To accurately grade copper coins, you must be able to describe the color of the copper. Over time the brilliant orange/red color of a freshly minted copper coin, such as a Lincoln cent, will diminish and fade to a deep chocolate brown color. As this degradation of color occurs, there are varying degrees in which both red and brown colors will exist simultaneously on the surface of the coin. This color designation only applies to uncirculated copper coins. All circulated copper coins are assumed to be "brown."
Additionally, depending upon the environment where you store your copper coins, the surface of the copper may turn different colors. These toned coins can be beautiful or ugly. Some copper coins may take on an iridescent golden red. Others may turn deep red with hints of blue and indigo. These beautiful tone colors usually happen over long-term storage in less than optimal coin holders. However, these beautifully toned coins are highly prized by collectors, and they will pay a premium.
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The Chemistry of Copper
Copper is an element with a chemical symbol of "Cu." Compared to other metals, copper is soft, malleable and ideal for the minting of coins since it exists in great abundance. Pure copper has a bright reddish orange color. Unfortunately, copper is also highly reactive to chemicals naturally found in our atmosphere. Oxygen, water vapor, and various acids react with the copper and cause it to tarnish. This oxidation, combined with other chemical reactions, results in its natural bright reddish orange color to gradually turn into a deep chocolate brown color known as patina.
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Copper Coin Color: Red (RD)
A copper coin when first struck exhibits a lustrous reddish orange color. These specimens are prized by coin collectors and carry a value premium over identical coins that are starting to turn brown. Most coin collectors will agree that if a copper coin has retained about 90% of its original orange-red color it will be designated as "Red" and abbreviated in its grade as "RD".
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Copper Coin Color: Red and Brown (RB)
Once oxidation and chemical reactions start to occur on the surface of the copper coin, its color will start to change from reddish orange to brown. This may include some areas of the coin that are approaching a chocolate brown color while other areas still have some of the original reddish orange color. A common measurement is that between 10% and 90% of the original orange-red color remains. This is termed as a "red brown" coin and is abbreviated as "RB" on coin grading descriptions.
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Copper Coin Color: Brown (BN)
When a majority of the coin's surface has reacted with the atmosphere such that the surface of the coin is almost entirely a chocolate brown color, this is considered a "brown" coin and is designated as "BN" on grading descriptions.Continue to 5 of 7 below.
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Colonial copper coins are another area where color helps determine the grade of the coin. It is not unusual to find Colonial copper coins in a variety of colors which may include olive, steel, green, magenta, mahogany, rose, purple, ebony, etc. Some of these different colors are caused by a reaction to the environment in which they were stored. Couple this with impurities contained in the copper and you will get a variety of colors on your colonial coins.
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Copper Coin Color: Green
Many ancient copper coins are recovered from being buried in the ground. The chemicals found in the soil react with copper and cause oxidation that is green in color. On ancient coins this is acceptable and usually noted with the coin's grade. Unfortunately, green oxidation on United States coins is usually considered damaged and renders the coin ungradable.
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Shades of Copper
Grading the color of copper coins is very subjective, especially when the color is on the boundary between shades. The chart at the left shows twenty coins representing the various shades of red (R), red and brown (RB), and brown (B). Remember that computer photos do not capture the exact color of the coins, so you may notice some inconsistency from one computer to another.