Acetone

You could soak it for weeks and it will not harm the coin. However, it should do its job within minutes. If the gunk does not come off or at least begin to come off after 5-10 minutes, then it is probably inorganic.

Not long ago someone posted a link to an article on how Acetone will ruin a coin if left in and the sun hits it. Naturally I just had to try this. I placed several coins in a very cleaned glass jar with very pure Acetone, Lab quality, from our labs. I placed the jar in my garage, in the dark for several days. Not much happened. I then placed the jar in a Southery window in the garage for several days. Still not much happened. I then placed the jar on my lawn, in the Sunniest place I could find and after several more days still not much happened. Had to move it or hit it with the lawn mower eventually.
Originally all coins were just plain coins except an old, beat up Mercury dime. Others were a Quarter, Roosevelt dime, Lincoln Cent and a Jefferson nickel. After several weeks of this nonsense all coins looked basically as they did when I first placed them in the Acetone. Possibly the Roosevelt dime looked a bit shinny but otherwise not much happened.
Of course the article about how Acetone ruins coins if left in the sun might have been referring to a different sun.

Acetone will only remove the finger oils, it will not change the surface of the metal if the oils have started to tone the coin or etch the surface.

Well, I missed the last post...
I went to a 40% 1976 quarter I have with a huge fingerprint
on the obverse. It already had a mark on the cheek when I
got it, then the fingerprint probably came from one of my
kids getting ahold of it. It is a common coin that I exposed
to the kids in lieu of less common coins... And it is a coin
that I was willing to experiment with on this subject.

Like I said, I missed the part about "no fingernail polish
remover". So that is what I used. Again, though, common coin.

It removed the entire fingerprint oil smudge. There was
however, some of the print left behind lightly in a central
area. Probably where the most pressure was applied. The
remainder was not oil or smudge though. It appeared to be
actual degradation or damage to the surface from the oils
being present over a long time.

So, as mentioned in previous posts, the oil is gone, damage
from 15 years of oil is there.

I guess time will only tell if the use of non-pure acetone
will be an issue.

I use acetone on a lot of copper coins that are brown only, and nickel coins to remove any contaminants on the coin before submitting to a TPG service. Remember to rinse well and pat dry. like meantion above finger prints that show on a coin is usally to late to totally remove and soaking the coin only really works for the first 10 minutes or so.

I have a friend who used to drink so much tomato juice that he had high-acid sweat. He could grab a gun or a knife and within minutes you'd see his prints, even thru bluing.

anyone put a proof in acetone?

sweet thanks