Originally Posted by Andy M.
Clay used in cooking vessels must be free of harmful contaminants while clay used to make flooring tiles or planter bases does not. Clays used to make non-food tiles could contain lead, other heavy metals or other harmful things. Not saying the all do, just that it's not regulated.
IF - and that is a big unlikely IF, any natural occurance of metals, such as lead found naturally in the soil were in the tile, baking doesn't leech lead or other material into a pizza dough. To the contrary, the tile draws moisture from the dough, leaving the dough crisp. It's not as if anyone were using it as a pot.