Mercury Dime - Is This Corrosion And Can It Be Restored?

I agree that the coin may always be a details coin but maybe it can be improved. I'm not a chemist so I don't know if the substance is organic or non-organic.

Whenever I try acetone and it doesn't help the coin I always follow up with a long soak in Xylene. Like acetone - xylene will not damage the coin but it is much stronger and can remove non-polar organic compared to acetone which removes polar organic.

WARNING: Xylene won't hurt the coin but can hurt you. Only use it in a well ventilated space and do not breath the fumes.

I played with your photos a bit. I see areas where the black gunk seems to have come off the surface.

Look closely at these areas:
A SSE-NNW line from just between the "G" and "O" of GOD upward just left of the chin
Just below the "R" and "S" of TRUST
About one-fourth of the way between the "I" of IN and the bottom of the "L" in LIBERTY

It looks like the gunk came off those areas without exposing surface pitting. There is slight discoloration where the patina was disrupted.

If so, you may have a chance to rescue this coin. It won't be "restored," but it may come out OK.

The acetone bath makes sense. There isn't any downside.

Before doing this next step, let's see what others think. This is potentially a risky step.

If the gunk can be loosened, gentle agitation may be enough to dislodge it. What you need is cavitation, rather than scraping or wiping. To accomplish this, one possibility is an ultrasonic cleaner. Most jewelry stores have these cleaners. Stay far, far away from auto parts cleaners. They are much too rough for what you need.

What does everyone think? Is an ultrasonic cleaner too rough? If not, what would everyone think about ultrasonic cleaning in distilled water? (I get squeamish about using acetone near anything with the potential for having a spark ignite the acetone.)

Nothing to add, that hasn't already been said....just waiting for "after" pictures....

Thanks so much for all the great feedback here. I guess I should have mentioned that I do not (as yet) own this coin. This question was in part to help me decide if it (might) be worthwhile to make a go at purchasing this coin, but only if the price was "right". Next time I'll clarify whether or not I own it, as I see many here assumed it was my coin.

I guess my thinking was that many potential buyers could be turned off to a coin like this and therefore it could go for under book value. Clearly it is a high(er) grade coin, with much remaining luster and other quality attributes. This is a semi-key date in the series and a date I've been certainly keeping my eye out for.

These deposits do appear to be raised, rather than incused into the coin surface, and I do see where it looks like they have come off and more or less left the patina stained - thank you for those observations. In my mind it would be a risky purchase at book value, but say picking this up at XF40 pricing could be worth the risk.

I have my eye on a few other potentials that don't have issues like this one, so we'll see. I agree an acetone dip is a very easy first attempt, other subsequent approaches like toothpick picking or ultrasonic cleaning seem more risky to me....but so many folks are saying toothpicks are fine, I could try that if I end up with this coin.

Has anyone used the PCGS restoration service at all? I am curious how they would approach a coin like this.

Indeed! Always good to do a final flowing rinse with fresh acetone.

After looking at it again, this maybe just dry up black ink, a soak in water for an hour or two should clear it up, if that's just black ink.