OK, maybe one of my blog-readers can help with this. I had had multiple explosions in the bisque kiln when I opened it last night to check on the dryness of the pots. The greenware was reasonably dry when it went into the kiln and I had planned on leaving it there at 160 F for about 20 hours, to insure they were fully dry. That's my routine and it works 99 percent of the time. After the pots are fully dry, I turn up the kiln and they bisque to cone 07. I rarely lose any pots this way.
The first photo you see is the bottom shelf of my bisque kiln as I found it this morning. Those thousands of shards were once two 10-inch slab plates, made into slabs by pounding them out with my fist on the wheelhead. They were about a half-inch thick and, I thought, pretty thoroughly dry.
The other photo is from the top shelf of the bisque kiln, former baking dishes built from thrown bottomless walls and attached to similarly pounded and rib-flattened slab bottoms, about 3/8 of an inch thick. You can see that the baking dishes blew apart at the joint, which makes sense to me and points to the likelihood that they were not thoroughly dry when they went into the bisque kiln. The joint was thicker than the walls or the floor of the pot. Somewhere between ambient temperature and 160 degrees F they blew apart.
I can understand that. My mistake. I've made those mistakes before and it just reinforces the lesson of starting with completely dry greenware. What I don't understand is the other pots (and there were several more than I'm showing you) and their thorough disintegration at that low temperature. Were they still too wet? Does the pounding of the clay set up multiple stresses that result in these explosions? Was the phase of the moon to blame?
I'm kind of at a loss. Any ideas?