Minecraft wiki entry for Beginner%27s_guide, associated text: Smelting takes some time, but you don't need to stay in the GUI, as the process continues while you go do something else. A furnace automatically uses fuel (one item at a time) from the fuel slot as needed, until either all of the items in the input are smelted or all of the fuel runs out. If the input items run out before the fuel, the furnace stays lit until the current fuel item is used up. You can put in more items to use up the rest of the burn time. While the furnace is lit, the furnace block becomes a temporary light source, displaying fire particles and making popping sounds.