Based on the venting, I'm guessing you would put your hot coals inside to heat it up.
Did you know there used to be gasoline powered irons?!?! Check out this video of one being lit up. HOLY COW! I would have burnt the house down with that thing.
It gives me a whole new appreciation for my iron and ironing...which I don't like doing.
Good thing we've come a long way baby:)