begin to specialize more in recording, investiga –
tion, and characterization and less in treatments, and stone replacement becomes more common, this raises the question: are treatments still needed? A colleague answered this question by suggesting, “the field has changed, but the stone has not, and in many cases it is crumbling.”
So yes, there is still a role for both preventive and active interven –
tions in the conservator’s tool kit, and important new treatment options have been developed over the past fifteen years. Examples include: 1) more-advanced methods of controlling clay swelling of stone, 2) cou –
pling agents for limestone consolidation, 3) latex solutions and laser sys –
tems for stone cleaning, 4) improved poulticing methods, 5) water-based
hydrophobic coatings, 6) less-brittle silane consolidants, and 7) nano-
particle solutions of lime for consolidation of fragile stone surfaces (Table 7.1). Inevitably, there is a time lag between development and wide –
spread application, and there is an onus on all researchers—whether sci –

Ston e Decay 79
PROOF 1 2 3 4 5 6 What Has Changed? Some Thoughts on the Past Fifteen Years 79
PROOF 1 2 3 4 5 6Table 7.1
Stone Conservator’s Tool Kit
Table 7.2 Conservation Scientist’s Took Kitentists or conservators—to ensure that their findings are implemented
appropriately. But the tool kit has undoubtedly changed, and further changes are on their way, as the tool kit for researchers (Table 7.2) has added new tools, including Focused Ion Beam/Environmental Scanning Electron Microscopy (FIB/ESEM), cryo-scanning electron microscopy (cryo-SEM), and wet-scanning transmission electron microscopy (wet-STEM), among many others. The context in which these conservation tool kits are used now includes the impacts of climate change, the Internet, and the rapid development of nanotechnology (Table 7.3).
Beyond the basic tool kit, there are signs of a new maturity in the
field of stone conservation. An awareness of the unintended consequences of some earlier interventions has, for example, resulted in a more cautious and incremental approach in the current generation of stone conservators. Interventions with wide application • Nano-lime particles suspended in alcohol
• Water-based hydrophobic coatings
• Spray-on latex for cleaning architectural
interiors
• Portable, large-scale laser systems for
cleaning
• Bioremediation
Interventions that are under
development• Coupling agents for limestone
consolidation
• Improved poulticing methods
• Treatments for clay swelling of stone
• Nano-particle-modified silane consolidants;
calcium alkoxides; calcium phosphate or
oxalate treatments
• Nanotechnology cleaning agents
Preventive conservation • Microclimate stabilization and shelters
• Mitigation of rapid environmental fluctua –
tions for immovable cultural property
• Environmental control for salt-laden struc –
tures based on computer models and observations
• Wind fences, trees, reburial, etc.
Documentation tools • 3D laser scanning to quantify surfaces
• Quantitative calibration of digital color
images
• Solving the lighting problem—repeat
photography
– PTM images – Color matching
Tools for damage monitoring • Laser interferometry• Laser scanning• Real-time crack monitoring• Time-lapse systems
• Linear Variable Differential Transformer
(LVDT)
Research tools • NMR• FIB/ESEM, cryo-SEM, wet-STEM• CT-scanning• Thermal analysis• Damage models• Heat and moisture transport models

80 Chapte r 1
PROOF 1 2 3 4 5 680 Chapte r 7
PROOF 1 2 3 4 5 6In many parts of the world we are now less likely to see the heavy-handed
use of biocides, waterproofing agents, and consolidants, and more likely to see emphasis on careful documentation, monitoring, regular maintenance, control of moisture, selective use of waterproofing agents and consolidants, stone replacement, and the design of minimally invasive treatments. The tradition of regular attention and maintenance is finally being seen as a more realistic alternative to the dream of a cure-all, silver-bullet stone pre –
servative. The preservation of our heritage in stone will ultimately benefit from our growing understanding of material behavior (Torraca 2009) and the maintenance necessary to sustain long-term performance (Brand 1995).
CONCLUSION
This volume opened with the suggestion that our knowledge of stone
was outstripping the practical application of that knowledge to stone con –
servation problems. We have seen that there have, indeed, been major
advances in our knowledge of stone behavior. This strong scientific foun –
dation has also been accompanied by an encouraging number of new conservation treatments, methods, and tools.
The key challenge for the future is that resources for applied
research, technology transfer, and long-term testing are needed. While progress in these areas has undoubtedly been evident over the last fifteen years, structural gaps remain between researchers and practitioners and between the old assumptions and rapidly evolving new knowledge. Scarce resources for stone research are not always being applied to best use. This may be a useful moment to rethink the structural problems inherent in traditional approaches to conservation projects and funding. In order to preserve our heritage in stone, it is time to build support for larger-scale and longer-term research and technology transfer projects. In a number of cases, we have exciting solutions to stone conservation problems, but we do not have the resources to properly test and implement these solutions.
Notes
1 EH = English Heritage; NPS = National Park Service, USA; TNO = Netherlands
Technical Organization.
2 Globish is a term proposed by a French academic for a subset of 1,500 English words often used for global communication (see http://en.wikipedia.org/wiki/
Globish ). The term “globish” is a blend of “global” and “English.”
3 Some more recent references in this book’s bibliography contain digital object identifiers (DOIs), which can be used to link to online resources using the following Web site: http://dx.doi.org/ .Table 7.3
Current Trends in Conservation Research• Impacts of climate change
• Rare events versus routine damage
• Use of volunteers in conservation assessments
• Internet (access to research and commentary)
• Biomimetic surfaces
• Nanotechnology

Chapter #
Chapter Title
Authors’ names
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Abu-Jaber, N., Z. Al Saad, and M. Al Qudah. 2006. Landscape, geology and lithology
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Adamo, P., and P. Violante. 2000. Weathering of rocks and neogenesis of minerals
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Adams, A. E., and W. S. MacKenzie. 1998. A Colour Atlas of Carbonate Sediments and
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Adolfs, N. C. 2007. Die Anwendung von Calciumhydroxid-Sol als Festigungsmittel
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PROOF 1 2 3 4 5 6A wide range of publications was scrutinized during the writing of this
book, and use was also made of databases such as AATA, BCIN, Scopus, ISI Web of Knowledge, and Science Direct; also, the Getty and UCLA libraries as well as many colleagues and other resources. Stone conserva –
tion is a multidisciplinary subject and thus relevant publications are to be found in many research areas. It is hoped that the following books and articles may provide the reader with a useful introduction to issues of research and application in the field of stone conservation.
This list is, by definition, incomplete and does not purport to be
definitive but merely aspires to be useful. The emphasis of this listing, originally created to aid the students of the 2009 Venice Stone Course,
is on stone, as mortars and grouts are covered elsewhere.
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Laboratory Tests: Final Report for the Research and Development Project . Appendix: Resources for Stone Conservation

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142 Appen dix: Resources for Stone Conservation
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Warscheid, T., and J. Braams. 2000. Biodeterioration of stone: A review. International
Biodeterioration and Biodegradation 46 (4): 343–68.
Introduction to Decay Mechanisms: Air Pollution and Environment
Brimblecombe, P., ed. 2003. The Effects of Air Pollution on the Built Environment . Air
Pollution Reviews 2. London: Imperial College Press; Singapore and River Edge, NJ: World Scientific.

