doehne, e. y price, c. stone conservation. getty. 2010

8/7/2019 Doehne, E. y Price, C. Stone Conservation. Getty. 2010

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The Getty Conservation Institute

Los Angeles

Stone Conservation

An Overview of CurrentResearch

Second Edition

Eric Doehne and Clifford A. Price

resea

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incon


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The 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 artsbroadly interpreted to include objects, collections,architecture, and sites. The Institute serves the conservation community through

scientic research, education and training, model eld projects, and the dissemina-

tion o the results o both its own work and the work o others in the eld. In all

its endeavors, the GCI ocuses on the creation and delivery o knowledge that will

benet the proessionals and organizations responsible or the conservation o the

worlds cultural heritage.

Research in Conservation

The Research in Conservation reerence series presents the nding o research

conducted by the Getty Conservation Institute and its individual and institutional

research partners, as well as state-o-the-art reviews o conservation literature.

Each volume covers a topic o 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

(Schaeer 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 ConservationResearch (Reedy and Reedy 1988).

2010 J. Paul Getty Trust

Published by the Getty Conservation Institute

Getty Publications

1200 Getty Center Drive, Suite 500Los Angeles, Caliornia 90049-1682

www.gettypublications.org

Gregory M Britton Publisher
http://www.gettypublications.org/http://www.gettypublications.org/


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vi Foreword to the Second Edition, 2010viii Preface to the Second Edition, 2010

x Foreword to the First Edition, 1996

xii Preface to the First Edition, 1996

xiii Dedication

xiv Introduction

Chapter 1 1 Stone Decay

1 CHARACTERIZING THE STONE

2 DESCRIBING DECAY

3 HOW SERIOUS IS IT? MEASURING THE EXTENT AND SEVERITY OF DECAY

5 Surace Techniques

6 Looking Beneath the Surace

8 All the Inormation We Need?

9 CAUSES OF DECAY

9 Air Pollution

14 Salts

20 Biodeterioration

Contents


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iv Contents

44 SURFACE COATINGS

44 Water Repellents

45 Anti-Grafti Coatings

46 Emulsions

46 Crystal Growth Inhibitors

46 Oxalate Formation

47 Lime and Biocalcifcation

47 Colloidal Silica

47 Biocides

48 Biological Attack on Treatments

Chapter 3 49 Do They Work? Assessing the Effectiveness of Treatments 50 CHARACTERIZING THE TREATED STONE

50 Properties That Change with Decay

50 Meeting Objectives

51 Standard Test Methods

51 LONG-TERM PERFORMANCE

53 Documentation o Field Trials

Chapter 4 54 Putting It into Practice: Conservation Policy

55 RESPONSIBLE USE OF SURFACE COATINGS AND CONSOLIDANTS

55 RETREATMENT

56 RECORDING

Chapter 5 58 Heritage 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


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Chapter 7 75 What Has Changed? Some Thoughts on the Past Fifteen Years

80 CONCLUSION

81 References

140 Appendix: Resources for Stone Conservation

152 Index

159 About the Authors

Contents v


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Petra, Angkor, Copn, Venice, Lascaux, Easter Island. Stone conservation

research may not be the rst thing that comes to mind when reading

these words, but it is because these places o irreplaceable cultural heri-

tage, and many other stone monuments, are eroding at a noticeable rate

that the subject o this volume is o such crucial importance. In the sum-

mer o 1994, the Getty Conservation Institute (GCI) invited Proessor

Cliord A. Price to provide an overview o research on the conservation

o stone monuments, sculpture, and archaeological sites. The purpose othe review, which was subsequently published in the 1996 book Stone

Conservation: An Overview of Current Research, was to inorm GCI

research policy in this eld and to highlight areas into which Getty

resources might useully be channeled. Today, a Google search or stone

conservation raises this book in the rst linka testament to its endur-

ing useulness to the wider conservation community.

Stone Conservation remains one o the most cited and down-

loaded o the GCIs books some teen years ater it was written. A

rereshingly opinionated work, its call to reorm the ocus and process o

research was subsequently echoed and reinorced by other authors and

i i i B i i h ll i i h b k i f d

Foreword to the Second Edition, 2010


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Foreword to the Second Edition, 2010 vii

2007 GCI scientist Eric Doehne, with the advice and collaboration o

Cliord Price, embarked on a new survey o the eld o stone conser-

vation research. The goal was to retain key characteristics o the rst

edition (notably its brevity, inormal character, and pointed suggestions),

while covering the recent explosion o new research, enlarging the discus-

sion o preventive conservation, and adding new sections on rock art and

other subjects. This required a parallel compilation o a new bibliography,

which included a review o more than six thousand abstracts and morethan three thousand PDF les o material published between 1995 and

2009. Topics ranged rom nano-scale measurements o salt damage by

materials scientists to conservators documenting the unintended conse-

quences o waterproong agents. The selected bibliography drawn rom

this research eort is included in this new edition as an appendix and

will be a useul starting point or many researchers.

With increasing reliance on the Internet and the rapid develop-

ment o 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 inrastructure or the conservation eld, arguably one

h i di i li d i i l h ll T


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Chapter #

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Authors names

Being a conservation scientist oten means acting as a bridgebetween

researchers and conservation practitioners, and between the many dier-

ent elds o research related to the preservation and conservation o

carved and worked stone, rom Stonehenge to the Sphinx, and rom ana-

lytical chemistry to X-ray tomography. Like the rst edition, this volume

is not a literature review. It is an overview that maps the landscape o

stone conservation, cites interesting and representative research, and is

intended to serve as a useul point o entry to the eld.I began the research or the second edition in May 2007 as an

eort to update and highlight the signicant changes that had taken place

in the eld since the rst edition and to encompass a much larger range

o publications. The text was largely written in 2008, with revisions and

editing completed in 2009. Such an endeavor unavoidably results in a

particular perspective. This tendency has been ameliorated by consulting

with an experienced, as well as linguistically diverse, group o 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

i b J h A h A i Ch l J D l d R d i V

Preface to the Second Edition, 2010


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Preface to the Second Edition, 2010 ix

Pique, Jerry Podany, Thomas Roby, Carlos Rodrguez-Navarro, Eduardo

Sanchez, Alison Sawdy, George Scherer, Stean Simon, Michael Steiger,

Marisa Laurenzi Tabasso, Giorgio Torraca, Vronique Vergs-Belmin,

Heather Viles, Norman Weiss, George Wheeler, Chris Wood, Konrad

Zehnder, and Fulvio Zezza. I would also like to acknowledge ormer

GCI interns and postdoctoral researchers Enrica Balboni, Ann Bourgs,

Tiziana Lombardo, Paula Lopez Arce, and Claire Moreau, who helped

teach me more about stone through our joint research. The students othe International Course on Stone Conservation have also been a source

o inspiration. The GCIs Beril Bicer-Simsir, David Carson, Giacomo

Chiari, Mara Schiro, and Jeanne Marie Teutonico provided important

support. I am grateul to Valerie Greathouse and Tina Segler or their

help in tracking down reerences and to Cynthia Godlewski and Cynthia

Newman Bohn or their excellent coordination and editorial assistance.

Two anonymous peer reviews o earlier drats o the book were thorough

and thoughtul. Finally, my coauthor has been brilliant in skillully aiding

my eorts to bind together the old and the new in this volume, and I

extend my kind thanks to him or his enthusiasm or this project.


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x Chapter 2

The sympathetic conservation o historic or culturally signicant stone

is a relatively recently recognized practice. In the past, the repair o dam-

aged sculptured stone objects was requently accomplished using more

intrusive means, such as iron dowels, staples, or clamps that oten marred

the appearance o the object and could lead to urther damage. For the

patching and lling o deects, lime mortar, cement, plaster o Paris,

sodium silicate, and various gums and resins were usedmaterials no

longer considered acceptable. Stone-cleaning processes involved harshacidic treatments ollowed, at times, by neutralization, which resulted

in the production o soluble salts that penetrated the stone and increased

the potential or uture 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

o the treatment.

The unsuitability o many o these treatments encouraged

research eorts to develop new materials and procedures or the preser-

vation o stone. Over the past twenty years or so, these studies have

resulted in the publication o a vast number o reports and papers, most

hi h d i h di d h i b

Foreword to the First Edition, 1996


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Foreword to the First Edition, 1996 xi

progressing and whether or not the current direction is proving ruitul.

