Author Topic: Best way to clean - Kushan with green deposits  (Read 426 times)

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Offline Hitesh

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Best way to clean - Kushan with green deposits
« on: September 10, 2021, 07:44:53 PM »
Hi - could anyone help me with the best way to clean this Kushan. I’m hoping below the ‘green’ there might decent details. Thanks in advance!
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Offline Seeker55

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Re: Best way to clean - Kushan with green deposits
« Reply #1 on: September 11, 2021, 04:04:28 AM »
I was trained as a chemist, and have had good success soaking bronze coins with green discoloration in a solution of sodium sesquicarbonate at room temperature.

Sometimes this condition is called bronze disease, but I don't particularly like that name. Here are a couple of articles about it:

The solution of mixed sodium carbonate (washing soda) with sodium bicarbonate (baking soda) in distilled water is easy and inexpensive to make and is a helpful soak to remove green corrosion and stop its progress. The soaking can be done for a few days at room temperature instead of the hot water method described.

Easy formula for sodium sesquicarbonate solution:

10.6 g of sodium carbonate (washing soda) and 8.4 g of sodium bicarbonate (baking soda) dissolved in 100 ml of distilled water
« Last Edit: September 12, 2021, 06:59:45 PM by Seeker55 »

Offline Figleaf

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Re: Best way to clean - Kushan with green deposits
« Reply #2 on: September 11, 2021, 10:05:54 AM »
And here is one more article from a fellow chemist and member of WoC, in a thread full of comments and questions.

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

Offline otlichnik

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Re: Best way to clean - Kushan with green deposits
« Reply #3 on: September 12, 2021, 05:39:36 PM »
If you have a powdery green (usually light green) surface that rubs off with fingernail or toothpick or even just toothbrush then you likely have bronze disease (BD).  BD is actually a self-sustaining chemical reaction due to the reaction between sulphates in the metal structure of the coin alloy and moisture and oxygen in the air.  If this is BD, which I kind of doubt given the photo, then you have a major problem and should do some research on BD treatments as your surface details will slowly eat away.

However, if (as I guess is the case here) you have a hard green surface that does not rub off easily, then you do not have BD you have patina, or the remains of it.  To me the coin looks like it was subject to chemical cleaning at some point.  This cleaning had limited effect, thinning the patina (which might have been thick and "crusty" once) and revealing bare, but now cleaned, surface metal in some places.  If this is the case, then your only options are to leave it as is, to clean mechanically (e.g. scalpel and microscope), or to use an aggressive chemical to strip the remaining patina (which in my personal opinion is never a good idea and can damage the coin).