If they weren't at least superstitious about things like souls and Gods, one would think Senku wouldn't have had any opposition to his plan. Or such would be Koumyou's thought on the subject.

But instead of going into all that, he's caught up trying to imagine a bronze... mirror. "I knew it could be polished up quite a bit, but into an actual mirror?"

Or had it been needed as a component, rather than the whole surface? That occurs to him, too, "Aren't old mirrors backed by silver? Is it something like that?"