I'm not sure if it still stands, but years ago the APS would refuse insurance for stamps stored in fireproof safes. The firebrick they used at the time was designed to release water as it heated up, and that would do more damage than usual. It protected documents pretty well, but would activate the gum on stamps and get everything stuck down even in a fire that they'd usually survive in a plain metal box.
But that was years ago, and the technology may have changed.
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