Clay // Why Clay?
What is Clay?
“One of the most common materials on our earth, clay is a remarkable substance. It is like no other artistic medium in the wide range of possibilities for creative expression it offers” (Speight, 1983, p. 10). Clay is the mud of the earth, the ground you walk on. Clay is formed under the surface of the earth at the site of a parent rock, typically a feldspathic rock. This rock breaks down over time from weathering. If the clay stays at the site of the parent rock it is considered primary clay. Primary clay holds little or no impurities. It is a white body known as kaolin. Once the clay has moved away from the mother rock it is considered secondary clay. Secondary clay refers to clay that has been transported. It has picked up other organic materials as it traveled by wind, down streams, or by glaciers.
Clay bodies can be broken down into three main categories: earthenware, stoneware and porcelain. Earthenware clay is a secondary clay. It most often holds iron oxide, giving it a reddish brown color. It is typically used at low fire temperatures of 1830-1980 F (1000-1080 C). Stoneware clay is composed of fire clay and ball clay (both secondary clays) as well as feldspar and silica. It is typically used for high fire as it matures between 2100-2400 F (1148-1316 C). The materials and high firing temperature vitrifies the stoneware clay body. When vitrified, the particles fuse together and become glass-like. Porcelain is a pure white clay body composed of kaolin, a primary clay known for its translucency. It is traditionally fired at high fire temperatures above 2300 F (1260 C).
Clay is found all over the surface of the earth. As a child, I found red clay walking along the shores of Lake Michigan. During a large storm waves washed away a large portion of the dune, exposing a large clay deposit. That was the start of my journey in clay. Clay is everywhere! If you are interested in finding a source near you, I recommend asking local artists in your town to lead you in the right direction.
Clay is a soft, plastic, and malleable material. It can be used to form objects in space, responding to every push and pull. Before firing, the clay is known as greenware. Greenware can be used and then reclaimed to bring it back to its original state. After the clay stiffens, it is considered leather hard. At this point you can make changes in the surface, and the piece can be altered or attachments made. Once the clay is bone dry, it is no longer wet and the form cannot be changed. It is very fragile and should be handled with care. The clay is then placed in the kiln to be bisque fired. After firing takes place, the chemical composition of the clay has changed, and it can no longer be reclaimed. The purpose of bisque firing is to burn out carbonaceous matter and to strengthen the wares before glazing. The piece is then glazed and fired again to its maturing temperature, which is dependent upon the clay body. After firing, the piece is now considered ceramic.
Bibliography & recommended readings
Leach, B. (1949). A potter’s book. New York, NY: Transatlantic Arts, Inc. (p.28-62)
Mattison, S. (2003). The complete potter. London: Barron’s Educational Series, Inc. (p.19-34)
Peterson, S. (2000). The craft and art of clay. Upper Saddle River, NJ: Prentice-Hall Inc. (p.127-152)
Spleight, C., & Toki, J. (1997). Make it in clay: A beginner’s guide to ceramics. Mountain View, CA: Mayfield Publishing Company. (p.1-12)
Rhodes, D. (2000). Clay and glazes for the potter. Iola, WI: Krause Publications. (p. 59-121)