If the furnace has an item in the input-slot, but no fuel,
looking at it from the outside, it will show "not cookable" or "item: 0%".
But from the inside, there is no indication
if the input-slot has a valid, cookable item or not.
It would be nice to have that bit of feedback inside as well,
e.g. by changing the background-color of the slot, and/or
by showing a text with the recipe / result over the output-slot.
Maybe even with extra info, about cooktime / burntime.