Over the centuries many world coins have featured — and continue to feature — edges adorned with lettering, geometric shapes, representational art, and abstract glyphs. Sometimes the edge design is intended as an anti-counterfeiting measure (a so-called “security edge”) or as a means of incorporating information that couldn’t fit elsewhere on the coin. But most of the time it is purely decorative. 

Most edge designs are incuse, as a raised design will quickly wear off or turn into mush as the coin circulates. Still, a few edge designs are raised and unprotected, while others are raised but housed in a slot-like recess. At least one design features incuse letters tucked inside a narrow slot (see below).

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With rare exceptions, edge designs are pressed into the edge of a blank before the strike. Edging is often sensibly combined with upsetting, a process that converts a blank into a planchet by pushing up a low proto-rim. 

There’s a very good reason why edging is performed before the strike. If the edge design was generated as a planchet expanded against an embossed or intaglio collar design, the two surfaces would be locked together. The complementary interlocking raised and incuse elements would prevent the newly-struck coin from being ejected. Any successful ejection would leave the edge design horribly smeared.

Some coins, like our recent Proof Sacagawea, Native American, and Presidential dollars, are struck in segmented collars. After the strike, the three segments retract and separate, freeing the coin and permitting ejection. This is not true of the circulation-quality versions, which have the edge lettering impressed into the edge after the strike.

Edge designs are typically pressed into the edge of a blank as the latter is rolled along and squeezed within a device that may consist of two edging dies (each responsible for 180 degrees of edge design) or a single edging die with a second component that provides the necessary resistance or pressure. Equipment design varies quite a bit, but one element will typically be fixed in position while the other will be mobile (sliding or rotating).

As with any step in the minting process, edging produces its own diverse array of errors. The errors may be due to faulty die fabrication, damage or breakage after the die is installed, or a faulty transfer of the design from the edging die to the blank.

Edging errors are quite abundant on the very thick (9 grams) Indian copper-nickel 5-rupee coins minted between 1992 and 2004. These carry a recessed security edge composed of a row of raised, alternately slanting bars separated by raised dots. The coin is later struck in a reeded collar. Since the security design resides in a slot-like recess, it doesn’t pick up the reeding.

John Paquette provides us with an unstruck 5-rupee planchet rolled and squeezed within a broken edging die. The breaks appear at opposite poles of the planchet. The two halves of the broken edging die were vertically dislocated, leaving the edge design lying in two different horizontal planes. The broken halves were also horizontally dislocated so that half of the planchet’s edge was squeezed more forcefully. The horizontal dislocation also caused the ends of the broken segments to telescope past each other at one pole.

The edge of an Indian 2004 commemorative 5-rupee coin shows only reeding. It was presumably struck on a blank that bypassed the edging die. Not all fully reeded 5-rupee coins originally lacked the security edge. The edge design and recess can be completely replaced by reeding if the coin is struck under abnormally high pressure.

Our final example comes from Mike Kirkham. It consists of a 10.5-gram copper-nickel planchet of unknown origin that carries the recessed, incuse security edge design (SARB R5) of a South African 5-rand coin (2004 to present). A normal 5-rand coin has a ringed-bimetallic composition and weighs no more than 9.4 grams. Had this planchet been struck with the design it was originally intended to carry, it would constitute an edge design mule.