[echoangle]

Just make sure that the metal you use has a high enough melting point.

[direwolf20]

Do people usually find big pools of metal on the ground in burned houses, or is everyday metal fine?

Especially inside a fireproof safe.

Wouldn't trust aluminium, solder, Wood's metal, gallium, or mercury, but apart from that...

[wtetzner]

Maybe a clay tablet (assuming it's safe from water)?

[Wistar]

Tungsten, perhaps.