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  for Metal Finishing since 1989
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Cleaning copper disc without abrasives

2006

I have a rare engraved copper disc that was slightly tarnished but not corroded. I was advised to clean it with a soft cloth and a mild solution of vinegar ⇦in bulk on eBay or Amazon [affil links] /salt/warm water and then rinse. It's been a disaster. The disc is now stained with brownish patches. I'm wondering if this can be repaired and what steps I should take. By the way, the disc is about 12" in diameter and can be fully immersed if necessary.
Thanks,

Ian Tregoning
- London, England

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Sorry for your troubles, Ian. As our readers have noted countless times, take anything rare or precious to a conservator! vinegar and salt in warm water is very mild on people but is not a mild solution on copper, it's extremely aggressive; it might be an okay first step on some bulky solid copper outdoor item, but not a delicate item. There are coarse abrasives and ultra fine abrasives, and I think you will find that you do need something that has a buffing action to get rid of the stains. However, from what I can see from the photo, it appears that this item is copper plated rather than solid copper, and that copper is missing and you're down to steel (or whatever the substrate is) around the periphery.

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

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You can use 5% ammonium citrate solution(50 gm citric acid ⇦this on eBay or Amazon [affil links] /1 lit water +add some ammonia,pH must be 9)!

Good luck!

Goran Budija
- Cerovski vrh Croatia
2006

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Your comments are much appreciated. If I use the ammonia ⇦this on eBay or Amazon [affil links] solution technique, how can I make sure it's at pH9?
Thanks again,

Ian Tregoning
- London, England
2006

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First of three simultaneous responses --

You must use ph test paper strips-any chemicals dealer must have it.Good luck!

Goran Budija
- Cerovski vrh Croatia
2006

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Second of three simultaneous responses --

I agree with Ted in having an expert do it but if you absolutely have to do it yourself, an alternative would be a quick immersion in warm chromic acid (any chrome plater should have hundreds of gallons already warm). It will brighten your copper to full luster but it will also "eat" the superficial layer so immersion time should be minimum just to clean it.

Guillermo Marrufo
Monterrey, NL, Mexico
2006

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Third of three simultaneous responses --

Try to find a local lab supply store, they'll be able to sell you pH paper. It isn't the most exact method, but better than nothing.

Try the mixture on a different piece first though - or follow Ted's advice and take it to a conservator; the last thing you want to do is accidentally make your problem worse.

Good luck!

Jim Gorsich
Compton, California, USA
2006