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.
Ceramic glaze is an impervious layer or coating applied to bisque ware to color, decorate, or waterproof an item. For earthenware, such as fired clay pottery, to hold liquid, it needs a glaze.
Potters apply a layer of glaze to the bisque ware, leave it to dry, then load it in the kiln for its final step, glaze firing.
Glaze firing requires that glazed items are very carefully loaded into the kiln. Pots should not touch each other 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.