Cleaning Your Coins
In most cases, the best answer is DO NOT CLEAN COINS
! While you might think they'll look nicer if shiny, collectors prefer coins with an original appearance. Cleaning a coin may reduce its collector value by half or more.
Cleaning coins is similar to restoring works of art - they're both jobs best left to professionals who have the knowledge and experience to know when it's advisable, what techniques will work best and how to use them properly.
Never abrasively clean coins. Even wiping with a soft cloth will cause small but undesirable scratches, which will reduce the coin's value.
If the surface of a coin appears to be tarnished, it is best left alone. The color change is the result of a natural process, which collectors call toning. Atoms on the surface of the coin have reacted chemically, often with sulfur compounds. The reaction cannot be reversed. "Dips" which strip molecules from the surface are available. Dipping is the quintessential example of a technique that should be used only by professionals, if at all. Additionally, natural toning sometimes increases the value of a coin (i.e. when it's considered attractive).
Dirt and other foreign substances adhering to a coin can sometimes be safely removed. Try soaking the coin for a few days either in olive oil or soapy water, followed by a thorough rinse with tap water. Dry the coin with compressed air or allow it to air dry. Do not rub the coin. Commercial coin cleaners may also be carefully used to more quickly loosen foreign substances.