Should the emphasis on stone conservation research be placed on devel-

opment o new materials and new application procedures? Has there

been signiicant work on the evaluation o the post-treatment stone

property improvements? Are the methods or evaluating stone properties

universally accepted? Do we need to conduct research on methods or

carrying out and assessing the long-term durability o treatments? Are

there problems in the process o conducting stone conservation researchthat bear on our ability to do the research eectively? Can these prob-

lems be deined; and, i so, what can be done to urther the eectiveness

o stone research? These are some o the many questions that Cliord A.

Price has considered in this review o the current status o stone conser-

vation research.

We asked Dr. Price to give us his subjective viewpoint on what is

being done right, what areas o current research should be continued or

accelerated, and what new directions should be addressed that would

promote an increase in the eectiveness o stone conservation. In the

course o preparing this review, Dr. Price has had extensive discussions

i h b i i i i h i i


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Chapter #

Chapter Title

Authors names

This volume was written over a short period during the summer o 1994,

ollowing a systematic study o the major publications o the last ve

years. Inevitably, the volume refects my own experience, expertise, and

linguistic abilities. An international working party could, no doubt, have

produced a more objective and comprehensive reportalbeit over a

longer span o 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 grateul or the help and advicethat 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 grateul to Sasha Barnes

or the help that she has given in rooting out reerences and to Julie

Paranics or help in the nal production o the volume.

The emphasis o this publication is on stone as a material. There

is little reerence to mortars, and no consideration o the structural per-

Thi l i d il d h

Preface to the First Edition, 1996


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Chapter #

Chapter Title

Authors names

We dedicate this book to the memory o John Ashurst, 19372008,

in recognition o his unparalleled contribution to stone conservation

through research, practice, and training.

Dedication


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Ater presenting his work at a recent stone conservation conerence, a

thoughtul researcher was responding to questions rom conservators.

A pained expression was evident on his ace as he said to the audience,

I eel as though I am explaining in great detail why I cannot help you.

This encapsulates the rustration elt 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 o long-standing conservation problems. The researchers 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 or action to ensure that history is preserved or uture gen-

erations is limited.

This volume takes a broad and sometimes critical look at the

present state o stone conservation and o the way in which research is

conducted. It looks rst at the deterioration o stone and ways in which

deterioration may be prevented or remedied. Then, it discusses some o

the actors that limit the eectiveness o research and makes recommen-

d i h h i h b d i I l d

Introduction


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The deterioration o stone is all too amiliar to anyone who has lookedclosely at a historic stone building or monument. While there are a ewstones that seem little aected by centuries o exposure to the weather,the majority o stones are undergoing gradual and episodic deterioration.This may not matter much i the stone is an undecorated part o a mas-sive wall. However, it does not take much deterioration o a carved pieceo stone or the sculptors original intention to be lost altogether. A high

proportion o the worlds cultural heritage is built o stone, and it isslowly but inexorably disappearing.

I we are to do anything to reduce or prevent this loss o our her-itage, we must rst be able to characterize the many stones involved. Weneed to be able to describe the decay and to measure its extent, severity,and rate. We then need to understand the causes and mechanisms odecay. Only then can we hope to understand the behavior o any particu-lar stone in a given environment.

CHARACTERIZING THE STONE

Chapter 1

Stone Decay


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2 Chapter 1

(1999)and Svahn (2006). Adams and MacKenzie (1998) provide a useulatlas o petrographic sections, while a more recent petrographic atlas andapplications o polarized light microscopy to building materials conserva-tion are presented by Bluer and Kueng (2007) and Reedy (2008).

In the process o characterizing stone, it is important to recognizethat while some stones have a similar composition, their behaviors mayhave ew things in common. For example, Istrian stone, Lecce limestone,and Carrara marble are all carbonate materials, but their contrastingmodes o deterioration depend more on their porosity, pore shapes, poresize distribution, and grain size than their chemical composition. Onedivision o stone types is based on the percentage and relative ratio o

pore-shaped and ssure-shaped voids (Croci and Delgado Rodrigues2002). A second division can be made on the basis o the degree o hygricswelling o the stone (Delgado Rodrigues 2001;Duus, Wangler, andScherer 2008), and a third division on the strength(Winkler 1985;Bourgs 2006). Subsequent divisions based on composition, texture, andhomogeneity enable urther distinctions to be made, but they may be lessimportant in rating overall perormance than the rst three. Those stoneswith high porosity, high rates o swelling, and low strength tend to be rel-

atively poor building materials (e.g.,Jackson et al. 2005).A review o the relationship between pore structure and other

stone characteristics is given by Bourgs et al. (2008). Gauri andBandyopadhyay (1999) review the interpretation o mercury porosimetrydata and cite a number o the seminal papers on pore structure determi-nation. Analysis o the positive correlation between the ractal dimension,stone pore surace, and the degree o natural weathering has shown thatincreases in the surace ractal dimension are a more accurate descriptoro the degree o weathering than pore size distribution (Yerrapragada,Tambe, and Gauri 1993; Prez Bernal and Bello Lpez 2000).


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Stone Decay 3

ize terminology (Stone Federation o Great Britain 1991), governmentalorganizations (Grimmer 1984), and research groups (Fitzner, Heinrichs,and Kownatzki 1997).

The ICOMOS-ISCS Illustrated Glossary on Stone DeteriorationPatterns(Vergs-Belmin 2008) helps dene and clariy usage across lan-guages and within the stone community, providing useul denitions oterms such as scaling, spalling, and faking. Weathering is generallydened as the result o natural atmospheric phenomena, while decay isany chemical or physical modication o the intrinsic stone propertiesleading to a loss o value or to the impairment o use, degradation isdecline in condition, quality, or unctional capacity, and deterioration

is the process o making or becoming worse or lower in quality, value,character, etc. Some interesting details o the history o stone glossariescan be ound in the introduction to the glossary.

A more guided approach than a glossary can be ound in work onexpert systems rom the late 1990s, with Van Balen (1996;1999) produc-ing an atlas o damage to historic brick structures as part o an expertsystem or elucidating environmental eects on brick. The atlas evolvedinto 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 o ash-ion over the past teen years, but the need to capture expert experienceand judgment has become ever more urgent, given the large number oconservation proessionals nearing retirement age.

Fitzner has produced an important classication o weatheringorms as a basis or mapping the deterioration across a building acade(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 o the number oparameters to be measured (Moraes Rodigues and Emery 2008) as well


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4 Chapter 1

that pollution is causing decay unless we have some way o correlatingpollution levels with decay? Second, we need to have some objectivemeans o assessing the extent and the rate o decay in order to decidewhether remedial action is necessary and, i so, how urgent the need is.Third, we cannot establish whether our remedial actions are having anyeect unless we can monitor the condition o the stone aterward.

I one accepts these eminently reasonable preconditions, then weare let with a situation where extremely ew monuments today (or evenpaintings) meet these basic conditions. Conservation decisions most otenrest upon a ramework o experience and general guidelines or treatmentcompatibility, instead o data on the actual behavior or rate o loss o the

monument. Conservation documentation or the majority o our culturalheritage appears to consist o a ew uncalibrated photographs takenunder dierent lighting conditions over a ew decades. Helping to ll thisvoid with more quantitative and reproducible approaches has been theobjective o many o the research projects cited in this volume: turningweathering or decay into numbers.

No single technique is sucient to measure stone deterioration,since decay takes many dierent orms. Some techniques, such as 3D

laser scanning and fuorescence LIDAR (light detection and ranging), lookonly at the surace, and they are well suited to decay that consists o agradual loss o surace, leaving sound stone behind. Other techniques,such as ultrasonic measurements, thermography, or magnetic resonanceimaging (MRI) are designed to probe below the surace, and these areuseul where decay consists o a loss o cohesion within the stone, or thedevelopment o detached layers, blisters, or internal voids.

Beore using more complex methods, simple visual examinationplays an important role in quantiying decay. A single examination canconvey the state o the stone at a particular moment, but it does not cap-ture the rate o decay. For this, a series o inspections is required, usually


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Stone Decay 5

.hpl.hp.com/news/2004/jan-mar/ptm.htmland http://www.southampton

.ac.uk/archaeology/acrg/acrg_research_PTM_amazon.html .

