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.
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’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.
Similarly, you should never scrub or scrape coins in order to remove tarnish or blackness. Products like steel wool or wire brushes will irreversibly damage your coins and remove their value.
If you live in a city, the tap water will be treated with chlorine. This chemical will discolor the face of your coin. To avoid this, either purchase distilled water from your supermarket, or clean coins under water from a purifier.
Do not use dish detergent to clean your coins; it’s too powerful and abrasive. Instead, use a weak, gentle soap, such as hand soap.
This practice works best on pennies produced before 1982. Pre-1982 pennies were made using real copper, whereas post-1982 pennies contain zinc, which will not be cleaned with ketchup. Be aware that ketchup is mildly acidic, and so may end up reducing the penny’s value.
This process will be most effective on older silver coins. It may be less effective on newer coins that contain less—or no—actual silver.
If the coin is still tarnished or dirty, try brushing it gently with a soft toothbrush. Be careful not to scratch the surface of the coin, however. If the coin does not look cleaner after a few minutes, place it back in the vinegar for a few hours. Very dirty old coins can even be left soaking in vinegar overnight.
