Just make sure that the metal you use has a high enough melting point.
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...
Maybe a clay tablet (assuming it's safe from water)?
Tungsten, perhaps.
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...
Maybe a clay tablet (assuming it's safe from water)?
Tungsten, perhaps.