Bradley, S. Preventive Conservation Research British Museum. 2005

8/3/2019 Bradley, S. Preventive Conservation Research British Museum. 2005

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JAIC 2005, Volume 44, Number 3, Article 2 (pp. 159 to 173)

PREVENTIVE CONSERVATION RESEARCH AND

PRACTICE AT THE BRITISH MUSEUM

SUSAN BRADLEY

ABSTRACTSince the early 1970s the conservation scientists at the British Museumhave pursued a program of research in what has become known as preventiveconservation. This object-centered research almost always was initiated as investigationinto the cause of deterioration, to gain an understanding of treatment and environmentalrequirements for stabilization. The majority of galleries and object storage areas are notequipped with air-handling systems; such systems are not always feasible in a buildingwhich is itself of such importance that it has been designated grade one listed byEnglish Heritage. Solutions have thus involved micro-environments tailored to theneeds of groups of objects. A continuing theme has been the role in deterioration played

by pollutant gases, particularly reduced sulfide gases and organic acids. This paperpresents an overview of these investigations in the broader context of preventiveconservation practice in the British Museum.

TITRERecherche et mise en pratique de la conservation prventive auBritish

Museum. RSUMDepuis le dbut des annes 70 les scientifiques de la conservationduBritish Museum (muse national britannique) ont poursuivi un programme derecherche en conservation prventive. Cette recherche centre sur les objets a presquetoujours dbut en tant qu tude sur les causes de la dtrioration, afin de mieuxcomprendre les ncessits de traitement et les conditions ambiantes idales pour lastabilisation. La majorit des salles d'exposition et de rserve ne sont pas quipes desystmes de climatisation; de tels systmes ne sont pas toujours possibles dans un

btiment qui est lui-mme d'une telle importance historique qu'il a t class dans lacatgorie de plus haute importance tablie parEnglish Heritage (hritage anglais). Lessolutions ont ainsi impliqu des micro-environnements conus en fonction des besoinsde groupes particuliers d'objets. Un thme constant de la recherche a t le rle jou

dans la dtrioration par les gaz polluants, en particulier les gaz sulfureux rduits et lesacides organiques. Cet article prsente une vue d'ensemble de ces investigations dans lecontexte plus large de la pratique en matire de conservation prventive au muse.

TITULOInvestigacin y prctica de la conservacin preventiva en elBritish Museum(Museo Britnico). RESUMENDesde principios de la dcada de 1970 los cientficosde conservacin delBritish Museum (Museo Britnico) se dedican al rea deinvestigacin conocida como conservacin preventiva. Esta investigacin centrada enlos objetos comenz como una bsqueda de las causas de deterioro para entender mejorlos tratamientos y requisitos ambientales adecuados para su estabilizacin. La mayorade las galeras y reas de almacenamiento no estn equipadas con sistemas de aire

acondicionado; estos sistemas no son siempre factibles en edificios que por s mismosson de tal importancia que han sido designados de grado uno en la lista de English


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Heritage (Patrimonio Ingls). As, las soluciones han involucrado micro-ambientesajustados a las necesidades de grupos de objetos. Un tema recurrente ha sido el rol delos gases contaminantes en el deterioro de los objetos, en particular el de los gasessulfurosos y los cidos orgnicos. Este trabajo presenta una visin general de estasinvestigaciones en el contexto ms amplio de las prcticas de conservacin preventiva

en elBritish Museum (Museo Britnico).

TTULOPesquisa e prtica em conservao preventiva noBritish Museum (MuseuBritnico). RESUMODesde o incio da dcada de 1970 os cientistas de conservaodoBritish Museum (Museu Britnico) tm seguido um programa de pesquisa que ficouconhecido como conservao preventiva. Esta pesquisa baseada no objeto foi quasesempre iniciada pela investigao sobre as causas de deteriorao, a fim decompreender o tratamento e as necessidades ambientais necessrios para a estabilizao.A maioria das galerias e reas de armazenagem de objetos no esto equipadas comsistemas de controle de ar; tais sistemas nem sempre so viveis em edifcios, os quais

por si s tm tamanha importncia que so classificados como grau um na listagem doEnglish Heritage (Patrimmio Ingls). Por conseguinte, as solues envolvendo micro-ambientes so desenhadas medida das necessidades de grupos de objetos. Um temaconstante tem sido o papel dos gases poluentes, em particular dos gases sulfricos ecidos orgncios, na deteriorao. Este artigo apresenta uma viso geral destasinvestigaes no contexto mais amplo da prtica da conservao preventiva noBritish

Museum (Museu Britnico).

1 INTRODUCTION

The importance of the interaction of environment and collections was recognized longbefore the then Keeper of the British Museum Research Laboratory published TheConservation of Antiquities and Works of Art(Plenderleith 1956). This book contains asection on The Influence of Environment based on research on the deterioration ofobjects in the collection carried out by the Museum scientists between 1922 and 1950.By today's standards the information is rudimentary, but it did give useful guidance oncollections care. In parallel to the work at the British Museum, research into the effectsof the environment on oil paintings and on Japanese art was being carried out in otherinstitutions. This work informed two important publications, The Museum Environment(Thomson 1978) and Characteristics of Japanese Art that Condition Its Care (Toishi

and Washizuka 1987), both of which have contributed to the framework of what hasbecome known as preventive conservation.

