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What makes Copper coins turn black

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What makes copper coins turn black?

Harry C. deleted

I am 12 and at school - Oswestry, Shrospshire, U.K.


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Usually copper turns black because it forms copper sulphide. This is produced when hydrogen sulphide (bad egg gas) or a solution of it comes into contact with the copper, especially if it is moist. If you hold a piece of copper long enough it will also turn black, but this is because your perspiration contains sulphur containing molecules that can break down in the moisture to form a solution of hydrogen sulphide, or similar solutions. the sulphur in your perspiration comes from proteins and the food we all eat - its not just from you!

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


April 13, 2009

I would like to know if there is a way to test copper that has turned black, as a result forming copper sulphide. I would like to test some copper I have and come up with a definitive answer as to the presence of copper sulphides and possibly amounts.

Thanks

Jose Colon
hobbyist - Sarasota Florida


April 17, 2009

Hi, Jose. Can you tell us where you are trying to go with this, like what your hobby is, what kind of component turned black, and why you are trying (I assume) to track the cause? Lab services can do SEM/auger analysis and determine the amounts, etc., but these testing services might charge more than you are interested inn paying. Good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


August 12, 2009

Hi, in my work as an electrician I often encounter PVC insulated copper cables that have a black coating. The black coating is undesirable as it results in a poor connection which generates heat when current passes through it. The composition of PVC is C2H3Cl but I cannot see a reference to a sulphur atom in there (unless a proprietary modifier has been added).

It seems the black coating may occur when moisture is drawn into the cable, possibly by capillary action in the interstices between the strands of copper.
Or, is there a possibility that a nearby sulphur compound in the vicinity of the exposed copper may liberate some of its sulphur into the air which may enter the interstices and travel up the cable?

Thanks for your help.

Peter Jackson
- Auckland, New Zealand

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