Why did my salt turn green?
My S.O. recently brought home (from a flea market) a cute little dish with a hinged lid made of some unknown metal (silver? tin?) and which was probably an ashtray. Since neither of us smoke, we decided to keep it on our dining table (after washing it very thoroughly) as a salt cellar.
It was working well for the past few months until last week, when we started keeping fleur de sel (as opposed to Kosher salt) in it. Within days, the salt turned green.
Common sense tells me that I probably shouldn't eat green salt, but I'm wondering if any chemistry-savvy hounds can enlighten me on what exactly happened here. I'm guessing that the fleur de sel reacted more than Kosher salt because it's moister. I'd welcome further thoughts, explanations, etc....
I suspect your dish is made of whitemetal, not silver. White metal is a mixture of (mostly) tin, copper, and antimony (also known as white lead). If you want to make sure, try to clean it as you would for silver. Whitemetal will become a distinct yellow-copper colour.
Fleur de Sel contains, of course, mostly sodium chloride (NaCl) as well as calcium and magnesium sulphates, among other things. As the salts, especially NaCl, are very soluble, they can easily mix with the moisture in the air, which effectively "frees" the ions to create other compounds with the metal.
It would be easy for this trace compound to spread from crystal to crystal within the salt.
From your description, the only compound I could come up with that was green was copper (I) chloride (CuCl). It is a white crystalline salt, but will turn green in moist air. If this were simple oxidation, your dish would be turning green as well (and maybe it is).
All of the other compounds I could come up with were blue (copper sulfate) or grey or white. That includes compounds of tin and antimony and silver with chloride or sulphate. None of these compounds--including copper chloride--are good for you, and over time are poisonous (what are known as cumulative poisons).
Throw the salt out, and don't use the dish for food anymore.