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Brownish red gloppy stuff in copper plating experiment

My son's science project involves copper plating nickel keys in an electrolyte of vinegar [in bulk on eBay or Amazon] and table salt. After about six hours of immersion, we have observed this reddish brownish lumpy residue being attracted to the keys. It does not adhere to the keys, and shakes off them in clumps. I'm guessing this is coming from the copper strips that we are using to plate with. This didn't happen the last time we tried to do this, and we are puzzled as to the cause. I have searched your sight for a solution, but haven't found one. I would appreciate any help.

Ann H.
Algonquin Middle School - Illinois, USA
2006

One can only theorize in this case. My theory would be, that since vinegar is such a weak acid, that some copper dissolved and then the acidity of the solution decreased and the copper could no longer be held in solution and has precipitated out as a copper oxide sludge. Take a small amount of the sludgey solution and put it in some fresh vinegar and see if it dissolves.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006

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