Clay used for pottery is indigenous to only certain regions of the world and every manufacturer uses the same base clay. United States clay body formulas are generally a mixture of several different types of clay, derived primarily from clays mined in Georgia, Tennessee, Ohio. Small amounts of darker clays that make up a formula, such as Van Dyke Brown, are found in New Mexico. Clay mined in California has a gritty coarse texture and therefore, is not usually used in formulas. Fine grogless clays from England are used for making porcelain clay bodies.
The earthenware clays generally vitrify at a lower temperature than the stoneware or porcelain clays. (Vitrification is the process whereby a clay body is heated until it becomes non-porous.) Stoneware clays fire to a higher temperature and is, therefore, a harder material than earthenware.
There are two types of firing: bisque firing and glaze firing, both done in kilns at temperatures ranging from cone 014 to cone 10. Bisque firing is the initial of first firing of a dried piece of pottery or ceramic, also known as greenware, performed before glaze firing. Glaze firing is the second firing; glaze is a colored liquid material that can be matte, gloss, or speckled. It is a silica that when heated fuses together and forms a glass-like coating.