With the global financial crisis the price of gold has risen as investors escape risky equities and invest in less risky precious metals, an endemic problem of gold (Au) bar/ingot forgery has arisen. Criminals are using the increase in the value of gold to their advantage by producing fake Gold bars.
The current method used by criminals is to plate Tungsten (W) with a thin layer of gold (about 0.1mm). They do this because Tungsten is a very similar density to gold but worth 1% of the value. By doing this, the criminal's get something that weighs the same as a gold bar.
In order to check if a Gold bar is legitimate, non-destructive testing (NDT) methods are applied to the bar in question. However, NDT inspectors are finding that the criminals clever method also defeats the current favoured non-destructive testing method of X-Ray Fluorescence (XRF) equipment as this only tests the material surface. Drilling a hole to take a sample for an assay analysis is not permitted as of course that would remove valuable material. So then some other non-destructive test is required to test for this condition.
John Hansen, MD of ETher NDE, a UK based manufacturer of Eddy Current NDT equipment and probes had the issue brought to his attention by a concerned contact who asked if eddy currents could be used to check for forged Gold (Au) bars as an alternative to the X-Ray NDT method.
John found that the Gold bars could be tested effectively using Eddy Current. He said, "I worked out in theory this was possible at 60kHz with a conductivity meter. The reason for this is that the conductivity of Gold is 45 Mega Siemens per m (MSm-1) (or if you prefer 77.6% IACS or resistivity 2.2 ƒÝƒà.cm) whereas Tungsten is 18 Mega Siemens per m (MSm-1) (or if you prefer 31.3% IACS or resistivity 5.5 ƒÝƒà.cm). However there is of course the skin effect to overcome (this is a phenomena that limits the eddy current concentration to the material surface). Well the good news here is that at 60kHz the traditional frequency for conductivity measurement the skin depth (average depth of penetration) in gold is 0.3mm and the gold plate is no more than 0.2mm."
A couple of days later John's contact phoned and reported that they had done some tests on forged and real gold bars and that using the Eddy Current conductivity meter worked very well.
Therefore using John's theory, ETher NDE has added the capability to their conductivity meter, the SigmaCheck software (should it be necessary in the future) to penetrate thicker plating.
Adding to this, one XRF supplier is very interested in adding Eddy Current Conductivity functionality to their XRF system probably a special for Gold Bullion use.
With the revelation that an Eddy Current conductivity meter can simply check for forged Gold bars, there is hope that this epidemic is halted before it spreads too far.
For more information on this article and the SigmaCheck, please contact John Hansen at [email protected].
For full product details see: www.ethernde.com.