The Building Conservation Directory 2022

152 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 C AT H E D R A L C O MM U N I C AT I O N S Others can be carried out using relatively inexpensive, portable equipment and performed, after a little practice, by the conservator. A suite of tests including particle-size analysis (sieving and hydrometer methods) and Atterberg limits testing (chiefly plastic, liquid, and shrinkage limits) are the most useful for conservators. Procedures are described in testing standards and manuals (see table, Earthen materials analysis resources). The data obtained through testing can then be interpreted with reference to soil classification systems to help define the soil’s behavioural parameters, for example soil texture, plasticity, and activity, and to allow comparison of data obtained from earthen supports and potential source material for the development of compatible repairs. Particle-size analysis comprises two stages. First, the sample is crushed lightly and wet sieved through a standard series of analytical sieves to establish the soil texture of the larger fraction. Material passing the finest sieve comprises the silt and the clay components. This is placed in suspension in a hygrometer jar and the silt:clay ratio is determined by measuring the density of the suspension over time. Together, the results establish the soil texture and classification of the sample. Distribution curve plotting and examination of the collected geological material allow comparison of the earthen support sample with local potential source soils and can also give invaluable qualitative and quantitative data on additives. Plastic and liquid limits determine soil behaviour in the presence of water, specifically the amount of water required to transform it from a solid to a plastic state, then from a plastic to a liquid state. The plastic limit is obtained very simply by establishing the moisture content of hand-rolled samples at the point at which they lose cohesion. The liquid limit is obtained using a Casagrande device to establish the moisture content at which samples are unable to retain a solid form (see illustration opposite). These two tests taken together provide data which enables the ‘activity’ and plasticity index of the soil to be determined. These parameters indicate its susceptibility to dimensional change or loss of cohesion in the presence of liquid or atmospheric moisture. Linear shrinkage is a particularly easy and useful test which determines the two-dimensional shrinkage of the support and potential source soils. This is done by placing a sample at around its liquid limit in a standard mould and calculating the per cent shrinkage on drying. Detailed mineralogical information can be obtained by thin section petrography and X-Ray Diffraction (XRD) to allow more precise comparison with potential source material and to better understand the influence of the clay fraction on soil behaviour, but this is not essential. EARTHEN MATERIALS TESTING AND ANALYSIS RESOURCES ASTM D422-63 (2007) e2: Standard Test Method for Particle-Size Analysis of Soils ASTM D4318-17 e1: Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils BS EN ISO 17892-12:2018 Geotechnical investigation and testing – laboratory testing of soil. Determination of liquid and plastic limits Day RW, Soil testing manual: procedures, classification data, and sampling practices , pp47–78, McGraw-Hill, New York 2001 Head KH, Manual of soil laboratory testing: vol 1: soil classification and compaction tests , pp59–95, 143–216, Pentech, London 1980 Many accounts have been written regarding the history, development, and decoration of British vernacular buildings, but the composition and characteristics of their earthen components are still rarely given much attention. In current conservation practice this also remains largely true, and here we have attempted to demonstrate that this deficit needs redress. The methodology we have proposed for testing is a little complicated and time- consuming, but the equipment required is relatively inexpensive, and once mastered, these tests provide the conservator with information on the nature of original materials, how they have been adapted, and which soils can be used to repair them. It is our hope that this will encourage conservators in the field to undertake conservation of this vulnerable heritage with the rigour and sensitivity it deserves. Recommended Reading Davies K, Artisan art: vernacular wall paintings in the Welsh marches, 1550–1650 , Logaston Press, Little Logaston 2008 Gowing R and Pender R, All manner of murals: the history, techniques and conservation of secular wall paintings , Archetype, London 2007 Hamling T, Decorating the ‘Godly’ household: religious art in post-Reformation Britain , Yale UP, New Haven 2010 Historic England, Wall paintings: anticipating and responding to their discovery , Historic England, London 2018 Historic England, Earth, brick and terracotta (Practical Building Conservation Series) , Historic England, London 2015 Teutonico, J-M, A laboratory manual for architectural conservators , ICCROM, Rome 1988 LISA SHEKEDE and STEPHEN RICKERBY are wall painting conservators and graduates of the Courtauld Institute, where Lisa’s master’s thesis was ‘English domestic wall paintings on earthen supports: materials and deterioration’. They undertake conservation projects in the UK and internationally, and teach and publish widely. (See Rickerby & Shekede, page 149) Analysis of the daub used at Alvechurch and material excavated from the original earth floor found that both were mineralogically similar, and both also contained quantities of brick dust, coal and charcoal, suggesting a common excavation source such as a local building plot or midden. Repair material was obtained from the earth floor and tensile strength and shrinkage were improved by the addition of sand, animal hair and straw. A Casagrande device being used to determine the ‘liquid limit’ of a soil sample – the moisture content level at which it is unable to retain a solid form. In this method, a soil paste is placed in the brass cup of the device and a groove is made down the centre. It is then subjected to a timed and highly regulated series of jolts until the groove closes.

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