Silver coins that have a grayish or greenish tint or crud on them had been exposed to salt water or a high salt content soil. I have also seen black staining on silver coins found in soil around pine trees.
If it is just a stain, it is not necessary to remove it, and can lower the value more if it is cleaned. If it has a crusty coating on it, it can be cleaned off with "non-sudsy" ammonia.
I use a small plastic container with a lid. I cover the coin with some ammonia and let it sit, usually overnight. Some coins can take longer.
The staining will start to disappear or the crud will flake off. Most coins will be left with a whitish haze, which I think is the black oxidation that is converted by the ammonia.
At that stage, I can brush or rub the coin with "Zud" cleanser(an Oxalic acid), or plain baking soda. This will leave the coin with a shiny surface that is not desireable to most folks, but that is something you must decide for yourself. If you can see the date and it is a more valuable coin, you should check first before any attempt at cleaning.
Note: Every coin will be different! Some will clean up nicely and some won't.
Good luck! HH
