Most numismatists advise against cleaning coins.

They often have patinas, toning and tarnish that can develop on the surface of a coin over time. Certain kinds of light tarnishing, called toning, are considered part of a coin's value. In the case of proof coins, it is very difficult to do anything that will not hurt the coin.

Wear Gloves and Only Handle by  the Edges!

Removing surface dirt from a coin is about the only cleaning that should be done. When handling or cleaning coins, you should wear cotton or polyethylene gloves (not latex). To remove surface dirt, wash the coin in lukewarm distilled water with a mild liquid soap. Do not scrub the surface. To rinse, use a cotton swab dipped in distilled water. Regular tap water contains chlorine, which can leave chloride on the surface of the coin that eventually leads to corrosion. After cleaning, use another cotton swab and acetone to remove any grease (this is called degreasing) that may remain on the surface. Because of its toxicity, acetone should only be used in a well-ventilated area. Allow coins to air dry on a paper towel.

Cleaning

Coin dips or metal cleaners (cloths, liquids or pastes) are not recommended. The dips contain acids that can cause corrosion if any remains on the surface. Most metal cleaners contain abrasives that can scratch the coin.

The approach to medals is slightly different. It is desirable to maintain the bright surfaces of silver medals, but care needs to be taken with the ribbons that are attached to them. It is advisable to wear cotton or polyethylene gloves (not latex) when handling medals. For surface dirt, use a cotton swab to apply a mild liquid soap to the surface. Rub gently to remove dirt and rinse well in distilled water. Let the medal air dry on a paper towel. Silver medals with light tarnish and bronze medals can be cleaned by rubbing a piece of soft cloth gently and evenly over the surface. After cleaning, degrease the surface of the medal using a cotton swab and acetone. Let the medal air dry on a paper towel.

Many coins and medals have protective coatings of either lacquer or wax to protect them from handling and environmental factors. Lacquering should be done by a specialist because, if applied improperly, it cannot only be unattractive but also can lead to corrosion.

Wax coatings are easier to apply and remove than are lacquers, but they provide less protection. To prepare a wax coating, mix equal parts of a good-quality paste floor wax and odourless mineral spirits (a solvent). Apply the mixture to the medal's surface with a soft cloth. Use a hair dryer to heat the wax so that it melts into all the surface recesses. Then, wait until the solvent evaporates and buff the surface lightly with a soft cloth. Wax coatings can be removed easily by applying a solvent (odourless mineral spirits).