conservation strategies for damp buildings and plaster: lacock abbey in wiltshire

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Conservation Strategies forDamp Buildings and Plaster:Lacock Abbey in WiltshireKaty Lithgow & John StewartPublished online: 16 Jan 2014.

To cite this article: Katy Lithgow & John Stewart (2001) Conservation Strategiesfor Damp Buildings and Plaster: Lacock Abbey in Wiltshire, Journal of ArchitecturalConservation, 7:2, 7-26, DOI: 10.1080/13556207.2001.10785291

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Conservation Strategies for Damp Buildings and Plaster: Lacock Abbey in Wiltshire

KA TY LITHGOW AND JOHN STEWART

Abstract

The transformation of the thirteenth-century nunnery of Lacock into a country residence at the time of the Dissolution has resulted in a complex structure, now with serious damp problems affecting the historic fabric. As owner of the Abbey, The National Trust has initiated an integrated conservation strategy to address the causes of decay. Surveys were undertaken to confirm the principal source of moisture and its distribution, and the types of soluble salts and microbiology present. A fuller understanding of the building has been afforded by monitoring the environment, groundwater variations and thermal transmission, and undertaking in situ quantification of microbiological growth through ultraviolet and infrared photography, and induced fluorescence, before and after irradiation with ultraviolet light. Urgent remedial conservation of fragile medieval wall plaster and limewashes has been carried out using limebased techniques, with a cellulose ether (Tylose) as an adhesive and a silica colloid (Syton) as a plaster consolidant for limited areas. Future work will entail comprehensive assessment of site drainage and necessary improvements.

Introduction

Lacock Abbey is renowned as the birthplace of W.H. Fox Talbot (1800-77), for his pioneering work on photography, and as the site of the quintessential English village. As suggested by its name, the house also incorporates the remains of a medieval abbey, founded in 1232 by Ela, Countess of Salisbury (1186-1271) as a nunnery for Augustinian canonesses. Despite suppression in 1539, purchase by Sir William Sharington in 1540, who remodelled the abbey as his private residence, 'Gothick' alterations in the eighteenth century, and repairs and

Journal of Architectural Conservation No 2 July 2001 7

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Katy Uthgow and]ohn Stewart

Figure 1 Plan of the monastic rooms discussed in this paper, with the results of the geophysical survey of buried archaeological features (resistance data) to the east of the complex. (Geophysical Surveys of Bradford)

restorations in the nineteenth and early twentieth centuries, the Abbey's cloister walk (see Frontispiece} and surrounding monastic rooms have survived, along with much of their original plaster and decoration (Figure 1) .1

After acquisition by The National Trust in 1944,2 building repairs were carried out with financial support and advice from the then Ministry of Works.3 However, the connection between the 'damp and peeling' state of the sacristy's plaster and the cloister's drainage was not made until 1978, with the assessment of the cloister as 'a giant sponge', collecting all the roof water from the surrounding buildings.4

8 Journal of Architectural Conservation No 2 July 2001

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Conservators from The National Trust and English Heritage recommended that work to the plaster awaited resolution of the damp problems, but repairs to roofs, masonry, glazing and fire-detection systems took precedence. In 1992, concerted investigations began into the state of the cloister and the causes ofits deterioration. By 1995, however, rapid deterioration of the sacristy's plaster meant treatment was necessary to prevent further loss, even though the drainage problems were not yet resolved. Conservation of the plaster in the chaplain's room was made necessary by modifications to the heating system, whose pipes run through the room.

Project organization

This lengthy and multidisciplinary project involves an extensive project team. The National Trust is divided into regions, which support the staff running the properties. The regions and their properties receive specialist assistance from advisers based at Head Office. The project is managed by the Region's Area Building Surveyor, who liaises with regional and property staff responsible for the conservation, curation and management of the property. The project is administered by the consultant architect, who is supported by the mechanical and electrical engineer in work on the heating system. A consultant archaeologist monitors all work, overseen by the Regional Archaeologist. Finally, there is close liaison with English Heritage, their grants officers and, in particular, the area architect, who attends annual project review meetings. This provides an outside perspective and continuity, as well as essential grant aid.

