One thing that I like to do is make things from scratch. Particularly things that not everyone makes or can make. Magnalium is a 50 50 alloy of aluminum and magnesium that is used primarily in pyrotechnics. It is quite brittle making it easy to break up into bits or powder.
The only magnesium I have burned is the ribbon so I wanted to see what I was getting into with a whole crucible of burning magnesium. I took about 3 cubic centimeters of magnesium and heated it with a torch until it melted and started to burn. It was one of the strangest sort of fires I have seen. There was no sparks or flame and little smoke. The metal just gave off a very bright white light and the oxide just expanded and piled up around the metal. The oxide pile was white and looked a bit like cauliflower. From that experiment I could see that the burning was not going to be much of a problem if I could put it out.
There are couple of YouTube videos of people making magnalium. In one of them a lid was kept on the melt to keep it from burning. That seemed like a good approach but I thought I would also try something else. Some references mention putting sulfur on the molten metal. The sulfur reacts with the magnesium to make a magnesium sulfide that will cover the melt and prevent it from burning. I am not real sure about that as magnesium sulfide is listed as melting above 2000C. This is way hotter than the melt so it would still be a powder. Commercially when casting magnesium they use sulfur hexafluoride gas to prevent burning. Using a gas would not work for my setup and I don’t have any sulfur hexafluoride anyway. I got to wondering if one could use charcoal granules to cover the melt. I know that molten magnesium will react with carbon dioxide but I do not think it will react with the carbon. So that is what I set out to do, cover with charcoal and keep a lid on it.
I wanted to use something for a crucible that was expendable and fairly tall so the exposed surface area would be small. In the YouTube videos a tin can was used. This is not a good idea. I have had cans burn through when melting aluminum. It would be a bit messy and waste my metal if there were burning metal running out of my furnace. Something a little heaver would be good. There was an empty torch propane tank laying around the shop so I cut it in half to use the bottom for my crucible. The inside was coated with kiln wash in hopes that the metal would not bond with the steel and make it hard to remove.
The magnesium I have is an old VW transaxle and the aluminum is from my ingot stock made from scrap metal melted down. I weighed out enough of each metal to fit in the crucible and cleaned up the magnesium to try and keep the dross to a minimum.
In the photo above is the piece of transmission case, aluminum ingots and the kiln wash coated crucible.
The aluminum is melted first and then the magnesium is added. This helps to keep the amount of burning down. At the same time the magnesium was added I added the charcoal. After covering it and waiting for a while for the magnesium to melt I stirred the melt to mix the two metals. The charcoal cover was working as there was little burning magnesium.
After that the heat was turned off and I went off to do something else while it cooled down.
I came back a while later to see how fast it was cooling off a got a surprise. Something was oozing out of the crucible and had even somehow gotten on top of the lid. It was black and lumpy looking a bit like a fungus growth.
I had no idea what the stuff was but it looked like using charcoal as a cover was not a good idea. The stuff coming out appeared full of bubbles and had a layer of metal just under the black coating. Under the metal was some white material.
After things cooled down enough to handle I could look at it a bit closer. Apparently the metal had foamed up and forced its way out of the crucible. There was magnalium in the crucible. It was brittle and I could easily break it up with light hits with a hammer but it was filled with bubbles.
The charcoal got mixed down into the melt when I stirred it and did not float back to the surface as I expected. Each charcoal particle produced some gas the filled the melt with bubbles. It is really quite beautiful and extremely light with all the bubbles. If you look close at the picture you can see the charcoal bits in some of the bubbles. I have no idea what gas was being given off by the charcoal especially since it had sat there at almost red heat for several minutes before I stirred it up. Next time I will skip the charcoal.