Old coins maintained for collections must be carefully cleaned with non-abrasive materials and a solution that does not damage the metal compounds in the coin. Improperly cleaning a coin can damage it and substantially reduce its value. Specialists knowledgeable in coin cleaning can clean coins without damaging them. Most expert coin collectors advise against cleaning coins at all, as discolorations and dirt may actually increase a coin's value, explains NemisMaster.com.
To avoid damaging the coin, clean off grime and dirt by soaking the coin in a solution of mild soap and distilled water. Only soak the coin; do not rub it with an abrasive material or mix it around, as this may damage a delicate coin. Use only distilled water, not treated tap water that may contain chlorine and other chemicals that can damage the metal. After bathing them, allow the coins to air dry. Never rub the coins dry with any material, as even the softest cloths can cause damage.
A heavily encrusted or corroded coin can be cleaned by soaking it in olive oil. The coins should be soaked undisturbed, sometimes for months, to remove the dirt without damaging the coin. Ultrasound machines can also be used to clean coins.