The Process, Continued - Firing
The glaze and the pot itself contract at different temperatures as they come out of the kiln and start cooling rapidly. The glaze contracts more quickly, and because it can’t force the pot to contract as quickly, it starts breaking up. This creates random cracks in the glaze--called crazing--that makes raku ware so beautiful and so recognizable. Some potters will rub India ink into those cracks to make them show up more clearly, but often the smoke from the burning wood chips does the job by itself.
Pyrometric cones are small slivers of specially formulated material that melt at specific temperatures. By placing appropriate cones in a kiln, viewable through peepholes, the potter can determine with some accuracy the temperature in the kiln, and whether the firing is proceeding on the desired schedule.
When the cone starts to melt, it bends over, and when the tip of it bends far enough to touch down at the base, you know the temperature in that part of the kiln has reached the temperature that cone was designed to indicate. Potters will usually put in several cones that span the range of the desired final temperature so that they can monitor how the firing is going and adjust to slow it down or speed it up.
For example, if I want to fire a gas-fired kiln with pots glazed in a copper red to Cone 10 (around 2380 degrees Fahrenheit), I will put in cones designed to melt at Cone 010 (a very low temperature), Cone 5, Cone 6, Cone 9, Cone 10, and Cone 11—one set in front of a peephole near the bottom of the kiln and one set near a peephole at the top of the kiln. I need to create a