Quality assessment of fine gold (part 2 of 2)

2012-APR-30

Modern methods of chemical analysis

One should keep in mind that the required sampling for chemical analysis will always damage the inspected item. Though fire assay can be used to test alloyed coins and jewellery of lower finenesses as well, this essay is focused on the testing of pure gold with finenesses above 995. Traditional techniques like touch stone tests as well as modern fast and non-destructive XRF- Fluorescence shall not be discussed here because of their limited accuracy.

Fire assay is still the most well established international standard for quality control of gold and gold alloys. Unaltered over the centuries in the first step, non-precious metals are separated from gold and silver by an oxidation treatment (Fig. 2).


Figure 2: gold oxidation treatment

Weighed samples and lead are molten in a ceramic “cupel” in a furnace under air. Lead oxide liquefies other high melting species like copper oxide and is soaked by the porous cupel. Finally a precious metal bead remains (Fig. 3), which is further processed in nitric acid to separate gold from silver (parting). The gold is weighed again and fineness is calculated basically from the two weights.


Figure 3: precious metals beads

Among the techniques of analytical chemistry, weighing is one of the most precise available methods. Using ultra sensitive balances with µg resolution, old-fashioned fire assay can still yield unrivalled levels of accuracy and precision in the hands of skilled assayers. For 999.9 purity items, however, an additional verification of purity can be performed in an indirect way, meaning by measuring the remaining impurities in the sample by optical emission spectrometry and summing them up. With to date ICP- or Spark-Emission instruments up to 40 individual impurities can be measured down to levels about and even below 0.001 mill.

Laboratory proficiency

Even with reference to the sophisticated quality demands in precious metals trading, the mere potential of combining old and new methods described above can lead to a highly trustworthy quality-control-scheme in fine gold production. However, real life performance of existing assay laboratories may be another issue and has to be proven and maintained. Quality management systems can help keep up proper maintenance of instruments, procedures and internal checks, but the litmus test of lab proficiency will always be the assaying of unknown reference materials. Reference materials of known composition may be synthetic alloys of high purity gold with deliberately added defined impurities or real life materials, characterised by the results from parallel assays of a group of special highly experienced and skilled laboratories.

For affiliation into the reputed “good delivery” list of the London Bullion Market Association, investment gold producers have to prove the proficiency of their quality control with reference to 24 checks supplied by the LBMA within the application procedure and with a smaller number in an annual check up. The acceptance criteria of this assessment procedure rank among the most challenging in laboratory proficiency testing schemes worldwide.

Click here to read the first part of this GoldMoney Laboratory Report about gold bar testing