The project team commissions investigations and considers their results and recommendations, which are turned into specifications for treatment by the architect and the relevant adviser. The specifications are then put out to competitive tender, as required by both English Heritage and National Trust auditing procedures.

Conservation strategies

The monastic rooms of the Abbey present a classic case of endemic dampness. The most significant features affected are the extensive remains of medieval plaster and wall paintings and, to a lesser extent, decorative masonry. These, and later limewash layers and their graffiti, are an important part of the archaeological record, and play a major part in the cloisters' 'spirit of place', which guides the management of the property. Effective preventive and remedial conservation of these


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architectural surfaces first requires a sound understanding of the build, ing, and of the dynamic environment of which they are part .

.Accordingly, there were three investigative components in this project:

• recording the surfaces and their physical context • determination of the condition of the surfaces and manifestations of

decay • monitoring the environment within which the processes of decay

operate.

The ultimate objective is, of course, to remove or mitigate the causes of decay in order to extend the life of the historical fabric. 5

Recording the site and building

In order to confirm and understand the sources of dampness in the build, ing, it was essential to characterize the geology and hydrology of the site, or the dynamics of the local ground water. This water has been analyzed, and soil stratigraphy has been identified from extracted cores. The most significant feature is a silty clay layer, which begins at a depth of 1.5 metres below ground level. The clay supposedly impedes the natural movement of the water table.

The Abbey itself has undergone a complex structural evolution, through its transformation from a monastic complex to a country residence. To understand the building's pathology, it is necessary to decipher its constituent parts through the review of archival documents and physical examination of the structure. This ongoing process is under, taken by the consultant archaeologist.

Ground levels have also been surveyed to establish the relationship between internal spaces and external topography. This will assist the evaluation of possible routes of moisture ingress into the historic fabric. For example, on the south wall of the sacristy, plaster loss is particularly extensive. This appears to be associated with a high external ground level caused by a flower bed that has its own historic significance, as it includes plantings by W.H. Fox Talbot.

Similarly, it is important to know as much as possible about the unseen structure, buried archaeological features (such as disused drains) or foundations of structures demolished before, at the time of, or since the Dissolution. For this purpose, a geophysical survey was undertaken between the house and River A von (see Figure 1). The survey measured


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electrical resistance within the ground and magnetic fields above it, indicating buried structural features.

The one survey still to be undertaken is a comprehensive review of the location of all drains, to clarify functional and blocked networks at Lacock Abbey. This will include the investigation of the major blocked medieval drain, partly visible at the base of the eastern facade, which connected the Abbey to the river.

Material analyses

In quantifying historical fabric, identification of the various building materials provides the basis for understanding deterioration and informs the eventual specification of repairs. Mortars and plaster, as well as wall painting pigments, are of primary concern at Lacock because of the apparently greater change in their condition compared to other features. These were subjected to materials analysis, and the information was used to inform the condition survey and treatment of the plasters and wall paintings. The results of mortar and salt analysis are discussed more fully below.

Condition surveys

Water, in its various forms, is an agent for different decay processes evident in the monastic rooms; hence a survey of the moisture content of the masonry was carried out. Moisture is defined in two ways- as the total percentage of water content in the masonry, and as the hygroscopic moisture content, or the percentage of moisture absorbed from the air due to the presence of hygroscopic salts in the wall.

In order to determine the moisture profile of a given wall, mortar samples were obtained by drilling and removing pulverized material in incremental depths throughout the height of selected walls (Figure 2, overleaf). Moisture content was determined on site by means of a calcium carbide meter; and hygroscopic moisture content was identified by subsequent laboratory analysis of part of the sample.6

The survey confirmed that groundwater was by far the most important source of liquid moisture: up to 50% of the pore volume of mortar was saturated near the base of walls. Fortunately, hygroscopic moisture is not significant.

The concentration of soluble salts in the masonry was also determined using mortar samples from the moisture survey. Three principal salt cations were identified quantitatively (see Figure 2).7 The coincidence of


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3.1

Katy Lithgow and]ohn Stewart

ClolalllrwaH. Sampling allll3.4 (0.13 mabova vauh)

:II ... ...

