Electrical conductivity testing of precious metal coins

In recent years gold and other precious metals have drastically increased in value, making the purity of the base components the most important attribute of any precious metal product. Therefore, precisely determining alloy composition and monitoring the trace and minor elements they contain is critical. Testing high-value coins and identifying fakes requires a reliable, accurate and non-destructive measurement technique.

When dealing with high-value coins, adulterated or counterfeit alloys can cause considerable losses and trading in fakes can entail legal ramifications. To avoid these risks, reliable analytical methods are necessary – but destructive analytical methods damage the coin’s material and value, which is unacceptable. Thus, non-destructive methods have become the standard for material testing valuable objects made of precious metals. In short, these methods must be predictable, reproducible and able to:

· Identify the alloy and the precious metal content

· Detect ignoble inclusions and forgeries

The various alloys and fine gold all differ in their electrical conductivity, making this particular physical dimension optimally suited for the quick analysis of precious metal bullion and coins.

Fig.1: Schematic drawing of counterfeit coins filled with a powdered tungsten alloy and covered with gold

The FISCHER SIGMASCOPE® SMP10 measures electrical conductivity using the eddy-current method according to DIN EN 2004-1 and ASTM E 1004. The phase-sensitive evaluation of the measurement signal enables determination of the electrical conductivity in a contactless way – even through non-conducting protective layers such as plastic packaging.

The SIGMASCOPE® SMP10 uses a measurement frequency of 60 kHz (or alternatively 120, 240, 480 kHz) and is therefore optimised for the testing of coins and thin ingots.

Fig.2: Measurement of the electrical conductivity of coins – fast and reliable

Gold coins such as the Kruegerrand consist of a precisely defined alloy which, itself, features an exactly defined electrical conductivity value. Authentication based on this characteristic is therefore quick and easily verifiable. Matching conductivity values can only be achieved by using a metal alloy with a dramatically different density, which is then evident in the visibly different coin thickness.

Electrical conductivity

Mean value

9.68 MS/m

Standard deviation

0.07 MS/m

Coefficient of variation


Tab.1: Authentication of a Kruegerrand coin which should exhibit a nominal electrical conductivity of 9.69 ± 0.32 MS/m

The SIGMASCOPE® SMP10 can reliably determine the alloy composition of gold coins and thin ingots via electrical conductivity, making it possible to test their authenticity without actually touching the items. For further information please contact your local FISCHER representative.

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