Usually copper turns black because it forms copper sulphide or copper oxide.
20+ Copper Turning Black. You said you had put water in copper vessel overnight, and it turned black downside. To restore copper color and remove black spots from your copper cookware, maintain it properly.
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As time goes on, though, it might start turning brown and then black. Traces of bromine, carried over with the gas, are removed by the copper turnings as black copper bromide if the level of the blackened copper in tube f rises appreciably. You said you had put water in copper vessel overnight, and it turned black downside.
Maybe the ones that turned black aren't of the best quaility copper.
With proper care, you can prevent your copper pots from turning black. Has anybody heard of copper turning black in an orgonite? It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. As time goes on, though, it might start turning brown and then black.