6000 MC 1.5% 1.Cl% 1.0%

5000 ...c 1.7% 1.3% 1.4% <1000

3000

2000

1000

0 1-11 11-22

lllotance from exltrior woll•rfll .. (em) MC - moisture cooten1 HMC - hygmscopie moisture content

COIIcxides

•sulphales

•tillatea

Clolalllr wall • Sampling sllll 3.1 (0.9 m from floor)

6000

5000 MC 3.0% 3.2% 4.0% ...c 1.7% 1.3% 1.4%

<1000 ll

3000 ... ... 2000

1000

0 1-11 12·21 22-31

lllotance from exltrior woll .,,. .. (em) MC • moisture content HMC· . content

Figure 2 Section through the cloister west wall, showing moisture and salt sampling points above and below the St Augustine painting. (Louise Bainbridge) The moisture content at the base of the wall (graph for site 3 .I) shows characteristic capillary moisture from the ground, increasing in depth. The sampling site below the roof (graph for site 3.4) is relatively dry, indicating little ingress of moisture from the external wall above the painting. Sulphate salt concentration is high at the base of the wall, but being less soluble than the other salts, it is not encountered in higher areas (a separation process known as 'fractionation').

the type of cations in the wall and salts in local soil indicates that the source of salts throughout the monastic complex is primarily from the ground.

The damp walls and floors within the monastic rooms offer fertile conditions for microbiological growth. To determine the severity of physical or chemical biodeterioration of the host substrates, a microbiological survey was carried out.8 This consisted of sampling in four distinct seasons to identify changes in microbial succession over a one-year period. A complete ecological succession of growths and higher plants was encountered - bacteria, algae, fungi, lichen and liverworts. The latter two are the most damaging.


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Environmental monitoring

The decay mechanisms of soluble salts and microbiological growth are influenced by the dynamics of the ambient environment. National Trust preventive conservation strategy is based on monitoring the local environment, and understanding its role in decay processes, in order to identify and implement the most appropriate improvements for the preservation of objects and buildings.

At Lacock Abbey, conventional monitoring of ambient temperature and relative humidity was undertaken with data loggers in seven locations over a period of one year. Surface temperature was also recorded on the Saint Augustine wall painting in the semi-enclosed cloister walk, to determine the incidences of frost and condensation. This has been followed up with further monitoring of the effects of screening the painting against light. Surface temperature has also been monitored since 1998 on the St Christopher painting in the chaplain's room to evaluate the effect of pipes from the heating system passing through the painting. Video infrared thermography was also employed to detect thermal differentials in the mass of the building fabric and across the St Christopher painting (Figure 3).

Figure 3 The paintings of St Andrew and St Christopher in the chaplain's room, with environmental monitoring probes in place, compared to a thermal image of the heating pipe passing through the wall painting, demonstrating its effect on the surface temperature of the painted plaster. Lighter areas correspond to higher surface temperatures, and show a temperature gradient across 75 em from 9°C to l9°C beside the heating pipe. Comparison with an image taken 30 minutes later shows an overall temperature rise of 2°C.


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Since capillary moisture from the ground was confirmed through these studies as the principal source of dampness in the monastic rooms, the variations in ground water on this complex archaeological site are of primary interest. Four areas are being monitored, in an attempt to understand the relationship between different parts of the structure and hydrological variations. Monitoring consists of recording ground water levels within piezometers, or narrow vertical pipes sunk into the ground after the extraction of a soil core.

Monitoring has determined that the water table is currently within the range of the wall foundations. Another characteristic is that water levels within the cloister court are more stable than on the terrace (near the River Avon), and are always higher than water levels at all of the other sites (Figure 4).

There is obviously a direct relationship between the ground water table and masonry moisture content. This relationship is being recorded by means of ongoing readings of masonry moisture. Selective holes drilled for the moisture content and salt survey have been filled with humidity· sensitive timber dowels in stainless steel fixings, fitted with electrodes. These allow the recording of changes in electrical resistance, which is a function of moisture content in the wall. Readings are relative and affected by salts within the wall.

