Depends entirely on the design and structure of your forge. Heat can be added in unlimited quantities, and so long as it cannot escape through any openings or through anything weakly insulating, it will simply accumulate … and accumulate … and accumulate, as you add more and more joules. The temp will get hotter … and hotter … and hotter. What your source of heat is, is irrelevant. This is how the interior of your car gets hotter than the surroundings on a sunny day, despite the source being the same, yes? Containment of the slowly-accumulating heat.

It’s like weight. It doesn’t matter how heavy a hippo is, if we keep adding hippo … after hippo … after hippo to a set of scales, we can eventually reach whatever weight, yes? Accumulation, not individual hippo weight, is what matters. Heat in a forge is no different, assuming your forge contains all the heat produced properly.

And they didn’t, they collapsed starting higher up. Check an unedited video.