Acetone and Copper

Many here and elsewhere seem to hold the belief that acetone can have an effect on the color of copper. I've also read that the apparent change in color that can happen on copper is not from the acetone itself causing the discoloration, but the acetone removing a film (contaminant) that is contributing to the coloration.

I've been using acetone on copper for many years now. I've used acetone on circulated and uncirculated copper, early 19th century to modern, from fully brown to fully red, on business strikes, matte and mirror proofs, PL and semi-PL business strikes, an early 20th century copper pattern and a late 19th century specimen (both with colorful toning), and various copper medals. I have never, not once experienced a change in color on copper from the acetone.

To those that believe acetone can change the color of copper - have you actually seen this happen yourself, or did you just hear about it somewhere? Has anyone here experienced this first hand that can share their experience (even better if you have before and after pictures)?

 

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Perhaps @GDJMSP would address this.

 

Yeah, Doug's been the main spokesman here for the dangers of acetone on copper. I'm skeptical from a pure chemistry perspective, but I don't have extensive experience with copper and acetone, so I'm trying to mostly listen.

 

Yep, sometimes "chemical perspective" doesn't cover all the possibilities of what might have happened.

 

I agree. Sometimes what you think is the "natural" color of a copper coin is not, and once the acetone removes a layer of stuff it changes the color. People think the acetone damaged the coin, when in fact the coin was probably that color all along and the acetone simply showed you the truth. I have had that once with an ancient. They had used some kind of marker to color in some spots. When I put it in acetone the marker came off, and I could see the spots. I think a lot of this "acetone discolors copper" is this in one way or another.

 

This much I can attest to: If you liberally REUSE your acetone, one possible source of color is redepositing the previous gunk on the new coin as a film.

Try this:

Next time you use acetone, after you're done, just let it evaporate all the way to dry. Then examine the residue left behind. Not pretty. I use clear glass for just this reason. I want to see what came off.

 

That is my suspicion as well.

Copper either reacts to acetone or it doesn't!

I've used it on so many copper coins at this point, with such a variety of conditions, and I've seen nothing that I would characterize as a change in color or the coin turning blue.

What I have seen the acetone do is dissolve surface contaminants. Does that change the appearance of a coin? Sure does. Can I believe that it could remove a surface contaminant that is altering the coin's color? Sure, but that is not the same as changing the color of copper.

 

There is a document which can be found in the internet which documents the effect of exposing copper to the presence of acetone.
Acetone will degrade to a mild organic acid under the influence of intense light.
This acid react with copper and form a metal salt.
Therefore, if your are going to expose copper to acetone, this exposure must be short, and under exclusion of light. I have done this myself, and it can lead to good results but care is needed - short and dark!

 

Yup. Acetone should be used once unless you've got the knowledge and skill to re-purify it. Even then, it may be more economical to purchase it new. Trying to distill it is a good way to blow yourself away.

Cal

 

Yep. It's not expensive. I've got gear suitable for distilling it, and in my younger days I probably would've tried that as an exercise -- but these days, I have to pay my own insurance bill, and I've gotten conservative.

 

So tell us, watcha distillin' there, buddy? Did I mention I have a "long position" in bottles?

 

I used quite a bit of acetone over the years, primarily for removing PVC plasticizer decomposition/reaction residue, although I have very little experience with high grade Red and/or PL copper. To date, I have had no noticeable color changes.

The thing that bothers me is that there are experienced numismatists that have experienced color changes and I have no reason to doubt their observations. Like @-jeffB, I have a lot of reservations about the chemical reactions between acetone and copper and it would be very interesting to read the scientific literature regarding this. So far, the only paper I know of, is the SUNY Stony Brook report (the paper @Eduard. references) They reported copper acetate formation, BUT only when the acetone completely evaporates. I dip the coin or soak for no more than 5 minutes then thoroughly rinse and I’m willing to bet that those reporting the color changes are doing something similar. So, I can’t see how the Stoney Brook report is applicable to what we do.

I think the comments from @Jaelus and @medoraman are interesting. Could the acetone be removing thin layers of organic residue that are influencing color. Or, could specific organic compounds combine with the acetone to form an insoluble material that redeposits on the surface? @GDJMSP, have you had any experience dipping “acetone colored” copper in xylene or redipping in acetone? What happened?

Finally, does anyone know of any scientific literature on this subject besides the Stony Brook report? Thanks.

 

All great info the last few posts. I have always taken fresh acetone from a properly stored bottle, so I would have never thought about those aspects, though I would have always been nervous to reuse any chemical.

 

Isopropanol. 70% in, 91% (the azeotrope) out; how much you need?

I'd never trust my lab technique enough to make something for ingestion. I wouldn't even touch the aspirin we made in freshman lab. I should probably have more self-confidence, but it's kept me out of trouble.

 

Borrrrrring.

 

Yup. The exciting stuff tends to be siren-inducing. And these days, if a passer-by sees elaborate glassware, their first thought is not "oh, he's probably just making some moonshine for personal use".

I'd just as soon let the boxes keep accumulating dust. I had mixed hopes and fears that one of the kids might develop an interest in chemistry, but that hasn't happened.

 

Although, now that I think about it, a Soxhlet extractor with a coin in the chamber ought to be a lot more effective than a simple soak or rinse. Hmm...

 

Yeah, and go whole hog and use pet ether. Might make a bigger boom than acetone.

Cal