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

Guessing this is beyond restoration. A details coin (ED).

Maybe it's worth a soak in acetone. If it's some hydrocarbon buildup then that might get rid of it

with llewllin...a LONG soak. Couple of days covered in a glass container w/lid then with a round toothpick swipe (probably gonna need a couple of toothpicks as they flatten out)...start at the edge...trying to push/shove it off...I do mine while still in the shallow bowl w/the acetone so as to not let it dry out while picking (once out of the acetone coin will dry out fast)...just might get some of that to pop off...might have to repeat the process...stuffs been on there for a long time...so it might take a long time to get it to soften up.
See how that goes...and if that doesn't work...might have to be more aggressive...?
If this were my beautiful 19-S coin...this is what I would do...but this is just my opinion.
Good luck and let us know how it goes.
smat

Agree with the suggestion to give the dime a looooooong soak in fresh acetone--several days at a minimum. Cannot hurt the coin, could loosen-up some of the crud. I soaked a foreign silver coin with similar looking, hardened gunk, and was able to gently reduce the intensity--and in some cases remove--a good portion of it. Total soak of three weeks.

I am pretty darn sure that an Acetone soak will do nothing for this coin . Acetone is not a miracle solvent that will remove everything .

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.)