Though debates rage about whether and how to clean your coins, most serious collectors agree on some basic guidelines.
The first principle is similar to the Hippocratic Oath for physicians: avoid damage at all costs. Most coins have already experienced wear and degradation of various kinds. Exposure to air, banging together in carrying bags, use in commerce and a host of other actions will result in nicks, scratches and corrosion.
Don’t make the situation worse once you acquire the coin for collecting or trade.
That principle implies a method for deciding when to clean a coin. If leaving the coin untouched will result in further degradation – because of the presence of corrosive chemicals, dirt or other material – then clean the coin GENTLY.
The goal is never to make them look ‘shiny and new’, but merely to prevent any further corrosion or damage from chemicals the coins may have come in contact with. The green stain on copper coins is a common example. This is copper oxide – in essence a kind of rust.
What you use to clean them with will depend on the kind of material you are trying to remove. But there are some common household ingredients that can be used safely.
Be sure to wash your hands and lay out a clean working area first. Test any method you use on an ordinary coin before using it on a collectible.
Ordinary liquid dishwashing detergent is useful for removing surface dirt. If soaking doesn’t work, apply a small amount to the surface and rub very gently with the thumb and forefinger. Do one coin at a time and keep them separate so they don’t scratch or ding one another.
Lemon juice contains a weak acid that is useful for removing oily smudges, including those produced from unwashed hands. Sometimes a short soak will remove material without the need for rubbing. Try that first. Keep in mind however that removing oil exposes the surface to air. The oil serves as a protective coat. That can lead to oxidation.
Coins should preferably be air dried, but if you must rub use an extremely soft cotton cloth and rub very gently. A better method and material is to use cloths made especially for cleaning eyeglasses, as they are non-abrasive.
Before using any kind of tarnish or stain remover, similar to the sort commonly used on silver spoons, for example, consult a coin dealer. You may actually lower the value of the coin by making it less tarnished. Always use the type especially made for coins.
Dealers and serious coin collectors will in rare cases use electrolysis to clean certain coins. Though, again, most coins are never cleaned or polished at all. Home kits exist or can be made for this purpose, but they should be used with extreme care.
To repeat: you may actually decrease the worth of your coin in attempting to ‘improve’ it. When in doubt, consult a dealer before cleaning any coin.