In my research I saw the importance of oxidation control, this will keep the expensive purity you paid for. This can still be done in open air, it just cuts down the number of times you can remelt bismuth and still get really nice oxidation colors on the crystals. What you melt the bismuth in is also important. Liquid bismuth will alloy with most metals in very small percentages, but even small adds up when your starting at 99.99%. Stainless steel works, but if you plan on remelting several times and still want good results you want a graphite crucible like the kind used in gold and silver casting.
Controlling the air in your crucible isn't actually too hard. A lid during heating will certainly slow down the skin formation. I plan on building a simple flow hood, not for the skin formation, but so I can attempt to determine the critical oxidation temps/rates. Since bismuth naturally oxidizes in air, there isn't going to be a hard temperature at which the colors form. It will probably be a time/temp/mass graph.
For this project I want precise temperature control which will require a scientific hot plate. Luckily a hot plate still works in a non oxygen atmosphere. A basic flow hood really isn't more than a box with a door and a bottle of shield gas, But I figure I will spend the 50% extra time to do it much closer to right. there will be pictures later, hopefully I get funded.
On a side note it appears you can reclaim that bismuth oxide scum by heating it to a high temp with carbon under a shield gas. This probably wont be useful unless you are manufacturing things with bismuth, and going through many melt cycles.
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