In the early 1970s, under the leadership of A. E. A. Werner, British Museumconservation scientists began a concerted program of research into the effects of theenvironment on the collection. This research was driven by the development of newgalleries and the need to establish whether or not control of temperature and/or relativehumidity was needed to safeguard the objects, to justify its inclusion in the projects.

There were several reasons not to attempt largescale climate control. First of all, beingdesignated grade one listed by English Heritage means that the British Museum

building is of such historical importance that it should be maintained in its originalform. Putting in air conditioning for galleries without affecting the structure is very


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costly, aside from the expense of installation, running, maintenance, and replacement ofthe systems. Secondly, the trend was away from single-object galleries, such as the runof upper floor galleries displaying Greek pottery, to thematic mixed-object gallerieswhere no one environment would be suitable for all of the objects. Finally, much of thecollection is stable in the ambient conditions in the Museum and air conditioning was

simply not needed. Hence, simple approaches and targeted control were favored for thedisplay and storage of those objects which were found likely to deteriorate in normalambient conditions.

The research focused on single objects or groups of objects which were deteriorating, toidentify the cause and a method of prevention. The role of relative humidity soon

became apparent. Determining an appropriate relative humidity and suitable method ofcontrol was part of the prevention strategy. Gases given off by the materials used instorage and display, and sometimes given off by the objects themselves, were found tocause corrosion of metal objects and to form mixed salts on the surface of some porousstone and ceramic objects.

In this paper an overview of the research between 1970 and 2005 on temperature,relative humidity, and pollutant gases is put in the context of the development of

preventive conservation practice in the British Museum.

2 TEMPERATURE AND RELATIVE HUMIDITY

2.1 COPPER ALLOY, WEEPING GLASS, IVORY AND BONE

2.1.1 Copper Alloy

In the 1960s and '70s the emphasis of conservation in the British Museum was on thetreatment of objects. Stabilization through treatments, such as soaking bronzes insodium sesquicarbonate solution or stripping to remove corrosion layers, was seen asthe appropriate response to deterioration. In the Museum Research Laboratory, studies

into the deterioration of copper alloy artifacts had shown that bronze disease could beprevented by controlling the relative humidity to below 35%, stopping the reaction ofnantokite with copper metal and moisture from the air to form paratacamite (Organ1963).

This research was implemented in the design of six new galleries opened between 1968and 1975. Dehumidified showcases were incorporated to display Middle and NearEastern copper alloy objects which were particularly prone to bronze disease. Theshowcases were controlled with desiccant dehumidifiers supplied by Munters Ltd.(Newey 1987). A dehumidified storage area was constructed for the copper alloyobjects not on display. Although it took some time to limit the use of chemicalstabilization methods to those objects for which a controlled environment could not be


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provided, control of relative humidity is now the primary method of preventing bronzedisease.

2.1.2 Weeping Glass

Weeping glass was shown to be stable if kept at an RH of 38-42%, and all of theweeping or crizzled glass was moved to a dehumidified showcase where the objectscould be viewed without being handled (Organ and Bimson 1957; Werner 1958). Thisdecision was based on the identification of potassium carbonate salts on the surface ofthe glass. At that time, analysis was restricted to those ions expected to be present, so itis not known whether some of the ions detected today on the surface of glass were

present in the 1950s (Bimson 2004). The critical relative humidity for potassiumcarbonate was determined as 42%, above which it takes up moisture from the air,forming surface droplets which give the appearance of sweating and provide an aqueousmedium into which more alkali ions can be leached from the glass.

2.1.3 Ivory and Bone

The collection contains ivory and bone objects from many different cultures of widelydiffering ages. Among the highlights of the collection are the Lewis chessmen, carvedfrom walrus ivory and dated to AD 11501200. Some of the chessmen were found to becracking and exfoliating and in 197071 all the pieces were examined by scientists

working in Dental Anatomy at University College London, who identified the cause ofthe problem ( Boyd 1971). The chess pieces which were deteriorating had been carvedin walrus ivory which had been boiled. This had caused denaturing which made theivory more susceptible to cracking in response to changes in relative humidity. In 1978the chessmen were exhibited in a showcase controlled by a ducted humidifier to 5060% RH. Recently this has been redefined to 4555% as the control achieved wasalways close to 50% and the chessmen remain stable.

Ivory objects from Nimrud which had been burnt in antiquity are not humidified. Theyhave been stable in an uncontrolled storage area that has naturally maintained 4045%RH and have been seen to react adversely to higher and lower relative humidity. The

most dramatic disruption occurred when an ivory lion head (ANE 132697) fragmentedfollowing a move to new storage. It was found to have fractured along old cracks whichhad been consolidated with polyvinyl acetate resin. Soluble salts, gypsum, and halitewere present in the cracks and their crystallization at the low relative humidity sustainedduring the move was considered a contributory factor in the deterioration of the object(Thickett and Bradley 1998).

