system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the material used to hold molten metal in a foundry and why doesn't it melt?

Every video of large foundries I've seen show these massive reservoirs of molten metal...but look like they're  (the reservoir itself) made of metal too. I get that different metals have different melting points, but what material holds the reservoirs' molten state? Some foundries are massive, so I'm having trouble picturing how they're made.
gpt
It depends on the material being melted, and the quantity.

When looking at things like bronze or aluminum (950C and 660C respectively) using an iron or steel crucible (just over 1500C melting point) gives a pretty large margin where the crucible won't melt.

Of course, what happens when you need to melt and hold iron? A steel allow crucible is used, lined with a special ceramic that can take the heat and insulate the crucible from the molten metal (that *could* melt through, if not for the ceramic insulation).

On some higher end ones, there is an extra layer of insulation between the internal ceramic crucible, and the external "support" crucible. Sometimes, the outside crucible is even water cooled to help keep it from melting.

TL;DR the reservoir is made from a special ceramic, the metal is just the outside container to provide support.