Cleaning includes washing, rinsing, polishing, scrubbing, dipping, or using any other process to alter a coin's surface to remove dirt, enhance luster, or otherwise enhance appeal.
Most all coin literature advises to never clean coins. This is almost always good advice, as countless thousands (millions?) of dollars in coin value are lost every year from newbies and non-coin enthusiasts who decide that cleaning their coins will increase their value. Thus, if you are reading this and wondering if you should clean your coins, DON'T. You will almost always decrease the value of your coins by doing so.
Dipping is a form of cleaning in which the coin is submerged in a liquid chemical designed to loosen dirt or grime and/or remove undesirable toning. High-grade AU coins are sometimes dipped to make the coins appear more lustrous and perhaps uncirculated, but an experienced eye can instantly detect a dipped coin.
Whizzing is a special form of cleaning in which a brush made of wire or a similar substance is applied to the coin to enhance luster. Whizzing achieves this by stripping off the natural surface of the coin to expose the lustrous, untoned metal beneath. Most whizzed coins are detectable without a loupe, especially to a trained eye, and the abrasion marks are readily visible by properly rotating the coin under a good light source. Because it is one of the most abrasive processes used on coins and can majorly detract from a coin's value, whizzing should absolutely be avoided.
the coin maxim of never cleaning your coins has recently been formally addressed with the advent of Numismatic Conservation Services (http://www.ncscoin.com/), which takes the perspective that coins, like fine pieces of art, may be properly restored if done so professionally. VAMworld.com does not advise for or against using NCS, but notes its existence for the sake of education regarding the contested cleaning issue.
Please note that die polishing has nothing to do with the cleaning of a coin. Die polishing is the process of cleaning the die that strikes the coin, not the coin itself. Please see the separate entry. If a coin listing says that the coin has "die polishing," the coin may be an interesting variety such as the 1921-S VAM-1i. If a coin listing says that the coin has been "polished," then the coin itself has been cleaned, and likely harshly if the term "polished" has been employed.
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