It's like stove and an oven combined, except heated and ignited by some nonmagical, inobvious mechanism. [Roy is intelligent enough to have deduced this much, based on shape and size and some results of his own experimentation.] Clearly what my world would take as reasonably rare magical advancement, this world takes as more common in these devices. I'd say they are of gnomish make, but I haven't seen any gnomes here.