Many specimens of native copper are obscured by black or green surface coatings which hide the beauty of the mineral. Black coatings on copper are cupric oxide -- tenorite to you mineralogists. Green coatings are generally combinations of copper sulfate (brochantite) and copper chloride (atacamite). On statues and pre-Columbian art, the green coating is termed patina and is generally regarded as a very desirable indicator of authenticity. On mineral specimens, the coating is generally regarded as offensive and most collectors want it off.
A number of techniques have been developed for cleaning copper. Some work better than others. Most involve the use of caustic solutions; so be careful. Work in a well-ventilated area, preferably outdoors.
At any rate, here is a short compendium of copper cleaning techniques ...
Make a paste using flour, salt, and vinegar. Brush it on, let it sit a while, then rinse it off. The acetic acid in the vinegar causes the tarnish to slowly dissolve. Stubborn coatings may require more than one application.
Another method of cleaning copper has been sucessfully used in at least one major museum. In a sealable glass container, mix one part caustic soda (sodium hydroxide) with three parts rochelle salt (sodium potassium tartrate). To this add 20 parts of distilled water and carefully stir until the chemicals are dissolved. Suspend the copper specimen in the solution with copper wire.
Some folk remedies for cleaning copper include scrubbing with buttermilk, using an ammonia/soapsud mixture, or (believe it or not) Toni permanent-wave solution without the neutralizer. Other folk remedies use catsup, olive oil, or baking soda with ammonia.
Some have used more drastic measures to clean copper. Most of these involve the use of very strong acids or potassium cyanide; however, these methods have been known to cause undesirable color changes in the copper (or you if you're not careful) and won't be detailed here.
Whatever method you use, be sure to throroughly rinse the specimen after cleaning and remember to dispose of all solutions safely. And once cleaned, consider protecting the freshly cleaned copper surface with lacquer to prevent further tarnishing.
[Ed. Note: If Toni cleans the corrosion off of copper, just
imagine what it does to your hair!]
Copyright © 1993 by Fallbrook Gem and Mineral Society, Inc.
The preceding article was originally published in the May 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).
Permission to reproduce and distribute this material, in
whole or in part, for non-commercial purposes, is hereby granted
provided the sense or meaning of the material is not changed and
the author's notice of copyright is retained.
Last updated: 18 September 2002