Clean old coins in a weak soap solution.

If the distilled water was too weak to effectively clean the surface of a dirty or encrusted coin, the only other non-damaging option is to use a weak soap solution. Pour a small amount of a weak liquid soap into a large bowl, and then fill the bowl with tepid distilled water. Grasp your coin by the edges, and swirl it around in the soap solution. Then rinse the coin under distilled water and pat dry with a clean cloth

Do not use dish detergent to clean your coins, it’s too powerful and abrasive. Instead use a weak, gentle soap, such as hand soap