That's a good question, but a very complicated answer. keep in mind I'm a backyard metallurgist, so take everything I say with a grain..
In alloy making, melting points are only for reference. You can alloy such and such a metal, say with a 4000F melting point, into molten iron @ 3300F. Why? It's called, well I actually don't know what it's called, but I call it the flux effect.
Just like molten flux can destroy a forge lining, several hundred degrees below it's melting point.
I would never use any metal crucibles to hold any other molten metals..
Although you can drastically reduce the melting point of alumina, and hold the liquid in a platinum crucible. Alumina, aluminum oxide, has a higher melting point than platinum, but the flux changes that.
The making of Ramaura Cultured Rubies
I live in rural St. Paul, next to Champoeg.