Acetone Mystery

Yeah, so did I. Dunno if this is a different study, or if I just inferred it the first time around.

 

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My bad, Doug -- I thought I'd highlighted the part of your quote about time not being a factor. (I've gone back and edited my post, so now it is highlighted.) That's the part that I haven't seen you mention before.

The paper talks about an 18-hour soak in acetone, with light exposure, in conjunction with high humidity. The damage to copper surfaces was apparent at the end of that time.

If I'm understanding your posts correctly (and I just spent some time going back through the first 20 or 30 of them), you're saying that you've seen coins discolor days or weeks after they were rinsed in acetone. I'm assuming "rinsed" means "not soaked for a long time". In that case, it seems like the paper has even less relevance to what you've observed.

I'm feeling an increasing temptation to get a roll or two of fresh Zincolns, divide them up into experimental groups, and treat them various ways:

1) no treatment
2) rinse with fresh acetone
3) soak in fresh acetone, in light and humidity
4) rinse with fresh acetone with a trace of acetic acid added deliberately
5) soak in fresh acetone with a trace of acetic acid added deliberately

Of course, since the color change "only happens sometimes", negative results from this test wouldn't prove anything. I'm not sure what test could; if the result depends on what's on the coin's surface, you'd need a set of coins with identical surfaces, and I don't know how to achieve that.

 

Glass tipped, I've got to look into those. I use small wood tongs that are almost tweezer sized.

 

Excellent advice to use Xylene instead of Acetone on copper coins. I think many people are not familiar with Xylene or maybe can't buy it in their state. Here in California they don's sell it because of some regulations. I went on eBay and bought a can of it from an out of state seller. We can still buy Acetone in California but not Xylene

 

I always use similar language (may) with my suggestion not to use acetone on copper coins. My comments are routinely rebuffed. I stopped defending myself. To each his own.

 

So, some of us have never seen acetone turn copper, but others have.

How can we be completely sure that xylene doesn't ever "turn" copper? I mean, you might have used it on hundreds of coins, but that doesn't mean that the three-hundred-and-first won't have some different surface condition that causes it to turn.

I can make good arguments for why xylene shouldn't be able to turn copper. But I can also make good arguments for why acetone shouldn't.

Besides, I can make good arguments against using xylene in general. It's more toxic than acetone, harder to dispose of, and increasingly harder to get.

I feel like it's worthwhile to keep having these discussions, because sometimes people do bring new information. I apologize if it gets tiresome.

 

I wish I saved my experimental 1981 cent that came out pink and end the argument. Of there will be detractors...

 

Yep, and we'll say "bad batch of acetone", or "the acetone stripped off something that was hiding the pink", or "user error". None of which are provable, and all of which may be wrong.

In light (sorry) of my latest re-read of that paper, I'm leaning toward "wet acetone produces vinegar more easily than I thought". If that's true -- if all it takes is acetone, water in the acetone possibly from high humidity, and ambient light -- that's enough for me to come over to the "don't use acetone on copper" camp, even if most of the time you'll get away with it.

Someone in the Desert Southwest might have a hard time reproducing the effect. Someone in South Florida or New Orleans might have a hard time avoiding it.

 

Excellent analogy. Finally someone is able to offer some reasonable explanations. I simply followed the advice of an experienced numismatist who posts on CT.

• Practice on junk coins.
• Acetone may turn copper coins pink.

I've tried acetone on several junk copper cents.
One of them turned pink.
I stopped using acetone on copper coins.
I share my experience to others here on CT.

 

@-jeffB the authors of the Stony Brook Univ paper are only proposing a mechanism. What we do know from their paper is that in the presence of light and moisture, AFTER the acetone evaporated 18 hrs later, copper acetate crystals formed on a 100% copper foil.

I think their proposed mechanism of Cu being a catalyst that helps acetone break down into acetic acid is valid. What they don't say is when does this reaction occur and what are the concentrations.

