Differences Between Mid-Range and High-Fire Clays
The main distinction between mid-range and high-fire clay bodies is implied in their names: the distinguishing factor is the temperature at which the clay matures. Mid-range clay bodies high to maturity between cone 4 and cone 7. High-fire clay bodies are usually considered to be those that mature between cone 8 and cone 11, although some porcelains go all the way to cone 14.
The ingredients used in mid-range and high-fire clay bodies are very similar within their type. The main difference is that, in relation to high-fire bodies, mid-range bodies will have either less refractory elements, more fluxing agents, or a combination of these two.
Stoneware Clay Bodies
Generally speaking, stoneware clay bodies fire to a gray, buff, tan, or light brown color. Color will vary with the same clay, depending on the kiln's atmosphere. Darker colors are possible with the addition of slips, such as Alberta Slip, or of coloring oxides.
Stoneware clays get their name from the dense, rock-like nature of the clay body when it is fired to its maturation temperature. There are some naturally occurring stoneware clays that need little modification. Usually, however, a stoneware clay body adds other ingredients for optimal performance. For example, ball clays may be added for plasticity, or fire clays may be added to raise the maturation temperature of the clay body.
Porcelain Clay Bodies
Porcelain clay bodies are known for their hardness, their extremely tight density, their whiteness, and their translucence when the pottery's walls are thin. Kaolin clays are the foundation of all porcelain clay bodies.
Kaolin is the purest form of clay. It is also so non-plastic as to be nearly unworkable if not mixed with other clays. Another difficulty is that porcelain clay bodies are very prone to warping during drying and in the kiln.
The purest porcelain bodies are fired at the highest temperatures used in pottery, usually between cone 11 and cone 14. However, many porcelain clay bodies are modified to make the clay more workable and also to bring the firing temperature down.