Quality assessment of fine gold (part 1 of 2)

2012-APR-30

Quality assessment of fine gold (part 1 of 2)

Introduction

Gold quality designation

Many countries have enacted consumer protection targeted regulations on quality and marking of precious metal goods. In addition, two internationally widely accepted regulations exist:

“Good delivery Rules” from the London Bullion Market Association (LBMA) for gold and silver investment bullion, and the Vienna “Convention on the Control and Marking of Articles of Precious Metals” for jewellery and other precious metal goods.

Both rules exclusively use fineness, defined as parts per thousand pure gold, from the total weight of the item for the specification of precious metal content. The older fineness declaration in carats is giving the fraction of 999 mills pure gold in an item in 1/24 units. Though customary in everyday talk about jewellery, it should not be used any longer, since it may be misleading and is not exactly compatible with the mill convention. For example the common fine gold quality 999.9 cannot be expressed on the carat scale.

Quality impacts of gold refining

Regardless of talking about gold concentrates from primary mining or precious metal scrap from manufactured items, to obtain commercially accepted purities for technical or investment purposes the raw materials have to be refined. Initial refining of mined gold, the Miller Process, sees molten raw gold treated with chlorine gas, converting most impurities (including silver) to chloride salts which can be removed from the gold as a liquid slag. The Miller Process yields finenesses from 995 to 997. Hydrochemical refining is accomplished by dissolution of raw materials in aqua regia and reductive precipitation of gold. It yields purities comparable to the Miller Process. Such quality is commercially accepted in 400 troy ounce bullion at the London Bullion Market (good delivery quality). For smaller bullion, however, 999.9 has become the standard fineness today. For some technical applications even higher purities up to 999.999 are required. These require a second purification step, predominantly done by electrolytic refining (Wohlwill Electrolysis).

Gold assaying

History

Native gold, known in Latin as elektrum – from the Greek word ήλεκτρον (ēlektron ) – always contains silver and some minor impurities. During the second millennium BC, man learnt how to alloy the native material with copper, to improve its hardness and make it better suited for coinage and man made artefacts. Regrettably, alloying has tempted criminals and monarchs throughout history into lowering the precious metal fraction in coinage and jewellery. Such fraud led to the development of assay techniques in early human history. Some archaeologists deduce from lead and precious metal findings in eastern Anatolia that the basic principle of fire assay may have been known already in the Bronze Age. Plinius the Elder described the use of “cupels” for the removal of non precious metals in fire assay in Roman times. Early assayers, however, mainly dealt with silver or with gold-copper alloys, and were unable to separate gold from silver. True assaying of gold became possible with the discovery of nitric acid in the Middle Ages, which selectively dissolves silver from gold-silver alloys. Since the 16th century, fire assay manuals have been published, giving detailed instructions on procedures and equipment (Fig.1).


Figure 1: gold bar fire assay

Physical testing

Different simple test-devices, relying on the measurement of physical properties like conductivity, are offered commercially – especially for gold alloys. Also, the density of fine gold is a well-defined physical constant. Nevertheless, the achievable precision in the measurement of physical constants is insufficient to detect minute fineness differences; sophisticated bullion fakes with a tungsten core cannot be identified by density because of the almost identical specific weight of tungsten and gold. Trained persons are able to discern such objects and true gold bullion merely by the sound of a hammered item. Devices with ultrasonic probes check sonic speed in the material and are suitable for rapid non-destructive fraud inspection. For coins released from national mints, dimensions and weight are sufficiently well defined that fakes can be easily spotted by experts.

Click here to read the second part of this GoldMoney Laboratory report about gold bar testing