I just received these two halves in the mail today from an auction. they where in 2x2's with a rubber band around them vertically and horizontally. Now they have a black stain on them. And yes I did remove the coin from the 2x2 to make sure it was on the coin. What to do? Sorry the pictures are not the best.
An acetone rinse may have some beneficial effect. If not, you're pretty much going to have to leave it the way it is unless you want to destroy the original surfaces. Rubber bands contain sulfur, one of the strongest coin staining substances. No cure for it once darkened that does not involve loss of natural toning.
Acetone rinse won't do anything. the only way to remove that discoloration would be to strip the surfaces with an acidic or basic chemical dip. And since the coin is worn you will be left with a dead appearing cleaned coin.
Yeah, a quick dip in hot water with a pinch of salt and a bit of baking soda and a square of foil would maybe remove that marking, but it would also clean the surface of the coin. I agree that acetone would be worth a shot if it was something less sulfuric.
The "stain" comes from sulfur in the rubber. It is sulfur in the air and on people's fingers that causes silver to naturally tone or tarnish; so your "stain" is chemically identical to the rest of the "natural" toning on the coin, which you would presumably prefer not to remove.
So the sad news is, you can't remove it without removing the rest of the toning and creating a "cleaned coin". It is up to you whether you prefer to own a coin that has "rubber band stripes" or a coin that is "cleaned", but this coin no longer has the option of being neither. If they were my coins shipped like that and I knew the coins didn't have the stripes when I bought them, I'd be ropable; their shipping methodology has ruined your coins.
Personally, I'd prefer leaving it as-is. I hate the cleaned look worse than I hate black stripes.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis