This is the hard green corrosion product that acetone will not remove. Many collectors will ruin the coin by trying to scrape it off with a sharp tool of some kind like a pin or razor blade. What you see is the result of a profession "spot conservation" that did not change the appearance of the rest of the coin. The collector was not charged because the chance to "fix" this problem improves the knowledge of the person doing the work. Oops, the after picture needed to be rotated counterclockwise.
Out of curiosity, how did you do this? (I understand if it is proprietary and you can't share... just wanting to learn) Are the very fine scratches a result of the process you used? I don't see them in the first picture, but it might be the angle or the light. Can you see that with the naked eye (obviously, these pics are highly magnified)?
I've heard some talk about using a rose thorn, because it's softer than the metal. I have no idea. I had some success with those plastic toothpicks with the single piece of floss, but I didn't try to pressure it much and did not quite remove it all. I would not recommend (me trying this) this with a rare valuable coin. I would rather pay the vig for it to be professionally done.
Excellent conservation! Obviously, it's a verdigris salt formed due to copper being in the alloy. As I recall, these are ~10% Cu. Organic solvents would have no affect, verdigris salts are insoluble in acetone, xylene, etc. Acetone was worth a shot if the verdigris was plasticizer-based because, with PVC degradation, the substrate residue is actually organic and soluble in most organic solvents. However, using only an organic solvent on plasticizer damage ignores the fact there is still HCl present. After PVC residue removal, it's always best to neutralize with alkaline water.
This would work great! https://www.kroger.com/p/kroger-alk...ntNgRAbvKEcZFdog8lhoCKxUQAvD_BwE&gclsrc=aw.ds Or you can make your own with baking soda too. You need just a few grains dissolved in a couple ounces of water to move the pH to >7. Soak for a few minutes, rinse well with fresh distilled water and then acetone to dry the coin.
Thanks for the chemistry lesson. As far as any tiny scratches you see at 50X: The work is done using a microscope, proprietary chemicals, and a toothpick. The chemicals do most of the job but theoretically it would be possible to scratch the coin's surface. Note that there are tiny scratches on the surface where nothing was done. Best of all, no one examining the finished conservation using 20X+ would have a clue that I "cleaned" the coin. PS I shall try to soak one of these green crusted coins in a basic bath as you suggested!
