Sources: Hodges (1964, pp. 19-41); Wulff (1966, pp. 136-171); Rhodes (1969, 1977); Hamer (1975); Rye (1981); Peterson (1995).
Ceramics are fired clay. Clays are composed of small particles, with diameters rarely exceeding 0.01 mm, formed by the weathering of certain rocks (Trench (ed), 2000, pp. 88-90). The main constituent is often the aluminium silicate, kaolinite, but clays have a wide range of chemical composition; other metal silicates and oxides are frequently present in many mineral forms. Clays that are used for ceramics must be plastic when mixed with water, but as steam is evolved and the clays themselves undergo several volume changes during firing, a piece made of pure clay would probably collapse, and so rigid materials (known as temper or fillers) must be present in the clay to provide support during the firing. Primitive ceramics, fired in an open bonfire with little or no control, usually have large quantities of filler, typ ically 20 to 30%, and the individual particles can be several millimetres across, although usually smaller. Many clays already have sufficient sand or grit particles and need no added filler, but otherwise a variety of materials have been added including crushed rock, notably flint, shell, straw, or reused ceramic (known as grog).