What Is Firing?
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 won’t look melted; their melting is on the molecular level.)
This final aim, however, is usually done in two steps.
Bisque firing the first time the pots go through high temperature heating. It is done in order to vitrify the clay pots enough that they won’t be harmed when glazes are applied, but not vitrified to such an extent that the glaze won’t adhere correctly.
The bone-dry greenware is very fragile and must be loaded into the kiln with a great deal of care. The kiln is closed and heating is slowly begun.
A very 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 heated too quickly, the water will turn into steam while inside the clay body, causing the clay to burst.
When a kiln reaches about 660°F (350°C), the chemically bonded water will begin to be driven off. By the time the clay reaches 930°F (500°C), it will have become 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 (945°C). At this temperature, the pot has sintered, making it less fragile while still porous enough to accept the application of glazes.
Once the desired temperature has been reached, the kiln is turned off. The cooling is also slow, to avoid the pots breaking due to stress from the temperature changes. After the kiln is completely cool, it is opened and the newly created bisqueware removed.
Once glazes have been applied to the bisqueware and have had 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 will it be opened and unloaded.