Lewin works with the cooperation of the government laboratory at Madini House, Nairobi, that signs-off the testing process so as to ensure that all supporting documentation is compliant for the purposes of export. This process occurs immediately prior to export and once tested by a Lewin representative, the product does not leave our possession until either assigned to Brinks into sealed containers, or, in the case of sample tranche deliveries, until it is placed directly into the hands of the buyer.
X-ray fluorescence (XRF) is based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. These characteristic scatter patterns are detected by the device and are shown on a readout which we record and archive for each tested sample.
Lewin utilises the Niton XL series of XRF analysers to determine surface composition of the product. Where this is agreed, the product may be drilled and the core sample analysed. Where the product cannot be drilled, we use specific gravity and ultrasound testing methods to determine uniformity of composition throughout the product:
The specific gravity of gold is the same as the ratio of its wet to dry mass – since water’s specific gravity is 1g/cm3. Pure gold’s specific gravity, or density, is 19.32 grammes per cubic centimetre. The raw gold we supply is typically of a purity ranging between 75% and 95%.
Once we’ve XRF-analysed the surface composition of a doré bar and therefore know what the gold is alloyed to, we know what specific gravity measurement should result from testing, if the bar is likely bona fide. For instance, at 92% purity, if the bar is primarily alloyed to silver, we expect a specific gravity of around 18.1. With a copper / silver alloy mix, we expect around 17.8.
In the diagram sequence below, we determine that the specific gravity of the bar is 18.12 by dividing the dry mass (1.000kg) by the wet mass (0.0552kg). With our XRF reading indicating a silver alloy, we conclude that the bar is likely consistent throughout with the composition measured on the bar’s surface – provided a very clever con has not been pulled off! (which we safeguard against in our 3rd test).
While we have an excellent relationship and high trust with mine owner suppliers, often, bars are traded between sellers in-country to accommodate supply chain amendments.
A very sophisticated con from a trader would involve a high purity surface composition and a tungsten core or insert, as tungsten’s specific gravity is very close to that of pure gold. We use ultrasound velocity and thickness testing to identify any aberrations in the bar’s structure. This method easily brings to light the presence of cavities or inserts.
In the example provided above, we already determined the specific gravity of a doré bar that is apparently consistently composed of 92% pure gold and silver. A further corroboration of the reliability of the three testing methods we use is to confirm that the volume of the tested bar is consistent with its calculated density.
A bar weighing 1.000kg and with standard upper surface dimensions of 80 x 40mm would be expected to have a thickness of 17.25mm. This can be determined very quickly with vernier callipers.