In my world, I wanted to smelt a ton of cobblestone to make stone bricks. I was planning on making a castle build. I knew that I didn't have enough coal, so I crafted a bucket, filled it with lava, and put it in the furnace. The entire bucket got consumed. At first I thought that the bucket was supposed to get consumed, but after watching a youtuber named Pixlriffs, I learned that the furnace gives back the bucket. I'm pretty sure that this is a bug, since I don't have an infinite source of iron.