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Brightener for copper art castings

Q. Hello, I'm an amateur artist experimenting with making pieces cast from repurposed pieces of copper that I salvage and then melt in my furnace and pour. When I get the pieces out and cleaned the best that I can there almost always is a layer of black stuff for lack of a better description, on the piece. Nothing I've tried has been successful in brightening the copper up so that I can then seal it to preserve its look.

I've tried [on eBay or Amazon (adv.)] undiluted and I've mixed hydrogen peroxide [on eBay or Amazon (adv.)] @ 1:2. That had some effect but was still very dull; vinegar [in bulk on eBay or Amazon (adv.)] had very little effect also other than a few days later green spots appeared. I've put a picture of one of the castings here.


It needs to be a dip or bath since there isn't a way to polish the pieces by hand because of the nature of the castings. They are done in fire ant mounds.
I really appreciate any help I can get, thank you!

M. Gaylor
- Gadsden, Alabama
June 6, 2023

A. Try electrochemical cleaning - connect your object to negative pole, piece of stainless steel to positive pole. As simplest electrolyte you can use baking soda [in bulk on eBay or Amazon (adv.)] solution (100 gms / 1 lit water), 6-12 V, 0,5 A/dm2. Ultrasonic cleaning can be an option too. Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia
July 4, 2023

A. Hi M.,
If Goran's idea isn't right for you or doesn't work for your situation, and you want to do it chemically ...

Brightening of copper requires an acid plus an oxidizing agent. You can google for "copper bright dip" to see if a metal finishing supplier can get you a proprietary one with all the desirable addition agents like wetters and inhibitors.

Although chemistry is for chemists, and it is dangerous for anyone not trained in it or not wearing the proper protective equipment including goggles [on eBay or Amazon (adv.)] and rubber gloves [on eBay or Amazon (adv.)], you can mix and use simple, but hazardous, stuff yourself. Try a tiny chunk in a small beaker of battery acid [affil links] plus hydrogen peroxide [affil links]. After you think you have it, try a slightly larger piece to confirm. The general idea is always to try your book knowledge on a tiny quantity -- you never mix up a whole tank full of witches' brew until you have sound reason to believe it will work in your situation.
Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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