If you’re a US cent coin collector you may have noticed that prices for uncirculated pennies vary depending on the coin’s color. That’s because all US cent coins—and many foreign coins—have copper in (or on) them.
Until 1983, with the exception of the 1943 wartime steel penny and a brief period in the mid-19th century, US cents contained 95% copper. Rising prices of the metal forced the Treasury to switch to zinc coins with a 2.5% copper cladding to maintain the coin’s traditional appearance. The only exceptions since are 2009 Lincoln Bicentennial collector sets that were made of the original 95% copper composition.
The Character of Copper
As metals go, copper is relatively soft, making it malleable and ideal for minting coins. But unlike gold and silver, the other metals commonly used in minting coins, it’s also highly reactive to oxygen, skin oils, acids, and water vapor. Naturally reddish-orange, these reactions cause copper to develop a patina that deepens to a deep brown over time. And this color change affects a coin’s collectible value.
From Red to Brown
When copper coins are graded for collectability, the grading service will add a color description to the overall quality score. RD indicates the coin retains 90% or more of its original finish. RB (reddish brown) is a broad spectrum from slightly tarnished to about 10% of the original finish remaining. B denotes a coin with less than 10% of the original finish existing. This is noted as part of the grade.
For example, MS 67 RD indicates a Mint State Grade 67 (almost perfect) coin with a reddish copper finish. This grade would put a coin at the top of its price range. A coin graded Extra Fine EF 45 RB would show minor wear and have a moderate amount of its original copper finish. A BN designation drags the value down to the lowest range of a coin’s type, but that didn’t keep a 1787 Connecticut Copper graded F 12 BN from bringing over $164,000 at auction since coins of that era are almost always fully tarnished due to their age.
And Then There’s Green
Fully oxidized copper surfaces turn green. Ancient copper coins that have been buried in soil for centuries almost always have a powdery green surface that is noted in grading but is only one factor in valuation given the rarity of many such coins.
Modern coins with a fully oxidized green finish are considered damaged, though, and are generally considered ungradable.
The Perils of Polish
While tarnished copper can be restored to its original finish, polishing a coin destroys its collectible value. Rare coins should never be cleaned without observing strict procedures, preferably by a professional coin expert. On the other hand, if you’re more concerned about appearance yhan value, polishing copper can bring it back to life. For tips on how to clean coins, see our June 2016 article “[Cleaned vs Uncleaned Coins].”
For a look at the ranges of color on US copper cents, see this graphic on Pinterest.
