As a kiln is firing up and cooling down, the changes in temperature make some profound changes in the clay. The clay goes from this soft, totally fragile substance to one which is rock-hard, impervious to water, wind, and time. The change is nearly mystical in its complete metamorphosis and might be deemed so if it were not so common.
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First Stage: Atmospheric Drying
When pottery is placed into the kiln, it is almost always bone dry. However, there is still water trapped within the spaces between the clay particles.
As the clay is slowly heated, this water evaporates out from the clay. If the clay is heated too quickly, the water will turn to steam right inside the clay body, expanding with explosive effect on the pot.
By the time the boiling point of water (212⁰F and 100⁰C at sea level) is reached, the atmospheric water should have all evaporated out of the clay body. This will result in the clay compacting and some minimal shrinkage. For more about shrinkage, read Why Clay Bodies Shrink.
To see a diagram of what happens to clay in the kiln, click "View Full-Size" below the thumbnail image.
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Second Stage: Burn Off of Carbon and Sulfur
Clay bodies all contain some measure of carbon, organic materials, and sulfur. These all to burn off between 572⁰ and 1470⁰F (300⁰ and 800⁰C). If for some reason (such as poor ventilation within the kiln) these are not able to burn out of the clay body, carbon coring will occur, weakening the clay body considerably.
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Third Stage: Chemically Combined Water Driven Off
Clay can be characterized as being a molecule of alumina and two molecules of silica bonded with two molecules of water. Even after the atmospheric water is gone, the clay still contains some 14% of chemically bonded water by weight. The pot will be substantially lighter, but with no physical shrinkage.
This chemically combined water's bond loosens when heated. Overlapping the carbon and sulfur burn off, the chemically bonded water escapes from the clay body between 660⁰ and 1470⁰F (350⁰ and 800⁰C). If the water heats too quickly, it again can cause the explosive production of steam inside the clay body. It is for all these changes and more that the firing schedule must allow for a slow build up of heat.
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Fourth Stage: Quartz Inversion Occurs
Potters call it silica, but silica oxide is also known as quartz. Quartz has a crystalline structure that changes at specific temperatures. These changes are known as inversions. One such inversion occurs at 1060⁰F (573⁰C).
The change in crystalline structure will actually cause the pottery to increase in size by 2% while heating, and lose this 2% as it cools. Ware is fragile during this quartz inversion and the kiln temperature must be raised (and later cooled) slowly through the change.Continue to 5 of 8 below.
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Fifth Stage: Sintering
Before the glass-making oxides begin to melt, the clay particles will already stick to each other. Beginning at about 1650⁰F (900⁰C) the clay particles begin to fuse. This cementing process is called sintering. After the pottery has sintered, it is no longer truly clay but has become a ceramic material.
Bisque firing usually is done at about 1730⁰F (945⁰C), after the ware has sintered but is still porous and not yet vitrified. This allows wet, raw glazes to adhere to the pottery without it disintegrating.
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Sixth Stage: Vitrification and Maturity
The maturation of a clay body is a balance between the vitrification of the body to bring about hardness and durability, and so much vitrification that the ware begins to deform, slump, or even puddle on the kiln shelf.
Vitrification is a gradual process during which the materials that melt most easily do so, dissolving and filling in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body.
It is also during this stage that mullite (aluminum silicate) is formed. These are long, needle-like crystals which act as binders, knitting and strengthening the clay body even further.
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The temperature a clay is fired to makes a tremendous difference. A clay fired at one temperature may be soft and porous, while that same clay fired at a higher temperature may be hard and impervious.
It is also imperative to note that different clays mature at different temperatures, depending on their composition. A red earthenware contains a large amount of iron which acts as a flux. An earthenware clay body can fire to maturity at about 1830⁰F (1000⁰C) and can melt at 2280⁰F (1250⁰C). On the other hand, a porcelain body made of pure kaolin might not mature until about 2500⁰F (1390⁰C) and not melt until over 3270⁰F (1800⁰C).
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There is another event that clay goes through, this time as it cools. That is the sudden shrinkage of cristobalite, a crystalline form of silica, as it cools past 420⁰F (220⁰C). Cristobalite is found in all clay bodies, so care must be taken to cool the kiln slowly as it moves through this critical temperature. Otherwise, pots will develop cracks.