The Effect of Carbon Spots on Copper-Nickel Coins
The term "carbon spots" refers to tiny black concentrations of corrosion. Oftentimes these are so small as to escape notice by the naked eye, though they may be seen with low-power magnification. Also called "flyspecks" by some in the hobby, these spots are actually slightly raised from the surface of the coin, as the corrosion forms around some particle of organic matter, such as paper dust (often present with coin albums and cardboard "2x2" stapled holders) or human saliva deposited unknowingly by a numismatist during casual handling. Oxygen, humidity, and other atmospheric elements react with the debris to form a minute mound of corrosion around it, and this is called a carbon spot.
Removal of the debris will usually stop the reaction, and thus worsening of the spot, then and there. This may be as simple as removing the offending particle. The resulting corrosion, however, will remain as an unsightly black speck that can range greatly in size from nearly microscopic to as much as a quarter-inch in diameter, depending on how much mass the contaminant possessed and how long the reaction was occurring.
Most often carbon spots will form on the surface of copper or bronze coins. The highly reactive nature of copper as a metal will often lead to their formation, but US copper-nickel coins as well as other copper-nickel coins from around the world are also quite susceptible. Most nickel alloys used for United States coinage are a combination of 75% copper and only 25% nickel. This includes the three- and five-cent pieces made since 1865 and the outer layers of our current dimes, quarters and halves, as well as those of the dollars coined 1971–99. The thick cents dated 1856–64 included 88% copper to only 12% nickel and, given their greater copper content, have an even greater susceptibility to developing carbon spots.
For copper-nickel coins displaying carbon spots, proper conservation can remove both the contaminants and the resulting spots. In some instances a pale ghost of the spot may remain, and removal of carbon spots will usually leave tiny bald patches in a coin’s toning. For these reasons, copper-nickel coins that undergo removal of carbon spots will typically have their toning removed as well during the conservation process. This is preferable to having such eye-catching gaps and is in the best interest of the coin’s long-term preservation.
Source: NCS
