A comparison of Specific Gravity, XRF and Ultrasound testing to authenticate gold bullion and silver bars

There is an array of test methods to authenticate gold bullion and silver bars which measure a range of chemical, physical and electromagnetic properties. Suitable testing should be non-destructive and conclusive. Non-destructive but inconclusive methods include magnetic testing and ping testing. Any results from inconclusive testing of bullion products need to be treated with caution for possible false positive validation. This article discusses three commonly used testing methods and their potential to produce false validations.

  • The time-honored specific gravity test
  • Testing by XRF instruments commonly found in dealer and pawn shops
  • Testing using an affordable ultrasonic test kit

The Specific Gravity Test

Ever since the Greek mathematician, Archimedes, was tasked to develop a test to determine whether or not King Heiro II of Syracuse had been given a crown made from fake gold some 2250 years ago, specific gravity testing has been a popular method to verify the authenticity of gold and silver objects.

Testing for specific gravity is relatively easy and can be done with an accurate set of scales. Place the gold or silver bullion on the scales and record the weight (1). Place a container filled with distilled water on the scales. Zero the scales. Using a rig with fishing line or a similar light weight material, suspend the gold or silver so that it is completely immersed in the water. Do not allow the bar or bullion, or anything used to suspend it, to touch the bottom or any other part of the container of water. Now record that weight (2). Divide weight (1) by weight (2). For pure gold the result (specific gravity) will be 19.32. For silver the result will be 10.49.

Unfortunately, the recent proliferation in the use of base metal inserts and cores to match the densities of gold and silver now make this test unsuitable as a stand-alone conclusive solution for detecting fakes. Tungsten can be used in exact proportions to imitate the mass and volume of gold in bullion. Lead can be used in exact proportions with minor amounts of another base metal, such as copper, to counterfeit silver bars.

XRF Analysis

X-ray Fluorescence (XRF) is commonly used by bullion dealers and pawn shops to provide an assay (test of purity) for gold and silver objects. In the case of bullion and bars the surface is analyzed to give a percentage of all the metals present, which can then be compared with the known specifications for that particular bullion product. Quality XRF instruments are moderately expensive, and therefore purchases by individual precious metals investors are often difficult to justify. However, many bullion dealers offer quick and convenient XRF analysis in their stores. XRF scanning and assay with a reputable instrument will give extremely accurate results.

Unfortunately, the downfall is that results from XRF analysis are limited to the outer surface of the object being tested. Typically the maximum depth of penetration into gold by XRF is just 0.01mm or 10 microns. A counterfeiter only needs to ensure that bullion and bars with base metal cores have a sufficiently thick coating of real gold or silver at the same purity as the genuine bullion product.

To overcome inconclusive results from specific gravity testing and XRF analysis, ultrasound is the test method of choice for all precious metal investors.

Ultrasonic Testing

Testing with ultrasound involves measuring both the velocity of sound in gold bullion and silver bars against the distance the sound waves travel. Sound velocity is dependent on both the elasticity and density of the material. Detection by differences in sound velocity between precious metals, and the base metals used as cores in fakes, guarantees identification of counterfeits. For example, whilst gold and tungsten have very similar densities, they differ greatly in elasticity.  Gold is very malleable and tungsten is very brittle. As such, the velocity of sound passing through pure gold is 3240 m/s, whilst for tungsten it is 5180 m/s.

With an easily affordable ultrasonic test kit the precious metals investor can easily calculate the distance that sound waves travel through gold bullion and silver bars.  Simply by presetting the known sound velocity of the precious metal being tested, the instrument returns a distance which is compared with the known thickness of the bullion or bar.  True thickness is known by product specification, calipers or even a simple ruler.

Secondly, sound waves emitted from an ultrasound transducer will reflect at interfaces between different materials. For example ultrasonic waves passing through gold that encounter tungsten will be reflected back to the transducer. This will give a meaningless result in the instrument that does corroborate with the true thickness of the gold or silver bullion or bar being measured.

Whilst specific gravity and XRF testing can often identify amateurish fakes of gold bullion and silver bars, due to recent and advanced counterfeiting methods, these tests are liable to give false positives. However, it is very difficult, if not impossible, to fake bullion and bars to give false verification by ultrasound.  Therefore all suspicious results recorded by ultrasonic testing warrant investigation for tampering with inserts, thick plating, or voids.