Im getting desperate.
Is there any danger in using a tile that youre not sure is food safe but using parchment as a barrier between the bread and stone?
Perhaps. None of us are going to be able to totally set your mind at ease or indemnify you from health risks.
That said, I've done it more than a few times. I'm not dead yet.
So do it at your own peril.
Most of the health concerns are probably not that serious. What I am getting at is the fact that when most of our grandparents grew up there were probably a whole lot fewer of these over concerned health advices. And many of them did still live to 80-90 years of age.
The biggest problem in our age is worrying to much about small things. What really kills you is cars, cigarettes, alcohol, fatness from over eating etc. not the trace amounts of lead you might find on your baking stone. And if you still are concerned just put a sheet of parchment in between you loaf and the stone.
I worked for 5 years for one of the world's largest industrial refractory manufacturers. I would strongly advise against using any refractory for cooking unless it has an MSDS or similar documentation stating that is is food safe. Although I cannot violate confidentiality on the components or manufacturing processes I _can_ tell you that we had strict separation between our industrial refractory processes and our medical/laboratory processes - and for very good reason.
Sometimes there are wolves in the woods. Even if saying so is considered "politically correct".
If you, as you say, use a parchment barrier under your dough, then there should be no material (hazardous or not) transferred to the bread by direct contact.
Two other things to consider, though.
-- Shrapnel! If the tile is glazed, then rapidly heating or cooling (like from the dough) it could lead to some of the glaze cracking off (gently or otherwise).
-- Vapours! Even after washing (or rather scrubbing) the thing thoroughly, its a good idea to bake it, on its own, until any smell has gone! The worry here is really contamination during transport and storage - and its likely most problematic with a material that is at all porous (I'm thinking principally of terracotta here.) But if its been varnished (or similarly coated), you want to burn off (or otherwise remove) that coating (if it is ever going to come off) before you put food near it.
Generally, if it is a bit of real (rather than "reconstituted") stone, ideally from a hard igneous or metamorphic rock, and you clean it and bake it thoroughly before use, then it isn't going to poison you. Its actually pretty hard to get things to dissolve quickly out of real rock, so don't worry about that.
One method I have used in restaurants to press sandwiches and bake bread was to wrap regular old bricks with foil...this should work just fine or covering your tiles with foil to prevent contact and vapors. Just an idea:)
You should have few problems with tile. Try to use quarry tile that is glazed, tile is baked at several zillion degrees and after exposure to temps that high everything nasty is burned off. In my oven I used several 10 X 10 tiles to cover the middle shelf. I had several cut to fit at the local Home Depot store which provided cuts at no charge.
i just bought some 12x12 saltillo tiles at homedepot, but none of my local home depots will cut them! they all say they used to do that but not anymore. without cutting, all i can fit is 1 tile in my oven. ugh...
If you can get the correct shape that you need in something such as pyrex you would be totally safe. Pyrex has almost identical heat capacity to granite though it is more conductive to heat so I can't be sure how well it would work.
What is the reason for using/wanting this stone?