ResearchHow to Clean and Care for Coins - for Coin Shooters
There is just one word in coin collecting: condition! A coin worth thousands of dollars can become nearly worthless through mishandling. Standards of coin condition values range through 70 grades, and the difference between one grade and another is a matter of careful examination with a magnifying glass. For the coin shooter - that is, those who find coins with a metal detector for a hobby - there is already a disadvantage: a coin that is unearthed from decades of unprotected rot in the soil comes out often indistinguishable from a lead slug. So a real challenge lies here, in that you're dealing with a specimen that's most likely been through plenty of abuse already. But with some proper care you can guarantee that you'll make the most of your find.
Rule number one: the worst enemy of a coin is fingerprint oil. Human fingerprint oil is a naturally corrosive substance, and a careless fingerprint on the face of a coin will show up as a big spot a few years down the line which no cleaning can remove. Always handle coins by the edges, and you're better off getting them into a plastic coin holder as soon as possible. An even bigger "don't do" is to use an abrasive cleaning method, particularly a wire brush polisher. If you ever have a need to get a good yelling-at, just bring a machine-polished coin to a coin dealer and try to sell it to him.
You may be lucky enough to find a coin that needs no cleaning at all. If you can clearly make out the surface details such as the date and mint mark, the coin is probably better off left alone. It is also natural for a very old coin in good condition to discolor slightly in a process called "toning". This is not a defect and in fact can make a certain coin worth more. When in doubt, never clean a coin until you've shown it to an expert and gotten their opinion. Fantasticly elaborate methods for cleaning coins exist that aren't normally accessible to the home hobbyist, right up to ultrasonic tanks and electrolysis, so again you might be better off taking it to a professional dealer first before attempting anything yourself.
Commercial jewelry cleaning solutions are usually too harsh for coins. These can leave tiny scratches on the coin's surface. The two best substances to use are olive oil or isopropyl rubbing alcohol. Occasionally soapy water made with a mild dish-washing liquid will do. Commercial coin dealers also sell special coin-cleaning solutions, but these are a bit pricey and you may decide the expense isn't worth it. Leave the coin soak in one of these mixtures 24 hours using only a plastic container. After this, remove it and rinse it off with distilled bottled water - never expose it to tap water. To dry the coin, either let it air-dry on a soft cloth or gently pat it dry with a soft cloth - never, ever rub a coin, no matter if it's with a tissue or a towel. After this, put it in a plastic coin holder and seal it shut, being careful not to leave moisture from your breath in the coin holder.
It's never a good idea to do this treatment more than once unless the coin is flat-out unrecognizable anyway. With coins, there's only so much that can be done to repair damage and then that's as good as they're going to get. If you still can't make out details like the date, you're probably out of luck as far as collector's value goes.
Remember that when you handle coins which may be prized by collectors, you have a responsibility to future generations to be as careful with them as you can. You are holding a piece of history, after all, and mistakes you make will leave a permanent record on the coin for the rest of its life!