Brent E. writes: How can one tell when an uncirculated older copper coin (e.g. early Lincoln cents) has been cleaned and retoned and when the toning is natural. The amount of luster seems to vary significantly on pre-1933 coins, or is that a function of cleaning?
When a coin is struck, the metal in the planchet (coin blank) momentarily melts and flows into the dies. The flow lines that are created by this process are what is responsible for mint luster. If you tilt a coin in the light you can see the mint luster travel in a circle around the coin. Under a microscope , one can actually see the radial lines that travel from the center of the coin outward. If you remove or damage the flow lines, the coin can no longer be considered uncirculated as broken flow lines are an indicator of metal being removed from the surface of the coin either by wear or cleaning. It is true that if coins are weakly struck there is less luster but the flow lines will still be unbroken.
Cleaning is bad for coins as it is easy to damage or remove the flow lines. On copper it is doubly so, as the color of a cleaned copper cent takes on a harsh orange color vs the red or red-gold color of a natural piece.
The subject of toning has been a running discussion among coin collectors for many years. Though this forum is too limited for a complete overview, here are a few tips:
– Since toning is the result of oxidation, that is air combining with the surface of the metal, the natural progression of oxidation would take a copper cent from red or red-gold to red-brown to brown.
– “Rainbow” toning with blues and other bright colors are the result of exposure to sulfur compounds. It is sometimes difficult to determine if the toning came from being in a high sulfur content cardboard holder for a long time or the result of chemical abuse. There is a difference, though in some cases even skilled numismatists have a difference of opinion.