Appen dix: Resources for Stone Conservation 145
PROOF 1 2 3 4 5 6Charola, A. E., and R. Ware. 2002. Acid deposition and the deterioration of stone:
A brief review of a broad topic. In Natural Stone, Weathering Phenomena,
Conservation Strategies and Case Studies, ed. S. Siegesmund, T. Weiss, and
A. Vollbrecht, 393–406. Geological Society Special Publication 205. London:
Geological Society of London.
Cooke, R. U., and G. B. Gibbs. 1993. Crumbling Heritage? Studies of Stone
Weathering in Polluted Atmospheres: A Report of Research on Atmospheric Pollution and Stone Decay for the Joint Working Party Between the Cathedrals Fabric Commission for England and the Joint Environmental Programme of National Power Plc and Powergen Plc . London: University College, Department
of Geography.
Fassina, V. 1987. Environmental pollution in relation to stone decay. Durability of
Building Materials 5: 317–58.
Livingston, R. A. 1997. Air pollution standards for architectural conservation.
In Saving Our Architectural Heritage: The Conservation of Historic Stone
Structures; Report of the Dahlem Workshop on Saving Our Architectural Heritage, The Conservation of Historic Stone Structures, Berlin, March 3–8, 1996 , ed. N. S. Baer and R. Snethlage, 371–87. Dahlem Workshop Reports.
Chichester and New York: John Wiley & Sons.
Meierding, T. C. 1993. Marble tombstone weathering and air pollution in North
America. Annals of the Association of American Geographers 83 (4): 568–88.
Searle, D. E., and D. J. Mitchell. 2006. The effect of coal and diesel particulates on
the weathering loss of Portland limestone in an urban environment. Science
of the Total Environment 370 (1): 207–23.
Smith, B. J., and P. A. Warke, eds. 1996. Processes of Urban Stone Decay: Proceedings
of SWAPNET ‘95; Stone Weathering and Atmospheric Pollution Network Conference Held in Belfast, 19–20 May 1995 . London: Donhead Publishing.
Introduction to Decay Mechanisms: Salt and Frost Weathering
Arnold, A., and K. Zehnder. 1991. Monitoring wall paintings affected by soluble salts.
In The Conservation of Wall Paintings: Proceedings of a Symposium Organized
by the Courtauld Institute of Art and the Getty Conservation Institute, London, July 13–16, 1987, ed. S. Cather, 103–35. Marina del Rey, CA: Getty
Conservation Institute. http://www.getty.edu/conservation/publications/pdf
_publications/wall_paintings.pdf .
Borrelli, E. 1999. Salts. Vol. 3 of ARC Laboratory Handbook, ed. E. Borrelli and
A. Urland. Rome: ICCROM. http://www.iccrom.org/pdf/ICCROM_14
_ARCLabHandbook02_en.pdf .
Burt, T. P., R. J. Chorley, D. Brunsden, N. J. Cox, and A. S. Goudie, eds. 2008. The
History of the Study of Landforms: Or the Development of Geomorphology. Vol. 4, Quaternary and Recent Processes and Form (1890–1965) and the Mid-
Century Revolution . London: Geological Society Publishing House.
Charola, A. E., and J. Pühringer. 2005. Salts in the deterioration of porous materi –
als: A call for the right questions. Restoration of Buildings and Monuments:
An International Journal = Bauinstandsetzen und Baudenkmalpflege: Eine
internationale Zeitschrift 11 (6): 433–42.
De Clercq, H. 2008. Performance of limestone contaminated with binary mixtures of
sodium sulphate and treated with a water repellent. In Hydrophobe V: Water
Repellent Treatment of Building Materials; Proceedings of Hydrophobe V, Fifth International Conference on Water Repellent Treatment of Building Materials, Royal Institute for Cultural Heritage (KIK-IRPA), Brussels, Belgium, April 15 and 16, 2008, ed. H. De Clercq and A. E. Charola, 107–15. Freiburg:
Aedificatio Verlag.
Doehne, E. 2002. Salt weathering: A selective review. In Natural Stone, Weathering
Phenomena, Conservation Strategies and Case Studies, ed. S. Siegesmund,
T. Weiss, and A. Vollbrecht, 51–64. Geological Society Special Publication 205. London: Geological Society of London.
Matsuoka, N., and J. Murton. 2008. Frost weathering: Recent advances and future
directions. Permafrost and Periglacial Processes 19 (2): 195–210.

146 Appen dix: Resources for Stone Conservation
PROOF 1 2 3 4 5 6Rodríguez-Navarro, C., and E. Doehne. 1999. Salt weathering: Influence of evapora –
tion rate, supersaturation and crystallization pattern. Earth Surface Processes
and Landforms 24 (2–3): 191–209.
Rossi-Manaresi, R., and A. Tucci. 1991. Pore structure and the disruptive or cementing
effect of salt crystallization in various types of stone. Studies in Conservation
36 (1): 53–58.
Sawdy, A., A. Heritage, and L. Pel. 2008. A review of salt transport in porous media:
Assessment methods and salt reduction treatments. In Salt Weathering on
Buildings and Stone Sculptures, 22–24 October 2008, The National Museum
Copenhagen, Denmark [Proceedings from the international conference], ed. J. S.
Albertsen, 1–27. Lyngby: Technical University of Denmark, Department of Civil Engineering.
Scherer, G. W. 2004. Stress from crystallization of salt. Cement and Concrete Research
34 (9): 1613–24.
Scherer, G. W., and J. J. Valenza II. 2005. Mechanisms of frost damage. In Materials
Science of Concrete VII, ed. F. Young and J. Skalny, vol. 7, 209–46. Westerville,
OH: American Ceramic Society.
Young, D., and D. Ellsmore. 2008. Salt Attack and Rising Damp: A Guide to Salt
Damp in Historic and Older Buildings . 2nd ed. Sydney: Heritage Council of
NSW, Heritage Victoria, South Australian Dept. for Environment and Heritage, and Adelaide City Council. http://www.heritage.nsw.gov.au/docs/HVC014_Salt
_Damp_tech_guide_FA_web.pdf .
Zehnder, K. 2007. Long-term monitoring of wall paintings affected by soluble salts.
Environmental Geology 52 (2): 395–409.
Zehnder, K., and A. Arnold. 1989. Crystal growth in salt efflorescence. Journal of
Crystal Growth 97 (2): 513–21.
Introduction to Stone Cleaning and Desalination
Allanbrook, T., and K. C. Normandin. 2007. The restoration of the Fifth Avenue
facades of the Metropolitan Museum of Art. APT Bulletin 38 (4): 45–53.
Andrew, C. M., M. Young, and K. Tonge. 1994. Stone Cleaning: A Guide for
Practitioners . Aberdeen: Robert Gordon University; Edinburgh: Historic
Scotland.
Ashurst, J. 1998. The cleaning and treatment of limestone by the lime method. In
Conservation of Building and Decorative Stone, ed. J. Ashurst and F. G. Dimes,
vol. 2, 169–76. Butterworth-Heinemann Series in Conservation and Museology. London and Boston: Butterworth-Heinemann.
Ashurst, N. 1994. Cleaning Historic Buildings . London: Donhead Publishing.
British Standards Institution (BSI). 2000. BS 8221-2:2000 Code of Practice for
Cleaning and Surface Repair of Buildings: Surface Repair of Natural Stones, Brick and Terracotta. London: BSI.
Clifton, J. R., ed. 1986. Cleaning Stone and Masonry: A Symposium Sponsored by
ASTM Committee E-6 on Performance of Building Constructions, Louisville, KY, 18 April 1983 . ASTM Special Technical Publication 935. Philadelphia:
ASTM.
Cooper, M., ed. 1998. Laser Cleaning in Conservation: An Introduction . Woburn, MA:
Butterworth-Heinemann.
Normandin, K. C., D. Slaton, N. R. Weiss, and J. Pearce, ed. 2005. Cleaning
Techniques in Conservation Practice. Special issue, Journal of Architectural
Conservation 11 (3).
Rodríguez-Navarro, C., K. Elert, E. Sebastián, R. M. Esbert, C. M. Grossi, A. Rojo,
F. J. Alonso, M. Montoto, and J. Ordaz. 2003. Laser cleaning of stone materials: An overview of current research. Reviews in Conservation (4): 65–82.
Sawdy, A., A. Heritage, and L. Pel. 2008. A review of salt transport in porous media:
Assessment methods and salt reduction treatments. In Salt Weathering on
Buildings and Stone Sculptures, 22–24 October 2008, The National Museum Copenhagen, Denmark [Proceedings from the international conference], ed. J. S.
Albertsen, 1–27. Lyngby: Technical University of Denmark, Department of Civil Engineering.

