Jean,
I have used quarry tiles which are a half inch thick, and come in
various sizes and shapes. They are unglazed and are able to
withstand the high heat of the oven. They are not very heavy and
tend to move around on the shelf so they frequently need straightening.
Because they are heavier, I prefer to use firebrick. They are about
one and a quarter inches thick and are used to line fireplaces,
kilns, fireboxes, and furnaces. Mine are 4 and a half inches by 9
inches and I can fit 6 on a shelf. I also use 3 of them in a broiler
pan on the bottom shelf to create steam; I pour a cup of boiling
water into the pan right after I load the bread into the oven. With
all that mass, I preheat my oven about an hour or more before baking.
I don't understand why, but I have been told that adding excessive
mass to the oven can create temperatures high enough that nearby
walls can get too hot and burn. I have a self clean oven which is
well insulated and I have never noticed any excessive heat on the
outside of the oven. Maybe someone else can explain this phenomenon.
Cheers,
Dave Glaze
Powell River, BC
Canada