Silicon Carbide

Silicon carbide neither melts nor burns away in a glaze. Instead, it oxidizes. Once the glaze has melted and sealed over, no air can reach it, so it takes its oxygen from the glass and the metal oxides around it. That reduces the metal oxides, and the reaction gives off carbon monoxide. The gas causes the craters and the borrowed oxygen causes the red, and both happen inside the glaze while the surface stays open to the kiln and re oxidizes. The amount decides which effect dominates. One part in two or three hundred pulls a copper red out of an electric kiln, and several parts in a hundred swell the surface open into craters. On its own, with no colourant present, it greys a glaze.