Append ix: Resources for Stone Conservation 147
PROOF 1 2 3 4 5 6Young, M., J. Ball, R. A. Laing, and D. C. M. Urquhart. 2003. Maintenance and Repair
of Cleaned Stone Buildings . Historic Scotland Technical Advice Notes 25.
Edinburgh: Historic Scotland.
Introduction to Stone Monitoring, Moisture, and Environment
Camuffo, D. 1998. Microclimate for Cultural Heritage . Developments in Atmospheric
Science 23. Amsterdam and New York: Elsevier.
Doehne, E., and S. Pinchin. 2008. Time-lapse macro-imaging in the field: Monitoring
rapid flaking of magnesian limestone. In Proceedings of the 11th International
Congress on Deterioration and Conservation of Stone, 15–20 September 2008,
Torun ´, Poland, ed. J. W. Lukaszewicz and P. Niemcewicz, 365–72. Torun ´, Poland:
Nicolaus Copernicus University.
Hall, C., and W. D. Hoff. 2002. Water Transport in Brick, Stone, and Concrete .
London and New York: Spon Press.
Massari, G. and I. Massari. 1993. Damp Buildings, Old and New. ICCROM,
International Centre for the Study of the Preservation and Restoration of Cultural Property, Rome. Rome: ICCROM.
Pender, R. J. 2004. The behaviour of water in porous building materials and structures.
Reviews in Conservation (5): 49–62.
Thornbush, M. J., and H. A. Viles. 2008. Photographic monitoring of soiling and decay
of roadside walls in central Oxford, England. Environmental Geology 56 (3–4):
777–87.
Tiano, P., and C. Pardini, eds. 2008. In Situ Monitoring of Monumental Surfaces:
Proceedings of the International Workshop SMW08, 27–29 October 2008, Florence, Italy . Florence: Edifir.
Introduction to Rock Art Conservation
Dorn, R. I., D. S. Whitley, N. V. Cerveny, S. J. Gordon, C. D. Allen, and E. Gutbrod.
2008. The Rock Art Stability Index: A new strategy for maximizing the sustain –
ability of rock art. Heritage Management 1 (1): 37–70.
Hygen, A.-S. 2006. Protection of Rock Art, The Rock Art Project 1996–2005: Final
Report from the Directorate for Cultural Heritage . Oslo: Directorate for
Cultural Heritage Archive. http://www.riksantikvaren.no/filestore/rockart
project-finalreport.pdf .
Levin, J., ed. 2006. Rock Art. Special issue, Conservation: The GCI Newsletter 21 (3).
http://getty.edu/conservation/publications/newsletters/21_3/index.html .
MacLeod, I. 2000. Rock art conservation and management: The past, present and
future options. Reviews in Conservation 1: 32–45.
Saiz-Jimenez, C., ed. 2006. Conservation of Rock Art. Special issue, COALITION:
CSIC Thematic Network on Cultural Heritage Electronic Newsletter (10–12).
http://www.rtphc.csic.es .
Whitley, D. S., ed. 2001. Handbook of Rock Art Research . Walnut Creek, CA:
Alta Mira Press.
Other important resources include the proceedings from numerous regional and international rock art conferences such as those held by the IFRAO (International Federation of Rock Art Organizations) and the Australian Rock Art Research Association (AURA). Some otherwise difficult-to-find material can be found in the California Rock Art Studies Bibliographic Database at the University of California, Berkeley
(Marymor, L. 2009. Rock Art Studies: A Bibliographic Database. Available
at http://bancroft.berkeley.edu/collections/rockart.html ), although it is not up to date
as of this writing. For the present overview, over four hundred bibliographic sources related to rock art conservation were collected and reviewed.
Introduction to the Conservation of Ornamental Stones
Fellowes, D., and P. Hagan. 2003. Pyrite oxidation: The conservation of historic ship –
wrecks and geological and palaeontological specimens. Reviews in Conservation
(4): 26–38.

148 Appendi x: Resources for Stone Conservation
PROOF 1 2 3 4 5 6Jiménez-González, I., C. Rodríguez-Navarro, and G. W. Scherer. 2008. Role of clay
minerals in the physicomechanical deterioration of sandstone. Journal of
Geophysical Research F: Earth Surface 113 (2): F02021.
Siegesmund, S., J. Ruedrich, and A. Koch. 2008. Marble bowing: Comparative
studies of three different public building facades. Environmental Geology
56 (3–4): 473–94.
Zehnder, K. 2006. Greying of black polished limestone: A case study to clarify the phe –
nomenon. Zeitschrift für Kunsttechnologie und Konservierung 20 (2): 361– 67.
Some Related EC Projects and Results
Bourgès, A., and V. Vergès-Belmin. 2008b. Comparison and optimization of five desali –
nation systems on the inner walls of Saint Philibert Church in Dijon, France.
In Salt Weathering on Buildings and Stone Sculptures, 22–24 October 2008,
The National Museum Copenhagen, Denmark [Proceedings from the inter –
national conference] , ed. J. S. Albertsen, 29–40. Lyngby: Technical University
of Denmark, Department of Civil Engineering. http://www.design.upenn.edu/
files/14-Bourges__Verges_Belmin_Desalination_SWBSS_2008.pdf .
European Commission Research, and P. Jacobs. 2009. SALTCONTROL: Countering
Erosion in Europe’s Historic Buildings. http://ec.europa.eu/research/fp6/ssp/
saltcontrol_en.htm .
Groot, C., R. van Hees, and T. Wijffels. 2009. Selection of plasters and renders for salt
laden masonry substrates. Construction and Building Materials 23 (5): 1743–50.
Malaga, K., B. Schouenborg, and B. Grelk. 2008. Combadura y dilatación de paneles
de piedra natural: Ensayo y evaluación de mármol y caliza [Bowing and expan –
sion of natural stone panels: Marble and limestone testing and assessment]. Materiales de construccion 58 (289–90): 97–112.
Price, C., ed. 2000. An Expert Chemical Model for Determining the Environmental
Conditions Needed to Prevent Salt Damage in Porous Materials: Protection and Conservation of the European Cultural Heritage . Research Report (European
Commission, Directorate-General XII, Science, Research, and Development) 11. London: Archetype Publications.
TU Delft (Delft University of Technology). 2009. EU Project Desalination: Assessment
of desalination mortars and poultices for historic masonry. http://www.citg
.tudelft.nl/live/pagina.jsp?id=267cbaf8-92c8-4204-92c0-97c32fff7eb5&lang=en .
Van Hees, R. P. J. 2002. Project ASSET: Assessment of suitable products for the
conservative treatments of sea-salt clay. http://www.onderzoekinformatie.nl/nl/
oi/nod/onderzoek/OND1282688/ .
Van Hees, R. P. J., S. Naldini, and J. Delgado Rodrigues. 2009. Plasters and renders for
salt laden substrates. Construction and Building Materials 23 (5): 1714–18.
Selected Stone Conservation Bibliographies
Baer, N. S. 1997. Materials of Art and Archaeology: Bibliography, Part IX; Stone . New
York: Conservation Center, Institute of Fine Arts, New York University. http://
www.nyu.edu/gsas/dept/fineart/faculty/baer/Bibliography9.doc .
Doehne, E. 2003. Building material decay and salt weathering: A selected bibliography.
Supplement to Natural Stone, Weathering Phenomena, Conservation Strategies
and Case Studies, ed. S. Siegesmund, T. Weiss, and A. Vollbrecht. Geological
Society Special Publication 205. London: Geological Society of London. http://
www.geolsoc.org.uk/webdav/site/GSL/shared/Sup_pubs/2003/SUP18182.rtf .
ICOMOS Centre de documentation = ICOMOS Documentation Centre. 2009. Stone:
Bibliography = Pierre: Bibliographie . Paris: ICOMOS. http://www.international
.icomos.org/centre_documentation/bib/stone.pdf .
ICOMOS-ISCS (ICOMOS International Scientific Committee for Stone) and V. Vergès-
Belmin. 2002. Bibliography (Selection). http://lrmh-ext.fr/icomos/consult/index
.htm [Bibliography].
Lewin, S. Z. 1966. The preservation of natural stone, 1839–1965: An annotated bibli –
ography. Art and Archaeology Technical Abstracts 6 (Suppl. 1): [183]–277.

