Clay differs from the inelastic earths and fine sand because of its ability, when wet with the proper amount of water, to form a cohesive mass and to retain its shape when molded. This quality is known as clay’s plasticity. When heated to high temperatures, clay also partially melts, resulting in the tight, hard rock-like substance known as ceramic material.
Classes of Clay
Clay can be divided into several classes, based on characteristics and at what temperature the clay must be fired to in order for it to become mature, or reach its optimum hardness and durability.
The three most commonly used clay bodies are earthenware clay bodies, mid-fire stoneware clay bodies, and high-fire stoneware clay bodies. All three are available commercially in moist, ready-to-use form. Clay bodies can also be produced by mixing dry clays and additives with water to create your own desired clay body.
Earthenware clays were some of the earliest clays used by potters, and it is the most common type of clay found. These clays are highly plastic (easily worked) and can be sticky. Earthenware clays contain iron and other mineral impurities which cause the clay to reach its optimum hardness at between 1745°F and 2012°F (950°C and 1100°C).
Typical colors for moist earthenware clays are red, orange, yellow, and light gray. Colors for fired earthenware includes brown, red, orange, buff, medium grey, and white. Fired colors are in large part determined by the content of mineral impurities and the type of firing.
Stoneware clays are plastic and are often grey when moist. Their fired colors range through light grey and buff, to medium grey and brown. Fired colors are greatly affected by the type of firing.
Mid-Fire Stoneware Clay Bodiesare formulated to fire to maturity between 2150°F and 2260°F (1160°C and 1225°C).
High-Fire Stoneware Clay Bodiesfire to their mature hardness between 2200°F and 2336°F (1200°C and 1300°C).
Ball clays are highly plastic and contain few mineral impurities. They fire to their mature hardness at about 2336°F (1300°C). When moist they are dark grey and when fired they are either light grey or light buff.
Ball clays do have a serious drawback. They cannot be used by themselves due to their excessive shrinkage during drying and firing. They are extremely useful, however, when added to other clays to increase workability and plasticity.
Fire clays vary widely in their characteristics. The hallmark is their high firing range. They mature at about 2696°F (1500°C). Although relatively free from mineral impurities, they tend to have spots of iron which lend a speckled appearance once fired.
Fire clays are often used in stoneware clay bodies to increase their maturation temperature and to give the fired clay a bit extra roughness, or "tooth". They are also used fuel-fired kilns to create cone packs (which monitor temperature), as supports for ware or shelving, and to seal doors.
Kaolin (Porcelain) Clays
Due to their mineral purity, kaolin clays are used for porcelain. Although kaolin clays do have some range in color, they are all very light in color. While moist, they will be light grey and will fire in the range between a very light grey or buff, to near-white and white.
Kaolin clays are not nearly as plastic as other clays and are difficult to work with. Pure kaolin clays fire to maturity at about 3272°F (1800°C). They are often mixed with other clays to both increase workability and lower the firing temperature. Many porcelain bodies are a mixture of kaolin and ball clays.