Can Extremely Long Acetone Soaks Damage A Coin Surface?

Acetone cannot chemically react with the metal of a coin.


The acetone is probably slowly removing the grime that has accumulated on your coin over the last 168 years, exposing the actual surfaces of the coin itself.
I have seen directions to not expose copper coins in acetone to direct sunlight, as photodegradation products of acetone can react with copper.

There is an academic research paper from 2001, which the anti-acetone crowd love to trot out, which demonstrates that acetone, when exposed to both bright shiny copper and UV light (including direct and indirect sunlight) for at least 18 hours, can photocatalyse into acetic acid which then is free to start to erode the copper. A control sample exposed to the same acetone in complete darkness showed zero reaction or evidence of acetic acid formation, thus proving it is indeed a photocatalytic effect.

The paper in question: https://www.academia.edu/7718029/Ph...ne_on_copper

So, is this likely to happen with your coins? I wouldn't have thought so. Your coin is not "bright shiny copper", it is oxidized copper, and it has to be raw metallic copper that acts as the photocatalyst. I'm also assuming you aren't sitting out in bright sunlight or under a UV lamp, so the light probably isn't bright enough to cause the effect over the "couple of days" duration. Finally, they needed a photomicroscope to see the damage to the copper, so visible-to-the-eye discoloration is unlikely to occur over that timeframe.

That being said, it is recommended not to use ultra-long soaks in acetone, for this reason, especially for red unc copper coins. Long soaks are also kind of redundant; if the goo doesn't come off in acetone after a quarter-hour at most, a longer soak is unlikely to improve things.

Oh yes, the paper also says the reaction happens in the presence of fully humid air, under which the acetone was allowed to evaporate to dryness. If the atmosphere is dry and there's not buckets of water lying around, and you don't allow the acetone to slowly and completely evaporate away, it's also unlikely to occur.

I would now try Verdi-Care , that may give you what you want.

Thanks for posting the link to the article, Sap.

Two comments:
1. I don't like that there was no control experiment without any acetone, but I don't think it affects the main conclusion on photodegradation of acetone and corrosion of the copper..
2. MORE IMPORTANTLY ... the analysis of the sample that was NOT exposed to sunlight still showed the presence of carbonyl carbon on the surface of the copper. That means that acetone molecules were stuck to the surface, and presumably as soon as exposed to light, the reaction to make acetic acid and corrode the copper would then have proceeded!
Having seen that, I would have expected the authors to do that obvious follow up experiment, but maybe they only had one shot at using the XPS equipment.

This highlights the importance of rinsing off any remaining acetone after your soak.

It should also make us think about things like the same reaction possibly occurring in CuNi coins (which are mostly copper).

I do not believe you can ever soak a coin for too long, as long as you do not let it evaporate and replace the bath with fresh acetone as it gets saturated.