Minecraft wiki entry for Beginner%27s_guide, associated text: The furnace GUI has only three slots and includes two icons to indicate time. The top left slot ("input") is where items to smelt are placed. The items are moved from this slot one by one as they are cooked and the products are placed in the right slot ("output"). To cook items, the furnace requires fuel, which is placed in the fuel slot at the bottom left. (The example below shows how to smelt 3 logs into charcoal.) The "fire" icon burns down to show how much of the current fuel item is left, while the arrow shows how far along the smelting of the current item is. Items cannot be partly-smelted: If smelting is interrupted (you pulled the item out of the input, the fuel ran out, or you broke the furnace), the input item remains unchanged.