DUX Bullion Testing

Test 1 – “D” for Density / Weight

Checking a bullion item’s mass (weight) with its volume (size) is called a density test and is the oldest and most common method to inspect bullion. For example, to check the mass of a silver Mexican Libertad coin, simply place the coin on top of a known good Libertad coin to ensure the diameter matches and then next to each other to ensure the coins’ thickness (height) matches. Then weigh both coins to ensure the mass is the same. Unless you find obvious inconsistencies the coins would have passed the density test.

About Volume:

Pure silver has a unique weight per unit volume (also called specific gravity/mass) of 10.49 times the weight (for the same volume) of water. Gold’s specific gravity/mass is 19.32 whereas platinum’s specific gravity or mass is 21.40. Thus because gold is 84% denser than silver a 1 troy ounce gold coin will be 84% smaller than a silver coin of the same mass.

As 1 liter of water weighs 1 kilogram and occupies 1,000 cm3. It is easy to calculate the volume that the bullion should occupy as we know the specific gravity/mass. For example:

• a 1 kg pure silver bar should have a volume of: 1,000 cm3 / 10.49 = 95.33 cm3
• a 1 kg pure gold bar should have a volume of: 1,000 cm3 / 19.30 = 51.81 cm3
• a 1 kg pure tungsten bar should have a volume of: 1,000 cm3 / 19.25 = 51.95 cm3
• a 1 kg pure platinum bar should have a volume of: 1,000 cm3 / 21.45 = 46.62 cm3

Determining an accurate measurement of volume is a bit complicated if the bullion is irregularly shaped but in practice this is rarely a problem for standard sized investment grade bullion as it can simply be compared to known genuine bullion of the same type.

About Weight:

Investment Grade bullion should be within a small range above its stated mass because of:

• Purity Compensation: A bar stating 100 oz of pure silver at .999 requires the testText to provide a total mass of 100 oz of silver. Hence the weight of this bar should be 100 oz / .999 = 100.1 oz whereby the extra 0.1 oz are non-silver metal alloys. A .99 oz bar should therefore be 100 oz / .99 = 101.01
• Production Process: There are slight weight variances in the production process which the mint must compensate for while insuring the minimum given metal is always provided for. This normally means that there are at least a few milligrams of extra metal in a coin or bar (especially silver). When striking coins for example the die will eventually start wearing out which can cause coins to be a few milligrams larger compared to a fresh die. To insure that coin are not underweight this compensation results in a few extra milligrams of metal

It is also possible for genuine bullion to be slightly below its stated mass due to excessive polishing or excessive scratching which can result in very small mass loss. This is very rate however.

So genuine bullion should normally be in a tight range than is at or slightly above the stated weight once adjusted for purity (or alloys). The weight difference, given the same volume, between a gold plated tungsten bar (specific gravity: 19.25) and a genuinepure gold bar (specific gravity: 19.30) is only 0.26%. This small density difference is the reason why tungsten is the preferred metal for counterfeit gold bars.

Interestingly Silver is harder to counterfeit using base metals as there is no inexpensive metal whose density is close to silver's 10.49 density. The closest material to silver from a specific gravity perspective is Molybdenum with a specific gravity of 10.19 followed by Bismuth with 9.79 Both of these materials are rare and expensive so nobody would try to fake coins with these materials. The next material closest to silver, lead, has a specific gravity of 11.35 but it is 8% heavier than silver. Copper, a likely material for fakes, has a specific gravity of 8.9 which is 15% lighter than silver.

Proper density testing requires precise, accurate scales and a good calibration to adjust for external factors such as altitude (e.g. sea level) which, by itself, can affect weight measurement by as much as 0.5%. If you are doing your own weighting you can avoid many of these calibrations by simply weighting the tested bullion to a genuine item of the same type and look for discrepancies.

We use scales whose accuracy and calibration are trade certified by Singapore Weight and Measures Office. We have also invested in advanced electromagnetic force compensation scales which are orders of magnitude more precise, and expensive, than traditional scales. The accuracy and precision of these scales greatly increases the confidence level of  test results.

Test 2 – “U” for Ultrasonic Thickness Measurement (UTM)

Despite high accuracy weighting Density tests can be fooled by advanced composite bars that combine metals of different densities to result in a near perfectly (e.g. 19.30 for gold) weighted and sized bar. Ultrasonic thickness measurement (UTM) however does not rely on metal density and is therefore an excellent complement to a density test.

