Minecraft wiki entry for Smelting, associated text: The furnace keeps track of experience for each item as smelting is completed for it, accumulating it in a hidden counter. It remembers the total earned experience even if a hopper is used to remove the items from the output slot. This earned experience is awarded to the next player who uses the interface to remove items manually, after which the counter is reset. (If the player takes some of the output but leaves some in the slot, the experience corresponding to those items is retained by the furnace and not awarded to the player.)