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How to clean copper sulphate stains from a 90% silver coin

I have several 90% silver coins that were stored in a plastic shopping bag in a damp/wet basement with several copper coins. The silver coins got "plated" with what looks like copper sulphate, i.e. leaving a greenish cast of varying degrees as well as large black patches on the silver coins. The black patches appear to have a thickness at their edges as if they would almost peel off, but they do not. What can I use to dissolve/remove the plating/tarnish with the least damage to the silver coins?

I have already tried putting the silver coins an aluminum pie pan with baking soda and boiling water achieving only very minor success. I was afraid to use vinegar [in bulk on eBay or Amazon] or anything stronger without seeking some expert advice. The coins are not necessarily numismatic grade but I would simply like to preserve the silver as much as possible.

I would also be interested in learning the chemical equation for the aluminum foil/baking soda/hot water/silver coin mixture.

Thank you in advance for any assistance.

Dale Purdy
hobbyist - Columbus, Ohio, USA
February 5, 2011

Try next web link www.cci-icc.gc.ca/crc/notes/html/9-7-eng.aspx -very good article about silver care and tarnish removal.Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia
February 9, 2011