Clean coins with Distilled Water
To clean coins without risking damage to the surface and a reduction of value, you should use only water (Distilled water preferably). Hold A Coin by the edges, and hold it under a gentle stream of lukewarm distilled water. (If you are still metal detecting then the use of a small Spray Bottle filled with distilled water can be ample to remove some shit)
Flip the coin over so that the reverse side is also wet, then gently pat the coin with a soft cotton cloth, this will remove some surface dirt without scratching the coin.
Town or city water usually has an element of CHLORINE and this can discolour the coin.
To Avoid this use either distilled water or clean water from a purifier.