Piezometer ground water level readings

i •• /---·+·)\·r·-,;c-·-·'(~·.-••-j_.!\,.c"iL ,.,.,./·r~r·--·: ~~=eourtl ..

Figure 4 Piezometer readings of ground water levels outside the sacristy and inside the cloister court. The consistently higher level of ground water within the cloister indicates that tanking of rainwater occurs here. (Paul Thomas)


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Monitoring of masonry moisture content, as well as of ground water levels, is intended to establish a datum of existing conditions. Monitoring will be continued to assess the effectiveness offuture interventions, such as improved drainage.

Wall painting and plaster conservation

Deterioration of the plaster throughout the monastic rooms has been recorded since 1978 and has continued, with layers of plaster detaching and crumbling, salt crystallization and copious microbiological growths. A condition survey commissioned in 1996 helped identify actively deteriorating areas of painted plaster that could not await drainage improvements.9 Conservation methods and materials were therefore required to cope with existing moisture levels. The following discussion focuses only on issues that relate to the causes of deterioration and consequent treatment.

North cloister walk

In 1894, during restoration work by Henry Brakspear for Charles Talbot, W.H. Fox Talbot's son, wall paintings were found in the two walled-up niches at the west end of the north cloister, over the lavatorium. The niches are believed to have been blocked in about 1550. The eastern of the two niches was partially uncovered in 1894 and recorded in a watercolour painting in 1898.10 This painting shows a nun kneeling on a grassy mound, in front of a bishop saint with a tree behind. Described in 1979 as being 'in poor condition, dirty and flaking', its treatment was carried out in 1983, including removal of the remaining overlying masonry on the western side. 11 Conservation revealed an inscription that identified the kneeling nun as Agnes Frary, abbess from 1429 to 1445. She is shown being blessed by the founder of her order, St Augustine. This identification suggests that the painting may be coeval with the rebuilding of the cloister in about 1432 (Figure 5, overleaf).

The technique was originally rich, as suggested by a tantalizing gilded crozier visible above walling in the adjacent niche. The wall painting to which the crozier belongs remains almost entirely covered, as the paint adheres better to the covering masonry than the plaster.

The exposed painting has deteriorated in the century since exposure, and possibly before, with substantial pigment alteration.12 Vermilion (mercuric sulphide) has converted to its blackened form (metacinnabar)


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Figure 5 The St Augustine wall painting in the north cloister walk, seen in 1993 (top), is compared with the watercolour made at the time of discovery, which shows masonry left in situ until1983, and suggests how the colour of the painting has changed since exposure.


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Conservation Strategies for Damp Buildings and Plaster: LAcock Abbey in Wiltshire

only in the area uncovered since 1894, as shown by comparison of the painting with watercolours made at the time of discovery (see Figure 5). Red lead (lead tetroxide) has changed to a black form oflead oxide across the whole paintingY A red glaze found beneath a fleur-de-lis may once have covered the whole background.

The verdigris leaves appear grey, because the overlying glaze is covered with salt crystals (sulphates and carbonates) and has discoloured. The fading of organic red glazes is caused by light, and the patchy darkening of vermilion is caused by high humidity. However, the mechanism of the oxidation oflead pigments is as yet unclear, since it occurs in areas that are both hidden and exposed. High relative humidity (RH), high pH through the presence of soluble salts, or the application of alkaline materials, such as limewashes or lime mortar, and an oxidizing agent, such as biological growths, have been suggested as causes. Indeed, two forms of fungi have been identified on the painting in the microbiological survey. The painting's external, albeit sheltered, location offers little protection against the continued effect of these factors, and provides the added hazards of airborne dirt, including bird droppings.

Trials'have been carried out to improve environmental conditions around the painting. A temporary screen has been erected in front of the niche during the closed season in winter, to determine its effect on RH and temperature, and to reduce exposure to light. Monitoring shows that the screen buffers extremes ofRH and temperature slightly, but high RH and freezing temperatures still occur, possibly with condensation. Although the location is visually sensitive, ideal protection would be a permanent glazed cover, easily removable for maintenance, with filters to eliminate ultraviolet (UV) light, enable blackout when the cloisters are closed, and control the microclimate behind. Covering the painting again would allow the decay processes to continue invisibly.

