Minecraft wiki entry for Block_of_Coal, associated text: Blocks of coal can be used as fuel in a furnace. One block of coal lasts 800 seconds (16000 ticks), which smelts 80 items. This is ten times the duration of a single piece of coal and 11⁄9 times as efficient (+11.11%) as nine individual pieces of coal—which would only smelt 72 items. The latter, however, prevents inactive furnace use; while smelting one item with one coal wastes seven potential items, smelting one item with one block of coal wastes 79 potential items.