Question:
What is Clay?
Mid-fire clays
Mid-fire clays are a type of hybrid earthenware/stoneware clay that matures in between the two temperature ranges. This means that it is possible to get the qualities of the higher fired stoneware, while saving money and fuel in the firing. Mid-fire clays typically mature around 2100o F (1150o C) - 2265o F (1240o C)
Clay
It is rare that a clay as dug from the ground will suit our needs for forming. Typically we alter the clay by mixing it with other clays, silica, feldspar and grog or sand. We call the resulting mixture a clay body. Another clay might be added to increase the plasticity of a less plastic clay or to alter its color and texture. Silica, is added to control shrinkage and to increase the thermal expansion of the body to help make glazes "fit" the body. Feldspar or other fluxes are added to decrease the temperature at which the body will fuse (vitrify) to become water tight. Grog is added to control shrinkage and cracking as the clay dries.
How to wedge
After selecting a clay, you must wedge (knead) it to remove air bubbles and insure uniform consistency. A clay body that has been mixed properly and extruded from a de-airing pug mill is homogeneous, and because a vacuum removes most of the air, it does not usually need as much wedging for its first use. There are two common kinds of wedging, the cut wedging method associated with potters in the Western world and the spiral wedging method associated with potters in the Eastern world. The spiral method is more efficient, particularly when wedging a large chunk of clay, but the spiral method does take longer to learn. Cut wedging is only successful when the clay is very soft, so it is only recommended if you are mixing a very wet and a very dry chunk of clay for later use.
Throwing Clays
Clay bodies
The clay body selected for throwing is as important as the type of wheel and wheel head used. There are three general groups of clay bodies - red or white earthenware, stoneware, and porcelain. Stoneware usually has the best working properties. Because it is not always as workable as an earthenware body, porcelain is generally considered a poor clay to use when learning to throw.
Experiment
The world of clay bodies is vast, you can use anything from clay found in the ground to clay purchased in bags. Clay can have many different attributes and will provide many different experiences depending upon the needs of the potter or ceramists. Many people begin their clay experience with terra cotta (red clay) or earthenware clay and then move on to stoneware then graduate to porcelain also hopefully raku clay. Different pre-blended specialty clay bodies are available at retail stores, check with your local pottery and ceramics supplier to see what suits your needs.
Physical properties
When moist, clay can be smooth, sticky or slippery, soft, and dense. Its shape can be changed without breaking the material--it can be modeled. We call this most important property of clay plasticity*. A sample of dirt we might find that becomes plastic when we wet it can be considered to be clay or to contain a large percentage of clay. When it is dry, clay can be hard, powdery, and fragile. When fired it is transformed to become very hard, strong, dense, brittle, and impervious to water. Read my full article: Physical properties of clay.
Ceramics Frequently Asked Questions
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What tools do I need to start potting?
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What safety aspect should I keep in mind when working with glazes?
What should I remember when firing the kiln?
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Are they Water or oil base stains?
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automatic kiln sitter, should I trust it?
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Ho do I avoid pitting and pinholes?
How do I decorate my ceramics?
How do I Bisque fire in an electric kiln?
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can I draw directly onto a piece?
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What should I remember when firing a new kiln?
how do I get the best results in glazing?
How do I prevent dirt on my greenware?
How do I accomodate Clay Storage in my studio?
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How do I load my brush for applying underglaze?
Why should I use pottery plaster?
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What are some Doing well Quotes?
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How do I store my throwing tools?
What is the history of Ceramics?
Why do I get black spots in my glaze?
How do I load my kiln for a glaze firing?
| Guru Spotlight |
| Patricia Walters-Fischer |
