I do not wish to nor will I get into a debate over the do's or don'ts about cleaning coins! Most everyone here knows that I am a staunch proponent of not cleaning coins which is evident from some of the arguments that have ensued on this issue and I have been a part of! However, I do believe that a light soaking in Acetone and a light rub with your' thumb or finger (if need be) to remove the black substance from the coin, does not constitute a true cleaning of the coin in the real sense. Since the black substance is either tar, dirty gum or other grime, the Acetone should remove it probably with no rubbing needed. If any light spotting is left behind in the areas where the black substance was removed, they should darken on their own with time. However, the coin believe or not can be placed into the ground for a few weeks or months and mother nature and the soil will do the rest. Once the coin is removed from the ground and the spots have darkened evenly with the rest of the patina on the coin, just rinse thoroughly with cold water and pat dry with a soft towel and it should be fine. Having collected and owned tens of thousands of coins and quite a few of them having unknown substances on them, I have tried just about every trick known and probably some unknown to man, some of them worked and some did not.
Frank