Minecraft wiki entry for Smelting, associated text: The furnace, blast furnace, and smoker share a similar interface: At the upper left is a slot for smeltable item input, below that is a slot for fuel input, and on the right is a slot where output items accumulate and can be removed by the player. Flames above the fuel slot act as a gauge showing the gradual consumption of the current fuel item, and an arrow in the middle gradually fills to show the progress of smelting the current input item.