Minecraft wiki entry for Automatic_smelting, associated text: The chest will not start receiving charcoal until both the furnace and the hopper feeding it fuel (level 2) are full. However, the hopper can be "stuffed" with four non-fuel items in its last four slots, as long as you leave the first slot open for fuel. Then the chest will start getting charcoal after two stacks of charcoal are produced, which will require 2-1⁄4 stacks of wood.