Regarding stainless steel, several wrote that it's nonreactive, but that's a
misconception. It's called "stainless" not because it doesn't react with
anything but because the reaction doesn't stain and pit the surface. Unlike
glass, stainless DOES allow some chemical transference, even though we don't
see visual evidence of it. It may not transfer any elements, or enough of
them, to harm sourdough yeasties, however. One experiment in growing a
soudough sponge in stainless should tell you whether your bowl will work
(some grades may be more rapidly reactive than others).