Surace Techniques

Surace techniques or quantiying rates o stone loss include the use oa microerosion meter, prolometry, close-range photogrammetry, laserscanning, and laser intererometry. The microerosion meter is a simplemicrometer device that measures surace height at a number o predeter-mined points relative to datum studs set into the stone. It was used, orexample, to monitor the rate o stone decay at St. Pauls Cathedral,London, over a twenty-year period, during which atmospheric sulur

dioxide levels in the region ell by 50 percent (Trudgill et al. 1992;Trudgill et al. 2001). Erosion rates on horizontal sites were ound to havedecreased rom 0.045 mm/year in the period 198090 to 0.025 mm/yearin 19902000.

Optical prolometry is a contact-ree technique that consists o theprojection o a grid o lines onto the surace at an angle o 45. Any irregu-larities in the surace are immediately evident. Aires-Barros, Maurcio, andFigueiredo (1994) have demonstrated its use, coupled with image analysis,

to construct a weatherability index. Similar optical methods include lasertriangulation, conocal microscopy, and digital holography.

A technique or monitoring surace roughness known as contactprolometry was utilized by Jaynes and Cooke (1987) to monitor thedecay o limestone when exposed to a range o dierent pollution envi-ronments. It measures irregularities by means o a stylus that is drawnacross the surace; movement o the stylus produces an electrical signal ina transducer.

Grissom has compared stylus prolometry, refected-light imageanalysis, and visual/tactile evaluation to assess the roughness o abrasive-cleaned stone. The results ound tactile evaluation to be the more practical
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6 Chapter 1

have made such systems less expensive and more practical in eld condi-tions (Keene and Chiang 2009).

Many stone decay processes can be evaluated by ocusing onsolution chemistry and mineral reactions. Microcatchment studies are auseul way to evaluate the chemical dissolution o stone suraces, wherethe ions in rain runo are measured to evaluate reaction rates (Halsey2000). Finally, atomic orce microscopy (AFM) and vertical scanningintererometry (VSI) have been used to monitor mineral reactions and theeects o biodeterioration (Davis and Lttge 2005;Perry, McNamara,and Mitchell 2005; Herrera, Le Borgne, and Videla 2009).

Looking Beneath the SuraceOutward appearances may be sucient in some instances, but they canbe deceptive. It is not unusual to nd a stone surace that looks perectlysound but which sounds hollow when tapped. Sooner or later, we need away o measuring what is going on beneath the surace.

Many techniques are available and some o the more importantare reviewed by Facaoaru and Lugnani (1993). These are typically dividedinto in situ eld methods and laboratory-based methods. Lab tests are

perormed on collected samples or on samples subjected to accelerated orarticial weathering.

In Situ Field Methods

Preeminent among ield methods is the use o ultrasonics todetect the presence o cracks, voids, and other inhomogeneities in stone(Mamillan 1991; Bluer Bhm 2004). This may take a variety o orms,such as using the longitudinal wave or the transverse component run-ning parallel to the surace. Galn and co-workers (1992)provide anearly case study demonstrating the reliability and cost-eectiveness othe technique.


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Stone Decay 7

ing the dicult challenge o long-term evaluation o stone treatments(Simon and Lind 1999; Favaro et al. 2006;Favaro et al. 2007).

The development and application o the drilling resistance mea-surement system (DRMS), also known as the drilling orce measurementsystem (DFMS), has provided an extremely useul and minimally destruc-tive method or evaluating the condition o stone and the perormanceo treatments or 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 drillbit with a sensor to measure the orce needed to advance the drill bit agiven distance. In principle, the DFMS can determine depth o deteriora-

tion and the penetration depth o consolidants, in situ, with a minimumo 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 historicbuildings (Finzi, Massa, and Morero 1992). It has seen wider applicationrecently by a number o researchers (Binda et al. 2003;Binda, Lualdi, andSaisi 2007;Huneau et al. 2008;Palieraki et al. 2008). The method is use-ul in detecting faws, voids, moisture, metal straps, and the thickness o

stone masonry.Inrared thermography has been used by a wide range o

researchers (Moropoulou, Avdelidis, and Theoulakis 2003; Grinzato et al.2004; Tavukuoglu et al. 2005) to study moisture in stone. To provideuseul results, a thermal contrast, such as solar heating or deliberate heat-ing by inrared lamps, is oten needed to identiy the dierent suracetemperatures related to dierences in moisture content. This method isknown as photothermal radiometry and has been developed to detectdelaminations and voids (Madrid, Coman, and Ginell 1993; Candor etal. 2008). Most building materials have signicant thermal inertia, andpractitioners using thermography on a casual basis will not necessarily


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8 Chapter 1

spectroscopy, hygric tests, and biaxial fexural strength measurements(Mamillan 1991;Snethlage, Wendler, and Sattler 1991; Bluer Bhm2004). Surace hardness measurements may be useul, and the salt con-tent o the slices may also be determined (Bluer Bhm2005).

The European Commission (EC) projects COMPASS andDESALINATION have developed a simple test or salt content basedon the hygroscopic moisture content (HMC) (Gonalves and DelgadoRodrigues 2006; Gonalves, Delgado Rodrigues, and Abreu 2006;Nasraoui, Nowik, and Lubelli 2009). Kaminski (2008) has proposed analternative gravimetric system and makes some constructive criticismso common aspects o the diagnosis and analysis o moisture and salts,

including misleading readings rom moisture meters based on electricalresistance or dialetric properties, dry drill powders showing lower thanexpected results, and salt solutionconditioned chambers not providingconsistent conditions or HMC measurements.

Jacobs, Sevens, and Kunnen (1995) proposed the use o comput-erized X-ray tomography (CT) to gain urther insight into the internalstructure o stone and the changes that occur during the deterioration obuilding materials. Procedures were developed to bring the resolution

down to grain-size level (about 100 microns or less).Mossotti andCastanier (1990) used CT scanning to show that or Salem limestone,capillary water reached the surace except under windy conditions, whenthe air/water interace moved into the stone. In the past decade, resolu-tion o the CT method has advanced signicantly (Bugani et al. 2008;Cnudde et al. 2009;Ruiz de Argandoa et al. 2009); however, it is stilldicult to see treatments and salts inside pores owing to the lack ocontrast and the small amount o material scanned. A promising way toovercome the limitations o x-ray CT is the use o high-speed neutrontomography (synchrotron radiation) or in situ dynamic analysis o wet-ting/drying, moisture transport, salt development, or the curing and eval-


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Stone Decay 9

has shown that i both layers are ully saturated with water at the start oa drying experiment, the stone will dry ater the plaster and soluble saltsin the stone will tend to be retained.

All the Inormation We Need?

With such sophisticated orms o investigation being pursued, one mightbe orgiven or thinking that no problems remain in the measurement ostone decay. There is, however, a long way to go. Stone decay is a com-plex phenomenon, and no single technique can disentangle and quantiyits causes and eects. Advances in experimental work, eld measure-ments, and theoryeach building on the otherare still needed. The

techniques that we have looked at thus ar are certainly useul, butthe methodical measurement o decay and our understanding o decayprocesses over time have not yet met the goal set orth earlier o conser-vation decisions being based on measurements instead o assumptions.

CAUSES OF DECAY

Beore we can take any action to prevent or to remedy the deteriora-tion o stone, we must understand what is causing that deterioration.Sometimes the cause is obvious; sometimes there may be several dier-ent causes acting at once. In an attempt to clariy the relative impor-tance and interdependency o individual causes,Verdel and Chambon(1994) have introduced the principles o system dynamics.1 Stone decaymechanisms and rates are reviewed in the proceedings o two Dahlemmeetings (Doehne and Drever 1994; Viles 1997), and both reports point

out areas where additional research is needed, essentially providing use-ul road maps or research. An interesting example o quantiying therelative importance o a range o actorsin this case or absorption


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10 Chapter 1

Air Pollution

Air pollution is, to the minds o many, the prime culprit in stone decay.Everyone has heard o acid rain, and it is easy to conjure up an imageo old buildings slowly dissolving in the rain. Needless to say, the truesituation is a good deal more complex, as reviews o the role o air pollu-tion and soiling in stone decay have ound (Charola and Ware 2002;Mitchell and Searle 2004;Brimblecombe and Grossi 2007;Siegesmund,Snethlage, and Ruedrich 2008). The emphasis o these studies has largelybeen on limestone, marble, lime mortars, and carbonate-cementedsandstones, as these are the most vulnerable to acidic pollution. However,soiling rom atmospheric particulates is a universal problem or all types

o stone.Until recently, all the attention was given to the direct eects oair pollutants on stone, and research ocused on the traditional pollut-ants: sulur oxides, nitrogen oxides, and carbon dioxide. All are naturallyoccurring, although human activity has greatly increased the amountsthat are to be ound in urban areas, as well as signicantly increasingbackground levels o pollution in rural areas. All are capable o dissolv-ing in water to create an acidic solution and so are capable o reacting

with calcareous materials.The direct eects o air pollution on stone received enormous

attention rom the mid-1970s to the early 1990s. This is due, at least inpart, to concerns about the eects o pollution on health, agriculture, andthe environment. Stone research in Western Europe and the United Stateswas able to ride on the back o these concerns and to benet rom theunding o large research programs.2

