By crack, is that hairline
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By crack, is that hairline cracks or cracked in pieces?
In my experience, complete breaks are generally due to too fast heating or cooling. Smaller hairline cracks within the piece are due to thermal shock after annealing. It sounds like it was a fairly thick piece — perhaps it wasn’t completely cool when you removed it after slumping??
Also, I typically hold at a lower temp in the first segment — 1000 degrees rather than 1220. Seems to work for me, but as always YMMV.