Sacristy

The sacristy retains a great proportion of its thirteenth-century lime plaster, bearing coeval medieval masonry pattern and later medieval decoration, above eighteenth-century dado repair plaster containing hair and gypsum. The repair shows that the capillary rise of moisture must have been a problem by at least the eighteenth century. There is also extensive graffiti, indicating that later limewashes date to the eighteenth and nineteenth centuries.


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Figure 6 Conservator Elizabeth Holford applying a facing over a dangerously detached area of plaster on the west wall of the sacristy.

By 1992, the plaster was covered with fungi, algae and lichens, and was detaching in curtains from the walls by up to six inches, as well as actively falling off (Figure 6). Loss and deterioration appears to be associated with the crystallization of salts within the plaster, and loss oflime binding the plaster, both associated with the movement of the moisture within the walls. In 1996, the condition of the plaster was documented graphically on photographic colour prints. 14 A specification was based on this survey, to provide accurate and consistent information to which conservators could tender. The tenders were evaluated by a panel according to a weighting process that considered experience and approach, as well as price. Treatment, which was grant aided by English Heritage, was carried out to the most unstable areas in 1996 and 1998,15


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Treatment of microbiological growths In addition to the normal conservation criteria of stability, reversibility and/or retreatability, and physical and chemical compatibility, the materials used had to be porous and capable of coping with damp conditions, as well as being appropriate to the archaeological presentation of the rest of the cloisters.

Fungi and algae had to be killed, to enable their removal and prevent contamination of conservation materials. Of the means available, removing the source of moisture was impossible in the time available, and chemical biocides risked introducing significant quantities of more soluble salts. Irradiation with ultraviolet light was chosen because it is less invasive, and was feasible as light-sensitive pigments and media were not present.16

Exposure to UV -C light for 115 hours at a distance of about 130 em killed the growths, and after ten days they could be brushed off. Finally, the wall was sterilized with a 50:50 mixture of water and alcohol.

Monitoring As UV irradiation has no residual effect, the conservators have monitored treated areas to determine the effectiveness of treatment and the rate of regrowth by photography under UV and infrared (IR) light. Live organisms that are invisible to the naked eye fluoresce when illuminated by UV light at around 340 nm and can be photographed on fast daylight film. They are also visible when illuminated under IR light and photographed through filters on false-colour infrared film. Photographed initially every 18 months, the limited regrowth has prolonged the monitoring interval to three years.

Coincidentally, The National Trust had offered this site for testing a fluorimeter, a device for monitoring microbiological growth.17 This handheld instrument detects fluorescence of the algae under specific, and therefore characteristic, wavelengths of UV light. Each cell produces a signal, which is collected digitally and plotted according to location and density, allowing comparison of condition over time. Initial trials confirmed that the rate of regrowth over 18 months, invisible to the naked eye, was low. However, there is as yet no benchmark that states how much growth is damaging.


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Katy Lithgow and]ohn Stewart

Re~adhering plaster To re~adhere the plaster, a temporary facing ofj apanese tissue was applied with an adhesive; this was needed to allow the plaster to be touched without provoking collapse (see Figure 6). Where a mixture of water and industrial methylated spirits (IMS) was insufficient to adhere the tissue, an organic adhesive was needed that was both effective and able to be removed completely from the plaster surface, since any organic adhesive can support microbiological growths. Effective removal of the adhesive is, however, a result of the physical methods and dexterity employed by the conservator, as well as the result of any chemical affin~ ity of the materials with a painted surface, especially on the uneven and porous surface of limewashed plaster. Therefore, empirical testing was used to select an adhesive. After test cleaning, the area treated with a 3% solution in water and IMS of a non~ionic cellulose ether, Tylose MH300, was found to show the least fluorescence of residues when photographed under UV light. 18 After more than four years, no increase in mould is evident on areas treated with Tylose.

Re~adhesion of flaking limewash Flaking paint has to be re~adhered after applying a facing, to prevent it from coming off when the facing is removed. Diluted lime could generally be injected behind flakes through the facing, but this mater~ ial was not effective on thin paint, which again required an organic adhesive. After testing a range of acrylic resins, the 3% solution ofTylose MH300 was again chosen for its performance and ability to retain surface porosity, as well as being compatible with the grouting process. Being water soluble, it may eventually lose adhesion under high RH, but it can be retreated.