UTM measures the thickness of a metal based on the time taken by the ultrasound wave to travel through and return to the surface of the bullion. Because ultrasonic waves travel through metals at a constant speed (called “celerity”) that is specific to a given metal it is possible to use UTM to determine whether bullion contains impurities or hollow areas by comparing the UTM measurement to the physical thickness of the sample.  Celerity values vary greatly by metal as follows:

• Silver: 3650 meter / seconds
• Gold: 3240 meters / second
• Tungsten: 5174 meters / second (Very different from gold)
• Platinum: 3260 meters / second

So a gold calibrated UTM test on a typical 10 oz Credit Suisse 8 mm thick 99,99% gold bar, would result in a matching UTM of about 8 mm (given a small 0.2 mm adjustment for the raised bar border). If the bar were to contain Tungsten the reading would be only 5.0 mm (3240 / 5174 * 8mm) as the sound wave would travel much faster through the tungsten resulting on a much lower UTM reading. Furthermore UTM also detect hollow areas inside the bar as these will cause ultrasound waves to bounce back prematurely.

Thus UTM and Density tests measure two completely different metal characteristics making it extremely difficult for a suspicious bar to pass both tests as these bars would be designed to fool one or the other. For DUX we use UTM readers having very accurate  probes of different physical size for optimum UTM measurements of both coins & bars.

You will find details of this test in the “UTM” section of your DUX Test Result which scans the bullion's interior. The sample below shows the results for a gold plated tungsten bar. Notice how, when testing UTM for gold (celerity 3240 m/s) the bar, whose physical height is 11.3mm,  will measure 7.0 mm because Tungsten's velocity (celerity 5174 m/s) is much higher causing the 38% deviation from Physical height.

As a confirmation of the Tungsten content it is possible to do a Tungsten calibrated UTM which would result in a near perfect match and would strongly confirm a Tungsten presence without the need for physically cutting the sample.

Test 3 – “X” for X-Ray fluorescence Spectrometry (XRF)

This test uses X-Radiation to ionize atoms on the surface of the bullion which results in the release of photons whose energy is characteristic of the atoms present. A spectrometer in the machine then sorts the fluorescence emission into an accurate picture of the metals on the tested surface.

X-Ray fluorescence results in a fast and accurate representation of the surface metal composition listing detected metals and +/- error estimates. The accuracy of the results are dependent on how long the metal are exposed X-radiation; we usually do 20 second of exposure times as the benefits of additional exposure become marginal.

Although extremely powerful in detecting surface compositions it is important to note that X-ray spectrometry cannot penetrate beyond the immediate subsurface (~20 microns). It is a surface only test. The test is powerful enough to detect gold plating but not strong enough to detect advanced hollowed out gold bars. So by itself X-ray-Spectrometry is not enough to reliably test bullion, but in conjunction with UTM and Density Tests this combined DUX tri-standard is very reliable and is industry leading in non-invasive bullion testing.

You will find details of this test in the “X-Ray fluorescence Details” section of your DUX Test Result. The  sample below shows such a test on gold plated tungsten. Notice the heavy surface nickel content which typical for certain plating processes. The test will not indicate Tungsten as X-ray can only analyse surface composition.

Putting it all together - The DUX Bullion Testing System

A reliable test standard requires tightly controlled, standardized procedures as well as traceability and a uniformity of testing parameters that do not vary subjectively based on the test operator.

The DUX bullion testing system is a set of procedures enforced by custom software that interfaces directly with the DUX testing equipment to offer maximum automation and traceability while minimizing the likelihood of human errors.

The basis of DUX are the X-ray spectrometers which, being powerful computers in their own right, store test results in tamper proof onboard memories providing excellent testing traceability. During an inspection the DUX system uploads X-ray spectrometry data directly into the central DUX database along with the spectrometer ID and reading IDs.

The operator then specifies the type of bullion that is being tested (e.g. a 10 oz Gold Credit Suisse bar) from our DUX product database which automatically loads the Weight, UTM, Metal and Purity parameters as well as the associated deviation tolerances and physical bullion particularities (e.g. raised bar borders which would affect the UTM vs. Physical Thickness metrics) for the tested bullion.

Once the Weight and UTM test results are entered the system then determines whether the results are within our acceptable tolerances and generates the DUX bullion Test Results. This standardization not only creates consistency across inspections but also enforces access control and process traceability by keeping track of “who did what when”.

Furthermore DUX test results will be retrievable online so that printed DUX test results can be verified directly with the online DUX database which keeps track of all test parameters as well as checksum and control numbers to detect potential errors or attempts to modify data.

To identify and mark the tested bullion we offer our test client the option to use out uniquely ID Tamper Proof Labels (TPLs) to link the tested bullion item with their respective online test results.