Introduction to Clay
Clay is produced in the decomposition of granite and igneous rock where the alkalis have washed out leaving behind quartz, mica, and clay. Primary clay is found where it was formed, it is not weathered and therefore - having fewer impurities - is whiter. Secondary clay has been moved by geological forces, by glacial action, by rock folding, by water, or by wind so the clay is finer in particle size. It has also picked up many impurities that alter the rate of shrinkage, the drying time, the firing temperature, and the colour. The most common impurity is iron oxide - we know it as rust - that gives most earthenware its red colour.
Earthenware makes up the majority of clay work. It is also known as raku, slipware, terra cotta, lusterware, maiolica, faience, or delftware. Earthenware is a secondary clay and is normally red but also comes in the colour of buff, sand, and white. It is normally fired to 1150 degrees celsius. It is not fired to vitrification which is the point where clay melts. It is slightly porous and generally glazed.
Stoneware is a secondary clay which has been fired to a temperature between 1150 and 1300 degrees celsius. It is hard, durable, almost impervious to thermal shock and is usually, buff, grey or red-brown in colour.
Raku clay is technically an earthenware that has been modified with the addition of grog. Grog is a ground fired bisqueware. In other words, clay that has already been through the firing. Depending on the size of the grog it can add texture to the clay but more importantly it also reduces shrinkage and helps prevent thermal shock that is part of the raku process. Raku is normally fired from 900 to 1000 degrees.
Porcelain is a primary clay generally composed of kaolin or aluminasilicate. This produces a product that is known for its whiteness, hardness, durability and translucency when thin enough. A distinctive ring is produced when it is struck. Porcelain is fired to the point of vitrification or melting at a temperature between 1250 degrees and 1400 degrees celsius.