Append ix: Resources for Stone Conservation 149
PROOF 1 2 3 4 5 6References from the French Literature in Stone Conservation
Bromblet, P., and G. Martinet. 2002. Joints, mortiers de pose et produits de ragréa ge:
Les différentes pathologies; Réflexions et préconisations. Pierre actual:
Matériaux, ouvrages, techniques (785): 66–79.
Bromblet, P., J.-D. Mertz, V. Vergès-Belmin, and L. Leroux. 2002. Consolidation et
hydrofugation de la pierre. Monumental: Revue scientifique et technique de la
Sous-direction des monuments historiques (2002): 200–243.
Bromblet, P, and T. Vieweger. 2005. Le laser de nettoyage de la pierre et la restau ration
des sculptures. Pierre actual: Matériaux, ouvrages, techniques (829): 86–95.
http://www.lrmh.fr/lrmh/telechargement/laserpb.pdf .
Orial, G. 2005. Les altérations biologiques et les biens patrimoniaux: Introduction.
Monumental: Revue scientifique et technique de la Sous-direction des
monuments historiques 2005 (1): 95.
Orial, G. 2005. Les altérations biologiques et les biens patrimoniaux: Les bactéries,
algues et lichens: Morphologie et altérations. Monumental: Revue scientifique et
technique de la Sous-direction des monuments historiques 2005 (1): 96–99, 117.
Orial, G., and F. Bousta. 2005. Les altérations biologiques et les biens patrim oni-
aux: Les traitements; Définitions, sélection des produits et mise en oevre.
Monumental: Revue scientifique et technique de la Sous-direction des monu-ments historiques 2005 (1): 107–12.
Vergès-Belmin, V., and P. Bromblet. 2000. Le nettoyage de la pierre. Monumental:
Revue scientifique et technique de la Sous-direction des monuments historiques
(2000): 220–73.
Vergès-Belmin, V, and P. Bromblet. 2001. La pierre et les sels. Monumental: Revue
scientifique et technique de la Sous-direction des monuments historiques
(2001): 224–62.
References from the German Literature in Stone Conservation
Siegesmund, S., M. Auras, J. Ruedrich, and R. Snethlage, eds. 2005. Geowissenschaften
und Denkmalpflege: Bauwerkskartierung, Natursteinverwitterung, Konservierungsstrategien. Zeitschrift der Deutschen Geologischen Gesellschaft
156 (1): 1–238. http://www.schweizerbart.de/papers/zdgg/list/156#paper55362 .
Siegesmund, S., and A. Ehling, eds. 2007. Rohstoff Naturstein = Natural Building
Stone Resources. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften
158 (3): 349–665.
Siegesmund, S., and A. Ehling, eds. 2008. Rohstoff Naturstein: Teil 2 = Natural
Building Stone Resources: Part 2. Zeitschrift der Deutschen Gesellschaft für
Geowissenschaften 158 (4): 667–1087.
Siegesmund, S., and R. Snethlage, eds. 2008. Denkmalgesteine Festschrift Wolf-Dieter
Grimm. Schriftenreihe der Deutschen Gesellschaft für Geowissenschaften 59:
1–326.
Snethlage, R., ed. 1995. Natursteinkonservierung I . Denkmalpflege und
Naturwissenschaft. Berlin: Ernst & Sohn.
Snethlage, R., ed. 1998. Natursteinkonservierung II . Denkmalpflege und
Naturwissenschaft. Stuttgart: Fraunhofer.
Useful Sources of Information for Stone Conservation Research
Free Online Research Databases (arranged in order of usefulness)
Google Scholar (useful for articles):
http://scholar.google.com/
Google Books (some books have searchable full text):
http://books.google.com/
WorldCat (search local libraries and post reviews):
http://www.worldcat.org /
Getty Conservation Institute Art and Archaeology Technical Abstracts (AATA):
http://aata.getty.edu/nps/

150 Appendi x: Resources for Stone Conservation
PROOF 1 2 3 4 5 6Canadian Conservation Information Network BCIN:
http://www.bcin.ca/
ICCROM Library:
http://library.iccrom.org/
The Laboratoire de Recherche des Monuments Historiques (LRMH) has a photo –
graphic and bibliographic database called CASTOR:
http://www.lrmh.fr/cgi-bin/qtp?typge=CZIE&lang=uk
The CRNS database CAT.INIST is useful, especially for finding missing abstracts for
older articles:
http://cat.inist.fr/?aModele=presentation
The Scirus database by Elsevier is similar to Google Scholar:
http://www.scirus.com/
Commercial Research Databases (by institutional subscription, in most
cases; arranged in order of usefulness)
Scopus by Elsevier:
http://info.scopus.com/
Science Direct by Elsevier:
http://www.sciencedirect.com/
ISI Web of Knowledge:
http://isiwebofknowledge.com/
Springer:
http://www.springerlink.com/
Geological Society of London Database:
http://www.lyellcollection.org/
Geological Society of America Publications:
http://www.gsapubs.org/
American Chemical Society:
www.acs.org/
JSTOR Non-Profit Archive:
http://www.jstor.org/
PowerPoint Slides Archive and Network
http://www.slideshare.net/
http://www.slideshare.net/icomos.uk
Online Network of Repositories
http://en.scientificcommons.org/http://repository.upenn.edu/
Online Building Conservation Education Site
(Practitioner support for building conservation accreditation)
http://www.understandingconservation.org/
Lists of Conservation Related Sites
Getty Conservation Institute List of Sites:
http://www.getty.edu/conservation/research_resources/othersites.html
ICCROM Database of Conservation Related Links:
http://www.iccrom.org/db_links.asp
Conservation Online (Cool) (site formerly hosted at Stanford University, now on a
new server at AIC):
http://cool.conservation-us.org/
American Institute for Conservation of Historic and Artistic Works (AIC):
http://www.conservation-us.org/

Appen dix: Resources for Stone Conservation 151
PROOF 1 2 3 4 5 6Robert Gordon University, Aberdeen, UK (links to heritage conservation and related
sites [last updated in 2005, but still quite useful]):
http://www2.rgu.ac.uk/schools/mcrg/sites.htm
UK National Conservation Centre:
http://www.liverpoolmuseums.org.uk/conservation/
Forum Restauro @ Conservazione:
http://www.forum-restauro.org/
A variety of documents are available digitally and can be found via their DOI (digital
object identifier). Similar to URLs, DOIs do not change as Web sites change. When the DOI for a document is known, the document can be located by accessing a DOI resolver, such as http://dx.doi.org , and entering the DOI.

