My understanding is that there are two major types of safes. Theft and fire.
Theft usually solid steel
Fire consists of three layers. Outside steel, sanwich core of a material that produces a type of water vapor at a certain temperature and the inner skin. They are meant to keep papers from charing and nothing more. They will come out a soggy mess but not burned to charcoal.

Fire safes are a no no for stamp colectors of mint items.

At least that is my understanding.
Wishing all enough,
Dick Staeffler R-3828