Bloating, and why I now use witness cones
The October glaze firing came out of the kiln with blisters on the base of three mugs and a slight swelling in the walls of a vase. Bloating: the body had been overfired, and the gases trapped in the clay pushed the surface out like dough.
The kiln controller said it reached cone 6. The pots said otherwise. The thermocouple in a shared community kiln is not exactly cared for, and it had drifted.
Witness cones
A cone pack costs almost nothing. Three cones in a wad of clay, placed where you can see them through the spy hole (or, in our case, where you can find them afterwards): one below the target, the target, and one above. If the target bends to about four o'clock and the one above stands straight, the firing was right.
The next firing, with a cone pack on the middle shelf, showed the kiln running nearly a full cone hot. The controller now finishes twenty degrees lower, and the bases are smooth again.