Minecraft wiki entry for Furnace, associated text: The main purpose of a furnace is smelting. Its interface can be opened by pressing the use item button on it. A fuel source (up to one stack of fuel items) is placed in the lower slot, and the items (up to one stack) to be smelted are placed in the upper slot. A furnace smelts items at a speed of one item every 10 seconds, or six items per minute.