PROOF 1 2 3 4 5 6note: page numbers followed by t
refer to tables.
Aachen concept, 45
accelerated weathering studies, 52–53acid rain, 10, 12–13
protection, testing effectiveness of, 50
rock art and, 61
acrylics, as consolidant, 42Acryloid B72, 42AFM (atomic force microscopy), 6, 17–18air pollution
biofilms and, 23
as source of salts, 15
study of, new approaches to, 75
air pollution, as cause of decay, 10–15
as ancient problem, 11
atmospheric sulfur dioxide, 5, 10,
12, 13
benefits of reduction, research on,
12–13
extent of problem, 11
factors affecting, 11
history of research on, 10, 26n2
indirect effects of, 14–15
memory effect in, 14
research needed in, 13–14
wet vs. dry deposition, 13
algae, 22, 23
biocides and, 48, 62
alkoxysilanes
as consolidant, 39–41
with B72, 42
need for research on, 53
recent developments in, 78
as surface coating, 45, 46
as water repellent, 44–45
alkylalkoxysiloxane, 45Altamira cave, 23, 58, 60, 62altitude, and differential stress, 25alveolization, 18–19, 26n7American Society for Testing and
Materials (ASTM), 51
aminoalkyl silane, 43ammonium bicarbonate, 34ammonium biocides, water repellents
and, 56
ammonium carbonate, 34ammonium oxalate, 46animal activity, and rock art damage,
60, 63
antibacterial treatments, 62. See also
biocides
antifungal treatments, 62. See also
biocides
anti-graffiti coatings
for rock art, 61
in stone conservation, 45–46
appearance, cleaning and, 29, 31, 32archaea, halophilic, 24Arte Mundit. See latex poultice method
ASMOSIA (Association for the Study of
Marble and Other Stones in Antiquity), 64
assessment. See effectiveness of
treatments, assessment of
Assessment of Desalination Mortars and
Poultices for Historic Masonry. See
DESALINATION
Association for the Study of Marble and
Other Stones in Antiquity (ASMOSIA), 64
ASTM (American Society for Testing and
Materials), 51
atomic force microscopy (AFM), 6, 17–18autotrophic bacteria, 23
bacteria
autotrophic, 23
biocides for, 48
in biological cleaning, 33
cyanobacteria, 23
in desalination, 35
halophilic, 19, 24
heterotrophic, 23
lime treatment and, 37
nitrifying, 12
role of, 20, 21, 22–23
Balfour Beatty Limited, 36
Balvac, 36barium carbonate coating, 38biaxial flexural strength measurements, 8BIOBRUSH (BlOremediation for Building
Restoration of the Urban Stone Heritage; EC project), 35, 47
biocides, 21, 47–48
current caution regarding, 80
endolithic microbes and, 22 in rock art conservation, 60, 62
biodeterioration, 20–24. See also algae;
bacteria; lichens; microbes; vegetation
balancing of appearance and
longevity, 21
factors in, 23
preventive measures, 23
recent research advances, 23–24
research reviews on, 21
terminology of, 21
biofilms
air pollution and, 23
effect of, 21
limestone and, 23
biological cleaning, 33biological stain removal, 47biomimetic surfaces, 43, 80tbioremediation, 21biotite, salt contamination and, 19black crusts
removal of, 29, 33, 34
research on, 11–12, 33
black fungi, 12BIOremediation for Building Restoration
of the Urban Stone Heritage (BIOBRUSH), 35
Bologna cocktail, 42bowing, of thin marble slabs, 26Brazil, conservation research in, 75, 77breakdown of treatments, need for
research on, 53
BRGM (Bureau de Recherche Géologique
et Minière), 64–65
British Geological Survey, 64British Museum renovation, replacement
stones for, 64
B72, 42, 52Bureau de Recherche Géologique et
Minière (BRGM), 64–65
Burra Charter, 54
calcite, transformation into calcium
phosphate, 43
calcite dissolution
biofilms and, 21
salt contamination and, 19
calcium alkoxides, 43
calcium carbonate, 36Index

Inde x 153
PROOF 1 2 3 4 5 6calcium hydroxide (slaked lime), as
consolidant, 36–37
calcium oxalate, 22, 46
calcium phosphate, transformation of
gypsum or calcite into, 43
calcium sulfate
converting back to calcium
carbonate, 33
removal of, 34
carbonate materials, cleaning of, 29carbon dioxide, as cause of decay, 10carbonic acid, 13, 14
lichens and, 22
caves, environmental control in, 23–24cement mortar, in rock art
conservation, 62
CEN (Comité Européen de
Normalisation) Technical Committee 346, 49, 51
characterization of stone. See also
documentation of current form of stone
after treatment, 50–51
before treatment, 1–2
chelating agents, heterotrophic bacteria
and, 23
chemical composition of stone, relevance
to preservation, 2
chemical dissolution, measurement of, 6China, conservation research in, 75, 77citation indices
need for development of, 78
usefulness of, 70–71
citation ranking, 71. See also H-index
clay minerals, salt decay in, 19clay-rich stone, alkoxysilanes and, 40clay swelling
and differential stress, 24
prevention of, 28
recent advances in, 78
research on, 25
cleaning, 29–33
assessing effectiveness of, 30–31
biological cleaning, 33
and damage, 30
efficiency of, water repellents and, 52
issues in, 29–30
laser cleaning, 31–32
latex poultice method, 32
recent developments in, 78
of rock art, 61, 62
target cleaning level, importance of
defining, 30, 31
targeting of specific types of dirt, 33
techniques, 30
climate change
impact on stone decay rates, 14–15
increased interest in, 75, 80t
close-range photogrammetry, 5coal, burning of, and air pollution, 11colloidal silica, as surface coating, 47Comité Européen de Normalisation
(CEN) Technical Committee 346, 49
COMPASS project (EC), 8computerized X-ray tomography (CT), 8conferences
on-line distribution of proceedings,
need for, 78
poor quality of papers at, 67, 72, 78
reviews, value of, 71
Torun Guidelines for Conferences in
the Field of Stone Conservation, 67, 68
conferences, suggestions for improving
attendance, funding of, 71
conference papers, selection of, 71–72
confocal microscopy, 5conservation, active. See also effectiveness
of treatments, assessment of
breakdown of treatments, need for
research on, 53
cleaning, 29–33
consolidation, 35–43
current caution regarding, 78, 79–80
desalination, 33–35
finite life of, and conservation policy,
54, 55
interaction of, research needed on,
55–56
past treatments, importance of records
of, 56
questions to ask prior to, 50
reversibility of, as realistically
impractical, 55
surface coatings, 43–48
conservation
key challenges for the future, 80
recent trends in, 79–80, 80t
Conservation of Ancient Stone Quarry
Landscapes in the Eastern Mediterranean (QuarryScapes) project (EC), 63
conservation policy, 54–57
finite life of treatments and, 54, 55
international uniformity, efforts to
develop, 54
parties playing role in, 54
recording of stone as it exists, 56–57
on retreatment, 55–56
on surface coatings and consolidants,
55
variety of approaches to, 54
conservation science
attraction of high-quality students to
field, 73
tool kit for, 79, 79t
training programs in, 73
conservators, lack of scientific
training, 73
consolidants, 35–43
acrylics, 42
alkoxysilanes, 39–41
application of, 36
aqueous emulsions, research on, 43
barium hydroxide, 38
blocking of pores by, 55
bonding with substrate, 39
current caution regarding, 80
and differential stress, 25 distribution of within stone, 39
emulsions, 43
endolithic microbes and, 22
epoxies, 41–42
evaluative studies of, 52
interaction of, research needed on,
55–56
lime technique, and related treatments,
36–37
organic polymers, 38–39
other materials, 43
recent developments in, 78
required characteristics of, 35–36
research needed on, 39
responsible use of, 55
reversibility of, as realistically
impractical, 55
for rock art treatment, 61
selection of, as empirical, 38–39
contact profilometry, 5cryo-scanning electron microscopy (cryo-
SEM), 79, 79t
crystal growth inhibitors, 18, 46crystallization damage. See salt damage
CT (computerized X-ray tomography), 8cyanobacteria, 12, 23cycloaliphatic epoxy resins, 41–42cyclododecane, as consolidant, 43
Dahlem Conference, 9, 26n1, 74n5
damage functions, 12damp-proof courses (DPCs), 34–35, 45databases
EUROCARE, 53
of field trials, 53
on Internet, and improvements in
research, 66
MONUFAKT, 53
of replacement stone, efforts to
establish, 64–65
of research materials, online, need for
development of, 78
of stone as it exists, 56–57
decay, causes of, 9–26
air pollution, 10–15
biodeterioration, 20–24
differential stress, 24–25
intrinsic problems, 25–26
research reviews on, 9
salts, 15–20
system dynamics approach to, 9, 26n1
decay, definition of, 3decay, describing
terminology, 2–3
types, 2
decay, measuring, 3–9
in evaluation of treatment
effectiveness, 50
inadequacy of current procedures, 4
purposes of, 3–4
quantities to measure, 3
research needed in, 9
subsurface methods, in situ, 6–7
subsurface methods, laboratory-based,
7–9

