Would it help to heat the water, maybe boiling distilled water soak for better molecular results? Something like steam cleaning or hot tanking engine parts.
I just think it's cool to imagine how this lovely peace dollar possibly sat in someone's closet, drawer, storage, basement, etc. for decades surviving the great depression and every other historical event until a family member most likely inherited it maybe in the late 60's-early 80's and it sat again for a long time. Meanwhile during all of this time the coin develops a nice patina from whatever environment it lives in. Then the same thing happens, but this time the second inheritor decides they would rather just have quick cash so they go down to the coin shop to get a few bucks not knowing the treasure they have... Now it is mine and I will get it certified to properly preserve it and it will sit again for decades!
On a side note, I have placed a single coin in a cup of water and microwaved it to get the water boiling...it boils pretty fast, but DON'T try it with two or more coins!!
I have heard of that, but never tried the hot water. Seems that @Kentucky would know more. Hey, Kentucky isn't metal in the micro a NO NO? lol
Something I wondered about and decided to try it. One coin under water just heats up quickly and can't get above 100 C because of the water. Two or more coins will kick sparks between them and I don't want to ruin my microwave.
If you have never done it, split a grape but leave the two halves attached by a flap of skin...put it on a paper towel in the microwave and nuke it...be ready to turn it off! I didn't cover it. https://www.google.com/search?q=mic...#fpstate=ive&vld=cid:d7d06242,vid:LY6xeFpBeMQ