82 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS UNDERSTANDING THE FIRESKIN The porosity of terracotta is dependent on a physical change known as vitrification that occurs during the firing process. The temperature of the kiln allows the finer clay particles on the surface of the clay unit to partially fuse or ‘vitrify’, resulting in the formation of terracotta’s characteristic fireskin. This hard, protective surface provides a weatherresistant barrier on the exterior of the terracotta, protecting the softer and more porous clay structure within. Our understanding of the importance of the fireskin is continuing to evolve and research by PhD student, Patrick Barry into its properties and significance is expected to be published in 2025. Pink- and salmon-coloured terracotta tends to be more susceptible to weathering and damage because these clays had to be fired at lower temperatures; as a result, the finer clay particles have not completely bonded together to form a solid surface layer. Brown-, red- and orange-coloured terracotta tends to be more resistant to weathering and impact damage, due to its being fired at higher temperatures. This means the surface layer of the terracotta is less porous and more resilient to weathering. MIND THE KNOWLEDGE GAP As part of my recent MA in conservation of the historic environment, I undertook academic research for a dissertation on terracotta to explore the state of current knowledge and awareness of terracotta conservation requirements across the UK and in different regions. With the assistance of terracotta manufacturers, conservation officers and contractors, a total of 11 case studies (based in London, West Midlands and the North West) were assessed as part of the research. Regional case studies provided me with a better understanding of the role of heritage professionals in respect of the care, replacement, colour matching and cleaning of the terracotta on recent terracotta projects. My research found that conservationaccredited professionals generally had the best understanding of the subject matter and material, while other professionals working in the field at least knew of Historic England’s PBC publications. Contractors and cleaning specialists were found to be the least likely to follow up with relevant training, relying on previous on-the-job experience. Inappropriate repairs and poorly matched mortar repairs, poorly colourmatched replacements and the use of inappropriate cleaning methods were found to be current issues in the conservation industry, and it became clear that many of the professionals working in this field are not referring to or adhering to current best practice guidance. The research highlighted the need for more training and collaborative working in the fields of repair, replacement and cleaning of terracotta. WHO’S RESPONSIBLE? Owners of properties have a duty of care to maintain and undertake repairs to their buildings as and when necessary. Owners of listed buildings and buildings within conservation areas are always advised to seek the advice of a conservation officer or heritage professional for guidance on the repair and maintenance of these buildings. Heritage professionals and many other specialists have an input into the repair, replacement and cleaning of terracotta. They include architects, surveyors, structural engineers and conservation officers, contractors, manufacturers and cleaning specialists. Some will be accredited in conservation, but many will have little or no formal training in the field. Conservation officers have a duty to assess listed building consent applications and planning applications in accordance with current legislation and guidance – which, in England is the Planning (Listed Buildings and Conservation Areas) Act 1990, National Planning Policy Framework and relevant development plan policies. As well as legislation, national and local plan policies, Historic England and the other national heritage bodies all produce excellent technical advice notes and good practice guidance. They can provide information on various heritage topics which can assist in assessing applications. RESIN REPAIRS My research found that the conservation industry has very different views on resin repairs. Some contractors and heritage professionals are wary of using them, referencing their brittle nature and their aesthetic appearance, while others consider them useful for repairs. Epoxy resin manufacturers promote their use, but without being unclear about their performance and longevity. Historic England’s PBC publication Earth, Brick and Terracotta, which is well known in the sector, was first published in 2015. Whilst it remains wholly relevant, there have been many new resin products introduced on the market since then, yet there has been no laboratory testing or field research conducted on them. Epoxy resin needs to be tested in both sheltered and exposed areas to assess its flexibility, breathability, and thermal performance over time, and to understand how it performs as a material in comparison to terracotta. Conclusive results would enable new guidance to be prepared on their use for repairs and their pros and cons. COLOUR MATCHING Whilst undertaking my research it became evident that colour matching both replacement units and mortar repairs is still an issue in the industry today. Conservation officers were not seeking details of repair, colour matching or cleaning methods, and these decisions generally defaulted to the architect or contractor. The use of a colorimeter and spectrophotometer may The scored and pitted surface of terracotta at the Royal Albert Hall is the result of poor abrasive cleaning carried out 30 years ago. Investigative works at the Natural History Museum Colour differences between the original and new terracotta units at London Road Fire Station, Manchester
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