When silver is exposed to air, the air oxidizes the silver, causing it to discolor. This tarnish is caused by contact with sulfur compounds such as hydrogen sulfide in the air. Some items that can cause tarnishing in silver include some paints, carpet padding, wool, felt, fuel, rubber bands and latex gloves. Some foods too, can affect the color of silver, especially eggs, onions and mayonnaise. Humidity and moisture can change silver’s color, as well as oily salts from fingers.
Careful handling and storage of silver can help prevent tarnishing. Storing silver in a moderate, controlled environment with low humidity works best. Numismatic, collectable coins should be stored in containers specially made for coin storage. Handling silver coins requires wearing white cotton gloves.
Removing tarnish from silver is not difficult. Try to catch it early, as it is more easily removed before it turns to light brown or black. With light tarnish, use a vinegar-based window cleaner or a liquid, non-abrasive, unscented hand sanitizer that contains no aloe. Rub it on gently with cotton balls, then dry with a cotton dish towel. This is the least abrasive method for cleaning sliver, although it shouldn’t be used on numismatic coins.
With silver coins, experts recommend that they be professionally cleaned. Coin dealers and collectors want to buy coins in their original condition. Removing tarnish from coins can harm the coins, leaving spots and scratches. However, if you must clean the coins, use the least harmful method. Very dirty or spotted coins can eventually corrode and become damaged. Again, use a professional service.
http://www.mycoincollecting.com/collecting/cleaning-coins.html
http://www.silversmithing.com/care.htm
