Clays and Pottery Essay -- Art Molding Papers

Clays and Pottery Ceramicists, working either on a wheel or building by hand, define three main classes of clay bodies or mixtures: earthenware, stoneware, and porcelain. These are divided on the basis of firing temperatures and the character (hardness, vitrification and durability) of the final material. Each clay body is composed of a mixture of clay minerals and other materials such as sand or fine gravel and "fluxing" agents which affect the color and texture when fired. Instead of discussing clays solely in terms of their chemical formulae, determined by x-ray diffraction, potters group clays into classes based on more general properties of the entire clay body, such as texture and color. One distinction potters make is between primary or residual clays, and secondary or sedimentary clays. Kaolin is the major primary clay which is used in ceramics. The use of the name "kaolin" for a clay body encompasses more than a body composed of pure kaolinite crystals, however, according to Rhodes, the composition of kaolin clays generally fall with in the bounds of kaolinite's composition: 46%silica, 39% alumina, 13% water (Rhodes p. 47). Kaolinite has the most basic 1:1 tetrahedral-octahedral clay structure, and maintains a simple and pure composition. So when kaolin is fired along with some silica and feldspar, it forms a highly refractory white solid. As the main ingredient in porcelain, Kaolin comprises, on average, about 50% of a porcelain mixture. Kaolin is not very plastic and thus presents a challenge to the potter in its pure state; however, some Kaolins are of sedimentary origin (such as those sifted from sands in Florida) and therefore have a smaller particle size which increases the plasticity. Generally though, kaoli... ...In dark colored ball clays containing highly expandible minerals, the increased amount of water in the system coming from the interlayer sites can lower the melting point. In addition, the pre-fired color of a clay body may be quite different from the fired color due to the fact that organic material often colors a wet clay, yet burns away during firing without leaving a stain. BIBLIOGRAPHY Chappell, James. The Potter's Complete Book of Clay and Glazes. New York: Watson-Guptill, 1977. Conrad, John, W. Contemporary Ceramic Formulas. New York: Macmillan, 1980. Rhodes, Daniel. Stoneware and Porcelain: The Art of High-Fired Pottery. New York: Chilton Company, 1959. Velde, Bruce (editor). Origin and Mineralogy of Clays: Clays and the Environment. New York: Springer, 1995. Velde, Bruce. Introduction to Clay Minerals. New York: Chapman and Hall, 1992.