I soaked it as suggested and the gunk came off pretty well, but left some green discoloration behind. I can't pinpoint the exact RIC # as there are so many varieties of these. It appears close to the below, but each one has subtle differences. RIC II.3# 859 RIC II.3# 3072 I also got a USB microscope for better pictures
Ouch. You could try sodium thiosulfate; that's usually good for horn silver and iron oxide, but might be worth a shot here.
I'll revisit in time as I learn more. For now, I carded it for safe storage. I've been moving everything in this collection into newly bought cards that are PVC free since everything has sat for over 60 years.
Sodium thiosulfate is pretty effective for silver coins as it tends not to damage the surfaces at all. This one is low-grade billon silver, but the point stands.
I fail to see why sodium thiosulphate should work? It’s not a magic bullet - it dissolves silver chloride and bromide, but that’s about it. Note that coins that are pitted due to corrosion, and then covered by AgCl will not be better of with this treatment either. That being said: I would soak this coin in synthetic citric acid, dissolved in distilled water, heated to boiling point.
I also fail to see why sodium thiosulfate should work against iron oxides, but in all my experiments it has been the one thing that works against rust. I don't know why, as the chemistry doesn't point to thiosulfate being effective against it!
https://www.ebay.com/itm/391427730454?hash=item5b22e92c16:g:lOQAAOSwjdFei9~W 4 oz bottle If you want a TON (5 pounds) it's a better value https://www.ebay.com/itm/2032199548...0OKfAd2CAELTO1oNbgFL|ampid:PL_CLK|clp:2334524
I just learned this, but NaSO is really good for treating cyanide poison. In case you eat too many almonds