The painting supposedly portrays St. Vitus, who was apocryphally boiled in a cauldron of oil, but that sure as shit looks like a bread oven to me. Vitus was a contemporary of Diocletian, and given the whole "what's fourteen centuries between friends?" ethos of the clothing (so common as to be normally unremarkable, btw) there's a reasonably good chance that the artist just said fuck it and painted the food preparation implement that looked the coolest to him.

There's also St. Lawrence (of Panisperna), who was martyred by roasting, but on a gridiron, not in an oven. Somebody with better skills than me will have to figure out what's really going on here.

As for the hose, that's probably just how they were made.