154 Inde x
PROOF 1 2 3 4 5 6 surface techniques, 5–6
decay rates, nonlinear, new awareness
of, 75
degradation, definition of, 3
DEMs (digital elevation models), 56desalination, 33–35
alkoxysilane consolidation and, 41
large-scale masonry projects, 34
source reduction, 34–35
DESALINATION (Assessment of
Desalination Mortars and Poultices for Historic Masonry; EC project), 8, 34
DFMS (drilling force measurement
system), 7
differential scanning calorimetry, 18differential stress, 24–25
research on, 25
digital elevation models (DEMs), 56digital holography, 5digital object identifiers (DOIs), 80documentation of current form of stone.
See also characterization of stone
current emphasis on, 80
current tools for, 79t
in rock art conservation, 60, 63
in stone conservation, 56–57
DOIs (digital object identifiers), 80dose response, as concept in air pollution
studies, 75
DPCs (damp-proof courses), 34–35, 45drainage, and rock art conservation, 61drawing, in recording of stone as it exists,
56
Dri Film 104, 42drilling force measurement system
(DFMS), 7
drilling resistance measurement system
(DRMS), 7
drilling resistance profile, in evaluation of
treatment effectiveness, 50
DRMS (drilling resistance measurement
system), 7
dust, and rock art damage, 60, 61
EBSPits (England’s Building Stone Pits),
64
ecological context, new emphasis on, 75
EDS (energy dispersive X-ray
spectrometry), 29
EDTA (ethylene diamine tetra acetic
acid), 32, 34
effectiveness of treatments, assessment of,
49–53
accelerated weathering studies, 52–53
criteria for, 49, 50
databases, efforts to develop, 53
long-term assessment, 51–53
natural exposure trials, 52
need for standardization in, 49
short-term assessment, 50–51
standardization of methods, efforts
toward, 51
tests designed for untreated stone and,
50–51electronic speckle pattern interferometry
(ESPI), 5–6
emulsions
as consolidant, 43
as surface coating, 46
endolithic microbes, 18, 22energy-dispersive spectroscopy, 7–8energy dispersive X-ray spectrometry
(EDS), 29
England’s Building Stone Pits
(EBSPits), 64
English, as lingua franca of science, 77
English Heritage, 64environmental concerns
biocides and, 47
cleaning and, 30
consolidants and, 43
and shrinking of conservator’s tool
kit, 78
environmental control
in caves, 23–24
new emphasis on, 27
in rock art conservation, 60
Environmental Geology (periodical), 19
environmental scanning electron
microscopy (ESEM), 17, 29
epoxies, as consolidant, 41–42epoxysilanes, 46EPS (extracellular polymeric substances),
23
ESEM (environmental scanning electron
microscopy), 17, 29
ESPI (electronic speckle pattern
interferometry), 5–6
ethanol, 62ethylene oxide, 48ethyl-silicate-based treatments, 43EUROCARE database, 53European Commission (EC), 34
anti-graffiti coating project, 45
BIOBRUSH project, 35, 47
COMPASS project, 8
DESALINATION project, 8, 34
interdisciplinary collaboration,
encouragement of, 67–70, 76
QuarryScapes project, 63
and rock art conservation research, 62
SALTCONTROL project, 46
STONECORE project, 37
European Cooperation in Science and
Technology, 32
European Norm (EN) standards, for
assessment of treatments, 51
European Science Foundation, 32evaluation of treatments. See effectiveness
of treatments, assessment of
extracellular polymeric substances (EPS),
23
feedback loops, as concept in air
pollution studies, 75
feldspars, salt contamination and, 19FIB/ESEM (Focused Ion Beam/
Environmental Scanning Electron Microscopy), 79, 79tfire, and rock art, 61flood protection, in rock art conservation,
61, 76
fluorescence LIDAR (light detection and
ranging), 4
fluorinated acrylic polymers, 45
evaluative studies on, 52
fluorinated polyurethane, 46fluoropolymers, as water repellent,
44–45
Focused Ion Beam/Environmental
Scanning Electron Microscopy (FIB/ESEM), 79, 79t
fractal dimension, characterization of
stone by, 2
freeze-thaw cycle, and rock art, 61frequency, as concept, in air pollution
studies, 75
frontal (in situ) polymerization, 43frost damage
to rock art, 61
to stone, 20
Funcosil, 41funding. See also research funding
of conference attendance, 71
as key challenge for future, 80
for maintenance, inadequacy of, 78
fungus
biocides for, 48
in biological cleaning, 33
control of, 24
and rock art, 61
Geological Survey of Norway, 63Gioia marble, barium hydroxide
consolidation and, 38
global warming. See climate change
glues, organic, in rock art conservation,
62
Google Scholar, 70government funding of research, decrease
in, 76–77
granite, laser cleaning of, 31ground-penetrating radar, 7gypsum
and rock art deterioration, 60
transformation into calcium
phosphate, 43
gypsum layer
latex poultice cleaning and, 32
removal of, 29–30
halophilic archaea, 24halophilic bacteria, 19, 24Hamar Cathedral, Norway, 28HCT, 43health and safety concerns
consolidants and, 43
and shrinking of conservator’s tool
kit, 78
heating, differential, and differential
stress, 25
heritage hydrology, need for research on,
76
heterotrophic bacteria, 23

Inde x 155
PROOF 1 2 3 4 5 6hexafluoropropene-vinylidene fluoride
elastomer, 43
high-speed neutron tomography
(synchrotron radiation), 8
H-index, 71. See also citation ranking
historic quarries
preservation, importance of, 63
research on, 63–64
HMC (hygroscopic moisture content), 8
holography, in recording of stone as it
exists, 56
homogeneity, characterization of stone
by, 2
honeycomb weathering, 18–19human visitors, and rock art
conservation, 60, 63, 76
humidity control, in preventive
conservation, 28–29
hydration damage, 15hygric swelling
characterization of stone by, 2
and differential stress, 24
hygric tests, 8hygroscopic moisture content (HMC), 8hygroscopic salts, halophilic microbes
and, 24
ICOMOS
Burra Charter and, 54
Principles for the Conservation of
Heritage sites in China , 54
Stone Committee, 2
ICOMOS-ISCS Illustrated Glossary on
Stone Deterioration Patterns , 3
India, conservation research in, 75, 77infrared thermography, 7inherent vice. See intrinsic problems
interdisciplinary research
increased funding of, 76
lack of, in current work, 67–70
strategies for encouraging, 72–73
in universities, increase in, 76
International Charter for the
Conservation and Restoration of Monuments and Sites (Venice Charter), 54
International Institute for Conservation
(IIC), 74
Internet
and availability of research, 77–78, 80t
databases on, and improvements in
research, 66
and dissemination of research, 70
needed improvements in web-based
tools, 78
intrinsic problems, as cause of decay,
25–26
ion chromatography, 16ISI Web of Knowledge (Science Citation
Index), 70
isocyanates, as consolidant, 43Italian Commissione NORMAL, 2, 51ivy, 20–21
journal impact factors, 70–71kanamycin, as biocide, 48
Kress Foundation, 77Kumar, 41
Laboratoire de Recherche des
Monuments Historiques (LRMH),
65
LACONA (Lasers in the Conservation of
Artworks), 32
language barriers, and research, 77Lascaux, caves at, 23–24, 58, 60, 70laser cleaning, 31–32
recent developments in, 78
of rock art, 62
laser holography interferometry, 7laser interferometry, 5laser profilometry, 5laser scanning, in documentation of stone
artifacts, 5, 56–57, 63
Lasers in the Conservation of Artworks
(LACONA), 32
laser treatment, as biocide, 48laser triangulation, 5latex poultice method (Arte Mundit), 32latex solutions, recent developments
in, 78
Lausanne molasse, 25–26Lecce limestone, 25lichens, 21, 22
biocides for, 48
prevention of, 28
removal of, 33, 61, 62
research on effects of, 62
and rock art, 61
limestone
acrylic consolidants and, 42
alkoxysilanes and, 40
consolidation, recent developments
in, 78
dissolution, bacteria and, 23
and osmotic swelling, 25
salt removal, 34
soiling, water repellents and, 52
linear variable differential transformer
(LVDT), 8
linear velocity displacement transducer.
See linear variable differential
transformer
long-term research and testing
funding for, as key challenge for
future, 80
importance of, 78
lack of resources for, 75, 78
long-term assessment, 51–53
loss compensation for stone, 65LRMH (Laboratoire de Recherche des
Monuments Historiques), 65
LVDT (linear variable differential
transformer), 8
magnetic resonance imaging (MRI;
NMR), 4, 8–9, 17–18
magnitude, as concept in air pollution
studies, 75
maintenance, routine current emphasis on, 80
importance of, 27
limited funding for, 78
marble
alkoxysilanes and, 40
bowing in thin slabs of, 26
Masonry Damage Diagnostic System
(MDDS), 3
mass balance methods, 13MDDS (Masonry Damage Diagnostic
System), 3
media, interest in stone conservation, 77Mellon Foundation, 73, 77memory effect, of air pollution, 14mercury porosimetry, 39methyl-methacrylate, 42methylphenyl silicone resin, 48methyltrimethoxysilane (MTMOS), 40,
41, 42
microbes
endolithic, 22
growth in replacement stones, 65
patinas and, 33
to produce sacrificial layer of calcite, 47
and rock art conservation, 60–61
surface coatings as food for, 48, 53
microcatchment, 6microclimate stabilization, in rock art
conservation, 60
microerosion meter, 5microflora, 12microscopy
AFM (atomic force microscopy), 6,
17–18
confocal microscopy, 5
cryo-scanning electron microscopy
(cryo-SEM), 79, 79t
ESEM (environmental scanning
electron microscopy), 17, 29
FIB/ESEM (Focused Ion Beam/
Environmental Scanning Electron Microscopy), 79, 79t
optical, 29
polarized light, 7
scanning electron, 7, 39
wet-STEM (wet-scanning transmission
electron microscopy), 79, 79t
mineral leaching, and rock art, 61minimally invasive treatments, current
emphasis on, 80
minimum intervention principle, 27mirabilite, 15moisture
effects of, need for research on, 76
transport, endolithic microbes and, 22
moisture control
current emphasis on, 80
for rock art treatment, 61
molds, in recording of stone as it exists,
56
monitoring, current emphasis on, 80MONUFAKT database, 53Mora poultice method, 32Mowilith DM 123 S, 62MRI. See magnetic resonance imaging

