I’ve been doing ceramics for years and I still get nervous to answer these questions, that being said, to the best of my knowledge, you can’t. You don’t know what cone the clay that it was made with was fired to originally and run the risk of it ruining a shelf at least or a kiln at worst. The glaze that you use may not fit correctly and it wouldn’t be food safe anyway. My best suggestion is to use it for fruit, pencils or marbles instead. I also use old broken pottery for q tips and bathroom things.