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How do you melt magnesium without causing its combustion?
I own a foundry capable of reaching temperatures around 2000 °F, and I recently bought some magnesium for melting before doing my research. I cannot find any information on how this process works or if it is even possible.
I have seen molten magnesium; it was covered with charcoal and other fluxes and in a furnace that had a gas shroud. (I think methane that burned any oxygen that got into the shroud atmosphere.) When an immersion thermocouple was put into the magnesium, the open spot immediately burst into flames. I recommend that you not try to melt magnesium; you certainly must get more information than you can get on the internet.
As with any science, advances are made by doing something stupid or accidental - the key is being able to do something intentionally stupid safely enough to do it again. That being said, wear proper safety gear - flame retardant clothes, blacksmithing gloves, a full face shield (i.e welding mask) not just safety goggles from a home improvement store. Once you're pretty sure you're not going to get hurt, then you can proceed.
Before you begin, you should know that this is not the cheapest way to do this, nor the most efficient, but it is effective, and will produce results you're looking for.
You will need:
Find an extra-tall, nonporous crucible (steel should be just fine), and place it in the cold foundry, making sure it has a sturdy base. Add the magnesium into the crucible, and fill to the top with argon. Begin heating up the foundry and crucible. Argon is heavier than air, so it will push the air out as it fills the container, which if you're using a charcoal foundry, will be evident by a ring of charcoal around the crucible momentarily going cold as the argon replaces the oxygen. At this point, sit back, and go about the melt like you would anything else.
If you're casting it, the trick is getting the mold to be encased in an argon bath as well, for that you will need a much larger container and a lot more argon.
Look into how they melt titanium – it has all the same issues.
Magnesium is best melted under a flux – these fluxes are normally based on magnesium chloride, with additions of barium-halogen compounds. Obtaining the flux could be difficult due to the toxicity of the barium compounds – it is even difficult to obtain when you are in the industry like myself.
Another option is a reactive cover gas like $\ce{SF6}$ or R134a, however these are expensive, and are extremely bad greenhouse gases.
