safety of using chamotte stone from hardware supplies store for baking

I'm going to disagree with Max and say that I would use it. My current baking stone is a slab of cordierite ceramic made for ceramic kilns, the same material many baking stones are made from. I got it because custom-cut kiln shelf is less expensive than a custom-cut baking stone.

However, there is a serious risk there. For both chamotte and cordierite, there is the possibility that the material contains traces of toxic heavy metals, particularly since they are good at absorbing metals from the environment. Supposedly, ceramic materials sold for food preparation are tested to make sure that they don't contain any, whereas ones used for other purposes are not.

My personal experience, though, is that (at least in the US) testing practices around things like baking stones are extremely lax, and I made the assessment that I was not reducing my risk by using a stone made "for cooking". I cannot back that with any science, though, so this is a personal assessment of risk. And if you're cooking for someone with elevated risk (such as someone with kidney disease), even the smallest risk isn't worth it.