A new addition to testing - Electrical Conductivity Measurement

Posted by Wai Leong James Lee on 17 Dec 2013


Since January 2013, we have been studying the feasibility of authenticating small to medium sized bullion items by measuring their electrical conductivity value. Fast forward to December 2013, we have incorporated electrical conductivity measurement into our non-destructive testing methodology for bullion. It is a simple yet powerful way to detect for inhomogenity or inserts present within the bullion item. Electrical conductivity measurement offers a faster and more possibly more reliable test alternative to ultrasonics to detect for inhomogenity within a bullion item. Here is why:

1.     Testing electrical conductivity is perfect for authenticating silver

Because silver is the most conductive element in existence, no other metals can be used to fake silver. Even if people try to mix and match metals and substances to form different alloys, it will be impossible to match the conductivity of silver because alloying a pure metal will cause its conductivity to decrease. In short, it is theoretically and practically IMPOSSIBLE to have a metal or an alloy to match the electrical conductivity of silver!

For gold, it might be theoretically to mix and match metals to obtain an alloy to match the electrical conductivity of gold, but it would not be possible to have an alloy that matches both the electrical conductivity AND the density of gold. Therefore, measuring the electrical conductivity as well as the density of the bullion is a truly definitive non-destructive test to authenticate pure gold.

  

2.       Tolerances are up to 16 times stricter than compared to ultrasonics

In-house statistical studies have shown that the tolerances obtained from electrical conductivity measurements are up to 16 times tighter compared to readings obtained from ultrasonic measurements. This makes our testing methodology much more reliable and sensitive (see point 3 on sensitivity).

 

3.       Gold and silver are extremely sensitive to impurities

A presence of small amounts of impurities can greatly affect the electrical conductivity of gold and silver. For example, from the chart below, we can see that the electrical conductivity of pure gold is at 45 MS/m. Ducat is a gold coin with 1.6% copper impurity. As seen from the chart, a mere 1.6% of impurity lowers its electrical conductivity to 25.3 MS/m. That’s more than a 40% reduction in electrical conductivity.

 Source: Fischer Instruments

 

4.       Able to test through plastic sheaths

The ability for our instrument to take readings through plastic sheaths means a lot to us because gold bullion from many major brands come packaged in plastic sheaths containing the mint’s assay certificates and labelled with their serial numbers. Removing the plastic sheaths to perform an ultrasonics test would mean destroying the mint assay certificate that comes along with the gold bullion, potentially affecting the salability of the item.

  

 

What about ultrasonics?

Currently, we are still using ultrasonics to detect for inserts/inhomogenity present in large bullion items, such as the 1000oz LBMA good delivery silver bars. This is because one of the constraints of electrical conductivity measurement is that it can only be performed up to a depth of 18mm of gold. Although electrical conductivity measurement is preferred for the reasons explained above, until new testing technologies or instruments are being developed, using ultrasonics to detect inhomogeneity within large bullion items will still be reliable enough for its intended purpose.