I have a sample of gallium (a few grams). I've noticed every time I melt the gallium and refreeze it, it forms this grey skin on the outside. When I remelt the gallium, this portion doesn't melt and remains a dull grey color. I have skimmed it off every time and collected it. I looked it up and gallium doesn't seem to create an oxide or other compound unless under extreme conditions. I have noticed this when melting it in a plastic bag, a glass beaker, and a plastic container. I have been using hot water to melt it, but the water doesn't contact the gallium, only the container. It seems I am slowly losing gallium from my sample and have a small amount of this grey material saved in case it can be recovered. Does anyone know what this is and how it can be reverted to elemental gallium?
I have no real knowledge of it, and I've found almost no literature on its handling and storage. From Scientific American 1878:"Unlike lead, however, it acquires only a very slight tarnish on exposure to moist air..." and "In aerated water it tarnishes slightly." My very very old copy of Cotton and Wilkinson (Adv. Inorg. Chem) states that it is a moderately reactive metal and will react with sulfur. And of course we know it readily alloys (I'd say "amalgamates" except that term is specific for mercury alloys) with a large variety of metals. The (III) oxide is white, so unless your scum is white, it ain't that. The (I, sub) oxide is brown-black so that's a possibility. As are mixed oxide /hydroxides. Since the original investigation says it does tarnish in air, I'd take that at face value. There is the obvious possibility that your sample is either contaminated or was sold adulterated. If it's been in contact with any other metal (especially Aluminum) then it is almost certainly contaminated. There's no practical way you'll be able to recover the metal - the expense will far exceed the price to simply replace it. The best way to check for any large contamination is to determine its melting point, it should be very very close to its stated MP, shouldn't even be off by 1°.