From time to time the use of copper in vault and safe doors has come up. I've generally associated this with Mosler doors, and in particular doors made from what they called Donsteel. Mosler also used copper in non-Donsteel doors. And it's been used in products from other makers.

The main point is to diffuse the heat from a torch in order to thwart such attacks.

I managed to find a Mosler patent that discusses this in the context of money safe doors. The "bad" news is that the patent was filed in 1960, while the practice actually goes back to the 1920s and possibly earlier. The patent mentions pouring molten copper into a door with appropriate cavities, and spinning it so the molten copper is forced into the spaces.

The patent is US 3,128,728. If I find any more patents, I'll post them here.