How cool is this stuff! Found it at a reclamacion store and paid $0.05 for it. First pass it turned a chalky grey black which was pretty cool in itself. Did a second pass and it turned silverish. Did a third pass and it looks like a very deep black, hard ceramic glaze. Interesting there were little pieces of it that never darkened/burned though. It’s sparked pretty good when being burned, but not dangerously so. What is it, is it some kind of slate or shale?