2.1.4 Control of Relative Humidity in Showcases and Storage Areas

For copper alloy objects susceptible to bronze disease, weeping glass, and ivory, thecontrol of relative humidity in both showcases and storage areas has been achieved by


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only dehumidifying to maintain a low relative humidity and only humidifying tomaintain a higher relative humidity. There has been respectively no counterhumidification or dehumidification, and no temperature control other than winterheating. This has meant that for the dehumidified cases the winter low internal relativehumidity has been accepted and for humidified cases the summer high relative humidity

has been accepted. The relative humidity inside the controlled showcases has driftedaccordingly, although the well-sealed showcases in use since 1988 buffer effectivelyagainst the peaks and troughs in relative humidity. The fact that no observable damagehas occurred to any of the copper alloy or ivory objects in a controlled relative humidityhas justified this approach. There have been instances of glass apparently starting toweep in the dehumidified case. Research into this problem is ongoing with the focus onoccasional very low relative humidity compared to the target of of 3842% RH, andhigh levels of indoor pollutant gases in the dehumidified case.

2.2 COMPLEX MATERIALS

2.2.1 Lindow Man

When Lindow Man, a freeze-dried bog body dated to the Iron Age, was put onexhibition, avoiding the seasonal extremes of relative humidity was thought necessary.Having no idea how the body would react to changes in relative humidity, Museumscientists and conservators were keen to stabilize the environment as much as possible.

The body had been immersed in a solution of polyethylene glycol 400 prior to freeze-drying and after treatment was acclimatized to ambient conditions in the Museum(Omar et al. 1989). Because low molecular weight polyethylene glycol was known totake up moisture from the air and sweat at high relative humidity, it was decided thatthe maximum allowed would be 60% RH. It was also decided to set the minimum to50% RH to limit the potential for dimensional changes.

To supply these conditions the showcase was constructed with an integral humidifier(Defensor PH5) and a dehumidifier (Munters M120), both controlled by a Sauter HSChygrostat. The humidifier was modified with a large funnel that covered the air outlet,connected by large diameter tubing to a hole in the showcase base. The selection of the

dehumidifier was based on a comparison of the operation of the condensation anddesiccant equipment. Desiccant equipment which had already been used successfully inthe Museum was selected. The dehumidifier was connected to the case following themanufacturer's instructions.

The showcase was monitored using an electronic RH/temperature probe which showedthe relative humidity was tightly controlled in the range 5357%, but the temperaturerange was 1830C. At 55% RH the moisture envelope surrounding the body was 918g/m3, a much wider range than anticipated. However, physically the body has remainedin good condition since it was put on display in 1989. The color post-conservation hasapparently lightened and because of this in January 2005 an examination of the body

was carried out. This showed the body to be in excellent condition, but some extrusionof deteriorated PEG had occurred. Color monitoring of Lindow Man while on display


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had indicated that fading occurred during the exhibition period between 1987 and 1997,when the light levels on the body were up to 400 lux. In 1997 Lindow Man was movedto the Late Bronze Age and Celtic Europe Gallery where it is displayed under amaximum light level of 50 lux. Since then the color change has slowed dramatically.

2.2.2 Ethnographic Objects

The Museum's Ethnography collection is now stored and displayed at a range of 4555% RH based on the large number of organic objects. There is no temperature controlin the main storage area, but on exhibition, air cooling has been installed to keep thetemperature in the galleries below 25C. Much of the collection is stable but this rangeof relative humidity promotes the deterioration of some objects.

When the deterioration of Maori objects made of New Zealand flax was investigated,the black dye used to color them, an iron/plant polyphenol complex, was found to be thecause (Daniels 1999). The dye was shown to become acidic causing hydrolysis of thecellulose fibers, and the iron in the dye caused oxidation. The research showed thatstoring the objects at a lower relative humidity would reduce the rate of hydrolysis.However this might affect the dimensional stability of the objects which haveacclimatized to the 4555% range since 1972. The decision was made to keep blackdyed New Zealand flax objects in the main storage area.

An investigation of the mechanism involved in localized browning of sugar objectsfrom Mexico showed the cause to be Maillard browning, a complex reaction of reducing

sugars and protein-containing compounds. The sugar commonly used in cooking andsweetening is sucrose which is not a reducing sugar. However, experiments showed therate of hydrolysis of sucrose to the reducing sugars, glucose, and fructose, increasedwith increase in relative humidity. The decoration on the objects was made from icingsugar which contains egg white, the source of the protein. As a result of this research,the objects were moved to a specially created low relative humidity, low temperaturestorage area (Daniels and Lohneis 1997).

2.2.3 Stone Sculpture

Control of both relative humidity and temperature which required full air conditioning,was justified for sculptures from the Great Stupa of Amaravati, India. The sculptureswere carved in a green-tinged, partially metamorphosed limestone, commonly known asPalnad marble, and were deteriorating by powdering and flaking, causingconsiderable loss of surface detail. Between 1960 and 1992, several campaigns ofscientific investigation were undertaken (Bradley and Freestone 1992) identifying five

potential causes of deterioration. The cleavage planes created by the folia caused thestone to split readily. The clay minerals in the stone were subject to softening andvolume change at high relative humidity, causing surface flaking. Disruption of thesurface through powdering was caused by a combination of crystallization and

dissolution of soluble chloride along with dissolution and re-precipitation of calcite byreaction with carbon dioxide and water and surface sulfation. This diagnosis suggested


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that the sculptures should be stored and displayed at a controlled temperature andrelative humidity of 1820C, 30-40% RH with filtration to remove sulfur dioxide.Under these conditions in storage and on exhibition in the Asahi Shimbun Gallery ofAmaravati sculptures, the sculptures have been stable. (Bradley 2003a).

