Minecraft wiki entry for Metadata_variants, associated text: Furnaces (as well as lit furnaces) with metadata values 0, 1, 6, 7, 12 and 13 would have no faces, using the side texture for all orthogonal faces and the usual top and bottom texture on the top and bottom. Furnaces and lit furnaces with data values 8, 9, 10, 11, 14 and 15 would appear normal, but were probably not legitimately obtainable.