Since the early 1990s, priorities have shited as progress has

been made in reducing sulur dioxide (SO2) levels in major metropolitanareas in Western Europe and the United States. Consequently, unds orresearch on air pollution on stone have steadily decreased and a number


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Stone Decay 11

Damage to stone by air pollution is still an important problem inparts o central Europe, China, India, Russia, and other industrializedregions (Larssen et al. 2006). For example, while scrubbers were installedto reduce SO2 near the Taj Mahal, a lack o water, power outages, and thecorresponding use o diesel generators were ound to reduce the eective-ness o the scrubbers and decrease air quality near the site. This outlinesthe importance o inrastructure development to monument health(Hangal and Harwit 1997). In the past decade the rapid development oIndia and Chinas economies has in some measure been built on burningcoal. This raises concerns or human health and corresponding concernsor well-known monuments, through both the direct and indirect eects o

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 thatdates rom antiquity. By examining the eects o pollution on individualhistoric buildings over periods o several hundred years, he has alsoattempted to correlate pollution levels with observed damage. This linksin with another widespread perception: that decay rates are accelerating

rapidly, despite alling levels o several major pollutants. There are insu-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 awarenesso the problem and to the act that stone loss through pollution is cumu-lative. Also, the reaction products o air pollution, such as soluble salts,oten remain on sheltered stone suraces and result in ongoing damage.

The direct eects o acidic pollutants on calcareous stones dependvery much on the immediate environment o the stone. I the stone is in

an exposed position where it is regularly washed by rain, the reactionproducts are washed away and the surace o the stone gradually recedes.I, however, the stone is in a relatively sheltered position, the reaction


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stone surace, but with inward growth predominating.Vergs-Belmin(1994) proposed a three-step process to explain the inward and outwardormation o 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 surace o the stoneand growing outward. Work byToniolo, Zerbi, and Bugini (2009) dividesblack crusts into three types, with marble substrates exhibiting dieren-tial preservation beneath each type.

While the vast majority o research on black crusts has ocusedon carbonate stone, interesting research on silicate stones has ound highsulation rates associated with diesel soot and accelerated rates o granite

kaolinization associated with black crusts (Simo, Ruiz-Agudo, andRodrguez-Navarro 2006;Schiavon 2007). In certain cases, black crustsorming on granites appear to be geochemically unrelated to the substrateand are thus accumulated rom atmospheric (Silva et al. 2009) and bio-genic sources (Aira et al. 2007).

Diakumaku and others have observed that some black ungi pro-duce small spherical particles that might, under some circumstances, beconused with fy ash (1995). Microfora are also capable o producing

sulates. In the opinion o these authors, the ormation o some blackcrusts in unpolluted environments may be attributable to biological ac-tors. In addition, Ortega-Calvo and co-workers (1995) have demonstratedthat sulate crusts may provide an ideal habitat or some cyanobacteriathrough the gradual dissolution o the sulate. Work by Mansch and Bock(1998) ound greater concentrations o nitriying bacteria and greaterstone decay rates associated with air pollution and black crusts. Gonzales-del Valle and others (2003, 219) have ound that building stones host an

active microfora that degrades ossil uel derivatives.Schiavon, Chiavari,and Fabbri (2004) ound organic compounds, such as polycyclic aromatichydrocarbons (PAHs), which appear to represent markers or present-


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Stone Decay 13

Some intriguing ndings have led to a better understanding o the inter-play between material, environment, and weathering rates: or example,tropical weathering has been ound to be less detrimental to marbletombstones than an acidic, polluted atmosphere (Meierding1993, 2000).

Some authors argue that sulur dioxide levels in certain citieshave decreased to the point where sulur dioxide is no longer a majorcontributor to decay. In other words, there may be a sae level oaround 30 g/m3, below which sulur dioxide is not a signicant contrib-utor to decay (Sharma and Gupta 1993).4 This view is not universallyupheld, with some experts nding that or many pollutants there is nosae threshold and that resulation o cleaned monuments is proceeding

apace in some places.5

One area where consensus is emerging is in the relative impor-tance o wet and dry deposition. Where sulur dioxide levels are high(urban areas), dry deposition appears to predominate on vertical suraces.On horizontal suraces and in rural areas, wet and dry deposition maybe o comparable importance (BERG 1989; Butlin 1991; Furlan 1992;Cooke and Gibbs 1993; Charola and Ware 2002). According to Grossiand Murray (1999), stones with a high specic surace area and/or a deli-

quescent salt content were ound to promote more nitrogen oxides (NOx)dry deposition.

Some recent ndings concerning the eects o air pollution havebeen unexpected, such as the observation o a decrease in dissolutionrom stone suraces blackened with diesel soot as measured in micro-catchment studies, apparently due to a higher mean surace temperatureresulting in aster drying(Searle and Mitchell 2006). This counterintuitiveresult suggests the importance o time o wetness in the damage to

stone, as conrmed by earlier research (Charola and Ware 2002). Recentdecay o marble in New York was evaluated using mass balance methodssensitive enough to detect a 2 nm surace loss (Livingston 2008).


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14 Chapter 1

as SO2. Yet despite several studies, the situation is still unclear.Some authors have ound synergistic eects or NOx on SO2reactions, 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 o theissue is that a catalytic eect or NOx on SO2 is present in dryconditions, but not in wet (Bai, Thompson, and Martinez-Ramirez 2006). In a larger context, research is showing thatthe geochemical cycle o nitrogen is being altered in ways,including the impact on historic stone, that are still poorlyunderstood (Gruber and Galloway 2008).

Whatisthemechanismbywhichsulfurdioxideisoxidizedtoproduce suluric acid? Does oxidation take place beore thepollution reaches the stone, or is it catalyzed by other pollut-ants on the surace o the stone? Is the oxidation catalyzed byother air pollutants, such as ozone, nitrogen oxides, or die-sel soot (see, or example, Rodrguez-Navarro and Sebastian1996)? Do bacteria in the stone play a part?

Towhatextentaretodaysdecayratesinuencedbypol-

lution levels o the past (the memory eect)? For example,sulate and nitrate salts that are already present in the stonewill continue to cause damage even i urther pollution wereeliminated altogether. The memory eect story is not yetcomplete (Vleugels, Dewols, and Van Grieken 1993; Prikryland Smith 2007).

Recentresearchhasexaminedtheroleofformates,acetates,

and airborne microbes (Kumar et al. 1993;Grossi et al. 2003;

Gibson et al. 2005;Maruthamuthu et al. 2008). Are otherimportant pollutants being overlooked?

Whataretherelativerolesofcarbonicacidversussulfuric,


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Stone Decay 15

crossing the deliquescence point o sodium chloride (~75 percent RH) areprojected to increase two- to our-old by the end o the century due todrier summers, which is likely to increase damage rom 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 o the most impor-tant causes o stone decay. Salts cause damage to stone in several ways.The most important is the growth o salt crystals within the pores oa stone, which can generate stresses that are sucient to overcome the

stones tensile strength and turn the stone to a powder. The deteriorationo many o the worlds greatest monuments can be attributed to salts,rom Angkor Wat (Siedel, von Plehwe-Leisen, and Leisen 2008) to Venice(Lazzarini et al. 2008), and rom Petra (Simon, Shaer, and Kaiser 2006) tothe Great Sphinx o Giza(Reed 2002).

There are many ways in which stonework can become contami-nated with salts. Air pollution is a major source o sulates and nitrates.Other sources include the soil, rom which salts may be carried into

masonry by rising damp; salts blown by the wind rom the sea or the des-ert; deicing salt; unsuitable cleaning materials; incompatible building mate-rials; garden ertilizers; and, in the case o some medieval buildings, thestorage o salts or meat preservation or even or gunpowder.

