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Do your old coins look as though the creature from the "Alien" series ate them? They're covered in green slime.
Relax--if your ancient coin is made of bronze, it just has a case of "bronze disease." Several blue-green or green patches will bloom on your old coin. If they're fuzzy, that's a sure sign of bronze disease. The fuzziness, rather than the color, determines if your old coins have this equivalent of athlete's foot.
Bronze disease starts when cuprous chloride in the coin mixes with water or humidity. Again, relax--you may not have exposed the coin to a muggy day, but it's a good idea to keep your ancient coins in plastic sleeves. Old coin values won't drop with normal wear and tear, but collectors and appraisers may not know about bronze disease. Fortunately, the value of old coins can be improved. You may even resell the coin for a higher price than you paid, because past collectors might have thought the defect was permanent.
N.B.: just wiping off the green fuzz won't work--that's only a cosmetic solution, you need some old coin tips on maintaining your bronze coins.
Barry & Darling Ancient Coins recommends soaking your old coin in distilled water for several days, changing water as needed. Don't use tap water. Tougher cases require strong chemicals, so you should consult an ancient and old coin expert. Strong chemicals may damage the patina.
One last tip: Many old coins were packed in earthenware jars in dirt, especially ancient Roman coins, and while all that ancient soil might be valuable to archaeologists, it can erode your old coins. You may not be able to correct this defect, but some experts recommend soaking coins in olive or lanolin oil. These are gentler cleansers than strong chemical cleaners.
Always wear protective gloves and goggles, as well as an apron, when working with strong chemicals. They're not quite as strong as Sigourney Weaver's fighting moves, but they pack a punch.