How do I clean coins?
The only way to clean coins is with a standard white pencil eraser. If you rub a new coin, you can remove the metal dust in this way. Use a soft cloth to remove the gum residue.
All other products attack the metal of the coin. You will always see impact in the short or long term (discoloration, gray and dark coins,...).
When coins arrive from the National Bank or Royal Mint of a particular country, they are never cleaned by me for the above reason.