The temperature needed to transform soft clay into hard ceramic is extremely high and is usually provided by a kiln. You cannot fire pottery in a home oven because ovens do not get up to the high temperatures of more than 1,500 degrees Fahrenheit that you need for firing clay. Greenware is fragile. To start, it must be bone-dry. Then, it must be loaded into the kiln with a great deal of care. The kiln is closed and heating slowly begins. 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, which can cause the clay to burst. The bisque firing continues until the kiln reaches about 1730 degrees Fahrenheit. At this temperature, the pot has sintered, which means it has been transformed to the point that it is 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" is removed. Potters apply a layer of glaze to the bisqueware, leave it to dry, then load it in the kiln for its final step, glaze firing. The glazed item is carefully loaded into the kiln for the glaze firing. It must not touch other pots or the glazes will melt together, fusing the pots permanently. The kiln is heated slowly to the proper temperature to bring the clay and glazes to maturity, then it is slowly cooled again. The kiln is opened and unloaded after it has cooled completely. This second kiln firing causes a remarkable change in the clay and glaze. It completes the transformation of pots from a soft, fragile substance to one that is rock-hard and impervious to water and time.
