I mean, obviously the vat must be made of a metal with a higher melting point. But then how did they cast the metal to make that vat?
In: Chemistry
they didnt. the “vat” (crucible) for tthe high temp metals is made out of ceramics or stone. You just find a rock and carve it out (or use chemically hardening ceramics)
* The vat can be made of something other than metal, like ceramic. Ceramic is a slurry that only gets hard once it dries, cures, and is fired.
* A lot of smaller smelting crucibles are made of graphite (carbon) which has a high melting point but is soft and easy to just carve out the desired shape rather than casting it.
* If it’s a metal vat, it didn’t necessarily have to have been cast from completely liquefied metal, it could have been just heated enough to soften and then pounded into shape.
It can if the molten metals are hot enough. Large crucibles can be lined with special fire resistant bricks to insulate the pot from heat or be made out of heat resistant materials like ceramics.
Think of space shuttle tiles- glowing hot on one side, easily grabbed by the hand on the other.
On a larger scale such at in the steel works I work at we have ladles and vessels lined with Magnesia Carbon bricks, sprayed with a refractory cement made of ceramics and various silicas. These ladles can withstand molten steel in excess of 1700 C / 3092 F. The slag pots we use are just steel and refractory sprayed.
Edit: by large scale I’m talking 300 tons of steel in a 140 ton ladle. Not small scale crucibles.
some materials can actually wind up with a higher melting point than what you started with once the material has hardened and cooled due to how the atoms rearrange and bond during the process. Ceramics are one of these material types, and much of our high end blast furnaces and molds and such are made from ceramics.
As a fun fact, this is also why some igneous rocks, once they have cooled and crystallized, actually have a higher melting point than the lava/magma they originally came from.
When heating and working with molten metal, such as when casting it, the crucible will usually be made of a ceramic with a much higher melting temperature than the metal being worked on. Graphite, silicone carbide, and several metal nitrides are ones I’m most familiar with.
Metallic crucibles are possible, they will also usually use a material with a higher melting point, it won’t have been cast to shape however. It would usually have been sintered; metal powder pressed in a mold at high pressure and elevated temperature to form a cohesive solid, then subsequently treated and coated to give it the properties to hold the liquid metal it’s made to handle. Tantalum and Tungsten alloys are examples I’ve worked with of sintered metal crucible’s.
It is possible for a crucible to safely hold a metal who’s temperature is above the crucible’s melting point. The furnace design, specifically the insulation, will have been specially put together to ensure energy flow through the crucible is sufficient that it never melts. This may be because of the furnace heating method needing a metallic crucible to work etc. usually someone doesn’t want to replace a furnace when a crucible and insulation pack are much, much cheaper.
The Crucibles where I work are ceramic, a mixture of special clay (similar to what’s in a dinner plate) and graphite, like in a pencil- about 90% graphite. They’re molded like clay, pounded by hand with mallets in some cases, and slowly dried for days, even weeks until they harden. They have to be absolutely dry before use, any water in them will cause them to quite literally explode at molten metal temperatures.
The melting point of graphite is 3600 degrees C. Iron is molten above 1600C, and copper around 1200C, so graphite crucibles are quite capable of not melting under molten metal temperatures.
Instead, what gets them, is either thermal expansion causing cracking over time (The crucible gets larger, then smaller each time it is heated then cooled) or the reactivity of the metals in the melt reacting with the graphite and generating slag. Aluminium is our least favorite, if the glassy surface of the crucible is damaged, aluminium in an alloy can get in and eat a graphite crucible from the inside out.
Then there are some other tricks, like an insulating sand liner behind the crucible, and water cooling behind that to keep the heat of the melt from cooking the heating coils, the hydraulics and the other stuff that surrounds the crucible.
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