2.3 RECENT CHANGES IN CONTROL OF RELATIVE HUMIDITY

IN SHOWCASES

The method of control used in showcases has improved in the last eight years. In newgallery developments, a new type of control system is being used instead of humidifiersand dehumidifiers. These controllers, developed by Glausbau Hahn Ltd., utilize a Peltiercell which cools air and hence dehumidifies without the use of refrigerants or dryingwheels. It is based on a thermo-electrical effect discovered by Jean Peltier in 1834. Thesystem can control to any range of relative humidity by feeding air into the showcase atthe required midpoint at a slow rate.

Units of different capacity have been used to control the relative humidity in individualshowcases, and from a central unit to groups of showcases in a gallery. The formerapproach has been used to control the relative humidity in individual small showcases inseveral galleries and in very large showcases in the Wellcome Trust Gallery whereethnographic objects are displayed. The latter approach has been used to control therelative humidity in all of the showcases in the North America gallery whereethnographic objects are displayed, and in the gallery where objects from Korea aredisplayed. Passive methods, such as use of conditioned silica gel (Artsorb or Prosorb),

are used to control relative humidity in only three showcases in the Museum because ofthe difficulty of maintaining the in-case conditions when the temperature in the galleryis not controlled and the labor-intensive nature of the installations. Currently there are103 controlled showcases in the Museum galleries.

3 INDOOR POLLUTANT GASES

When Werner (1972) published a short paper on the corrosive effects of materials usedin showcases, he could not have imagined that 36 years later, scientists in the Museum

would still be conducting research into that issue and that there would be an annualconference on indoor air quality. The development of a simple test to screen materialsused in the storage and display of objects (Oddy 1973) was followed by research intothe gases given off by materials (Blackshaw and Daniels 1978), the conditions underwhich gases are given off, adsorption of gases by objects, and the formation ofuncharacterized mixed salts on objects. The role of the external pollutant gaseshydrogen sulfide and carbonyl sulfide in the tarnishing of silver has also been animportant area of research in the Museum.


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3.1 ACCELERATED CORROSION TESTING OF MATERIALS

3.1.1 The Oddy Test

Simple corrosion test methodology in which the test material and a metal coupon wereheated to 100C for 3 days was adapted to carry out the first testing of showcasematerials. Preliminary studies were conducted to determine the optimum testtemperatures using both pure metal and metal alloys. From these tests it was decidedthat 60C for 28 days was optimal for testing. At 100C the test materials degraded

beyond recognition, and a lower temperature of 50C did not cause enough evolution ofgases to corrode the test coupons in a reasonable time. Pure silver, copper, and leadcoupons were seen to adequately represent the metals of antiquity based on theanalytical data available in the Research Laboratory and in the literature at that time.

Initially only the fabrics used inside showcases were tested, but it soon became apparentthat a much wider range of materials such as paints, woods, adhesives, sealants, andfittings needed to be tested. Two materials emerged as highly problematic for use inshowcase construction, wood and wool. The problem of acetic acid being given off bywood and corroding lead had already been published by Scott during the early years ofscience in the Museum (Scott 1922), and had been known in antiquity (Rackham 1968).In the tests it emerged that wool fabrics always caused silver to tarnish. This was

because the sulfide linkages in the protein chains degraded, giving off reduced sulfurgas. Wool was banned from use, but it was not possible to ban the use of wood as at thattime all of the showcases in the Museum, including new showcases, were constructed of

wood.Since its publication in 1973, the Oddy test has undergone modifications (Oddy 1975,Blackshaw and Daniels 1979). In 1992 a major review of the methodology was carriedout by two of the Museum scientists. They conducted an interlaboratory comparison ofthe testing and found that there was a great variation in test results obtained ( Green andThickett 1993). As a result of this study, a revised methodology was developed with astep-by-step guide to carrying out the tests (Green and Thickett 1995). A number ofspots tests or quick tests were introduced to cope with the many occasions when therewas not enough time to carry out the corrosion test (Daniels and Ward 1982; Zang et al.1994). All of the test methods were brought together in a booklet with an introductory

chapter on why testing is necessary (Lee and Thickett 1996; Thickett and Lee 2004).

3.1.2 The 3-in-1 Test

Carrying out large numbers of accelerated corrosion tests is time-consuming, andfollowing the publication of a test methodology where one test contained all three metalcoupons (Bamberger et al. 1999), a 3-in-1 test was developed for use in the BritishMuseum (Robinet and Thickett 2003). The test methodology devised by Bamberger etal. was not used after finding that the method of deploying the coupons by bendingthem over the edge of a beaker resulted in contact with water condensation, inducingcorrosion that would not normally be observed. The method set out by Robinet utilized


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a reliable supply of disposable silicone stoppers which fitted the quickfit tubes used forthe accelerated corrosion test. The coupons were inserted into slits in the stoppers andthe rest of the test set up was as before. For a six-month evaluation period the 3-in1 testand the normal test were run on every material which came in for testing. Currently the3-in-1 test method is used for the routine testing of materials for use in storage or

display of the collection. More complex methods for evaluating materials have beensuggested and one has been published (Reedy et al. 1998). This method requiresequipment, expertise, and time that are not available in most museum sciencelaboratories.

