How To Clean Old Coins
Do you know how to clean old coins? In most cases, you SHOULD NOT CLEAN COINS. While you might think uncleaned coins look nicer if they are shiny, collectors prefer coins with an original appearance. Cleaning coins may reduce its collector value by half or more.
Cleaning coins is similar to restoring works of art - they're both jobs best left to professionals who have the knowledge and experience to know when it's advisable, what techniques will work best and how to use them properly.
If you do want to undertake this effort, never abrasively clean coins. Even wiping with a soft cloth will cause small but undesirable scratches, which will reduce the coin's value.
If the surface of a coin appears to be tarnished, it is best left alone. The color change is the result of a natural process, which collectors call toning. Atoms on the surface of the coin have reacted chemically, often with sulfur compounds. The reaction cannot be reversed.
"Dips", which strip molecules from the surface, are available. Dipping is the quintessential example of a technique that should be used only by professionals, if at all. Additionally, natural toning sometimes increases the value of a coin (i.e. when it's considered attractive).
Dirt and other foreign substances adhering to a coin can sometimes be safely removed. You can try soaking the coin for a few days either in olive oil or soapy water, followed by a thorough rinse with tap water. Dry the coin with compressed air or allow it to air dry. Do not rub the coin. Commercial coin cleaners may also be carefully used to more quickly loosen foreign substances.
Cleaning coins can affect their value in some cases. You will need to tell if a coin has been cleaned in order to assess its condition.
If the coin has been cleaned with an abrasive, the coin will have hairlines. Also, abrasive cleaning often leaves some crud in the recesses of the coin (untouched dirt or left over abrasive).
If the coin has been dipped, it may or may not be detectable. Although it is possible for such an original coin to exist, it is unlikely. Also dipping can strip the luster off of the coin, with the end result that there is no luster where you would expect it to be for a coin in said condition (XF and better coins).
A natural coin has a particular appearance which reflects the history of its storage. Haphazardly stored coins tend to have a "dirty" appearance to the toning. Coins that have lived for a long time in a coin cabinet tend to have spectacular colored toning.
Coins stored in a clean metal vault (such as an old style "piggy" bank) may stay white (or red) for a long time. Coins stored in albums develop either the familiar "ring toning" (slide type albums) or the much less desirable "one sided toning" (all cardboard albums). Coins stored in mint bags often show spectacular rainbow toning, similar to that seen on coins stored in coin cabinets.
Copper/bronze/brass coins that have been cleaned have an unnatural color, often looking like a toned gold coin. Even after they re-tone, they tend to be uneven and a slightly odd color (watch out for dark areas). Red in the recesses of that VF copper coin is generally not a good sign! Naturally toned, *circulated* copper tends to be very uniform in color, although they might be dark and dirty around the lettering and similar protected areas. Un-circulated copper may tone very unevenly (especially proofs), so do not automatically count this against such a coin.
Exactly the other way around, silver coins that have been cleaned tend to be extremely uniform in color after they re-tone, including the tops of the letters and protected areas. Silver coins with natural toning will usually show some variation in the color at these places. Be aware that a uniform slate gray color can be produced on silver very easily with a number of chemicals. Finally, a heavily toned and subsequently dipped silver coin will tend to have a gray appearance caused by surface roughness rather than tarnish. This can be detected by careful examination with a strong magnifier.
The ANA advises that sudden "hard line" changes in color do not occur on naturally toned coins. Naturally toned coins exhibit a gradual change in color or darkness. In any event, it’s mostly a matter of looking at a lot of coins and forming your own opinions. Assuming that you are buying coins for your personal collection, in the final say, it is your opinion that really matters.