Today is the first day of a very busy pottery class session. I'll be teaching 8 classes a week during January, with lots of beginning students. I always start my new students with a short explanation of how we choose the clay bodies that we use, so I thought I'd go ahead and post it here as well:
The clays that we use to make pots are not a single clay dug from the ground. Rather, they are are a mixture of clays, binders and texturizers that are mixed to a specific formula or recipe. We call these mixtures 'clay bodies'. Every company that mixes clay bodies has their own proprietary formulas. For better or for worse, a cone 6 Buff body from one company will not be the same as a cone 6 Buff body from another company.
Clay bodies are formulated and purchased according to 3 main criteria: color, texture and firing temperature. Color is determined by the amount of iron oxide present in the clay body. Porcelain has no iron, white stonewares may have trace amounts, and stoneware bodies can have a small amount to make the body a light tan color, or a large amount to make it dark brown or brick red. The color of the clay body is important because it will affect the color of your glazes. The darker the clay, the more it will darken the glaze.
The texture of a clay body can range from very smooth to very gritty. Grit is obtained by using fire clays in the formula, or by adding grog (ground fire bricks) or silica sand. Grog and sand are available in a variety of sizes, but usually range from 20 mesh to 100 mesh when added to clay bodies. The smoother the clay body, the less forgiving it will be, meaning it will shrink, warp and crack more, therefore requiring good throwing and assembly skills. Large, thick or uneven pieces like sculptures generally require a much grittier body than wheel thrown pots to keep them from cracking during drying and firing.
Last, and probably most important, a clay body must be formulated to fire at the right temperature. If you are using cone 6 glazes, you must use a cone 6 clay body. If you are using cone 06 glazes, you must use a cone 04 clay body. If your clay and glazes are formulated to mature at a different temperature, they generally won't 'fit' together very well, and a multitude of problems can arise, from crazing to shivering. And if you over-fire a clay body, it can melt into a puddle all over the inside of your kiln. See HERE.
So there you go. If you need help selecting a clay body, the tech at your local ceramic supply store can help you out. They know their clay bodies very well, and can help you pick the right one for your project.