Well, that was a dirty shame.
Working with the soapstone mold, dropped the back pane on a piece of rock and it broke along a known (it was previously visible) fracture line. I was lamenting about it on FB and Dafne (the great and powerful), suggested I mend the rock with “plumber’s epoxy.” Apparently regular epoxy won’t hold up to the heat well, and plumber’s works better. So I hit both Fred’s and Ace and neither had straight up epoxy; at Ace they had “plumber’s epoxy putty” which you slice off a chunk of and then massage like play-dough and press into place.
I could, if the plumber’s epoxy stops working, cut off the pane or just get a different piece of stone and use that but this seems to be working fine for now. The fracture is pretty high up in the sprue, so it’s possible that cutting and sanding the top would be fine. But let’s see how this patch goes.