How To Clean Coins?
Published on 8/14/2020 by APMEX
Age may not affect coins as much as it does people. But just like us, they pick up toning and scarring throughout their lives. Numismatists that drive the market know what they want, and it’s not cleaned coins. And long-term, cleaned coins lose value.
Cleaning coins can substantially harm their value. It’s always a bad idea to clean your own coins, and in the rare case a coin needs it, it needs to be in the hands of experts. PCGS and NGC have conservation and restoration services that will take care of those rare exceptions.
Cleaned vs. Uncleaned Coins
To understand why cleaned coins lose so much value, we need to understand a little bit about metallurgy.
When metals are exposed to the air over time, their surfaces tend to oxidize, forming a layer that collectors call toning in the context of coins. In other contexts, this layer might be called patina or tarnish. The Statue of Liberty is one of the most famous examples — its bright green color comes from a layer of oxidation over the original copper surface.
Copper and Silver are particularly reactive metals, and their surfaces tend to change color dramatically over time. Copper takes on a greenish hue when stored in a very reactive environment, but it often tones to a brown or dark brown color. Silver usually gains more gray, and it may get darker toning as well in the recessed areas. Some may turn very black if stored in reactive environments, while others may get golden or rainbow toning. Nickel will gray out over time as well, gaining darker toning in recesses. Gold is the least reactive coin metal and usually only mellows slightly to a softer yellow. There are notable exceptions to this, like the deep red coloration of one Pompeiian Gold hoard, famously referred to now as Boscoreale toning.
The toning creates a surface that keeps oxidation from the underlying metal. This is known to collectors as the “skin” of the coin. Whether manual or chemical, any method of cleaning a coin will change that surface and often take the skin off entirely. It removes metal.
That changes the way a coin looks. Cleaned coins may be referred to as “dipped,” after the most common method of cleaning. Dipped coins can usually be quickly distinguished from undipped coins by looking at the luster. A natural coin will have a “cartwheel” pattern to the shine, and dipped coins will lose luster and reflection.
Over time, a dipped coin may tone oddly or unnaturally. Repeated dipping can even pit the surface, creating a duller sheen or even a grainy, porous surface.
Cleaning will alter the coin. And that can take thousands of dollars off the price.
Grading Services and the Rise of Slabbed Coins
PCGS and NGC, the two best-known grading services, started in the 1980s to solve the problem of inconsistent coin grades — PCGS in 1986, NGC in 1987. Before grading services existed, coins were much less consistent in grade. An MS-65 coin and an MS-64 coin may have thousands of dollars of difference between them, and a layperson (or even many coin collectors) can barely tell the difference. That led to issues with grading from one dealer to another.
The coin grading services aimed to standardize the grading of coins. That led to the homogenization of the numismatic field over time, as whatever the coin grading services looked for became the standard. And these coin grading services specifically call out cleaned coins.
Grades are usually bumped down on coins that have been cleaned. The “slabs” used to seal a coin will have a note if it has been cleaned, and many collectors do not want a cleaned coin if an original example is available.
In any case, it’s best to consult an expert about cleaning your coins. Occasionally it may be called for, but it should only be done by expert hands. The major coin grading companies offer services for just such an occasion.
The question “How do I clean my coins?” is best answered, “You don’t.” Let that Silver dollar stay tarnished. It’s better that way.
APMEX is not responsible for damage if collectors should clean their coins. We encourage you to contact professional services, such as PCGS or NGC, should you wish to clean your coins. We do not advocate or promote the cleaning of coins.