Another idea if you want to hide your seed phrase from metal detection. It is NOT to use metal plates. To do this, you can use plates from
1 - fiberglass or glass fiber laminate (without a metal layer of copper, so that the detector doesn't detect it), which has mechanical resistance, inertness to chemicals, moisture and water, resistance to thermal effects, but I suppose it will be vulnerable in a strong fire.
or
2 - ebonite, which is resistant to alkalis and acids, resistant to water, lends itself well to engraving (machining). Doesn't burn, but chars at high temperatures, which may make the text on the surface unreadable. So slightly brittle that it increases at low temperatures.
You can search for other materials with similar properties. These, of course, are a little exotic, but quite accessible if desired (depending on the location) and can be used to achieve OP's goals. You should also know that both fiberglass and ebonite come in different grades and have different characteristics. In general, if the goal is to make the seed phrase invisible to a metal detector and there is no threat of burning, then this option can be used.
Why not using a polished stone instead or a granite/marble plaque used to decorate graves? I know it's quite difficult to engrave but it's very resistant to water and fire (even more than most of metals IMO) and it can't be detected by metal detectors, another option could be to use some cement, it's easier to engrave and to manipulate.
Maybe because special equipment is required to apply text on a granite or marble slab? You're not ready to show your seed phrase to an engraver, right. Also, there is a size issue. A marble or granite plate will be larger than a fiberglass plate, for example. Otherwise, it will be fragile.
Cement is easily destroyed, especially if the manufacturing technology of this is not followed. Then you should prefer some polymeric compounds (based on marble, basalt, granite, quartz) that are used in the manufacture of stone countertops or sinks. But I can't say how polymer resins will behave in a fire.
In general, as I have said more than once: the choice of storage method and material is limited only by your imagination and goals.
1 - fiberglass or glass fiber laminate (without a metal layer of copper, so that the detector doesn't detect it), which has mechanical resistance, inertness to chemicals, moisture and water, resistance to thermal effects, but I suppose it will be vulnerable in a strong fire.
I guess it depends on the type of fire and the flame in the building. Some basic googling shows that house fire temperatures start from 1100°C and can go up to 3000°C depending on the flame. Wikipedia and some other sources say that fiberglass starts softening up at 1200°C and starts melting at 1700°C. Maybe it could survive the weakest of flames, but it stands no chance against the worst types of fires.
What do you think I focused on?
You highlighted this text in bold and simply described what I said in more detail. But thanks for your clarification.
~snip
You can search for other materials with similar properties. These, of course, are a little exotic, but quite accessible if desired (depending on the location) and can be used to achieve OP's goals. You should also know that both fiberglass and ebonite come in different grades and have different characteristics. In general, if the goal is to make the seed phrase invisible to a metal detector and there is no threat of burning, then this option can be used.
I have nothing to add to this.