Grouting After this preparation, detached plaster was re~attached by grouting, injecting an adhesive material through the facing into voids. The grout consisted of: non~ hydraulic lime putty diluted 50% with limewater; sand, marble dust or brick dust to prevent cracking; and about 5% trass. Trass is a German volcanic material containing silica and alumina that reacts with non~hydraulic lime allowing it to set behind plaster without exposure to atmospheric carbon dioxide. A grout based on lime will still allow moisture to move freely through the wall. The water~based grout and


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water-soluble facing softened the paint and plaster layers, allowing them to be manipulated back into position. Pressure was then applied over grouted areas to keep the plaster in place while the grout set, and flakes were pressed back with a Melinex-covered pad of cotton wool.

Preconsolidation with Syton W30 A few areas of extremely powdery plaster were considered impossible to grout without preconsolidation. It was feared that synthetic adhesives traditionally used for this would reduce the porosity of the damp wall, and result in further detachment through salt crystallization at the interfaces created by the adhesive. The conservators proposed Syton as a preconsolidant. This is a colloidal dispersion of silica in water, analogous to the silica content of stone and plaster (i.e. sand and clay) .19 Held in suspension by a tiny amount of sodium hydroxide, the particle size controls its ability to penetrate. Silica is deposited in the plaster pores through evaporation of the water vehicle, forming a porous and chemically stable network around particles. Developed for industry as a binder and surface coating, Syton has been tested primarily on the Continent as a binder for conservation mortars and grouts, and as a consolidant for stone, mortar and plaster.20

In the United Kingdom, most conservators depend on academic institutions and museums for the scientific testing of materials. Unfortunately, in this instance, none of the institutions approached were able to test Syton. Empirical tests were therefore developed on exposed areas of the base coat, applying different dilutions and particle sizes, and testing the adhesion of fallen render re-attached with the lime grout on consolidated and unconsolidated plaster. These tests suggested that a 50% dilution in water (or 7% solids content) ofSyton W30 was effective (though even greater dilutions can be used), retaining porosity, whilst avoiding any visual change. This version has a larger particle size, but contains less salt than other grades. It was applied by injection to the void, as its particle size is considered too great to enable application through a painted surface without depositing a disfiguring film.

Because it is a new material with unknown ageing properties, the application ofSyton was limited to areas where preconsolidation was deemed essential for grouting to work. Documentation and monitoring of these areas will allow long-term evaluation of Syton's performance. To date (six and three years after treatment), no undesirable effects are evident.


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Presentation The edges of medieval plaster, exposed through loss within the render, were supported by thin fillets oflime and sand mortar coloured with sands to match the base coat, leaving the base plaster visible. More graffiti and medieval line decoration were revealed by cleaning associated with the removal of the facing and the microbiological growths.

Chaplain's room

The chaplain's room, so-named because it was used by chaplains attached to the Abbey, is located at the west end of the south cloister walk. Although the walls have been damaged by later partitions and cupboards (now removed), and by heating pipes driven through in the nineteenth century, around 70% of the medieval plaster survives. There are wall paintings of St Christopher carrying the Holy Child (c.1275) and the Crucifixion of St Andrew (fifteenth century) on the north wall, and the remains of a crucifixion on the east wall (medieval, but undated). The vaults retain most of their medieval plaster outlined in red, and the ribs are decorated with single red masonry lines. There are also medieval graffiti on the north wall depicting hares, nuns and grotesques, which probably postdate the thirteenth-century work. 21

Impact of nineteenth-century heating system While the first phase of conservation was being carried out in the sacristy, pieces of plaster fell off the wall painting ofSt Andrew.lt transpired that this happened annually, and the pieces were carefully collected and stored by the property staff. It became apparent that this damage coincided with turning on the domestic quarters' central heating system, whose pipes passed directly above the wall painting.