The Oddy test and 3-in-1 test are pass/fail tests which can be carried out by a scientist orconservator who is trained in the test procedure and has a good understanding oflaboratory practice. These tests provide a simple way of ensuring that the risk to objectsfrom indoor pollutant gases given off by materials used in storage and display isminimized.

3.2 CARBOXYLIC ACIDS AND ALDEHYDES

Indoor pollutant gases are those which are given off by the materials used inside abuilding. The gases of concern in conservation are those which react on object surfacesto form corrosion. The lower carboxylic acids, up to C4, were identified in emissionsfrom a range of hardwoods, softwoods, and wood products by gas chromatography(GC) headspace analysis and shown to cause corrosion of lead (Blackshaw and Daniels1978). In practice the main gases of concern are acetic and formic acid, possibly

formaldehyde and acetaldehyde, and the reduced sulfur gases, hydrogen sulfide andcarbonyl sulfide.

3.2.1 Acetic and Formic Acids

For manufactured wood products, the most important indoor pollutant is formaldehyde.When composite wood products such as particle boards and Medium Density Fiber

board (MDF) were developed, they were quickly taken up by the furniture industrybecause of their low cost and easy working properties. However, these products give off

large quantities of formaldehyde derived from the adhesives used in their manufacture.Formaldehyde was found to affect the health of people and was eventually identified asa carcinogen. Because of this, a passive test for measurement of formaldehyde wasdeveloped and was used in several museums including the British Museum; at the timethere was no easy quantitative test for acetic and formic acid.

Use of the formaldehyde test put undue emphasis on formaldehyde as a main source ofcorrosion of metals, although laboratory tests in the Museum showed that atconcentrations of 0.5 and 5 ppm it did not corrode lead or copper test pieces at 50% RHand temperatures of 15, 25, and 35C. Slight corrosion of lead test coupons occurredusing the same experimental set-up at 100% RH. This suggested that at higher relative

humidity, conversion of formaldehyde to formic acid occurs (Thickett et al. 1998).Other researchers suggested that formaldehyde was a more serious problem (Hatchfield


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and Carpenter 1987). Researchers at the University of East Anglia showed thatformaldehyde could be oxidized to formic acid at ambient temperature. However theexperimental levels of oxidant were high compared to what normally would be expectedin the air (Raychaudhuri and Brimblecombe 2000). Another mechanism suggested isthat of the high-temperature Cannizaro reaction (Schmidt 1992), but this seems unlikely

to occur at ambient temperature.

The wood industry changed formulations and lowor zero-formaldehyde MDF emerged.These products still gave off copious acetic and formic acid and failed the acceleratedcorrosion test. For more than thirty years in which the accelerated corrosion test has

been in use, most of the woods and wood products tested have corroded the lead testcoupon. Even ancient wood can corrode lead. An eighthcentury BC lead figurine (1880-12-16-46) formed from tiny lead and ivory squares had a wood core which was found to

be the source of the regular corrosion of the lead (Duncan 1986a).

In the British Museum the corrosion product identified most frequently on lead objects

by X-ray diffraction (XRD) is hydrocerrusite, PbCO3. Pb(OH)2. The corrosion of leadto basic lead carbonate has been described as a two-stage process via an intermediate,lead acetate, which reacts with water and carbon dioxide in the air to form basic leadcarbonate. In the last few years, more findings have been made of acetateand formate-containing corrosion products on lead and other metals, and of mixed salts on porousstone, ceramics, and glass. These occur where composite wood products or untested

paints are in use. Recent improvements in analysis by ion chromatography (IC) in theMuseum have identified the presence of carbonates in salt mixtures which would

previously have been identified only as formate and/or acetate. Measurement of theacetic and formic acid levels using the diffusion tube method described by Gibson et al.(1997a) have shown acetic acid to always be present at a higher concentration thanformic acid. The prevalence of formatecontaining corrosion and salts suggests thateither formic acid is more reactive than acetic acid at some object surfaces, or oxidationof formaldehyde to formic acid is occurring.

3.2.2 Conditions for the Formation of Mixed Salts

From a number of incidents of rapid formation of mixed corrosion products and mixedsalts on objects, it was suspected that although acetic and formic acid may have

adsorbed onto the surface of some objects, the formation of corrosion and efflorescencewas dependent on high temperature and high relative humidity, either promoting out-gassing from wood, or promoting the reaction of the adsorbed gases on object surfaces.The formation of acetateand formate-containing corrosion and salts has beeninvestigated on a range of materials including Egyptian copper alloys, limestone,marble, glass, and enamels. On Egyptian bronzes a previously uncharacterizedcompound, sodium copper carbonate ethanoate (acetate) was identified andcharacterized (Bradley and Thickett 1999; Thickett and Odhlya 2000).

On an Egyptian limestone stela (EA1332) a salt efflorescence containing methanoate(formate), nitrate, and chloride in the ratio 3:2:1 was identified. This is likely to be the

corrosion product characterized by Gibson et al. (1997b). During conservation the stelahad been poulticed to remove salts and was returned to its oak storage box before fully


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drying. It is likely that the moisture promoted a reaction between acetic acid from thewood and the soluble salts in the stone forming the mixed salt. The soluble salt profilesthrough the thickness of this stela and one which had not been treated with water anddid not have salt efflorescence present were compared. In addition to the expectedchloride and nitrate, acetate and formate ions were present in amounts of 0.010.06%

w/w throughout both stele (Bradley and Thickett 1999). This shows that gases given offby the oak storage boxes had been adsorbed not only on the surface of the porouslimestone but throughout its structure. A white efflorescence had formed on the surfaceof marble relief (MLA OA 10562) following leakage from a water pipe. This wasidentified as a calcium acetate formate hydrate (Thickett 1995), a mixed salt which had

previously been identified on shells (Tennent and Baird 1985). The relief was mountedin a glass-fronted wood box that was a source of acetic and formic acid.