The growth o damaging salt crystals is usually attributable tocrystallization, caused by the evaporation or cooling o salt solutionswithin the stone. In the past, there was much reerence to hydrationdamage, building on the act that some salts can exist in more than

one hydration state. The prime example is sodium sulate, one o themost damaging o soluble salts, which can exist as the anhydrous saltthenardite (Na2SO4) or the decahydrate mirabilite (Na2SO410H2O)


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16 Chapter 1

been deposited. Is there not ample room or the crystals to develop inthe pores, without the necessity o orcing the pore walls apart? However,this simplistic view overlooks the dynamic aspects o stone decay (Yu andOguchi 2009). A stone may be ed constantly with salt-bearing moisture

rom the soil, or example, so that salts are constantly accumulating atthe point o evaporation. Detailed analyses o this situation are given byLewin (1982) and by Hall and Ho (2007) and in a useul new bookby the Italian engineer Edgardo Pinto Guerra, Risanamento di muratureumide e degradate (Restoration o Damp and Deteriorated MasonryWalls)(2008). Work in Rhodes shows that the amount o salt is cor-related to the severity o damage to the stone (Steanis, Theoulakis, and

Pilinis 2009).Several tables o salt levels that are considered potentiallyhazardous or porous materials have been published in Germany(Wissenschatlich-Technische-Arbeitsgemeinschat r Bauwerkserhal-tung und Denkmalpfege e.V. 1999), Austria (sterreichischesNormungsinstitut [ON] 2006), and France (Ministre de la culture et dela communication 2003). Simply measuring the concentration o salt instone captures only part o the issue, since substrate characteristics (resis-

tance to salt weathering) as well as the severity and requency o envi-ronmental fuctuations are important in determining rates o salt damage(Doehne 2002). Any proposed international norm or salt levels in porousmaterials would have to take these actors into account, in addition toaddressing the issue o identiying a method or measuring salt levels inbuilding materials that is less costly than ion chromatography.

Modeling by Hall, Ho, and Hamilton (2008) shows that in theUK rising damp can typically transport several hundred liters o moisture

per year, per linear meter o stone, which can easily result in the accumu-lation o salts even rom dilute groundwater solutions. The accumulationo salts and whether they crystallize on the surace or as a subforescence


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Stone Decay 17

pick up moisture rom the air and subsequently lose it (causing damageby crystallization). However, the conditions under which salt mixtureswill cause damage are much more diicult 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;DeClercq 2008;Franzen and Mirwald 2009). In an example that highlightsthe importance o understanding material behavior, recent work hasshown that the type o salt is critical in determining i damage mayoccur. A pillar at Angkor Wat with severe erosion at its base was oundto contain the same amount o salt in damaged and undamaged areas,leading to questions about whether salts were or were not the cause o

the damage. Thermodynamic calculations subsequently showed thatthere were dierences in the salt type present that explained the damagepattern, with highly hygroscopic salts that did not crystallize oten beingpresent in the undamaged zone and salts that crystallized requentlybeing present in the damaged zone (M. Steiger, personal communication).Computer programs can now predict the sae ranges o temperatureand 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 mostimportant being that the programs can predict only what will happenunder equilibrium conditions; they say nothing about the rate at whichit will happen (Prokos and Balaawi 2008).

The second important area o research is concerned with the mech-anism by which damage occurs. Some o the papers are quite daunting, butthe ideas are essentially quite simple (Hamilton and Hall 2004; EspinosaMarzal and Scherer 2009). Consider a crystal bridging a pore and exerting

a pressure on the pore walls. I it is to grow any urther, and thereby dodamage, it is necessary or the surrounding solution to be able to get inbetween the crystal and the pore walls. I the pressure gets so high that this


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18 Chapter 1

(Hamilton, Hall, and Pel 2008), and dierential scanning calorimetry(DSC) (Espinosa Marzal and Scherer orthcoming). Some additional workon direct measurement o the disjoining pressure using AFM is also neededto help clariy some o these issues.

The size o 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 (lessthan 30 nm) (Rijniers et al. 2005). Since most types o stone have ewpores in this range, it is predicted that most salt weathering damage takesplace during nonequilibrium conditions, such as rapid drying. Another

way that damage increases is when the stone pores ll with salt, whichmodies the pore size distribution, essentially creating small pores wherecrystallization pressure can occur, even under equilibrium conditions. Thismay help explain the sudden onset o some salt weathering problems,since damage may not start until the pores are ull o salt.

Other researchers have looked at the initial nucleation andgrowth stages o crystals in pores, with a view to inhibiting nucleationor modiying the shape and size o the crystals that orm by using trace

amounts o the chemicals commonly used to inhibit mineral scaling onpipes in industrial applications (Selwitz and Doehne 2002; Rodrguez-Navarro, Hernandez, and Sebastian 2006; Cassar et al. 2008; Ruiz-Agudo, Putnis, and Rodrguez-Navarro 2008). In some ways it is a riskystrategy, or it has long been known that crystallization pressure is relatedto the degree o supersaturation o the solution rom which the crystalsgrow. I nucleation is inhibited or postponed, this will lead to even higherlevels o supersaturation, so that damage (i and when it does occur) may

be more severe than it might have been. Modiers may also behave dier-ently when in solution than when absorbed to stone suraces.

A urther aspect o recent research concerns the role o wind in


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Stone Decay 19

by dierences in the duration o drying periods (Huinink, Pel, andKopinga 2004). The researchers ound that short drying periods tend toresult in the accumulation o salts on the surace (resulting in more uni-orm erosion). For longer drying periods (slow evaporation rates), salts

accumulate in sheltered areas with lower evaporation rates (tending toexpand pits and resulting in honeycomb patterns). Experimental labora-tory work has also shown that wind (Selwitz and Doehne 2002) andrelated rapid drying (Lombardo, Doehne, and Simon 2004) increasesdamage rates due to increases in salt supersaturation, and that variableweathering rates related to wind can result in honeycomb patterns(Rodrguez-Navarro, Doehne, and Sebastian 1999). Recent modeling o

the eect o wind on the Sphinx ound that areas o rapid erosion corre-lated with areas o high wind riction and enhanced drying (let shoulderand the top o the haunches) (Hawass 1998; Hussein and El-Shishiny2009). Lab experiments and work at sites such as Petra in Jordan showthat wind speed strongly infuences the rate o damage and pattern o saltdistribution (Balaawi 2008). Pore blocking by salts also appears to bean important actor in controlling the pattern o salt weathering damage(Espinosa Marzal and Scherer 2008b; McCabe, McKinley, and Smith

2008; Espinosa Marzal and Scherer orthcoming) and may result ingreater crystallization o salts as subforescence.

Is crystallization the only way in which salts can cause damage?It seems not. It appears that they can also cause damage through stressrom dierential thermal expansion, since sodium chloride, or example,expands at about ve times the rate o calcite at surace temperatures(Nocita 1987;Holmer 1998; Smith et al. 2005). Schaer (1932) attrib-uted this idea to Scott Russell. Salts also have a role to play in the weath-

ering o stones that contain clay minerals (Snethlage and Wendler 1997;Rodrguez-Navarro et al. 1998; Scherer 2006;Scherer and Jimnez-Gonzlez 2008), in some cases enhancing the swelling potential o these


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20 Chapter 1

decay mechanisms can be ound in Scherer (2004). Salt Weathering onBuildings and Stone Sculpture conerences were held in Ghent in May2007 and in Copenhagen in October 2008 (Albertsen 2008); the nextconerence will take place in Cyprus in October 2011. Finally, a recent

review o salt weathering calls or new eld research on building materialbehavior and soluble salts (Gulotta et al. 2008).

Closely related to the issue o salt damage is the issue o damagerom rost. The topic has been reviewed by Scherer and Valenza (2005)and Matsuoka and Murton (2008). In France, the standard on rost resis-tance o natural stone (Norm XP B 10-601, see LERM 2006) gathers allthe tests to be perormed and gives the appropriate thresholds, according

to the destination o the stone in the building and according to localclimate. Created in 1984, the standard is regularly revised to t with eldobservations and climate change.

Inevitably, urther questions remain. Why are certain types ostone much more vulnerable than other types to salt damage? Why arecertain salts much more damaging than others? Is damage caused mostlyby relatively rare environmental events (rapid cooling, drying, or conden-sation) or cumulative everyday stresses (humidity cycling)? What are the

long-term eects o various conservation treatments, such as desalinationor consolidation, on salt damage? How can desalination and preventiveconservation eorts be enhanced? Can general agreement be achievedregarding the undamental mechanisms o salt weathering? Can the saltdamage process and weathering orms such as taoni be accurately mod-eled using existing knowledge? How does the hydration o salts progress,and how are crystallization pressures sustained in situ?And, above all,how can the great undamental strides o recent years be converted to

practicable applications?

