According to coin expert Susan Headley for About, the easiest process for cleaning old coins is to gently rinse the coins in a bath of warm tap water and mild dish-washing detergent. It is important to make sure that the hands are thoroughly washed to remove all surface oils and dirt that may potentially contaminate the coins. Headley stresses that cleaning old coins is not recommended unless absolutely necessary.
Headley warns that attempting to clean old coins using improper methods may significantly reduce their monetary value. Many people mistakenly believe that the tarnish or seasoning on old coins should be removed, so that they are restored to an attractive shiny appearance, but it is the very presence of this natural oxidation or "toning effect" that increases their value during appraisal. Attempting to clean old coins using products such as silver dips, polishes or chemical solutions can effectively ruin them. Ideally, old coins should be protected from the elements, including direct human touch and exposure to air and water. Headley states that the only time when it might be appropriate to clean a coin is when a beginner collector wishes to remove germs and gunk from a cheap collection for hygienic reasons.