Hey Kevin, I can’t swear by this yet, but I seem to have a decent mixture, made from clay, and white blasting sand. I have made samples, and heated them red hot with a torch without deterioration. My latest formula is equal parts perlite, sand and clay, I’m thinking the reduced density will be useful.
I would say find a local brick type clay, and experiment. It isn’t rocket science, and if it does crumble, you can replace them later. Of course, the bricks will need to be made well in advance, and dried slowly in some sheltered place, then can probably be dried good in an oven, then fired in the stove in place.