I'm not at my modding computer right now, but are there by any chance keywords on the forge furniture objects that might indicate this?
You're certain they haven't gone and placed those same hazards on the vanilla forges?
Now that you mention it, there are. Mainly in the scripts, there's one named 'Blacksmithforge'. In it's properties there's a property named FlameDamage and it's type is spell. I don't really know how exactly to put it in other objects and make it work though. And no, they haven't putted hazards on the forges they have scripts on them instead.