Pete wroteSure thing! Like they would need to worry about anyone using a torch to break into something that big! That is interesting. So I guess that copper has a high enough melting point that torches would not be able to melt it?
The idea of the copper layer is not that it melts at a high temperature (it doesn’t, it melts at a much lower temperature than steel), but that it conducts heat much better than steel. If you try to attack the door with a torch, in order to melt the steel you have to really heat it, but only right where you are cutting. The copper takes that heat and redistributes it over a wider area, cooling the spot where you’re trying to cut, preventing you from effectively using the torch.
That was the theory. I don’t believe anybody other than Mosler worried about this or marketed copper-layered doors. And even Mosler gave up on it; my notes suggest it was introduced around 1921 and they stopped marketing it by 1930. So it might have been more marketing hype than a reasonable deterrent. And that much copper wasn’t cheap then or now. Plus, copper is not an easy metal to machine.
I haven’t ever found out much about the technical details, like how the “sandwich” was made and held together, or whether they used copper or some other alloy (beryllium-copper is an even better conductor of heat but it’s a dangerous alloy due to the beryllium and in the 1920s it may have been too exotic/expensive; brass/bronze is easier to machine).
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