Now that I read it again, the comment was about storing the master password in a safe, in which case, you could stamp it into a metal plate or some other solid

Now that I read it again, the comment was about storing the master password in a safe, in which case, you could stamp it into a metal plate or some other solid that has a high enough melting point. Steel seems like a good choice of melting temperature, with titanium even better. Or try tungsten, for a much higher melting point.