Context 143 - March 2016

22 C O N T E X T 1 4 3 : M A R C H 2 0 1 6 JAMIE FAIRCHILD Cleaning brickwork, faience and terracotta Past damage caused by inappropriate treatment has led to masonry, and particularly brickwork, faience and terracotta, being cleaned more carefully in the past 25 years. In 1988, the first series of the English HeritageTechnical Handbooks were published under the authorship of John and Nicola Ashurst. Volume 2, Brick, Terracotta and Earth includes two pages of advice on the cleaning of terracotta and faience, commencing with ‘Cleaning systems to be avoided’. Move forward to the Icon Stone andWall Paintings Group seminar in October 2015 on ‘The Conservation of Architectural Terracotta’. As we were leaving, a fellow delegate commented (in reference to cleaning methods): ‘that was very interesting but I feel I know much more about what I can’t do than what I can.’ One might be forgiven for thinking that nothing has changed in three decades. However, established cleaning methods have in fact evolved significantly, but so too have our expectations. In December 2015 Historic England published Earth, Brick and Terracotta, the final book in its Practical Building Conservation series. This is perhaps a useful moment at which to take stock of these changes. This article describes masonry of fired clay (thus excluding bricks made from other materials), with glazed elements (including glazed bricks) labelled ‘faience’ and unglazed elements as ‘terracotta’ (although unglazed, frequently possessing a fireskin). Although unglazed bricks in particular vary greatly in composition, the broad approach and precautions apply. During the past 25 years the conservation field has become more circumspect towards masonry cleaning generally, and the cleaning of sandstone and terracotta in particular.This is principally a reaction to past damage inflicted by unsympathetic or inappropriate treatment. The underlying issue is the greater susceptibility of these substrates to ‘discoloration’, alteration of the mineral composition as distinct from the presence of overlying residue. Such alteration is normally driven by water saturation or migration, transporting salts fromwithout (frommortar, fixings, pollution and cleaning chemicals) or mobilising constituent salts/compounds contained within the fireclay. The fireskin of brick or terracotta (and porous or cracked faience) is susceptible to forming strong physical/chemical bonds with pollution residues and applied coatings. The removal of overlying residue is invariably much easier than reversing discoloration. Even so, the effort required and the risk of substrate damage increase sig- nificantly when cleaning is taken towards the maximum. The dangers of aggressive or excessive cleaning were recognised in the cautionary advice published in the first English Heritage Technical Handbooks and much earlier in Weathering of Natural Building Stones (Schaffer, 1932) and Building Stones (Warnes, 1926). Considerable care has been taken with the cleaning of certain of our most valuable buildings, recorded in the Ministry of Works case studies detailed in Cleaning Historic Buildings , for example. However, it has taken time and the introduction of policy (principally PPG15 Planning Policy Guidance: Planning in the Historic Environment, 1994) for this caution to be widely accepted and enforced, at least for listed buildings. Retaining sound surface takes precedence over the level of clean or whether cleaning shall be carried out at all, and a greater degree of discoloration is now accepted as patina. The nature of soiling and the rate of accumulation Recommended reading Historic England (2015) Practical Building Conservation: Earth, Brick and Terracotta, Ashgate, London Ashurst, N (1994) Cleaning Historic Buildings, Donhead, Dorset Ashurst, N (2011) Cleaning Brickwork and Terracotta, Conservation Directory Fairchild, J (2016) Abrasive Cleaning Methods for Masonry , Conservation Directory Pavia, S and Bolton, J (2000) Stone, Brick and Mortar ,Wicklow A terracotta frieze (1887) to the front elevation of Cutlers Hall, London EC4M, before and after cleaning.The upward facing surfaces have a substantial layer of particulate matter.The majority was removed dry, with soft brushes, prior to light and diffuse rinsing (the spray becoming more finely atomised at lower pressures and high temperature, 20bar and 150–155°C respectively) with a proprietary superheated water system. Localised repairs, principally of cracks, and re-colouring of old repairs were also carried out. (Photo far right: Cliveden Conservation) Exterior white glazed brickwork at basement level.The sample was carried out with a proprietary swirl abrasive system and 140 micron calcite abrasive. Alternatively, water and brush agitation was also effective but very slow. Certain of the bricks retain a green hue (assumed to be organic in origin) and the glaze of these is finely crazed. Superheated water had only a nominal result with removal of the primary (brittle) residue and offered no perceptible removal of the green stain.