Clay Basics (from About.com)

Not All Clay Is the Same

What is clay? Is it simply dirt? Well, yes and no. "Dirt" covers a lot of ground, so to speak. We can break dirt into several sections: topsoil, clay, inelastic earth, and rock. Topsoil contains a lot of organic material, which makes it good for growing plants. Clays and inelastic earths are the results of decomposing rocks, in which the particle size is extremely small. Rocks include bedrock and boulders all the way down to fine sand.

Clay Is...

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

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.

• Low-Fire Clay Bodies
• Dry Earthenware Clays and Slips

Stoneware Clays

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 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).

• Mid-Range and High-Fire Clay Bodies