Author Topic: What’s the black coating?  (Read 762 times)

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Offline Hitesh

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What’s the black coating?
« on: August 01, 2020, 01:51:50 PM »
Hi all - I have an Afghan silver that I’d like to clean if possible.
Not sure what the black deposits are. I’ve already tried soaking it in olive oil for an year or so to no avail. Would appreciate any help.

Thanks!
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Offline THCoins

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Re: What’s the black coating?
« Reply #1 on: August 01, 2020, 03:32:01 PM »
The black coating could be silver chloride. If so, you could try to dissolve this is a watery ammonia solution (keep well ventilated !)

Offline Hitesh

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Re: What’s the black coating?
« Reply #2 on: August 26, 2020, 08:40:49 PM »
Thanks! Will try it out soon and revert with the results.

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Offline theshredplayer

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Re: What’s the black coating?
« Reply #3 on: August 29, 2020, 03:08:32 AM »
One of the tougher adhesions to clean, and doesn't come off with one method easily. It is commonly seen in the silver that comes out of the Indian Pakistani and Afghani soil, so technically it is find patina. It is often a thick layer, so try to very carefully scrape a bit on the Kabul side using a sharp Xacto. Shouldn't damage the coin since it is a thick layer. If you see a darker orange or any shade of orange under, try using  Baking Soda+Aluminum Foil+Boiling Water method with a drop of Ammonia.

It might be best to try formic acid first, long soak will get it off. Stan Goron's favorite method. He has "millions" of these Afghan/Barakzay Rupees, maybe not millions, but certainly the largest collection of Afghan coins. 

Soaking in Oil will never get it off, even if you leave it in for 10 years.
« Last Edit: August 29, 2020, 03:26:39 AM by theshredplayer »

Offline theshredplayer

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Re: What’s the black coating?
« Reply #4 on: August 29, 2020, 03:35:40 AM »
I can share a picture before and after cleaning of a similar adhesion. It might take me days to find one .

Offline Figleaf

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Re: What’s the black coating?
« Reply #5 on: August 29, 2020, 08:55:07 AM »
This sounds like a tried method. I'd be quite interested in "before" and "after" pictures.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline theshredplayer

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Re: What’s the black coating?
« Reply #6 on: August 29, 2020, 08:05:35 PM »
Typical hoard patina especially from Punjab, looks like this. I might have some old photos as well.

Offline theshredplayer

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Re: What’s the black coating?
« Reply #7 on: August 29, 2020, 08:36:19 PM »
I am pretty sure it is not only AgCl, it is attracting a green found on copper. That's why scraping a little will make it clear which method will work the best. Formic Acid is safe enough to try, it doesn't target silver itself as much as the coating. Brushing under tap water and re-soaking is the best. Below is a picture of Before of the Punjabi Hoard Patina. 

Offline theshredplayer

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Re: What’s the black coating?
« Reply #8 on: August 29, 2020, 08:38:27 PM »
Here are the afters in the link. Diluted Formic acid and brushing is all I used. Hitesh's 2 coins have adhesions thicker and tougher than mine. 


Edit: "after" picture added at same size as "before" picture
« Last Edit: August 30, 2020, 12:00:44 AM by theshredplayer »

Offline Figleaf

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Re: What’s the black coating?
« Reply #9 on: August 29, 2020, 11:09:36 PM »
Formic acid (acide formique, mierenzuur) is pretty hard to buy. Wouldn't another acid (e.g. diluted lemon juice) have the same result?

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.

Offline theshredplayer

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Re: What’s the black coating?
« Reply #10 on: August 29, 2020, 11:51:21 PM »
Lemon Juice should work. I have used it on Baktrian Tetradrachms with Find patina in the past. Food Grade Granular Citiric Acid is a good alternative, which is readily available. Also known as "Tartari" to the desis, is used to curdle milk for Paneer(cheese) etc. The good thing about Citric acid is that you can adjust its strength. 
« Last Edit: August 30, 2020, 12:05:20 AM by theshredplayer »

Offline Hitesh

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Re: What’s the black coating?
« Reply #11 on: September 26, 2020, 08:57:49 PM »
I was able to haggle someone at my alma mater high school to get me ammonia and Formic acid. Expecting delivery soon and will update you all with the post processing images.

Thanks everyone.
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Offline FosseWay

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Re: What’s the black coating?
« Reply #12 on: September 26, 2020, 10:06:54 PM »
Formic acid (acide formique, mierenzuur) is pretty hard to buy.

You're welcome to the giant ant hill in my garden if you want to farm them  ;)

("Giant" refers to both the hill and the ants, btw.)

Offline Paulsoumyaj

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Re: What’s the black coating?
« Reply #13 on: September 28, 2020, 09:22:45 PM »
Hi all - I have inherited this 1940 George VI that matches KM#556 parameters and weighs 11.6gms.

I’d like to clean if possible but not sure how and if the coin will be damaged in the process.

Would appreciate any help.

Thanks!

Offline Figleaf

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Re: What’s the black coating?
« Reply #14 on: September 28, 2020, 09:34:01 PM »
Looks chemical to me. If so, you may be in luck. First try to wash in lukewarm water. This will probably have no effect, but you never know. Next stop would be rubbing with acetone. This will remove chemical stuff. Wash off remnants and dry thoroughly. If this helps, repeat as often as you wish. It will not damage the coin. If it doesn't help, the next step options would damage the coin.

Peter
An unidentified coin is a piece of metal. An identified coin is a piece of history.