there is a rumor about a bucket of lava in the furnace
Just put one in with a stack of 64 stone.
The bucket disappeared and fueled it, now to see how long it burns for...
Edit: Down two pixels in 20 stone.
If you happen to be smelting full stacks of iron and have a lava pit, this is a good thing.
Edit II: Looks like it'll do about two stacks to a bucket.