From Sturdy's website:
http://www.sturdysafe.com/sturdy_002.htm
"Sturdy Fire Safes are made with a first layer of heavy steel (7 or 8 gage). The second layer consists of a 2300 deg. U.L. listed ceramic wool. The third layer is a U.L. listed 1000 deg. glass blanket. The fourth layer consists an inside sealed steel liner. Which compresses the two insulating blankets to a total thickness of two inches or more throughout the safe."