The Building Conservation Directory 2022

INTER IORS 5 151 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 2 inside and out. In situations where the original fabric is concealed by later lath and plaster or wallpaper coverings, it is not uncommon for wall paintings to be unwittingly destroyed during domestic renovation work. It is also an alarming truth that earthen daubs and plasters are to this day regarded by many people, builders and homeowners alike, as an element of vernacular building which is better stripped out and replaced than retained. For all these reasons, earthen supports remain very much at risk, and our domestic wall painting heritage continues to diminish. CONSERVATION CONSIDERATIONS Failures are often multiple and, by the time conservators become involved, condition can be catastrophic. Treatment requirements are often major and urgent, but at the same time interventions can be exceptionally difficult to implement, often involving considerations which are not encountered in more familiar lime-based wall painting technologies. For example, where grouting is used in a repair, the absorption of water can result in distortion, slumping and surface staining. As light panels of earth and plaster retain water over a longer period than most other substrates, the increased weight means that the risk of collapse during or following treatment is high, especially given the low tensile strength of earthen supports. Repairs applied too wet or using incompatible materials can cause damage to the original fabric as they shrink back on drying, and they often fail entirely. In the long-term, differences between repair materials and the original in density, porosity and water vapour permeability can create a range of additional problems. Risks resulting from the use of incompatible materials are also compounded when earthen supports constitute one element of a delicately balanced and easily destabilised composite structure. Unfortunately, there is little technical/ scientific research on the study of earthen support materials to guide conservators, and treatment using inappropriate materials remains the norm rather than the exception. There remains a pressing need for accessible diagnostic tools and analytical procedures to help develop conservation practice in this area. HOW CAN ANALYSIS HELP? Compatibility is key, but although material for earth construction was often sourced in the vicinity of the site, soil formation processes often result in soils with radically divergent characteristics even within a small geographical area, and it is never safe to assume that all locally obtained earth can be used for making repairs. Unless substantial fallen (unpainted) original earthen material can be recovered at the site and reused, compatibility can only be determined with any degree of accuracy through systematic examination, testing and analysis of both the original (using unpainted material) and potential repair soils. The main types of information conservators need regarding the composition, behaviour and susceptibility to deterioration of earthen supports can be divided into two broad categories: • those which govern the texture and mechanical properties of soil (ratios of sand-, silt- and clay-sized particles, and particle morphology) • those which determine the chemical behaviour of soil and govern its shrink/swell potential and capacity to absorb and adsorb water (chiefly a function of the clay types present). Laboratory testing procedures borrowed from the field of geotechnical engineering and standardised by various international organisations (such as ASTM international, the British Standards Institute and the International Standardization Organization) can provide much of the data required. Standards are available for a large range of testing methods. Some require access to advanced analytical equipment such as laser diffraction particle-size analysers. An early devotional domestic wall painting from Cothay Manor, Somerset. Cracking, bulging, separation and loss of lime plaster from its daub support are common problems. Components of earthen analysis in progress showing sedimentation of the finer fraction, plastic limits testing, higher fraction material separated through sieving, and linear shrinkage tests Analysis of the Cothay Manor daub established that it was closely related to material obtained from the nearby banks of a tributary of the River Tone. Ten per cent of the daub is composed of added sand and gravels to combat shrinkage. Fibres were also added to improve the soil’s properties. Gold room daub 4.33% River bank earth 7.36% COTHAY: % LINEAR SHRINKAGE Diameter (mm) %