CLAY BODY FORMULAS

Clays that are formulated and mixed by potters or sold by ceramic supply companies are called clay bodies. A clay body is a combination of clays, fluxes, and fillers. Each serves a function to help determine forming characteristics, drying shrinkage ,surface texture, fired absorption, fired shrinkage, glaze interface, and fired clay color.

Clay body formulas, as in cake-baking recipes, can encompass subtle variations, which bring distinctive forming and firing characteristics to the whole mix. For example, if a me­dium to high temperature clay body requires flux to help bring other materials in the clay body into a melt, there are many types of feldspar that could fill that requirement. However, you must decide which of the available feldspars will be appropriate for the clay body. The best clay body formulas contain appropriate raw materials and the correct ratios of clays, fluxes, and fillers to achieve their desired result. The use of metallic coloring oxides suc h as manganese diox ide, iron oxide,chrome oxide, and cobalt oxide can produce a dark-colored clay. In a white clay body, cobalt oxide can produce a blue clay. In a white clay body, chrome oxide can produce a green clay.

TYPES OF CLAYS

Clay, the first of the three essential ingredients of a clay body, is grouped depending on refractory qualities, particle size, oxide composition, loss on ignition, shrinkage rates, absorption rates, and other defining characteristics. The basic clay groups found in clay body formulas arc fireclays, ball clays, kaolins, stoneware clays, bentonites, and earth­ enware clays. We’ll go into further detail about ball and fire clays next week.

Understand that not every clay group will be used in every clay body formula. For example, some earthenware clays, due to their relatively high iron content and low maturing range, would not be found in porcelain clay body formulas, which require low-iron-content, refractory kaolins to achieve their white fired color.

Within each major group of clays are subgroups that further define a particular clay characteristic, such as plastic kaolin (such as grolleg clay) and non-plastic kaolin (such as English China clay). Further, each group and sub-group contains many individual brand names of clay. Some of the many ball clays are Tennessee ball clay #9, Taylor ball clay, Kentucky ball clay OM #4, Zamek ball clay, Kentucky Special, and Thomas ball clay.

Each group of ball clays, stoneware clays, kaofjns, ben­tonites, fireclays, and earthenware clays contributes specific attributes to the total clay body formula. Those qualities include particle size, green strength, fired strength, fired color, shri nkage, plasticity, deRocculating potential (also known  as Zeta potential), texture, forming abilities, and low amounts of warping in drying and firing stages.