Terra cotta

'Terra Cotta' (Italian for 'cooked earth') is red burning earthenware, generally unglazed. Terracotta is normally used to make sculptures, tile, planters, garden and architectural ware. If ware is glazed the ware is often referred to as 'red earthenware' rather than the term 'terra cotta'.
Red clays have more flux impurities and fire to a harder stronger matrix than white burning materials at the same temperature. Still, terra cotta bodies fire to a porous matrix at cone 06-04 and do not have anywhere near the mechanical strength of vitrified stoneware bodies. Without significant additions of expensive frits it is impossible to vitrify a body at these temperatures. However many terra cotta clays do develop rapidly after cone 04 and turn from red to brown in the process. It is possible to produce fired ware that rivals stoneware in strength at cone 02-1, however few people do this because the clay is so volatile, slight overfiring will produce warping or bloating. In addition to cost one of the primary advantages of the terra cotta process in the warm red colors of the raw clay surface. In addition glazed low fired terra cotta remains red whereas at higher temperatures the glaze matures the surface and turns it brown.
Some terra cotta pieces may be glazed on the inside. Because terra cotta ware is weak and porous it is very important that the glaze and body thermal expansions match. The clay-glaze interface is not well developed (the glaze is not stuck on as well as stoneware) so a measure of resistance to chipping and crazing can only be achieved by a well melted glaze of low enough thermal expansion to resist crazing. In the past inexpensive lead compounds were used on terra cotta because they contributed exactly these properties plus they gave very bright and vibrant colors. Today boron glazes are employed. While safer to use they do not have the ideal set of properties that lead based compounds had.
'Majolica' refers to the use of a terra cotta clay with an opaque white glaze decorated with colored overglazes. Today red clays are used in this process because they provide maximum strength at low fire. In the past white low fire materials were not available.

In Bound Links

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• (Articles)

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• (Glossary) Earthenware

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