156 Inde x
PROOF 1 2 3 4 5 6MTMOS (methyltrimethoxysilane), 40,
41, 42
multidisciplinary research. See
interdisciplinary research
nano-lime technology, 37, 78
nano particle-modified silanes, 40NAPAP. See U.S. National Acid
Precipitation Assessment Program (NAPAP) studies
National Center for Preservation
Technology and Training (NCPTT), 74, 77
National Science Foundation (USA),
research funding, 77
NCPTT (National Center for Preservation
Technology and Training), 74, 77
NGOs, research funding, decrease in,
76–77
nitric acid, 14
autotrophic bacteria and, 23
nitrogen oxides
as cause of decay, 10
research needed on, 13–14
NMR. See magnetic resonance imaging
nonlinearities, as concept in air pollution
studies, 75
Norway
Hamar Cathedral, glass envelope over,
28
and preservation of ancient quarries, 63
rock art preservation efforts, 59, 61, 62
NSF (National Science Foundation),
research funding, 77
oligomeric alkylpolysiloxane, 45optical microscopy, 29optical profilometry, 5osmotic swelling, 25oxalate patinas, 22oxalic acid
effects of, 22
lichens and, 22
PAHs (polycyclic aromatic hydrocarbons),
12
painted stone, laser cleaning of, 31Paraloid B72, 42
irreversibility of, 52
Peclet number, 16, 26n6peer review
increase in, 76
need for, 72, 78
perfluoropolyether, 46photogrammetry, 56photographs
in quantification of decay, 4–5
in recording of stone as it exists, 56
photothermal radiometry, 7polarized light microscopy, 7polycyclic aromatic hydrocarbons
(PAHs), 12
polymerization, frontal (in situ), 43polymers, as consolidant
interaction with solvents, 55 research needed on, 39
polynomial texture mapping (PTM), 4–5
in recording of stone as it exists, 56
polyureas, as consolidant, 43polyurethanes, as consolidant, 43pores. See also voids, surface
blocking of by consolidants, 55
size and distribution, characterization
of, 50
structural changes in, due to
consolidation, 39
Portland brownstone, 24, 25potassium carbonate, 33potassium sulfate, 33poultices
in desalination, 34
DESALINATION project on, 8, 34
latex poultice method, 32
recent developments in, 78
surface voids and, 34
urea and glycerol poultice, 33
preventive conservation, 27–29
current tools, 79t
importance of, 55
minimum intervention principle, 27
new emphasis on, 27
of rock art, 63
variety of approaches to, 27
Preventive Conservation of Stone Historical
Objects (Domaslowski), 27
Principles for the Conservation of Heritage
sites in China (ICOMOS), 54
profilometry, 5protective shelters, 28PTM (polynomial texture mapping), 4–5.
See also RTI
in recording of stone as it exists, 56
QuarryScapes (Conservation of Ancient
Stone Quarry Landscapes in the Eastern Mediterranean) project (EC), 63
Queen Nefertari wall paintings,
approaches to conservation of, 76
radar, ground-penetrating, 7raking light photography, in recording of
stone as it exists, 56
recording of stone as it exists. See also
characterization of stone
current emphasis on, 80
current tools for, 79t
in rock art conservation, 60, 63
in stone conservation, 56–57
Reflectance Transformation Imaging
(RTI), 4. See also PTM
regulations, and shrinking of
conservator’s tool kit, 78
Reigate stone, 25–26replacement stone
databases on, efforts to establish,
64–65
issues in selection of, 64, 65
preservation of historic quarries
and, 63replicas, in recording of stone as it
exists, 57
research
importance of practical dissemination,
70
language barriers and, 77
limited interest of scientific community
in, 77
previous, limited access to, 78
research, interdisciplinary
increased funding of, 76
lack of, in current work, 67–70
strategies for encouraging, 72–73
in universities, increase in, 76
research, recent
increased quality of, 66, 76, 85
Internet tools, needed improvements in,
77–78
in nonwestern nations, 75
ongoing problems in, 78
scholarly review articles, increase in, 76
trends in, 75–77
research, shortcomings of, 66–70
conference papers, poor quality of, 67,
72, 78
dubious general applicability of studies,
1, 67, 78
lack of interdisciplinary perspective,
67–70
lack of standards of nomenclature or
testing procedures, 67
overly-theoretical work, 69, 70
publications, poor quality of, 66–67
superficiality, 69
research, suggestions for improving,
70–74
conference attendance, financial
support for, 71
conference papers, selection of, 71–72
emphasis on quality over quantity,
70–71
interdisciplinary research, 72–73
peer review, necessity of, 72
research managers, value of, 71
scholarly review articles, emphasis on,
73–74
training programs for conservation
scientists, 73
researchers
current tool kit for, 79, 79t
networks of, in Europe, 76
training programs for, 73
research funding
cutbacks in, 66
encouragement of interdisciplinary
collaboration through, 72–73
for interdisciplinary research, increase
in, 76
as key challenge for future, 80
trends in, 76–77
in United States, 77
research publications
increasing number of, 77
poor quality of research in, 66–67, 78
retreatment

