Non-Destructive Testing Guidelines
The only reliable method of ensuring the requisite fineness and quality of gold bars is to source them from an LBMA GDL refiner.
The only way to accurately determine the fineness of a gold bar is to take a representative sample and analyse by Fire Assay and/or an appropriate instrumental technique. Representative sampling will always result in the destruction of the bar.
Energy Dispersive X-Ray Fluorescent Spectroscopy (EDXRF)
Elemental Surface analysis only. Can be used to verify possible fineness if the sample is known to be homogenous.
Due to the Mass Absorption Coefficient (MAC) of gold, the penetration of X-Rays is very low; generally, less than 10 microns so this technique can be deceived by simple plating.
Effective in identifying materials in the bar that are not gold, for example substitute inserts, air pockets or other trapped material. False readings can be made if obturation is poor.
No information on fineness.
Very quick and easy way of determining whether the bar might – or might not – be gold. Not definitive.
Only accurate to certain depth, only tests the area of the probe surface.
Very limited information on fineness, however, can provide limited information on the assay if the matrix is constant.
At present there does not appear to be a definitive non-destructive testing solution that can be endorsed and so the best risk mitigation of sub-standard assay remains the Good Delivery eco-system of refineries and chain of custody.
As techniques are developed and refined, it will become necessary to review these Guidelines.