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Leave your old coins as they are. Have a coin expert inspect your old coins. Clean only worthless or filthy coins.

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Although it may seem counterintuitive, the best course of action when dealing with dirty coins is simply to leave them as they are. If a coin is in good condition, with only a small amount of smudging or tarnish on the face or back, it will be worth more to a collector in this condition than if you clean it. Nearly all types of cleaning will substantially decrease the monetary value of your coins, especially if the face or back is damaged in the process. If you’re not an expert yourself, but are wondering if your old coins may be worth any money, take them to an expert before cleaning. The expert will be able to advise you as to whether or not you should undertake cleaning the coins. If the coins are unique or valuable, the expert will likely advise you not to clean them. A numismatist—an expert on money and coins—will also be able to advise you as to the worth of your collection. The more valuable a coin is, the less reason you have to clean it. If you have coins that have little value and you are not planning to collect or sell to a collector, you may wish to clean these for purely aesthetic reasons. It’s also up to you whether or not to clean extremely dirty coins. If a coin is so blackened or tarnished that its face is all but invisible, you may decide to accept the risk and clean it. If you’re in doubt as to whether a coin has any value, should be cleaned, or is worth keeping in a coin collection, always take the coin to an expert before you try to clean it. It would be a shame to find out that you’ve cut the value of a rare coin in half by cleaning it.