So, I emptied the kiln this afternoon - a ^6 firing. One mug cracked, some mugs OK, some stuck to their coasters. Spent a while with the angle grinder and a diamond disk and cleaned the mugs. While working on the last one, I glanced at the first one I'd fixed and there, from top to bottom, was a crack. That made 2 cracked mugs from one firing.
I'd also made a spatula rest Mark IV. Mark I is in daily use but is ^04 and I wanted a stoneware one. Mark II had a glaze failure and it has bubbles all over it. Mark III jumped out of my hands on the way to the kiln and shattered. Mark IV emerged from the kiln in glorious perfection. I smoothed it's feet, washed it and carefully carried it into the kitchen. The other half picked it up and studied it - he is after all the one who will use it - and it slipped out of his hand and crashed into the sink.
I know what I'll be making at pottery class tomorrow...... more mugs and Mark V spatula rest.
Any suggestions as to why the mugs cracked? One was rolled and formed into a tube, the other was slip-cast. Both are ^6 clay, both had ^6 white gloss commercial glaze on the inside and outer rim, both had (different) commercial ^6 glaze on the outside. Both cracks appear to be on cooling, sharp edged. I've had three mugs crack all with the same glaze, one hand-made, the other two slip-cast from different moulds. I'm beginning to suspect the glaze. Will have to make some more samples.