Acetates and formates have been found on the surface of glass and enamels. Many gasesreadily adsorb onto the surface of glass, and it is highly likely that acetic and formicacids were taken up during long periods of storage in wood cupboards or display in

wood showcases.

Like many objects around the world, those in the British Museum have traditionallybeen stored in wood cupboards, resulting in long-term exposure to emissions of acidsand aldehydes, which have been adsorbed onto the surface or even throughout thestructure of many of the objects. This appears to be a substantial problem, but there arenot that many instances of corrosion or salts on objects containing acetate or formate.On investigation, incidents of formation of corrosion or salts containing acetates andformates have involved the presence of water, applied during conservation, leaking ontothe object, or in high relative humidity.

From the analysis of acetic and formic acid levels in showcases and store cupboards, anempirical relationship between high humidity and temperature and the rate of out-gassing from materials has emerged (Bradley 2003b). In the non-air-conditionedgalleries of the Museum there is a seasonal variation in levels of acetic and formic acidand aldehydes, with winter levels considerably lower than summer levels. Since wood isstill in use in showcases, albeit wrapped in a barrier film to reduce out-gassing (Thickett1998), acids and aldehydes are present. However an examination of at-risk objects ondisplay showed that they were not being affected by the gases and lead coupons inshowcases did not corrode (Bradley and Thickett 1999). Even very high levels of aceticacid in cupboards used for the storage of Egyptian copper alloy objects did not corrode

lead coupons. The relative humidity was at or below 45%. In general, the presence ofmoisture is needed for mixed corrosion products or mixed salts to form on objects;when conditions are favorable, formation is rapid. Further work is needed to establish ifthese types of reactions can be eliminated by keeping objects at a low relative humidity.

4 REDUCED SULFUR GASES

The reduced sulfur gases hydrogen sulfide (H2S) and carbonyl sulfide (COS) areimportant because they form silver sulfide on the surface of silver objects causing them

to tarnish, form black sulfides on copper alloy objects, and react with lead pigmentscausing blackening. There is no need for any material which emits these gases to be


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used in the storage or display of collections since they can be easily identified duringthe accelerated corrosion test. However even though only materials which do not tarnishsilver are used in the Museum, reduced sulfur gases are present in the external air atconcentrations of parts per trillion and are present in the unfiltered air in the Museum.Very low concentrations of these gases can cause silver to tarnish. In the British

Museum research into these gases has focused on the tarnishing of silver since this hassuch an impact on conservation effort and time, with many objects requiring regularcleaning. Silver used to be lacquered to reduce the rate of tarnishing, but removinglacquer was difficult and residues have recently been shown to promote tarnishing(Thickett and Hockey 2003). Another approach was to use silver cleaners whichincluded tarnish inhibitors, usually mercaptan based (Wilthew 1981, 1987).

4.1 TESTING TARNISH PREVENTION PREPARATIONS

Between 1975 and 1987 different types of tarnish-prevention preparation were tested foruse in the Museum. The preparations included vapor-phase inhibitors, protective papersand cloth for wrapping or enclosing silver, and absorbent materials. The testing took theform of exposing a cleaned silver coupon and the material under test in a closedcontainer with a source of hydrogen sulfide. Other researchers used either artificiallyhigh levels of hydrogen sulfide or carbonyl sulfide (Franey et al. 1985). Theeffectiveness of the preparations was determined by the number of days taken fortarnish to be visible on the silver coupons. In these tests all of the preparations retardedthe formation of tarnish when compared with the time taken for the control coupons totarnish. However the most effective preparations were 3M silver protector strip,Tarnprufe cloth, four ICI catalysts, of which 75-1 was most effective and CharcoalCloth (Bradley 1983; 1985; 1989; Duncan 1986b; 1987). As 3M protector strip wasdesigned to be used in smaller closed storage boxes and Tarnprufe cloth, whichcontained zinc acetate as a sulfide scavenger, was designed to enclose silver, neitherwas suitable for use in the display of silver objects. ICI catalyst 75-1, a highly porouszinc oxide (wurzite) bonded with cement, designed to remove hydrogen sulfide from

North Sea gas, and Charcoal Cloth were the most promising preparations for use inshowcases to reduce or remove reduced sulfur gases.

In 1993 a more rigorous investigation of silver tarnishing to quantify the efficiency ofthe tarnish inhibitors and identify the nature of silver tarnish in the Museum began (Lee

1996). The high hydrogen sulfide concentrations used in previous experiments formed atarnish layer on silver was very much easier to remove than that formed naturally in theMuseum galleries and in the laboratory air. As a result it was decided to useconcentrations of reduced sulfur gases as near to ambient levels as possible. There was a

precedent for conducting corrosion experiments with silver at naturally occurring levelsof hydrogen


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Table .