Thermal imaging showed that switching on the heating system produced a thermal gradient of l0°C within a radius of approximately 7 5 em of the heating pipe, reaching up to 22 oc beside the heating pipe (see Figure 3).22 It appears that the sudden temperature changes, combined with the different coefficients of thermal expansion in the materials of the wall, weakened areas of plaster, which eventually fell off when the system was turned on.

Following an emergency repair, detached plaster was re-adhered and some fragments replaced in 1996, again with lime-based materials, using similar techniques to those employed in the sacristyY Although the pipe


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passing through the wall paintings is no longer used (to the benefit of the paintings), it is currently retained, as the central heating system is early and therefore historically important.

Environmental monitoring Modification of parts of the central heating system to humidistatic control, for the benefit of showroom contents, means that flow temperatures are reduced, pipe insulation improved, and the pipe passing through the St Christopher wall painting decommissioned. 24

Background heat from the pipes will therefore decrease, and consequently humidity levels and mould growth may increase. A humidistatically controlled electric radiator is now used within the chaplain's room during the closed season and shoulder months to maintain the status quo. The condition of the wall paintings, their surface temperature and the environment of the room are being monitored to assess the impact of these changes.

Stabilization of plaster Work to the old pipes around the chaplain's room was an obvious risk to the medieval plaster due to their close proximity. Treatment was therefore undertaken to put the plaster in the best condition to withstand this and the impact of environmental changes, with a conservator in attendance during work to the pipes. Again, a condition survey to identify detaching and vulnerable areas formed the basis for a specification. The objectives, methods and materials used are similar to those used in the sacristy, that is, to stabilize the plaster with materials that can tolerate moisture movement pending improvements to drainage. Treatment was preceded by UV radiation and removal of algae in the northeastern window recess. In addition, the visual impact of old and stable, but disfiguring, repairs was reduced by refinishing and limewashing.

Conclusion

The treatment of the painted plaster has so far been dictated by its rapid deterioration and the impact of repairs to the heating system. There is a great deal of conservation work still to be done to deteriorating plaster elsewhere in the sacristy and throughout the cloisters, as well as to

deteriorating stone carving, painted bosses, and original lime-ash and cobbled flooring.


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Katy Uthgow and John Stewart

The timing of such treatments must depend on the surveys of the medieval and later drains, and remedial work carried out to them, to reduce the moisture content of the ground and fabric, and therefore the microbiological growths. Even so, treatments will still have to cope with the high and fluctuating moisture levels, salts, microbiological growths and soiling that exist in what is, in effect, an enclosed archaeological site, as opposed to an inhabited building. The work undertaken has included some innovative materials to cope with these conditions, and it is hoped that this project will provide a stimulus for further research into them. Environmental monitoring will continue as changes to the heating system and drainage take place. However, investment in drainage work should be a more cost-effective form of preventive conservation than the repeated treatment that will be inevitable without it.

Biography Katy Uthgow BA(Hons) (Cantab), MA(Cantab), DipCons, CAPT, AMUKIC After studying history of art at Cambridge University, Katy Lithgow trained in the conservation of wall paintings at the Courtauld Institute of Art, where she worked for two years, before joining TheN ational Trust as an Area Housekeeper in 1991. Since 1995, she has worked as Adviser for Wall Painting Conservation and assists the Housekeeper in preventive conservation.

John Stewart BA, MSc, AMUKIC John Stewart is an architectural conservator and historian, who has worked on projects in North America, Europe and the Near East. He was formerly senior conservator at the British Museum prior to joining The National Trust, where he is now Adviser on the Conservation of Archaeological Sites and Monuments.

Acknowledgements The authors wish to thank the following for their contributions to this project and help in preparing this paper: National Trust regional staff Mary Greenacre, Martin Papworth and Tim Cambourne; historic buildings consultant Paul Thomas; archaeologist Jane Harcourt; Dr Graham Morgan of the University of Leicester; Dr Rachel Wakefield of Robert Gordon University, Aberdeen; Louise Bainbridge of Seymour and Bainbridge Architects; and Dr Jagjit Singh of Environmental Building Solutions.


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Notes 1 The National Trust, Lacock Abbey, Wiltshire, Geophysical Surveys of Bradford

National Trust, London (1999). 2 The National Trust is the largest conservation charity in Europe. It was founded in

1895 to preserve places of historic interest or natural beauty in England, Wales and Northern Ireland.