Biodeterioration


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Stone Decay 21

Biological growths on stone are both a blessing and a blight.Colorul lichens and creepers, such as ivy, can contribute an air o ageand romance to a monument, and their removal can leave the stone look-ing stark and denuded. Nevertheless, many organisms contribute to the

deterioration o stone, and it is necessary to nd the right balancebetween appearance and longevity. The discussion surrounding this topichas become more complex and nuanced, as evidence has accumulatedthat complex biolms in some situations may help to stabilize ragilestone suraces and in other cases may strongly accelerate decay (Uchida etal. 2000; Chiari and Cossio 2004;Caneva et al. 2005; De Muynck, DeBelie, and Verstraete 2010). For example, in laboratory experiments, bio-

lms have been shown to result in a 4070 percent decline in dissolutionrates o calcite (Davis and Lttge 2005). In more recent work, the contri-bution o bacteria to dissolution or protection has been related to theamount and type o ood or microbes present, such as nitrate versusammonium ions and organic carbon species (Jacobson and Wu 2009).Research on the action o biolms on silicate stones (granite and basalt)has shown they may enhance dissolution rates in some situations (Wu etal. 2007; Wu, Jacobson, and Hausner 2008). While additional work is

needed, research in this area suggests that some surace patinas may bean eective natural protection or carbonate stones, while other biolms,particularly in polluted environments, may be deleterious.

Bioremediation and biocides are related topics o recent researchthat are discussed later in the section on surace treatments in chapter 2.

The biological degradation o rocks is well known and has beenstudied or a long time: it is one o the weathering mechanisms responsi-ble or the ormation o soil. The deterioration o stone in buildings and

monuments through the action o biological organisms has also beenacknowledged since the mid-1960s, but the topic has received increasingattention over the past decade. Some o the literature is concerned pri-


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22 Chapter 1

Much o 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

o lichens, conirming that their eects are both physical and chemical.Mechanical damage is caused by penetration o the hyphae into thestone and by the expansion and contraction o the thallus (the vegetativepart o the ungus) under changes o humidity. Chemical damage, how-ever, is more important and may arise in three ways: by the secretion ooxalic acid, by the generation o carbonic acid, and by the generationo other acids capable o chelating ions such as calcium. Field examples o

damage rom lichens to stone monuments have recently been described incontexts ranging rom Persepolis to the Alhambra palace and theJeronimos Monastery (Mohammadi and Krumbein 2008;Sarr et al.2006; Ascaso et al. 2002).

The secretion o oxalic acid, which reacts with a calcareous stoneto produce calcium oxalate, is o particular interest. A number o authorshave noted the presence o calcium oxalate on the surace o stone monu-ments, where it can orm part o a coherent and seemingly protective

layer known as scialbatura. Del Monte and Sabbioni (1987), or example,have argued that scialbatura is caused solely by lichen activity, whereasLazzarini and Salvadori (1989) have enumerated other possible causes,including the deliberate application o a protective coating. Correlatingthe environmental limits or lichen growth with the distribution ooxalate on Trajans Column, Caneva (1993) ound that the oxalate dis-tribution pattern was the opposite o that expected or lichens and thuslichens were perhaps not the best explanation or the columns patina.

Analysis o rock outcrops suggests that some oxalate patinas may be rel-ics o past paleo-environments that were more suitable or lichen growthduring an interval o greater surace moisture (Moore et al. 2000).


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Stone Decay 23

have appeared determined to see bacteria and nothing else. A troublingnumber o authors have noted high numbers o bacteria in decayingstone, in comparison to low numbers in sound stone, and have concludedthat the bacteria cause the decay. However, an alternative explanation

could be that decayed stone presents a preerred habitat or the bacteria.Bacteria that attack stone chemically all into two groups: auto-

trophic bacteria derive their carbon rom carbon dioxide (CO2), and mayderive their energy rom light (photolithotrophs) or rom chemical redoxreactions (chemolithotrophs). Heterotrophic bacteria, by contrast, utilizeorganic compounds on the stone to derive their carbon. Autotrophic bac-teria include those that are capable o oxidizing sulur and nitrogen com-

pounds to produce suluric acid and nitric acid, respectively. They are onemore means, thereore, by which air pollutants such as sulur dioxideand nitrogen oxide are turned into sulates and nitrates. This underlinesthe diiculty o separating out the individual causes o stone decay; sev-eral dierent actors may play integral roles in the overall decay process.The question remains o whether bacteria or catalyzing metal com-pounds, or example, are the main routes o sulate production. How-ever, i oxidation by both bacteria and metal compounds is rapid by

comparison with the rate at which sulur dioxide arrives at the stonesurace, then the arrival rate will be the rate-determining step, not theroute taken. The synergistic eects o air pollution and bioilm orma-tion have been researched, with the inding that there is strong evidencethat bioilms enhance the absorption o air pollutants (Young 1996;Mansch and Bock 1998).

Heterotrophic bacteria produce chelating agents and organicacids that are weaker than the inorganic acids produced by the sulur-

oxidizing and nitriying bacteria. They have received comparatively littleattention, but their role in deterioration is well established nonetheless(Lewis, May, and Bravery 1988; Saarela et al. 2004;Gorbushina 2007;


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24 Chapter 1

limit the introduction o ungal strains through the use o a ormalde-hyde oot-wash treatment or visitors resulted in the growth o aormaldehyde-resistant strain o white Fusarium solani ungus (Bastianet 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 o a new ventilation system (Brunet, Malaurent, and Lastennet 2006;Lacanette et al. 2009). Computer modeling o the airfow at the Lascauxcave suggests that reducing the airfow may help avoid uture damage(Malaurent et al. 2007).

The role o halophilic microbes (mostly archaea, with some bacte-

ria) is important in stone decay (Laiz et al. 2000; Saiz-Jimenez and Laiz2000). A signicant and open question is i hydroscopic salts may raisemoisture levels to the point where halophilic microbes increase in abun-dance, setting the stage or urther microbial development o adjacentareas o stone.

Dierential Stress

While air pollution, salts, and biodeterioration capture the lions share

o attention, there are advances in our understanding o other, otenrelated decay mechanisms that are worth some consideration. Reviewingthe recent literature on stone conservation, it is clear that there is animportant trend in decay mechanism research that is ocusing on whatis called here (or want o a better term) dierential stress. This decaymechanism includes the eects o wet/dry cycling, clay swelling, dieren-tial hygric stress, dierential thermal stress, and stress rom dierentialexpansion rates o material in pores (such as salts or organic material)

versus in the stone. The general idea is that treatments, salts, water lms,or biolmsanything that causes the stone surace to react dierentlythan the interiorcan result in a shear stress, crack propagation, and,


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Stone Decay 25

Organic material in pores, whether it is a polymeric consolidantor originating rom biological sources, may expand signicantly asterthan the stone on wetting (Laurenzi Tabasso 1995; GCI and IHAH 2006).Work by Yang, Scherer, and Wheeler (1998) highlighted the importance o

making sure consolidants have thermal expansion properties similar tothe substrate being treated. Some recent work on thermal damage tostone reveals that it is an important decay actor, and stress may resultrom dierential heating, such as when areas o stone undergo short-termcooling events rom shade (Weiss et al. 2004; Gmez-Heras, Smith, andFort 2006; Hall and Andr 2006; Sumner, Hedding, and Meiklejohn2007; Gmez-Heras, Smith, and Fort 2008). This eect 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 cloudblocks the sun is substantial.

