Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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What makes Copper coins turn black
Q. What makes copper coins turn black?
Harry C. [surname deleted for privacy by Editor]I am 12 and at school - Oswestry, Shrospshire, U.K.
2003
A. Usually copper turns black because it forms copper sulfide. This is produced when hydrogen sulfide (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 sulfur containing molecules that can break down in the moisture to form a solution of hydrogen sulfide, or similar solutions. the sulfur 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
2003
Q. I would like to know if there is a way to test copper that has turned black, as a result forming copper sulfide. I would like to test some copper I have and come up with a definitive answer as to the presence of copper sulfides and possibly amounts.
Thanks
hobbyist - Sarasota Florida
April 13, 2009
A. 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 in paying. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 17, 2009
Q. 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 sulfur 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 sulfur compound in the vicinity of the exposed copper may liberate some of its sulfur into the air which may enter the interstices and travel up the cable?
Thanks for your help.
- Auckland, New Zealand
August 12, 2009
A. Hi Peter. Chinese wallboard with far too much sulfur ⇦this on eBay or Amazon [affil links] is a widely recognized reason for copper piping and wiring to turn black.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Is there a way to determine whether the copper cent turned black because of a person's perspiration or the actual chemical hydrogen sulfide.
Amanda Anderson- Selma Oregon
September 13, 2017
A. Hi Amanda. I don't think I've seen pennies turn black from perspiration. Unfortunately your question is rather abstract. You could start by analyzing the black coating to determine what it is; then with the analysis in hand you could try to determine what environment could lead to a coating of that material (there are other sulfide compounds besides hydrogen sulfide). But who are you, why do you want to know, and is it worth the cost of SEM evaluation to try to find out?
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2017
November 1, 2018
Q. I sell kits that allow you to make a portrait of Abe Lincoln from old and new pennies. How can one artificially turn a copper penny darker (though not black)?
All the best,
Maury
- Austin, Texas
A. Some old school coin guys use to use a pickling solution to clean pennies. It contained sulfuric acid. Over a long period of time the pennies would turn black.
Harvey Masciale- Cape Canaveral, Florida
October 28, 2020
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