Looks like there may be a major change bearing down on my pottery life. For years I have used a procelain-type of clay called 6-50, made by Tucker's in Toronto. It fires very white and smooth, throws very well, hand-builds not too badly for my platters and flat dishes, and I have gradually worked out my problems with glazes and firing.
But I have had to avoid making ovenware.
Too many pie plates came back cracked.
The other on-going problem has been that occasionally someone will report that their teapot has cracked. I have had to replace a number of them. Last year I didn't hear from even one customer about that, so I was starting to think maybe I was homefree. Not so. Right after Christmas a young woman called and said her teapot had cracked..... she had only used it for one year.... she was conscientious about pre-warming.... Bad words ran through my head (but stopped before exiting) and a heavy growly feeling descended on me. Going into the Studio became a not altogether happy thing.
Co-incidentally, there was discussion on Clayart, a list I follow, about ovenware. I asked my question: "Is it possible to make ovenware at cone 6?"
Of course I got some of the expected answers: it was my glaze, it was the shape of the piece, nobody else ever had a problem, my customers were obviously dumb users, yada yada. In the middle of this I did get a useful response talking about thermal shock. That rang the bell.
I always did say it was the clay that cracked, not the glaze. And the cracks did not occur at 'edges' or 'corners', they occurred in the middles of flat surfaces or walls.
Apparently, thermal shock can happen after numerous small shocks.
The only answer: switch clays.
I'm going to test the other Tucker's white cone 6 clays but I am not optimistic. I've already tried them and both gave me other problems. But I'll try again, and if neither of them work for me, I'll have to try another company's clays. I want to support Tuckers' - they have helped me a lot in the past, but enough already. I need to make pie plates and casseroles.
I'm trying to see this as just another adventure! I'll keep you posted.