A “struck-through” error occurs when a foreign object is struck into a coin, leaving an impression. Such errors are always more compelling when the nature and origin of the foreign object is readily apparent.

A persistently popular category is the “struck through reeding” error. Shown above is the reverse face of a 1971-D Kennedy half dollar that was struck through a long piece of reeding.

Kennedy half dollar: The shot heard around the world in 1963, a bullet from an assassin's weapon that ended the life of U.S. President John F. Kennedy, is still remembered on the annually produced half dollar struck in his honor since 1964. How much are Kennedy half dollars worth?

Relatively little space has been devoted to the source of this detached reeding. It’s clear that most reeding strips are derived from previously struck coins. Since the edge of a normal coin is unlikely to break off spontaneously, we can assume that some sort of press malfunction or planchet defect lies behind such errors.

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Most reeding strips probably represent sheared off “fins.” A fin is a thin, vertical flange of metal that extends from the rim/edge junction. Shown above is a modestly developed fin on a 1984-P Washington quarter dollar. Other fins are much larger. I’ve seen fins on cents that exceed a centimeter in height.

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Fins can be generated by excessive ram pressure (the ton­nage delivered to a planchet of normal thickness). When ram pressure is too high, coin metal is forced into the narrow gap between die neck and collar. Fins can also be generated by a tilted (vertically misaligned) die. The die’s downward-tilted pole generates increased effective striking pressure in that area. Fins are also generated when two or more discs of coin metal are stacked on top of each other. The increased aggregate thickness between the dies results in increased effective striking pressure. Finally, a fin can form gradually if a coin is struck numerous times in-collar by a die pair that has an unusually small minimum die clearance.

Fins are easily torn when a coin is ejected. The severed fin can remain behind in the striking chamber and get struck into the next planchet.

A 1998 5-rupee coin of India shows a reeding impression that was probably generated by a sheared-off fin. A normal 5-rupee coin has two bands of narrow reeding separated by a recessed security design. The security design is pressed into the planchet’s edge during upsetting while the reeding is generated during the strike. In this coin, the reeding impression is significantly wider than a normal band of reeding. This would indicate that the reeding strip was probably derived from a coin struck so hard that the recessed security design was obliterated while metal above and below it was extruded into a fin.

Detached reeding is also generated when the lower edge of an expanding coin grazes the upper margin of a partially deployed collar that is stiff but still somewhat mobile. This contact between coin and collar can occur when an intrusive coin or planchet on the upper face generates asymmetrically and prematurely applied striking pressure to the coin beneath it. It can also occur in isolation, when a centered planchet is converted into a “forced broadstrike” (Collectors’ Clearinghouse, Jan. 10, 2011). In either case, the upper edge of the collar’s working face can shear off the lower portion of the reeded edge to form a thin piece of detached reeding. Shown above is the obverse face a Mexican 1-peso coin with a nearly detached section of reeding that lies opposite a partial brockage on the reverse (upper) face.

Several less common sources of detached reeding exist. I have seen a number of coins where part of the edge was neatly shaved off. Such damage can occur im­mediately before or after the strike. The Aug. 20, 2012, column featured a double-struck 1985-P quarter dollar in which the entire edge was shaved away after the first strike but before the second strike.

The edge of a newly-struck coin can also be sheared off if that coin is nudged into a slightly off-center position and caught between the hammer die and a collar frozen in the “up” position. The resulting thin, crescentic strike clip is then available to be struck into a subsequent planchet (Collectors’ Clearinghouse, June 15, 2015).

A last possibility would involve the edge of a brittle coin breaking away. A number of such coins are known (Collectors’ Clearinghouse, March 14, 2011).