Firing clay transforms it from its humble, soft beginnings into a new substance, ceramic. Ceramics are tough, strong and similar in some ways to stone. Pieces of pottery have survived for thousands of years, all due to clay that met fire.
The temperature needed to transform soft clay into hard ceramic is extremely high and is usually provided by a kiln. You can't fire pottery in a home oven because it cannot deliver the necessary temperature.
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Firing is the process of bringing clay and glazes up to a high temperature. The final aim is to heat the object to the point that the clay and glazes are “mature”—that is, that they have reached their optimal level of melting. Pots and other clay objects don’t look melted; their melting is on the molecular level.
This process is usually accomplished in two steps.
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Bisque firing refers to the first time the pots go through high-temperature heating. It is done to vitrify the clay pots enough that they won’t be harmed when glazes are applied, but not to such an extent that the glaze won’t adhere correctly.
The bone-dry greenware is fragile and must be loaded into the kiln with a great deal of care. The kiln is closed and heating slowly begins.
A slow temperature rise is critical. During the beginning of the bisque firing, the last of the atmospheric water is... driven out of the clay. If it is heated too quickly, the water turns into steam while inside the clay body, causing the clay to burst.
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When a kiln reaches about 660 F, the chemically bonded water will begin to be driven off. By the time the clay reaches 930 F, the clay becomes completely dehydrated. At this point, the clay is changed forever; it is now a ceramic material.
The bisque firing continues until the kiln reaches about 1730 F. At this temperature, the pot has sintered, making it less fragile while remaining porous enough to accept the application of glazes.
After the desired temperature has been reached, the kiln is... turned off. The cooling is slow to avoid breaking the pots due to stress from the temperature change. After the kiln is completely cool, it is opened and the newly created bisqueware removed.
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At this point, potters apply glazes to the bisqueware. After glazes are applied and have a chance to dry, the ware is carefully loaded into the kiln for the glaze firing. Pots cannot be allowed to touch at all or the glazes will melt together, welding the pots together.
The kiln is heated slowly to the proper temperature to bring the clay and glazes to maturity and then slowly cooled again. Only after the kiln has cooled is opened and unloaded.
The changes in temperature in the kiln cause a... remarkable change in the clay and glaze. Pots change from a soft, fragile substance to one that is rock-hard, impervious to water and time.