Counterfeits from Spark Erosion Dies

Posted on 7/1/2007

Skip Fazzari outlines the process of creating counterfeit coins using electric discharge machining (EDM) and how to spot these insidious fakes.

By Skip Fazzari, NCS Authenticator and Senior Conservator

Several books and articles have taken the position that counterfeit coins made using spark erosion dies are easy to detect. Some are, especially the first products made by this process. Yet a recent inquiry into what is new in the field of electric discharge machining (EDM) for the tool and die industry provided an eye-opener regarding what may be coming our way in the future.

Let’s take a quick look at the spark erosion process for making counterfeit dies. Basically, it’s electrolysis. Two conductive substances are placed near each other in an electrolytic bath and a current is passed between them so that a spark jumps across the gap to complete the electric circuit. The spark (electric discharge) produces localized temperatures that melt away metal in a controlled fashion to duplicate the original object in reverse. A conductive substance is needed for the original model, so a real coin is generally used for this application. Since the original model is partially destroyed, we probably will not see any superb copies of extremely rare coins made this way. But a faker could alter the date of a common Flying Eagle cent, for example, to use as the model to produce an 1856 cent. Incidentally, as I recall, the first spark erosion counterfeit I ever saw was a coin of this type and date in the late 1970s!

The model, which can be a genuine coin or a graphite copy (a few steps from the original) is submersed in an electrolytic bath comprised of a non-conducting (dielectric) solution such as mixtures of silicon, ionic water, etc. The “bath” cools the work by disbursing the heat generated by the electric spark, provides insulation in the spark gap, and helps clear away the waste products of the reaction as the metals are dissolved.

When the process begins, a cyclic DC current is applied to the two pieces (coin/model and steel die blank), causing a spark to arc across the gap. The repetitive electric discharges remove bits of metal from the softened die steel (work piece) being formed. The spark arcs across the closest gap. As the process continues, the point of impact will move around the surfaces of both pieces, wherever the distance becomes the shortest, until the entire coin design is etched into the die steel. Each spark pulse produces a tiny crater. Close control of the spark distance is important as the smaller the gap, the smoother the resulting surface.

The minutely pitted surface is one of the diagnostics used to detect fakes made using EDM dies. Since the surface of the die is pitted, this will produce tiny raised pimples on the coin. The industry is working hard to find methods to reduce or polish away these tiny defects, even to the point of polishing the work piece in a radial direction.

Coins made recently using spark erosion dies can be very deceptive, especially if they are given a patina and slightly circulated in a trouser pocket with other pieces. Fakes that remain in a fresh, as-struck condition will often exhibit sharp edges and rims unlike the genuine article. Fortunately for authenticators, whenever a counterfeit coin or die is made, each “step” between the original coin and the finished counterfeit will allow for defects or at the least, a slight degradation of the original coin’s design. As with any counterfeit made using fake dies, detection is often confirmed by the presence of repeating bagmarks, scratches, pimples, and depressions. Counterfeits made by this process lose some design detail and lack Mint-quality luster.

There are some deceptive EDM counterfeits in the marketplace, so be sure to consult one of the major coin grading services if you have questions about the authenticity of a coin. Since counterfeiters will usually strike many pieces with the fake dies, chances are that professional authenticators will have seen an identical example of your coin that aroused someone else’s suspicions.

This article previously featured in Numismatic News