Laboratory Evaluation of Tarnish Inhibitors. Results of XPS Analysis of Silver

Coupons Expressed as Atomic Weight Percent. sulfide (Pope et al. 1968). Laboratory air was selected as the most readily availablesource of low level concentrations of reduced sulfur gases and had the advantage of

providing a natural mix of hydrogen sulfide and carbonyl sulfide for the experiments.

A small aquarium pump was used to pump air through a series of 2L capacity

containers. Two holes were drilled into the screw tops of the containers, one for the inlettubing and one for the air outlet. The containers were connected by rigid Teflon tubingand polythene T-connectors. The arrangement ensured that the pathway to each of thecontainers was exactly the same length, volume and had the same number of joins andthat each container received an equal air flow. Eleven materials were evaluated in thisexperiment but only the results on the two most effective ICI catalysts, now namedPuraspec 5040 and 2040, and on Charcoal Cloth are reported here (Table 1).

The materials were placed in individual containers with the cloth cut to cover the baseand the Puraspec pellets added to form a monolayer. Analar silver coupons werecleaned by abrasion and degreased in high purity acetone before being pierced and

weighed. One coupon was then suspended from the screw top of each container, all atthe same height. Controls were set up with a silver coupon suspended in a containeralone and one suspended in the area of the apparatus in the laboratory.

The experiment was started in July 1994 and completed in April 1995. When theexperiment was stopped the control in the laboratory was a dark yellow brown color andthat in the container was distinctly yellowed. The silver coupons exposed with theCharcoal Cloth and Puraspec 5040 and 2040 showed no visible change, but at 60xmagnification, black spots could be seen.


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4.1.1 Analysis of Silver Test Coupons

Prior to analysis the coupons were individually stored in bags made from a film withlow permeability to gases along with sachets of Ageless oxygen scavenger to preventfurther change. The coupons were analyzed by X-ray photoelectron spectroscopy (XPS)

with a Kratos XSAM series 800 using Mg and Al K radiation at 300 W using 7.5mmslits and 80/40 eV pass energies. The results in Table 1 (Johnson Matthey TechnologyCentre 1995) show that the silver tarnish contained oxides, chlorides, and sulfates inaddition to the expected sulfide. On the control coupons, sulfide was present at lowerconcentrations than oxide and sulfate. Despite efforts to minimize handling of thesamples the presence of sodium suggested handling contamination on some of thecoupons and an adjustment was made to the chloride measurements to reflect this. Highlevels of carbon were present on all of the coupons, particularly those from the Puraspec2040 and Charcoal Cloth containers. These coupons were kept in the protective bags forsix months before they were analyzed and this was most likely the source of thecontamination. In this experiment, where the materials were used in a passive mode,

Charcoal Cloth performed as effectively as the Puraspec 2040 and 5040 in reducing therate at which silver tarnished.

4.2 REDUCED SULFUR GAS MONITORING

As part of a broader pollutant gas monitoring campaign in the Museum, hydrogensulfide and carbonyl sulfide levels were monitored using diffusion tubes supplied by S.Watts of Oxford Brookes University, Oxford, UK. The tubes and silver coupons were

deployed in Gallery 70, Rome City and Empire, in the gallery, in two showcasescontaining silver objects, in a control case containing only glass, and outside of theMuseum building for 28 days. The showcases were different sizes but were constructedto the same specification, and the case inserts, dressing fabrics and label materials had

been tested to ensure that they did not out-gas reduced sulfide gases. The analysis ofreduced sulfur gases was interesting, as carbonyl sulfide was present in greaterconcentration than hydrogen sulfide at all of the locations monitored. The levels insideshowcases were almost as high as the levels in the gallery and externally (Table 2).Only the coupon exposed externally was visibly tarnished, particularly at the edges.Taking the measurement


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Table .

Carbonyl sulfide and hydrogen sulfide gas concentrations measured between 31

January and 28 February 2001

errors into account, this location had the highest hydrogen sulfide and carbonyl sulfidelevels.

These coupons were analyzed by XPS and Static Secondary Ion Mass Spectrometry(SSIMS). The SSIMS analysis was carried out using a Millbrook Chemical Microscopein large area (2.25 mm2), static mode for both negative and positive ions. The primary

beam is provided by a raster scanned gallium liquid metal ion gun with low energyoptics for secondary ion extraction into a 300 Da quadropole mass spectrometer.Although both methods are surface analysis techniques the information they provided isdifferent. In XPS the elements present are separated according to their binding energy(eV) and hence sulfur as sulfide and as sulfate are discriminated. The analysis isquantified as atomic concentration percent. SSIMS is a non-quantitative technique andthe output is a mass spectrum detecting fragments as well as ions. Sodium, chlorine,oxygen, zinc, sulfur as sulfide and as sulfate, silicon, and carbon were detected on thesurface of the silver coupons.

4.3 THE NATURE OF SILVER TARNISH

In this program of monitoring, the nature of silver tarnish in the British Museum wasconfirmed as a mixture of oxide, chloride, sulfate, and sulfide. Carbon was again amajor contaminant on the surface of the coupons with fragments of up to C6 in chain

length present as determined by SSIMS. In this work, handling of the coupons wasdramatically reduced by mounting them on aluminum stubs for presentation in the


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SSIMS equipment and storage in aluminum containers prior to exposure and analysis.Hence carbon species in the atmosphere were indicated as the primary source of thiscontamination (Hallett et al. 2003). Carbonyl sulfide was shown to be the predominantreduced sulfur gas in the air and not hydrogen sulfide; hence, the strategy of basing atarnish prevention system on a material designed to remove hydrogen sulfide was

flawed.

