firing, process of treating clay or other plastic ceramic materials with heat to produce a hard, durable but brittle material such as pottery. Primitive potters baked their clay in an open fire, but for firing at higher temperatures and for the use of glaze, a kiln is needed. In general, pottery is fired once to harden it into biscuit ware, then a glaze is applied and fused with the clay by a second firing. China painting, enamel work, and stained glass also require firing. Temperatures of firing vary from about 1,100°F (590°C) for fixing paint on glass to about 2,800°F (1,540°C) for producing hard porcelain. Certain ceramic materials, such as those used for rocket nose cones, are fired at still higher temperatures.