When exposed to high temperatures this plaster material expands due to the vaporization of moisture held within it, sealing the door shut. This helps to protect the contents against temperatures in excess of 350 degrees F for the specified period of time (usually 1 to 2 hours). Look for an Underwriters Laboratories label. If it doesn't have one, it is not a real fireproof file or record safe.
Products of other enterprises
This group of safes are usually the least expensive of all safes, since they are rarely designed to afford significant protection against burglary or surreptitious entry. Their construction usually consists of a thin sheet metal skin with a poured-in gypsum plaster insulating material. Fireproof files may also be considered in this category.
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Fire / Record Safes