4.4 GALLERY TRIALS

Meanwhile the evaluation of the passive deployment of Puraspec 5040 was continued ingallery trials. The pellets were deployed inside a large Lshaped showcase displayingsilver objects and were visible. Silver coupons were put into this case and

Table .

Gallery trial of passive deployment of Puraspec 5040. XPS analysis of silver

coupons expressed as ratios of element to silver into a rectangular case with silver objects and no Puraspec. Silver coupons were alsodeployed in two control locations, the gallery, and a showcase containing glass objectsalone which would not have been a sink for reduced sulfur gases.

The coupons in the control locations tarnished after 42 days. All of the coupons were

then analyzed by XPS. The results are presented in Table 3 as ratio atomic percent.There was more sulfur present on the silver coupon exposed in the showcase containingglass objects than that containing silver objects and no Puraspec. Hence the trial showedthat silver objects are likely to tarnish at a slower rate in showcases containing othersilver objects. This is presumably because the silver objects act as a sink for reducedsulfur and other gases. The Puraspec 5040 in passive mode did not reduce the rate oftarnishing. The results of this experiment were later found to have been affected by thehigh air exchange rate of the complex L-shaped showcase, more than 8 air changes perday.

A trial of a positive pressure system was undertaken replacing Puraspec 5040 with

Puraspec 2030 which was suggested by ICI Katalco. This product is also formed fromzinc oxide and contains aluminum oxide which oxidizes carbonyl sulfide to hydrogen


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sulfide which is in turn absorbed by the zinc oxide, and a copper compound which actsas an indicator, turning from green to black when it takes up sulfides. A simple filter

bed was prepared in a tubular container using Puraspec 2030 and activated carbon cloth.It was decided to incorporate the cloth since it absorbs a broad range of pollutant gasesand pumping air into a case could lead to a build up of pollutants at the surface of the

objects. The filter bed was connected through the base of a showcase displaying onlysilver by plastic tubing and a small pump. Silver coupons were deployed to monitor therate of tarnish formation and these were visually tarnished after 60 days. Again theywere analyzed by XPS which showed that the pump had been effective at reducingsulfide formation (Table 4).

On the basis of this research, current thought is to use positive pressure systems in allshowcases where silver objects are displayed.

5 CONCLUSIONS

Greater understanding of the interaction of the museum environment with the collectionhas been achieved through research by the scientists in the British Museum. Increase inknowledge has been incremental, with improvements in scientific technology providingthe capability to extend the work into levels of detail unimaginable in 1970. So thatactual object needs are met in a cost-effective way, Museum scientists conductedobject-based research designed to inform the preventive conservation strategy (Bradley1996), rather than advocating wholesale implementation of conditions taken from theliterature. Full air-conditioning of the building is not appropriate given the historical

significance of the building itself, the range of materials and their levels ofdeterioration; and the mixed object displays which


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Table .

Gallery trial of pump system to deploy of Puraspec 2030. XPS analysis of silver

coupons expressed as ratios of element to silver are used to interpret the collection in themed galleries. However, when large numbers ofobjects are identified as needing environmental control, a global gallery or storage areaapproach is justified.

The research has been utilized both in the British Museum and in the wider museumcommunity informing showcase and storage unit design. It has prompted manufacturersto move away from wood to metal storage units, and glass and metal showcases, and todevelop in-case conditioning systems which are superior to those based on humidifiersand dehumidifiers which we used for 25 years. Design of new galleries and storagefacilities incorporates conservation requirements such as air cooling and filtration which

are justified by the research. Most importantly, in new galleries and new storage areas,the objects are displayed and stored in safe conditions that are a vast improvement overthose which existed in 1970 when this research was begun.

ACKNOWLEDGEMENTS

I would like to thank all the members of the Conservation Science team who haveworked over the years to develop our understanding of deterioration and the Museumenvironment. In particular Lorna Lee (ne Green) and David Thickett have made a

substantial contribution to this research. I would like to acknowledge the help of theconservators, curators, museum assistants, designers, particularly Geoff Pickup and

buildings staff who point out incidents of objects changing, help with measurements ingalleries and stores, and implement solutions. I would also like to thank SheridanBowman for reading the manuscript and suggesting improvements.


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AUTHOR INFORMATION

SUSAN BRADLEY, Head of Conservation Science and Analytical Chemistry in TheBritish Museum Department of Conservation, Documentation and Science, has a degree


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in chemistry from the University of London. In 1972 she joined the Museum to work onconservation problems, researching conservation methods for waterlogged wood andleather, the deterioration and conservation of stone, metals, ceramics, and glass, thestorage of collections, and the museum environment. In 1988 she became Head ofConservation Research Group, leading a small team of scientists on object-centred

research. Currently her main interests are the deterioration of glass and enamel and themuseum environment. Address: Department of Conservation, Documentation andScience, The British Museum, Great Russell St., London WC1B 3DG, UK

Bradley, S. Preventive Conservation Research British Museum. 2005

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