A cold spot near the flue took the load. Eleven planters came out with hairline cracks at the base, all in the same corner of the same shelf.
The twelfth planter is on the shelf now. We moved the thermocouple, and the next firing gets a slower climb through quartz inversion.
Eleven cracked. The twelfth is the one you can buy.
Terracotta fires low, at cone 04, in its own load. On the way up and on the way down it passes quartz inversion, at 1,063°F, where the silica in the clay changes shape and the pot shrinks a little, all at once. If one side of a pot is cooler than the other at that moment, it cracks.
The thermocouple was reading the middle of the kiln. The corner by the flue ran 60 degrees cooler on the way down, so the bottom shelf crossed inversion later than the top. Now we cool from 1,100 to 1,000°F at 100 degrees an hour, damper closed.