• Is it immediately upon immersion?
• Is the concentration of acetic acid high at the beginning, or after 18 hrs as the acetone is just about completely evaporate?
• At what point does the copper acetate form? This is a big thing because it's the part where Cu from a coin can can physically become involved in the reaction.
• How do the conditions of the experiment relate to numismatic usage? The experiment used a static sample.
• If I stir or swirl the acetone container or coin, will there be enough acetic acid to concentrate near the coin surface or will it be quickly diluted into the solvent?

I think the research is solid and their proposal is reasonable, but most collectors don't let their coin sit out long enough for the acetone evaporate. Nothing in that paper suggests that a short soak and rinse will lead to the formation of concentrated acetic acid.

I believe the members who have posted that they have had copper change color after soaking in acetone. But I feel that it's more likely due to technique (soaking too long, improper rinsing, redeposited organic residue) or a questionable surface under the residue. So far, the science doesn't support acetone being a problem for copper if used for a short soak and rinse.

I've soaked hundreds of copper coins over the years (primarily as a precaution to remove PVC plastic over residue) and have had the same results as you @-jeffB no discoloration

 

I agree, based on current knowledge.

That's... not a very good argument for safety.

Until recently, you could buy all the methylene chloride you wanted, and painters used it all the time, and it's a lot more dangerous than xylene. Same thing for benzene; it was a hardware-store staple for decades, until I believe the 1980s when research tipped the scales against it. Same thing for carbon tetrachloride.

I used benzene and carbon tet in home chemistry experiments as a kid. They didn't kill me, and I hope they didn't leave any ticking time bombs deep in my liver or bone marrow. I wouldn't encourage my kids to experiment with them today, and if they did, I'd have much better ventilation and safety procedures in place.

 

But gunk dissolved in the acetone will be deposited back on the surface. You NEED to rinse the coin after an acetone soak

 

But that depends on how much gunk was there, right? Acetone has very low surface tension, so very little of it stays on the coin anyhow.

Having said that, best practice is to rinse in acetone, or do the three-container process @GDJMSP and others have described. After the final step, let the acetone evaporate off the coin.

I see some people on the forum recommending a water rinse after acetone -- I don't remember if you're one of them. I still think that's a bad idea; if there was something on the coin that's soluble in acetone but not in water, a water rinse could knock it right back out of solution, possibly landing back on the coin. And, of course, water evaporates more slowly than acetone, and if it happens to have anything dissolved in it -- say, salt from your skin -- it can deposit that on the coin.

 

Yes. That's why after soaking in acetone, you soak it a second time in fresh acetone, and then a third time in fresh acetone. The final soak is, in effect, a rinse.

That protocol has been cited here so many times by so many people its shouldn't need (or NEED if you prefer) repeating.

 

One word I haven’t seen in this post is ‘hygroscopic’. Acetone is hygroscopic, meaning that it will draw moisture from its surroundings (assuming that it doesn’t evaporate first). In my opinion, that would explain the discoloration. If acetone was not hygroscopic, it would not be able to hold the acetate ion in solution and would not cause the discoloration mentioned.
On the other hand, xylene is nowhere near as hygroscopic as acetone at room temp. And xylene will not catalyze into any corrosive compounds that I’m aware of. And xylene and acetone have similar solubility profiles. Yes, xylene is slightly more toxic than acetone, but even water is poisonous if you consume too much of it. Working with xylene in a ventilated area for short periods should not pose any health concerns to healthy individuals.
So, IMO and based on the discussions here, it sounds like xylene should be the solvent of choice.

 

AGAIN. Has anyone tried my suggestion of BAMBOO TONGS? They do not leave anything on the coin and you can securely hold the coin while rinsing it for what ever time you feel necessary. Even after rinsing in running water I place mine in Distilled Water. By the way I use two sets of tongs one specifically for Acetone and the other specifically for the Distilled water.
Happy New Year to ALL of the CT readers/participants. Let's leave 2020 as a nightmare to forget.
Phil