When I tried smelting items in a furnace (copper ore, gold ore, iron ore, among other things) using a lava bucket, once the initial stack of items were done and removed, I tried putting more in the furnace to smelt them down. All that happened, though, was the fuel level rising back up to full, despite being at least 4/10 of the way down before I put the new items in, and the items wouldn't smelt.