I've gone through 2 stones (I think they were 12") - and used to put them right on the metal above the burner. Both cracked the first or second time I used them. Used the second one in 2 pieces for a while, but finally threw it out. Now, I set the timer on the microwave to remind me to turn whatever I'm cooking half way through the cook time. Seems to work for me, but I have found the oven cooks better if I set it for 25 degrees hotter that the directions call for, and often have to cook things 2-5 minutes longer than called for.