I'm new to home pizza making. First time was a success--After researching on Chowhound, I used a pizza stone, which I oiled first and then preheated at 500 degrees for at least an hour with the oven rack in the lowest position. The pizza came out just fine-- in fact delicious!. Second attempt--I did the same thing , except did not reoil the stone. Did not think this was necessary since I've read about people just leaving their pizza stones in the oven on the bottom rack all the time when baking other things. When I went to put the pizza on the stone after the hour of preheating, I was shocked to see that the stone had completely cracked through. Any ideas why this may have happend and how to avoid it in the future?