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. 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. Terracotta, which means quite literally “baked earth”, is one of the most popular types of earthenware. Mid-fire stoneware clay bodies are formulated to fire to maturity between 2150 F and 2260 F (1160 C and 1225 C). High-fire stoneware clay bodies fire to their mature hardness between 2200 F and 2336 F (1200 C and 1300 C). 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 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 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.