3 Arnold Root, unpublished text of a talk prepared for The National Trust's Conservation Conference 2000. The Ministry ofWorks was the predecessor of English Heritage, the Government's lead body for the care of historic buildings, landscapes and archaeological sites in England.

4 Root, op. cit. (2000) citing a report made in 1978 by David Sumpster, Historic Buildings Architect for the Department of the Environment.

5 The National Trust's conservation policy is described in Historic Buildings. The Conservation of their Fixtures, Fittings, Decorations and Contents, National Trust Policy Papers, London (1996).

6 Following Building Research Establishment, Rising Damp in Walls: Diagnosis and Treatment, Building Research Establishment Digest 245, BRE, Watford (1981).

7 Salt analysis was carried out by Dr Graham Morgan, School of Archaeological Studies, University of Leicester.

8 Microbiological survey by Dr Jagjit Singh, formerly of Oscar Faber. 9 The condition survey was carried out by the wall paintings conservator Tobit Curteis.

10 The watercolour is signed Louisa Chatham Strode and dated May 1898. 11 Unpublished reports by E. Clive Rouse (1979) and John Dives (1983). 12 Paint analysis by Catherine Hassall, formerly of University College London. 13 Lead white and red lead can darken by conversion to lead sulphide through the action

of atmospheric sulphur dioxide, but at Lacock X-ray diffraction showed that the conversion is clearly to a lead oxide. Conversion to lead dioxide (plattnerite) is well known on English wall paintings.

14 Condition report by the Cliveden Conservation Workshop. 15 Conservation work in the sacristy and in part of the chaplain's room was carried out

by Christoph Oldenbourg and Elizabeth Holford. This is described in a number of treatment reports held by the region and head office.

16 This technique had been used previously by Christoph Oldenbourg and Madelaine Katkov at Abbey Knockmoy and Clare Island Abbey in Ireland. For a survey of the role of ultraviolet light in microbiological monitoring and control, see Wakefield, R. and Brechet, E., 'On-Site Methods for Detection and Monitoring Microbial Colonisation of Stone Sutfaces', Zeitschrift fUrGeomorphologie, sup. Vol 120, 2000, pp. 115-31.

17 The fluorimeter is being developed by Dr Rachel Wakefield of the Robert Gordon University in Aberdeen, with European collaborators. See Wakefield and Brechet, op. cit. (2000).

18 Tylose MH300 is a methyl hydroxyethyl cellulose, considered resistant to microbiological attack, though it will support mould growth, and is principally used as an adhesive in paper conservation. It is made by Clariant (formerly by Hoechst Chemicals) and has no known associated health hazards.


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19 Syton is manufactured by DuPont (UK Ltd). It has no specific hazards associated with it. The technical information in the text is from the distributors, Marlow Chemicals.

20 The chemistry of silicon-based compounds in conservation is complex. Some products pose significant health and safety risks, and need to be selected and used with great care on historic artefacts to ensure that irreversible and damaging effects do not occur.

21 Rouse, op. cit. (1979) and Tristram, E.W., English Medieval Wall Painting, The Thirteenth Century, Oxford (1950), p. 557.

22 This survey was carried out by South Bank University for Paul Thomas, historic buildings consultant, as part of his environmental monitoring of the cloisters.

23 The emergency repair was carried out by Tobit Curteis in 1993, followed by further treatment by Christoph Oldenbourg in 1996. The rest of the sacristy plaster was conserved by Tom Organ in 2000.

24 For an account of The National Trust's strategy of conservation heating, see:

26

Staniforth, S., Hayes, B. and Bullock, L., 'Appropriate Technologies for Relative Humidity Control for Museum Collections Housed in Historic Buildings', in: Roy, A. and Smith, P. (Eds.), Preventive Conservation: Practice, Theory and Research. Contributions to the 1994 Congress, IIC, London (1994), pp. 123-28.

Journal of Architectural Conservation No 2 July 2001

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conservation strategies for damp buildings and plaster: lacock abbey in wiltshire

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