Numismatic and History Discussions > Coin Photography, Conservation and Storage

Patina on cleaned coins turns from brown to green

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DzikiZdeb:
I am not very knowledgeable about patina and can't really explain how cleaning some coins changes the color of the patina from brown to green. Is the brown top layer independent of the green bottom layer and is removed during the cleaning process, or are there some chemical processes taking place that expose green copper compounds more?

Examples:

1.

Before:


https://www.acsearch.info/search.html?id=3289701

After:


https://www.acsearch.info/search.html?id=2996915

2.

Before:

https://www.acsearch.info/search.html?id=6784349

After:

https://www.acsearch.info/search.html?id=7854846

3.

Before:

https://www.acsearch.info/search.html?id=3305905

After:

https://www.acsearch.info/search.html?id=3247660

418teapot:
Hey DzikiZdeb

I'm very new to cleaning and patina, but here are some observations to consider:

Coin 1:
While the reverse shows more detail after cleaning, the obverse does not. It seems like no material was actually removed here.
Also notice how the bronze-looking overcleaned high spots in the before image have also turned greenish in the after photo.
My best guess would be that the cleaning was aggressive (potentially involving chemicals), stripped the original patina, and the coin was re-patinaed in green afterward.

Coin 2:
This one looks like a pure cleaning job, with brown in the low spots having been removed. The color is also nearly unchanged.
The only thing that has me surprised, is that the originally smooth surface of the portrait seems to have gone rough after the "cleaning".
(That being said, I have never cleaned a silver coin / coin with intact silvering layer.)

Coin 3:
The before observe looks like rock hard encrustations. That must have been a tough cleaning job.
From the reverse the green patina is a bit surprising, but it's hard to tell.

Ron C2:
In my opinion, all 3 of those was chemically stripped with acid or lye and repatinated artificially.

Lech Stępniewski:

--- Quote from: Ron C2 on March 09, 2022, 07:44:54 am ---In my opinion, all 3 of those was chemically stripped with acid or lye and repatinated artificially.

--- End quote ---

But look at the first coin. How can you make an artificial patina worn out in exactly the same places as before? Probably it could be done with time consuming manipulations but why make the job so difficult.

Mark Fox:
Dear Board,

As someone who has seen a lot of coin photos over the years (including those from the auction houses in question) and who has just been doing some numismatic photography of his own, I am fairly certain the photographer(s) are at least partly responsible for the observed color changes.  The photos of the two Roman provincials (after the coins were expertly cleaned and possibly lightly smoothed), simply do not reflect their true colors, whether it be due to a (green?) filter, photo editing, or some other process.  Further support for this view can be found in Lech's observations.   

Hope this helps.


Best regards,

Mark Fox
Michigan

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