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My father collects coins as a hobby and lately has observed some green spots in some of his coins so he told me to ask here to see if anyone knows what it is.

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The coins are made of silver (999 parts out of 1000 according to the description) so at first he thought that it might be some silver oxide but the silver oxide is dark brown and not green.

He looked it up in the Internet and he has read that it might be copper chloride but it doesn't seem right to him due to the purity of coin.

Is this feasible? Can the spots be copper chloride? If not, what can they be?

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    $\begingroup$ Unless for some reason you prefer to breathe chlorine at home (which would be very peculiar indeed), I find the copper chloride hypothesis quite improbable. More likely, this is basic copper carbonate, a typical component of patina formed on copper as it gets oxidized by air. $\endgroup$ – Ivan Neretin Dec 29 '15 at 14:15
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    $\begingroup$ Indeed, as Ivan noted, it is most likely copper patina. Silver is usually obtained as a byproduct in copper refinning, so a copper impurity is likely. $\endgroup$ – permeakra Dec 29 '15 at 14:25
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This phenomenon is relatively common among jewellers and silversmiths and they call it verdigris. This verdigris can be copper oxide, copper acetate, copper carbonate and even copper chloride. The former options are more likely, but copper chloride can actually be a possibility if you live close to the sea.

Perhaps relevant to your father: verdigris can be cleaned by applying a mildly acidic solution to it, such as lemon juice or vinegar. The formation of verdigris can be prevented by keeping the metal dry as this kind of corrosion is typically formed by contact of the metal with moisture.

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