Minecraft wiki entry for Smelting, associated text: The smelting process can be automated with hoppers on the top and bottom of the furnace. For larger smelting jobs, a third hopper on the side of the furnace can feed in fuel and, in case of lava being used as fuel, any empty buckets come out of the bottom hopper. This automatically feeds and empties the furnace so that different materials can be smelted in the same batch with no loss.