There are several materials that are referred to as clay. The properties which make them different include: Plasticity, the malleability of the body; the extent to which they will absorb water after firing; and shrinkage, the extent of reduction in size of a body as water is removed. Different clay bodies also differ in the way in which they respond when fired in the kiln. A clay body can be decorated before or after firing. Prior to some shaping processes, clay must be prepared. Each of these different clays are composed of different types and amounts of minerals that determine the characteristics of resulting pottery. There can be regional variations in the properties of raw materials used for the production of pottery, and this can lead to wares that are unique in character to a locality. It is common for clays and other materials to be mixed to produce clay bodies suited to specific purposes. A common component of clay bodies is the mineral kaolinite. Other mineral compounds in the clay may act as fluxeswhich lower the vitrification temperature of bodies. Following is a list of different types of clay used for pottery.
- Kaolin, is sometimes referred to as China clay because it was first used in China. Used for porcelain.
- Ball clay An extremely plastic, fine grained sedimentary clay, which may contain some organic matter. Small amounts can be added to porcelain to increase plasticity.
- Fire clay A clay having a slightly lower percentage of fluxes than kaolin, but usually quite plastic. It is highly heat resistant form of clay which can be combined with other clays to increase the firing temperature and may be used as an ingredient to make stoneware type bodies.
- Stoneware clay Suitable for creating stoneware. This clay has many of the characteristics between fire clay and ball clay, having finer grain, like ball clay but is more heat resistant like fire clays.
- Common red clay and Shale clay have vegetable and ferric oxide impurities which make them useful for bricks, but are generally unsatisfactory for pottery except under special conditions of a particular deposit.
- Bentonite An extremely plastic clay which can be added in small quantities to short clay to increase the plasticity.