81 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 MASONRY 3.2 TERRACOTTA WAS used extensively in the late 19th and early 20th century, both as a cladding for steel-framed buildings and as finely moulded decorative details for enhancing ordinary brickwork. As a form of fired clay it is an extremely robust material, but it is difficult to clean without damage and inherent defects in the new construction systems with which it is often associated are common. Many of these issues are still poorly understood. This article aims to highlight current sources of best practice, the availability of specialist knowledge and forthcoming research. IS THERE SUFFICIENT GUIDANCE? Historic England’s excellent 2015 Practical Building Conservation (PBC) volume, Earth, Brick and Terracotta outlines the material, its historical use and the causes of deterioration and decay. These include: structural issues; environmental issues; inappropriate treatments; accidental and incidental damage; structural repairs to be undertaken with dowelling; patch or fracture repairs; non-structural repairs to be undertaken in a lime mortar or with an epoxy or polyester base resin; replacement units to colour match; caring for the material; and cleaning of the material avoiding abrasive, acid and alkali cleaning. British Standard Guidance BS 7913:2013, Guide to the Conservation of Historic Buildings, provides guidance on managing the care of historic buildings. The guidance refers to the requirement for minimal intervention so as not to harm the significance of a heritage asset. It also highlights the use of appropriate materials for conservation works and, in reference to repairs, it advises they should be ‘identifiable’ and all works must be undertaken by ‘competent persons’. Nicola Ashurst’s 2011 publication Cleaning Historic Buildings outlines the damage which can be caused by both abrasive and chemical cleaning of terracotta buildings. Ashurst identified that fine particle abrasive cleaning methods and techniques cause damage to the surface of the material resulting in ‘pitting and scoring’. In turn, this affects the appearance of the building, as seen at The Royal Albert Hall. Previous chemical cleaning methods, in particular Victoria Law Courts, Birmingham: designed by Aston Webb and Ingress Bell, construction started in 1887 when the terracotta revival was at its height. TERRACOTTA Lessons not learnt and the application of current best practice in conservation DEVINDER K MATHARU the use of concentrated chemicals and long dwell times, have resulted in terracotta units marked with streaks, as seen at the Natural History Museum. Academic research in Britain and the USA highlights how inappropriate cleaning methods have resulted in irreversible damage to terracotta buildings. Conahan’s thesis (1999) concluded that cleaning terracotta, even just once, with a cleaning solution makes its surface more porous. Research by Cummings and others in 2003 concluded that abrasive cleaning had the greatest impact upon surface texture. Steam cleaning has a lesser impact, while the use of laser cleaning risked harsh discolouration of the surface. Other academic research highlights the impact of inappropriate cleaning methods and the impact upon the fireskin, porosity, surface texture, colour, and light reflectance of the material.
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