Terracotta

Terracotta translates from Latin as ‘burnt earth’ and is a type of clay-based ceramic or earthenware material that has been used for sculpture, pottery and architectural purposes by many civilisations, from the ancient Greeks, to the Egyptians, the Chinese and Native Americans. The Chinese and Indian cultures used terracotta as a form of elaborate roof decoration for temples and other prestigious buildings. It was also commonly used for buildings in Victorian England, and the American architect Louis Sullivan used terracotta to create ornamentation designs.

It is formed by a mixture of clay and water that is fired and then either left unglazed, or painted, slip glazed or glazed. If terracotta is to be painted, gesso (a type of primer) is applied first.

Terracotta can be used structurally or non-structurally on both the exterior and interior of buildings. Some of the typical uses that terracotta has had in construction include; chimney pots, air bricks, copings, planters, water and waste water pipes, roofing tiles and shingles, capitals and other architectural details and ornaments.

Terracotta is formed by moulding an appropriate refined clay to the required shape by pouring or pressing it into a plaster or sandstone mould and leaving to dry. It is then placed in a kiln and fired, typically at around 1,000 °C. The characteristic red-brown colour of terracotta is the result of the iron content in the clay reacting with oxygen during the firing. The terracotta is then slowly cooled and finished.

The performance of terracotta is influenced by its porosity. It has poor resistance to tension and low shear strength but is strong in compression.

Terracotta can fail due to; poor manufacturing or installation, weathering, atmospheric pollution that causes salt formation, freeze-thaw cycling, and so on. Poor installation can be due to improper loading, or the mortar used being too strong, which transfers stress to the terracotta block.

By the 1920s, a process known as mechanised extrusion was capable of mass-producing terracotta blocks in standard forms for flooring, roofing and cladding applications. However, it could not compete with more modern building materials such as concrete, structural steel and plate glass, and the changing aesthetic preferences of minimalism and Modernist architecture meant that its use declined throughout the 1930s.

See also: The history and conservation of terracotta.

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• Practical Building Conservation: Earth, Brick and Terracotta.
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