Work by Warke and Smith (1998) ound that climate chambersimulation studies do not take into account the important eects o radi-ant heating and thus are not representative o eld conditions. The bal-ance between thermal and hygric damage is addressed in work at Petraby Paradise (2002; 2005). Research on clay swelling has advanced signi-

cantly based on the research at Princeton University (Wangler and Scherer2008; Duus, Wangler, and Scherer 2008; Jimnez-Gonzlez, Rodrguez-Navarro, and Scherer 2008), where it was ound that shear orces cancause buckling o wetted stone suraces and that intracrystalline swellingo clay is the primary mode o swelling or Portland brownstone, despitethe proportion o swellable clay being only 1 percent o the stone. Theclay is present as a cement at sand grain boundaries, permitting the claysucient leverage in brownstone. Osmotic swelling (salt-activated

clay swelling) was ound to be important in sepiolite-rich Egyptian lime-stone (Rodrguez-Navarro et al. 1998). Understanding the relative roleand dynamics o dierential stress as it relates to air pollution, biodeteri-


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26 Chapter 1

and alls o perpetually in great scales. He comments wryly that Reigateis a stone that would saw and work like wood, but not durable, as ismaniest (as quoted in Prudon 1975). An alternative point o view notesthat the decay o Reigate stone is mainly conned to surace layers and

has not been responsible or structural ailure (Lockwood 1994).Work on Swiss molasse and similar stones has included the use

o grouts or the extensive detached areas oten ound on buildings and anew treatment or reducing the swelling o clays (discussed in more detailin chapter 3) (Jimnez-Gonzlez and Scherer 2004; Rousset et al. 2005).

One intrinsic issue that researchers have puzzled over or severaldecades is the bowing o thin marble slabs on emblematic modernbuildings, such as the Amoco building in Chicago, the Grande Arch dela Dense in Paris, and Alvar Aaltos Finlandia city hall in Helsinki.Substantial recent research has ound that dierential expansion ocalcite enhanced by moisture, microstructure, and dierential residualstrains in the marble is the main cause o 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 o complex systems over

time. It is an approach that uses internal eedback 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 or the Challenges o Modern

Society, the United Nations Economic Commission or Europe (UNECE), the USNational Acid Precipitation Assessment Program (NAPAP), the UK National

Materials Exposure Programme, and the German Bundesministerium r

Forschung und Technologie (BMFT).


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Chapter 2

Putting It Right: Preventive and Remedial Treatments

When conronted with decaying stonework, ones immediate instinct is todo something about it. Traditionally, this has meant doing somethingto the stone: perhaps patching it up with mortar, applying some kind oprotective coating, or cutting out decayed stone and replacing it with newstone. Regular maintenance is vitally important, wherever practicable;William Morris (1877) wrote o the need to stave o decay by dailycare, and in a textbook or conservators encouragingly titled Preventive

Conservation o Stone Historical Objects,Domaslowski (2003) persua-sively argues that routine maintenance is an oten-underappreciatedaspect o preventive conservation. Now, however, there is an increasingemphasis on doing something not only to the stone itsel but also to theenvironment in which the stone is ound. This refects a growing aware-ness o the importance o preventive conservation, o the principle ominimum intervention, and o the need to limit the use o materials thatmight prove harmul to either the stone or to the environment. Also, now

that there is a better understanding o decay mechanisms, a conservationstrategy can be designed to reduce the rate o damage by ocusing onpoints o leverage that can mitigate some decay processes. An interesting

l i h l i l lli i hi i i


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28 Chapter 128 Chapter 2

Preventive conservation measures o more immediate eect areusually concerned with keeping water out o the stone and with control-ling the relative humidity and temperature o the air around the stone.This is relatively easy or stone artiacts within a museum, and it may

also be easible or stone masonry that is exposed on the interior o abuilding (Price and Brimblecombe 1994; Price 2007). It is less easy orstonework on the outside o a building, although a dramatic example othis approach is provided by the glass envelope constructed over the ruinso Hamar Cathedral in Norway.

More modest protective shelters are requently used on the out-side o a building to protect those eatures that are particularly impor-tant. They may be part o the original design (or example, a canopyprotecting a statue in a niche), or they may be a later addition. As anextreme measure, they may enclose the eature altogether. Their purposeis to reduce the amount o rain that reaches the stone and, insoar as ispracticable, to stabilize the temperature and moisture content o thestone. I the shelter is a later addition, it is likely to be visually intrusiveunless it is so small as to serve little purpose.

Few studies have been undertaken o the design requirements o

such shelters, and it is possible that their beneits are more psychologi-cal than actual. This has been evaluated in practice in only a ew cases(Agnew et al. 1996;Aslan 2007). One case study is at the site o theHieroglyphic Stairway, in Copn, Honduras, where a simple canvasshelter has prevented lichen growth and the swelling o the clay-containing stone due to requent rainstorms (Doehne et al. 2005; GCIand IHAH 2006). A second case study, which calculated protectiveindices or several styles o shelter at the archaeological site o Joya de

Ceren in El Salvador, ound that evaporation was reduced and thermaland relative humidity stability improved in several cases (Maekawa2006). A urther useul study was undertaken or a pavilion at


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Stone Decay 29 Putting It Right: Preventive and Remedial Treatments 29

and the environmental conditions modied to reduce the incidence o saltcrystallization events (Laue, Bluer Bhm, and Jeannette 1996). There isincreasing evidence that drying rates are important and that even a smallreduction in drying rate can result in salts crystallizing on the surace as

relatively harmless eforescence (Selwitz and Doehne 2002). This was thelogic behind the suggestion that a row o trees be planted to help protectsalt-laden structures at the site o Port Arthur in Australia (Thorn andPiper 1996).

The remainder o this chapter is devoted to research related toactive conservation: doing something directly to the stone itsel. In keep-ing with the title o this volume, this chapter is not a handbook o repairtechniques. Inormation on the routine practice o stone conservation isavailable elsewhere (Ashurst and Ashurst 1988; Ashurst and Dimes 1998;Ashurst 2007; Snethlage 2008).

ACTIVE CONSERVATION: CLEANING

Cleaning is oten one o the rst steps to be undertaken ater a condition

survey has been completed. As expected, carbonate materials are the mostreactive to acidic pollution and thus have received the lions share oattention in studies o stone cleaning. By removing the dirt, one can bet-ter see the condition o the underlying stone and thus judge what urtherconservation may be necessary. Cleaning may also serve in some circum-stances to remove harmul materials rom the surace. However, the pri-mary reason or cleaning will oten be the dramatic change in appearancethat can be achieved. A dirty building or monument does not look well

cared or, and the dirt may well obscure both ne detail and major archi-tectural eatures. Nonetheless, there are those who would argue thatcleaning contravenes one o the undamental principles o conservation


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reached, it means that the original surace is completely gone (JosDelgado Rodrigues, personal communication).

A wide range o techniques is available or cleaning stone, rang-ing rom those that are intended or use on large acades to those that

are intended or meticulous use on nely carved and delicate sculpture.Techniques are reviewed by a range o researchers and practitioners:Fassina 1994; Andrew, Young, and Tonge 1994; Ashurst 1994; Cooper,Emmony, and Larson 1995;BSI 2000; Vergs-Belmin and Bromblet 2000;Rodrguez-Navarro et al. 2003; Normandin et al. 2005; Worth 2007. Thisis an area where much progress has been made in the past twenty years,although only a portion is reported directly in the literature. The basictechniques have remained largely the same, although they have becomemore rened. This refects an increasing awareness o the damage (andconsequent litigation) that may be caused by inappropriate or overenthu-siastic cleaning and also o the environmental issues posed by the use ocertain chemicals or excessive quantities o water (Maxwell 1996; Young,Urquhart, and Laing 2003). With some exceptions, such as latex cleaninglms, developments have largely come about through care and attentionon-site rather than in the laboratory. These lessons rom the eld 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 denition o the target cleaning level beore the workbegins. For example, in the present urban environment, uncleaned lime-stone suraces may range in color rom white (where water runo hastaken place) to dark brown and black, depending on the amount o accu-dark brown and black, depending on the amount o accu-mulated dirt. All o these suraces dier substantially rom the originalreshly cut suraces, and establishing a target level o cleaning is not an

easy task when a single building may contain a wide range o suraces.A number o authors have emphasized the damage that can be

caused by cleaning: loss o surace, staining, deposition o soluble salts,


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Kozub, and Dham 2008). Vergs-Belmin (1996b) gives a particularly use-ul overview o methods or evaluating cleaning treatments or stone.Recent work has shown that quantitative measurements o color changeater stone cleaning vary considerably, mainly due to the action o hygro-

scopic salts (Vergs-Belmin, Rolland, and Leroux 2008). Precautionsshould be taken to account or the infuence o salts when making suchmeasurements. When discussing color change due to cleaning, it shouldbe made clear that once aged, the stone surace can never be returned tothe reshly cut color. Color can be used as criteria or cleaning only whena reerence surace is dened and taken as a target or the cleaninglevel to be reached in the intervention. Color changes related to lasercleaning are dealt with in the next section.

