Author Topic: Cleaning green patina on old copper coins  (Read 1080 times)

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Offline SNK

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Cleaning green patina on old copper coins
« on: April 04, 2020, 04:44:34 PM »
Hello fellow numismatists.
My name is Sneh, from Nashik, India. I am a collector of coins, particularly of British Indian period and later. A few weeks ago, I received a large lot containing all sorts of copper coins and they have been sitting in an oil bath since then. When I took them out to inspect, I noticed that although most of the coins had acceptable black patina regularly throughout the surface of coin, some coins had  patches of light green patina across the surface. When I read about it online as well as talked to a few people in this hobby, they explained that the black patina isn't harmful, the green patina is. I have tried some things such as the soda carb-bicarb solution provided here on this site for bronze disease and dipping the coins in mild acids, or vinegar or stuff like brass cleaners. None have worked out so far. Either the green patches stay the way they are, else the coin looks too damaged (in case of cleaning by acid and vinegar). Can anybody guide me as to the best possible way of treatment for these coins. Also, These aren't bronze coins. All of them are copper coins as they are in Victorian period and earlier. I have attached a pic of the green patina coin as a sample.

Any help would be much appreciated. Thanks in advance.

Offline bruce61813

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Re: Cleaning green patina on old copper coins
« Reply #1 on: April 15, 2020, 03:43:11 PM »
First, a green patina is not in itself bad. Green is a normal oxide of copper, how there is a blue-green coloring that is symptomatic of Copper Disease (or Bronze Disease) and that is bad. But it is not smooth, and outbreaks look fuzzy, not smooth. 

What level are you trying to clean then coins too? I know that collectors in the East usually prize something that tries to look as if it was just minted, no old. I have never like oil soaks, it takes years, and the final result is often not an improvement. I was informed that you have tried the soda mix soak, but for how long and at what temperature? This method is designed to neutralize CD and remove the salts that cause it. But it will not remove stabile green copper oxide (malachite), also the initial solution should be heated (at least  60 - 70 deg C) for at a half-hour. Between soaks, the coins should be scrubbed with a still toothbrush, a bamboo skewer can be used to help remove dirt, mud, or clay that has become wedged on close spaces. Part of the reason for heating the soda solution is to soften the clay. Remember that what we see as just dirt or clay, is a mix of clay, copper oxide and possibly lime.

Here are the steps
I would try:

1. Remove all oil, was the coins in very warm water with dishwashing soap.
2. Make a mix of Sodium Carbonate & Sodium Bi-carbonate, mix 17 gms into 350 ml of hot water with the coins. You can mix the water, coins and soda mix together and boil it in a pan for about
    5 minutes, then let it sit and cool. I would allow the soak to continue for an hour,  drain the water, then give the coins a light scrub, check for areas of clay or dirt build-up and us the skewer to work it loose. Then repeat the process. Coins can be rubbed in a paste made from baking soda and dishwashing soap, apply to the coin and rub using a soft cotton cloth.

Bruce




Offline Kopper Ken

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Re: Cleaning green patina on old copper coins
« Reply #2 on: April 20, 2020, 01:23:38 AM »
I leave my copper coins that have verdigris on them in a container of virgin olive oil for a period of time.  I usually leave it over the winter months on the radiator, turning it periodically.  Always comes out nice.   You can "tamp" it with a cloth to remove the oil.

KK