Assaying of Gold Jewellery – Choice of Technique DR CHRISTOPHER W
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Assaying of gold jewellery – Choice of technique DR CHRISTOPHER W. CORTI World Gold Council, London, E nglan d Introduction A s sa yi n g o f gol d je we lle ry Ancient man first learnt the craft of coin, w hen melt ed, resu lts in the gold plating alloying with the underlying Why assay? metalworking several thousand years Why do we need to assay gold ago, making jew ellery and other base metal with a consequent colour jew ellery in modern times? Why? – decorative artefacts from native gold change. Because gold jewellery is sold on the and copper, well before the Bronze How ever, as well as these rough and ready methods of testing basis of its gold content. We desc ribe and Iron Ages. This native gold jew ellery in terms of its Fineness, in contained silver and other metals as precious metals, the ancient Greeks parts per thousand gold, or Caratage, impu rities, but it did not take long for knew of 3 quantitative methods for where 24 carat represents pure gold. man to deliberately alloy gold w ith assaying gold and silver and all 3 are still in use today: the touchstone, fire In many countries, there are laws that copper to improve properties during govern the actual caratages of the second millennium BC. All this assay and density methods. We will jew ellery that can be sold and the naturally led to the early use of gold examine each of these techniques allowable tolerances on the gold and silver as coinage around the 7th later in terms of their relevance in today’s world. content. In many countries, the law Century BC. insists that each piece of jewellery is Definition of ‘assaying’ tested – or assayed – and mark ed w ith Forgery of coinage the gold content. This is commonly It was not long, however, before the At this point, it is appropriate to define the term ‘Assaying’. Assaying known as Hallmarking when done by appearance of debased or plated an independent assay laboratory. coins. Andrew Oddy (1) notes that is the technical term used for the quantitative chemical analysis of a Rushforth (4) has discussed this aspect coin forgery was commonplace in the in some detail. Since the price you pay ancient world, a fact which is material, or object, for one specific metal or chemical element of is primarily governed by the gold demonstrated by the number of content, undercarating, whether by s u rv iv ing cou nt erfeits made of particular interest. It is often associated with the analysis of ores accident or by intention, is tantamount silver-or gold-plated base metal. to fraud, particularly where there is Forgeries exist for virtually all the and metals; in the context of gold jew ellery, assaying means the hallmarking legislation giving precious metal coinages struck in consumer protection. As has been antiquity. Gilding of silver and base determination of the gold content of the item . remarked previously (5), metal (i.e. coating the object with a gold-rich surface layer) was a technique w ell known to ancient man (1-3). It is not surprising, therefore, that governments of antiquity were very concerned that their coinage was not debased or counterfeited and many enacted law s to protect it, with severe penalties for those convicted of forgery. To detect forgery, coins needed to be tested. There was little indication in these early laws of how testing of coins was to be car ried out. Cutting of coins was one method used and many examples of coins with cuts are known, made to ensure that the coins were not plated. Acoustic ‘ringing’ of coins by dropping them onto a hard surface was also practised, as was melting of the coin and its subsequent solidification. A plated Figure 1 - Undercarating of gold jewellery in the USA (AJM Magazine, 1997) 20 undercarating is a major problem in cost and accuracy, factors such as In several of the techniques, the some countries. Even the USA is not speed of measurement, consistency accuracy of the method depends on ex em pt from the problem, Figure 11 of measurement, whether it is the use of reference alloy standards, The important point is that destructiv e or non-des tructiv e, size of either to calibrate the equipment or knowledge of the gold assay is sample, etc also play a part in run against the test sample. Optimum essential to the jew ellery selecting the appropriate technique. accuracy in some techniques requires manufacturer, the retailer and the The need for an accurate, quick a large number of reference alloy customer. That jewellery is not technique, using cheap equipment standards. undercarated or debased can only be and a low skill base to operate is a For hallmarking purposes, we determined by testing – by assaying continuing requirement in the should also note that the the item. Assaying ensures that the industry. allowable techniques and proce- gold