Die deterioration doubling (DDD) is a common form of mint-made error on many United States and Canadian coins that results from degradation of the die used to strike the coin.
Due to a combination of improper preparation and treatment of the dies, excessive wear, and overuse, die deterioration doubling has occurred on nearly every series issued by the US mint in recent times. Certain coins are more susceptible to it than others, due to various factors, but it does occur on most series of coins.
Causes
Annealing Errors
During the annealing process of die making, two main errors can occur, resulting in less than perfect dies. It is impossible to tell without expensive and lengthy tests whether these errors occurred, but they will lead to significantly weaker dies and a higher tendency to deteriorate quickly. The first thing which can happen to a die in this process what metallurgists call decarburization. Decarburization is the result of an improper mix of metals and chemicals in the annealing furnace where the dies are sent to soften. If there is not enough ambient carbon in the furnace, the steel will give up some of its carbon and will become soft. The result is a thin surface of inadequate metal whose chemical composition has been altered and is below standard.
Another less obvious error can also occur when heating the dies. After being taken out of the annealing furnace, the dies must cool for 24 to 36 hours. They are usually given an oil bath to cool them more quickly, but the dies must still cool for a period of several hours. When the dies are cooling, if they are not stored properly or not allowed to fully cool before being used, the result is a much softer die. When the die is subjected to the hubbing press again and work hardened again, it will be softer and more susceptible to wear. If the die is softer because it was not fully cooled, it will not harden to the same standard as a die which has received proper treatment. However, there will not be a noticeable difference between a good die and a bad die; the change in the structure of the die is not apparent until it is used for a period of time.
Overuse of Dies
In modern presses, a die strikes approximately 120 coins a minute. This is a large number of coins produced in a short time, and obviously the die cannot last forever. However, the process of wearing the die is only hastened by the metals used in coins. Nickel, one of the main metals used in today’s coins, is quite hard and causes wear quickly. Copper has been used for centuries because of its malleability and the ease with which it makes coins. However, it too wears the dies when they are used for too long. An infamous example is the 1955 "poorman’s double die." This coin is sold as a replacement for the 1955 doubled die cent, but it is no more than Die Deterioration Doubling, caused by wear on the dies.
When a coin is struck, the planchet is not heated. Although the planchet would be softer and more malleable, the extra time and expense would prove too great for the Mint. The planchet is therefore struck at room temperature, and the only thing which makes the coin form is the tremendous pressure used to strike it. With a metal such as nickel, which is harder than a normal coin metal like silver, gold or copper, the pressure must be greater. When a nickel coin, or any coin, is struck, the metal must "flow" into the contours of the front and back dies. It is through the atoms of the metal flowing into the dies that flow lines are created. However, when metal flows over a sharp corner in the die, like the edges of a mintmark or words, it tends to roll the detail out. It wears on the die, and a little detail is lost with every strike. Die Deterioration Doubling is most prevalent on the date and mintmark because these fine details are alone in the middle of the field, and the metal must flow into these without the help of other valleys nearby. When the metal rolls into the mintmark or date, it wears away the corner of the die, and after long enough will appear on the coin as Die Deterioration Doubling.
In addition, the mint has a practice of polishing the dies and using them longer. When a die begins to show evidence of wear, a mint technician will polish the die and reuse it. However, the die is still rapidly deteriorating, and polishing it cannot avoid the problem of wear. So, after many thousand coins in rapid succession, the wear on the die has gotten so bad that it appears as Die Deterioration Doubling.