Good Delivery
Physical Market Guidelines
The following guidelines have been designed to inform and assist those who work in the Good Delivery physical market, particularly in relation to electronic weighing and non-destructive testing.
Weighing Guidelines
In general, a good electronic balance is one of the most accurate and precise measuring devices in any laboratory or vault. The current generation of electronic balances report mass with 7 significant digits and determine that mass of gold bars with a readability to 0.0003 tr oz (for large gold bars).
The purpose of this guide is to provide some insight into how electronic balances work, factors that affect mass measurement, and discuss the best practices for mass measurement of precious metal bars.
Non-Destructive Testing Guidelines
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 can only 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.
Ultrasound
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.
Eddy Current
A very quick and easy way of determining whether the bar might – or might not – be gold. Not definitive.
Only accurate to a 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.