Laser Cleaning

Using lasers to clean stone is now routine, and large-scale commercialapplication o laser cleaning has become more common over the pastiteen years (Dajnowski, Jenkins, and Lins 2009). Its great attractionis that it does not entail any physical contact with the stone and solends itsel to the cleaning o very delicate suraces. There are no sol-

vents or water to redistribute potentially harmul salts. The techniqueis selective and sensitive in terms o the degree and control o removal.The principle is essentially simple: a laser beam impacts the surace,and the energy o the inrared beam is dissipated by the sudden heat-ing and expansion o light-absorbing material on the surace, such asparticles rich in carbon, and the nearly instantaneous vaporization omoisture in the surace layer, which acts to remove surace dirt.Spraying the surace with water just beore laser cleaning can enhance

the eectiveness o the treatment (Siedel, Neumeister, and Sobott2003). For light-colored stones with dark surace deposits, the inraredbeam continues to be absorbed while the stone remains soiled and


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chemical; at comparing the perormance o lasers with other cleaningtechniques; and at identiying possible hazards to the operator (Vergs-Belmin et al. 2003; Bromblet, Labour, and Orial 2003; Rodrguez-Navarro et al. 2003). The use o a laser requires special caution when

cleaning suraces with traces o polychromy (Fassina, Gaudini, andCavaletti 2008). A set o conerences devoted to the use o lasers in artconservation (Lasers in the Conservation o Artworks, or LACONA) hastaken place every two years since 1995: or example, Liverpool in 1997and Madrid in 2007. A European Cooperation in Science and Technologyproject on the topic o artwork conservation by laser, unded by theEuropean Science Foundation, ran rom 2000 to 2006 and resulted ina handbook available or download (http://www.cost.es.org/library/publications/05-40-Cleaning-Saely-with-a-Laser-in-Artwork-Conservation ).

Further development o equipment has taken place, identiying,or example, the appropriate means and timing o delivering the laserpulse to the surace o the stone (Margheri et al. 2000; Mazzinghi andMargheri 2003; Dogariu et al. 2005;Siano et al. 2008). An importantissue with laser cleaning is the color o the cleaned surace. In some cases,a yellow surace layer is revealed, which in some examples is related to

previous restoration treatments (Vergs-Belmin and Dignard 2003;Zaropulos et al. 2003; Gavio et al. 2004; Gavio et al. 2005; Vergs-Belmin and Laboure 2007; Andreotti et al. 2009). Color changes aterlaser cleaning may happen due to modications in the substrate (pinkeldspars, or instance), to modications in any covering colors, or tochanges in deposited dirt particles. The last situation may indicate thatthe target cleaning level has not been reached.

Latex Poultice MethodAn important challenge or stone conservation has been the cleaningo large, public interiors, such as cathedrals, while allowing them to
http://www.cost.esf.org/library/publications/05-40-Cleaning-Safely-with-a-Laser-in-Artwork-Conservationhttp://www.cost.esf.org/library/publications/05-40-Cleaning-Safely-with-a-Laser-in-Artwork-Conservationhttp://www.cost.esf.org/library/publications/05-40-Cleaning-Safely-with-a-Laser-in-Artwork-Conservationhttp://www.cost.esf.org/library/publications/05-40-Cleaning-Safely-with-a-Laser-in-Artwork-Conservation


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Biological Cleaning

Hempel (1976) was one o the rst to raise the possibility o biologicalcleaning. He had been surprised by the eectiveness o a clay poulticecontaining urea and glycerol and proposed that microorganisms were at

least partially responsible. Kouzeli (1992) has reported avorably on thetechnique in comparison with pastes based on EDTA or ammoniumbicarbonate.

Biological cleaning, in general, has been little researched (Ranalliet al. 1996; Ranalli et al. 2000). Gauri has demonstrated the use o theanaerobic sulur-reducing bacterium Desulovibriode suluricans in remov-ing the black crust on marble (Gauri et al. 1992). He has argued, moreover,that the bacterium was converting calcium sulate back into the calciumcarbonate rom which it was originally ormed (Atlas, Chowdhury, andGauri 1988; Gauri and Chowdhury 1988). Konkol has demonstrated thatusing an enzymatic cleaner derived rom the ungus Trametes versicolormay reverse biological staining o marble (Konkol et al. 2009). Eorts toremove lichen rom concrete through the use oThiobacillus bacteriahave been evaluated by De Muynck, De Belie, and Verstraete (2010).Comparison o sulate-reducing bacteria treatment versus conventional

chemical cleaning procedures on a marble element o the Milan Cathedralis reported by Toniolo et al. (2008) and Cappitelli et al. (2007a).

Targeting the Dirt

Gauris work is interesting because it takes account o the nature o thedirt. It is true that this may be implicit in other cleaning techniques (e.g.,the use o complexing agents to increase the solubility o calcium sulateor the use o hydrofuoric acid to dissolve silica), but it is disappointing

that only a ew developments in cleaning techniques have fowed out othe extensive studies on black crusts. One example is the work oVergs-Belmin, Pichot, and Orial (1994) determining the point at which to stop


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The removal o water-soluble salts sounds tantalizingly easy, but it canprove dicult in practice. Salt reduction may be a more appropriateterm (Redman 1999; Sawdy, Heritage, and Pel 2008; Pel, Sawdy, andVoroninaa 2010).

Salt reduction is relatively straightorward in the case o smallartiacts, which can, or example, be immersed in water or enclosedcompletely in a poultice, though even here problems can arise throughthe railty o the surace or the presence o 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 rom themasonry o a building or monument. In an early desalination study,Bowley (1975) demonstrated that it was possible to extract a worth-while quantity o salt rom masonry through the repeated use o claypoultices, although little would be gained in the long run unless onecould eliminate the source o urther salt. An excellent review (Vergs-Belmin and Siedel 2005) makes it clear that larger-scale masonry desali-nation needs urther study.

Desalination o masonry is usually attempted through the use ofpoultices, which may consist o a range o materials (e.g., clay, sand,

and paper pulp) (Auras 2008). In those instances where calcium sulateis to be removed, additional materials may be added in order to increaseits solubility. Clearly there are overlaps here with cleaning, especially inthe removal o black crusts. The additives may include EDTA and itssodium 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 owarning may be appropriate: I a limestone is heavily sulated, the cal-

cium sulate may be all that is holding it together, and total removalcould be disastrous.

An EC project, Assessment o Desalination Mortars and Poultices


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with the accumulation o salts and damp has a mixed record in somechurch monuments (Henry 2006, p. 277).

Finally, the use o bacteria in desalination may merit urtherattention. Gauris use o sulur-reducing bacteria to eliminate the black

crust has already been mentioned, and Gabrielli (1991) gives an anec-dotal account o using the reducing atmosphere created by cow dung toconvert nitrate salts into elemental nitrogen gas. One wonders, however,i other salts are added at the same time. Removal o 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 o the ECBIOBRUSH project (BIOremediation or Building Restoration o theUrban Stone Heritage; May et al. 2008). However, these studies oundthat any eects o the bacteria were masked in many cases by the eecto the material used to apply them and that there were practical prob-lems in supporting the weight o the application material on large areas.One is let with the eeling that additional development is needed beorepractical biological cleaning can be readily applied. In contrast, biocalci-ication appears to be at a much higher level o development (see Limeand Biocalciication section below).

ACTIVE CONSERVATION: CONSOLIDATION

Where stone is severely weakened by decay, some orm o consolidationmay be necessary to restore some strength. Ideally, one might hope tomake the stone at least as strong as it was originally ( Snethlage 2008;Scherer and Wheeler 2009), so it might resist urther decay, but even the

strength to resist the battering o the wind or the wing o a bird may beenough to prolong survival.

It all sounds so easy. One just has to nd something that will


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consolidant-cum-preservative. It is a wonder we have made as muchprogress as we have. An enormous variety o materials have been triedsince time immemorial (Bar 1860; Egleston 1886), each with its ownadvocates (Palmer 2002).

One has to start somewhere, and one o the properties that aconsolidant must have is the ability to penetrate the stone. This, in turn,requires a low viscosity and a low contact angle. Next, the consolidantneeds to stien or set once it is in place in order to strengthen the stone.These requirements can be met in three ways: rst, one could think oapplying a substance that is liquid at high temperature and stiens as itcools downwax or instance. In practice, it is hard to get a low enoughviscosity without excessive heat, and wax tends to be sticky and to pickup dirt. The consolidation might become risky in areas having signicantexposure to the sun. The second approach is

doehne, e. y price, c. stone conservation. getty. 2010

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