content meets legislative Today, there are a number of dures are defined in various ISO requirements , protecting the consum er modern instrumental analysis Standards and allowing manufacturers and techniques available as well as the 3 retailers to operate their business on a ancient techniques mentioned lev el playing field. F or m anufactu rers , earlier. Are these old techniques still there is also a need to assay jewellery, relevant in today’s w orld? work-in-progress and recycled scrap Unfortu nat ely, for mos t jew ellers, it is during manufacture for quality control difficult to make an informed purposes. Sometimes, there is a need comparison of techniques. Whilst simply to distinguish different there are articles in the literature on caratages of component parts. For some techniques, there have been buyers of scrap jewellery, there is a few attempts to provide an overall need to determine its caratage to review. The Assaying and Refining enable a fair price to be paid without booklet (8) publis hed by World Gold excessive risk. Council was a recent attempt and this review seeks to update that Accuracy o f assaying publication in a way useful for Thus, there are several reasons why jewellers so that they can make an determination of gold content is informed choice. important. The accuracy of measurement required, however, Th e te chn i que s o f a ssa yi n g varies according to need. For Table 1 lists the more common Hallmarking purposes, gold content techniques available for assaying of is defined in parts per thousand, so gold jew ellery, each of which we the measurement technique used shall discuss in turn. In some of the must be capable of measuring to 1 modern instrumental techniques, not part per thousand or less, i.e. in parts only can the gold content be per ten thousand. On the other hand, measured, but the other alloying to distinguish between, say, 14 carat metals and impurities can also be and 18 carat components in a mass analysed simultaneously. This is of manufacturing line requires a lower some importance in a manufacturing accuracy, say only 1-2 wt. %. situation and is an additional At this point, it is w ort h not ing that advantage. the accuracy of the assay is not only determined by the accuracy of the Table 1. Assaying techniques for gold jewellery assaying technique, but also by the representativeness of the sample on which the measurement is taken. The Technique Destructive Comment only truly accurate assay value of a Anc ient Fire Assay Yes – sample ta ken Weighing piece of jew ellery is obtained by Touchstone Yes, rubbing ta ken Colour comparison testing the whole piece destructively. This is obviously not a practical Density No Archimedes method proposition and so some Parting Yes – sample ta ken Weighing compromise has to be made. The Modern Electronic Gold Pen No – surface only Capacitance decay sample size required to assure X-ray Fluorescence representativity is almost a science in Spectrometry (XRF) No – surface only Measures emitted X-ray s itself (6,7). Atomic Absorption Yes – sample in solution Atomic absorption Spectroscopy (AAS) or as solid cathode lines measured Choice of technique The methods of assaying are also of Inductively Coupled Atomic emission lines Yes – sample in solution interes t for other reas ons . A part from Plasma Spectrometry (ICP) measured 21 1. Fire Assay (Cupellation) This ancient technique is first referenced by the Egyptians in a Cuneiform tablet dating back to around 1360 BC, Figure 2, and is based on a gold refining technique. In its present form, it remains the most accurate technique for gold assay and s erv es as the standard technique against w hich all others are com pared. It is covered by ISO standard ISO 11426:1993 and is the reference technique used by all the Assay Offices w orldwide. The technique depends upon tw o weighings, so analytical accuracy is related to the ability to weigh to high accuracies, i.e. the precision of the analytical balance. The principle is based on the removal of all base metals from a small weighed sample by oxidation in the cupellation stage to leave a pure gold-silver alloy, followed by removal of the silv er by dissolut ion in nit ric acid – the parting stage – to leav e a residue of pure gold which is then weighed to allow calculation of the gold content. Figure 3 - Fire Assay (a) Sampling a gold ring by scraping (b) Weighing sample on an analytical balance (c) Wrapping gold sample and silver addition in lead foil (d) Samples in refractory cupel placed in furnace (e) Cupels aft er removal from furnace showing gold-silver beads (f) Rolling the flattened gold-silver bead (g) Dissolution of the silver from the ‘ cornet’ in nitric acid In practice, a sample of the jew ellery is taken, usually by leave a small gold-silver alloy bead, scraping, Figure 3a, and weighed, Figure 3e, w hich w ill als o contain any Figure 3b.