Inde x 157
PROOF 1 2 3 4 5 6 research needed on, 55–56
with surface coatings, 44
Réunion Internationale des Laboratoires
et Experts des Matériaux, systèmes
de construction et ouvrages. See
RILEM
reversibility, principle of
and cleaning, 29
consolidants and, 55
Paraloid B72 and, 52
vs. practical reality, 55
surface coatings and, 55
review articles
benefits of, 73–74
on biodeterioration, 21
on decay, causes of, 9
increase in, 76
Reviews in Conservation (periodical),
74n5
RILEM (Réunion Internationale des
Laboratoires et Experts des Matériaux, systèmes de construction et ouvrages)
25-PEM Working Group, 49, 51
59-TPM (Traitement des monuments en
pierre) Working Group, 51
Risanamento di murature umide e
degradate (Pinto Guerra), 13
road dust, and rock art damage, 60road salt, salt damage from, 60rock art
definition of, 58
important sites, 58
rock art conservation, 58–61
appropriateness of standard stone
conservation techniques for, 60
deterioration factors, common, 60–61
documentation of art as it exists, 60, 63
growing interest in, 59
microbes and, 60–61
need for stability evaluation methods,
59–60
research needed in, 59
traditional practices, 59
vs. traditional stone conservation,
59, 60
treatments, 61–62
vegetation and, 61
as young science, 58–59
Rock Art Stability Index, 59–60root damage, to rock art, 61RTI (Reflectance Transformation
Imaging), 4. See also PTM
rubbings or rock art, as documentation,
63
Russia, conservation research in, 77
St. Trophime (France), 36
SALTCONTROL project (EC), 46salt damage, 15–19. See also desalination
accumulation of salts, 16, 24
crystal growth inhibitors, 18, 46
factors affecting, 18–19
hazardous salt level tables, 16
humidity control and, 28 indoors, 15
mechanisms of damage, 17–18
from mechanisms other than
crystallization, 19
recent research advance in, 16–18
research needed in, 20
research reviews on, 19–20
to rock art, 61
in rock art deterioration, 60
safe temperature and humidity ranges,
17
from salt mixtures, 16–17, 28
salt type and, 17
sources of salts, 15
salt levels, measurement of, 8, 16sand, blowing, mitigation of, 63sandstones, alkoxysilanes and, 40San Petronio Cathedral, 42satellite images, in building degradation
monitoring, 27
scanning electron microscopy, 7, 39scholarly review articles. See review
articles
scialbatura , 22, 46
Science Citation Index (ISI Web of
Knowledge), 70
scientific community, limited interest in
preservation research, 77
scientific method, conservation research
and, 67
Scopus, 70Scotch tape test, 7, 50scratch repair, in rock art, 61self-cleaning surfaces, research on, 43shelters, protective, in rock art
conservation, 28, 60
silanes. See alkoxysilanes
silica, colloidal, as surface coating, 47silicate stones, biofilms and, 21silicone polymers, need for research
on, 53
silicones
evaluative studies of, 52
as surface coating, 44–45, 46
sodium bicarbonate, 34sodium chloride
and differential expansion, 24
global warming and, 14–15
and MTMOS, 41
water repellents and, 45
sodium sulfate
heptahydrate, 15
hydration states, 15
and MTMOS, 41
testing absorption of, in treated stone,
50–51
soiling rates
from air pollution, 13
water repellents and, 52
soil microbes, consolidation and, 37sol-gel treatments, 41solvents, interaction with polymers, 55South Korea, conservation research in,
75, 77
sponge test, 7stains, biological, removal of, 47standards, lack of, and research quality,
67
standards committees, recent activity
by, 77
stereophotography, 56stone, need to “breathe,” 39Stone Conservation for the
Refurbishment of Buildings (STONECORE; EC project), 37
stone production technology, evidence of,
in historic quarries, 63
strength of stone, characterization of
stone by, 2
streptomycin, as biocide, 48strontium isotope analysis, 60sulfation rates, water repellents and, 52sulfur dioxide (SO
2)
atmospheric, and stone decay, 5
atmospheric, reduced levels of, 10,
12, 13
research needed on, 14
synergy with nitrogen oxides, 14
sulfuric acid, 14
autotrophic bacteria and, 23
sulfur oxides, as cause of decay, 10surface coatings
anti-graffiti coatings, 45–46, 61
biocides, 47–48
biological attacks on, 48
colloidal silica, 47
crystal growth inhibitors, 18, 46
emulsions, 46
as food for microbes, 48, 53
lime and biocalcification, 47
oxalate formation, 46
responsible use of, 55
retreatability, 44
reversibility of, as realistically
impractical, 55
types of, 43–44
water repellents, 44–45
surface cohesion, tests for, 7surface hardness measurements, 8, 50surface treatments, and differential
expansion rates of materials, 24–25
surfactants, 40SWAPNET (Stone Weathering and
Atmospheric Pollution Network), 10
Swiss Expert Centers, 77Swiss molasse, 25–26Sydney sandstone, 24synchrotron radiation high-speed neutron
tomography, 8
synchrotron X-rays, 17system dynamics approach, 9, 26n1
tartrates, 43
technical analysis, dubious general
usefulness of, 1, 67, 78
temperature control, in preventive
conservation, 28–29
TEOS (tetra-ethoxysilane), 40, 41, 44terminology

158 Inde x
PROOF 1 2 3 4 5 6 of biodeterioration, 21
for describing decay, 2–3
standard, lack of, 67
testing, long-term
funding for, as key challenge for
future, 80
importance of, 78
lack of resources for, 75, 78
long-term assessment, 51–53
testing procedures, lack of standards
for, 67
tetra-ethoxysilane (TEOS), 40, 41, 44
texture, characterization of stone by, 2thenardite, 15thermal buffering, vegetation and, 20, 61thermal expansion, differential
and differential stress, 24
salt damage from, 19
thermal variation, rapid, 24thermography, 4, 7thermo-mechanical analysis (TMA), 173 D laser scanning, 4
in documentation of rock art, 63
threshold, as concept in air pollution
studies, 75
TMA (thermo-mechanical analysis), 17tool kit of stone conservator, 78–79, 79tTorun Guidelines for Conferences in the
Field of Stone Conservation, 67, 68
training programs for researchers, need
for, 73
ultrasonic pulse velocity testing, 50ultrasonic testing, 4, 6–7ultraviolet radiation, as biocide, 48United States, research funding in, 77universities, increased research by, 66, 76urea and glycerol poultice, 33U.S. National Acid Precipitation
Assessment Program (NAPAP) studies, 13
vegetation
and rock art conservation, 61
and stone conservation, 20–21
Venice Charter (International Charter for
the Conservation and Restoration of Monuments and Sites), 54
vertical scanning interferometry (VSl), 6video holography, 5–6vines and creepers, 20–21visual examination, in quantification of
decay, 4
VOC (volatile organic compound)
regulations, consolidants and, 35
voids, surface. See also pores
characterization of stone by, 2
differential expansion rates of materials
in, 24–25
poultice cleaning and, 34
size of, and salt damage, 18–19
volunteers, use of
in rock art conservation, 60, 63, 95
in stone conservation, 80t
VSI (vertical scanning interferometry), 6
wall paintings
new approaches to conservation of, 76 salt damage to, 28
surface coatings for, 46
water
effect of, need for research on, 76
protection from, as preventive
measure, 28
water repellents, 44–45
ammonium biocides and, 56
current caution regarding, 80
evaluative studies of, 52
recent developments in, 78
testing effectiveness of, 50
water uptake, tests for, 7, 50
weathering
definition of, 3
new approach to research on, 75
The Weathering of Natural Building
Stones (Schaffer), 20
Westminster Abbey, 25–26wet/dry cycling
and differential stress, 24
drying rates, and salt damage, 29
humidity control and, 28
rapid drying, benefits of, 39
wet-scanning transmission electron
microscopy (wet-STEM), 79, 79t
wetting and humidity cycles, research on
effects of, 8–9
wind, salt decay and, 18–19
X-ray photoelectron spectroscopy
(XPS), 45

PROOF 1 2 3 4 5 6Eric Doehne holds a BS in geology from Haverford College (Pennsylvania)
and MS and PhD degrees in geology from the University of California, Davis. He joined the Getty Conservation Institute in 1988 and worked there as a research scientist until 2010. Doehne’s projects included a wide range of research collaborations, including the desalination of historic masonry in New Orleans, the conservation of magnesian limestone build –
ings in Yorkshire, and coral-red gloss on Greek vases. In 2010 he founded Conservation Sciences, LLC—a consultancy focused on materials science for art, architecture, and archaeology. He specializes in consulting, teach –
ing, and investigations about the composition, behavior, and treatment of such historic inorganic materials as stone, glass, and ceramics. Doehne’s particular interest is in stone decay mechanisms, for instance, the role of soluble salts in the deterioration of architecture, wall paintings, and sculpture.
Clifford Price is Emeritus Professor of Archaeological Conservation at
University College London (UCL). He earned an MA and PhD in chemis –
try at St. Catharine’s College, Cambridge, and worked first at the Building Research Establishment. In 1983 he was appointed head of the Ancient Monuments Laboratory with English Heritage, and in 1990 he moved to the Institute of Archaeology, UCL. His research has been concerned pri –
marily with the decay and conservation of stone. He has been involved in the conservation of several English cathedrals and has undertaken consul –
tancies at sites around the world. He retired from UCL in 2007. About the Authors

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