Of all the metals used for United States coins, copper is perhaps the most chemically volatile. While silver, gold and (to a lesser degree) nickel are reactive to some degree, copper is the most likely to oxidize, tone and change in color. With that in mind, numismatists have devised specific designations to describe a copper coin’s color.
Copper coins, when newly struck, display a bright, vibrant orange-red color. Copper piece retaining full original mint color are designated as “RD” or “Red.” This designation is listed immediately after the coin’s grade, i.e. MS 65 RD. In order to qualify for the RD label, virtually all of the orange-red color must be intact save for some extremely minor spots or flecks.
As copper coins begin to oxidize and tone, their color generally turns to a brownish hue. Coins that still retain some (but not all) of the original mint red, the designation “RB” or “Red-Brown” is applied. The RB designation covers a wide range of colors, ranging from “near-miss” coins that are mostly red to coins that are predominantly brown with minor amounts of red left.
If virtually all of the original mint red has vanished, the designation “BN” or “Brown” is used. A Brown copper coin can still have traces of mint red, but only very small amounts. Even after a copper piece has turned brown, it can still retain strong luster. In addition, sometimes toned copper coins can take on attractive purple, blue, green and turquoise coloration. These colors may knock a Red coin down to Red Brown or Brown, but they can enhance a coin’s eye appeal, grade and value.
Why do copper coins shift in color? For coins, there are two common triggers: moisture and sulfur. As coins circulate, they come in contact with humidity and moisture—not to mention human hands. Even if a red copper coin is plucked from circulation, it still might tone if stored in a reactive holder or album. Many early coin albums contained sulfur, which can rapidly accelerate a copper coin’s transformation.
It’s important to note that copper can oxidize incessantly to the point of making a coin porous, corroded and/or environmentally damaged. Defects like green oxidization (also known as verdigris) and rough surfaces are considered highly detrimental for copper coins and will greatly diminish a coin’s value. To some degree light porosity is tolerated for 18th century copper coins, but later issues are expected to be devoid of such flaws.