Authenticity testing of gold bullion using electrical conductivity

Gold and precious metals have always been popular investment objects. However, due to the sky­rocketing prices of precious metals in recent years, the production of counterfeit gold bars has become a booming industry; more and more sophisticated fakes emerge ever more frequently, leading to much uncertainty in the market and investor wariness. This is why the demand for fast, reliable and non-destructive testing has also been on the rise.

To determine the authenticity of gold bullion – non-destructively and therefore without loss of value – its electrical conductivity is measured using the eddy current method according to ASTM E 1004. This method utilizes the physical fact that fine gold, various alloys and foreign material will all differ in their conductivity, there­fore allowing forgeries to be detected.

Fig.1: SIGMASCOPE® GOLD B for testing gold bullion and ingots – even through a plastic cover

For this purpose FISCHER has developed the SIGMASCOPE® GOLD B, which can determine the electrical conductivity of gold bars up to about 1 kg in weight. By taking measurements from both sides toward the middle, the entire depth of the bullion can be measured, testing the genuineness of the fine gold. Hidden enclosures of non-precious metals with a comparable density (e.g. tungsten) can be detected unambiguously with the SIGMASCOPE® GOLD B – and the piece revealed as counterfeit. The phase-sensitive evaluation of the measurement signal enables determination of the electrical conductivity without surface contact – even through non-conductive protec­tive layers such as plastic packaging.

Fig.2: Schematic drawing of a counterfeit gold bullion showing a core of tungsten sticks hidden by a real gold skin

The SIGMASCOPE® GOLD B is designed to operate with three measurement ranges for different penetration depths of the electrical field. A penetration depth at least to the very centre of the bullion is necessary in order to detect even tiny inclusions at its core. For small ingots (e.g. 1 oz / 50 g) the SIGMASCOPE® GOLD B also provides thickness compensation; that is, the thickness of the bar is entered into the instrument and accounted for in the measurement results. Any influence of the conductivity value by various thicknesses is avoided by operating with the correct measuring range and the thickness compensation.

Fig.3: The electrical field to determine the conductivity penetrates through the gold bullion, forgeries show different conductivity values and can therefore be easily detected, wherever they are located.

Measuring the electrical conductivity is a precise yet quick method for testing – non-destructively – the genuineness of valuable items made of precious metals. X-ray fluorescence analysis (XRFA) is a suitable complement for accurately determining their composition. For further information please contact your local FISCHER representative.

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