Clays and Mineral Contents
There are several materials that are referred to as clay. The properties of the clays differ in: Plasticity, the malleability of the body; porosity, the degree to which the fired pottery will absorb water; and shrinkage, the degree of reduction in size of a body as water is removed. The various clays also differ in the way in which they respond to different degrees of heat when fired in the kiln. Each of these different clays are composed of different types and amounts of minerals that determine the resulting pottery. There are wide 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. The two essential components of clay are Silica and Alumina which combine to form Aluminium silicate, also known as Kaolinite. Other mineral compounds in the clay may act as Fluxes to lower the melting point of the silica during firing. Following is a list of different types of clay used for pottery that are available in different regions of the world.
- Kaolin, is sometimes referred to as China clay because it was first identified in China.
- Ball clay An extremely plastic, fine grained sedimentary clay, which may contain some organic matter. It is usually added to porcelain to increase plasticity.
- Fire clay A clay having a slightly higher 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 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 make it more plastic.
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