Its primary mineral is kaolinite; clay may be generally described as 40% aluminum oxide, 46% silicon oxide, and 14% water. There are two types of clays, primary and secondary. Primary clay is found in the same place as the rock from which it is derived, it has not been transported by water or glacier and thus has not mixed with other forms of sediment. Primary clay is heavy, dense, and pure. Secondary or sedimentary clay is formed of lighter sediment that is carried farther in water and deposited. This secondary clay, a mixture of sediment, is finer and lighter than primary clay. Varying additives give the clay different characteristics. Clay comes to a production potter in one of two forms as a powder to which water must be added, or with water already added. Large factories purchase the clays in huge quantities as dry materials, making up the clay batch as needed each day.
Glazes are made up of materials that fuse during the firing process making the pot vitreous or impervious to liquids. (Ceramics engineers define vitreous as a pot that has a water absorption rate of less than 0.5%.) Glazes must have three elements: silica, the vitrifying element (converts the raw pottery into a glasslike form) is found in ground and calcined flint and quartz; flux, which fuses the glaze to the clay; and refractory material, which hardens and stabilizes the glaze. Color is derived by adding a metallic oxide, including antimony (yellows), copper (green, turquoise, or red), cobalt (black), chrome (greens), iron, nickel, vanadium, etc. Glazes are generally purchased in dry form by production potters. The glazes are weighed and put into a ball mill with water. The glaze is mixed within the ball mill and grinds the glaze to reduce the size of the natural particles within the glaze.
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