Assaying of 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 , 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 , in contained 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. 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. Some additional silver is platinum group metals present in the added before wrapping it in a pure sample. A reference gold sample of lead foil, Figure 3c, and placing it in a k now n gold content is normally run special porous crucible called a in parallel with the sample to cupel. This is placed into a furnace at eliminate experimental errors. around 1100°C (2012°F), Figure 3d. On cooling, this bead is hammered The silv er is added to dilu te the gold flat and rolled to a thin strip to content to 25% or less (known as increase its surface area, Figure 3f. It Figure 2 - Cuneiform tablet of King Inquartation). The lead melts and is coiled loosely into a ‘cornet’ and Burraburiash of Babylon (1385-1361 BC), alloys with the sample and added the silver dissolved out in 2 stages in recording assaying of a consignment of gold nit ric acid, F igu re 3g. This is why from Egypt (from reference 1). silver and then all base metals are oxidised to form a liquid slag. This silver is added to the original sample. slag is absorbed into the cupel to For successful parting of the silver, it is necessary for the gold content t o be

22 Table 2. Fineness of gold in gold jewellery alloys obtained by touchstones was quite widespread in cupell ati on (ISO 11426) prehistoric times. It is arguable that this ancient Alloy Cupellation Italy Cupellation Germany method is essentially non- Test A lloy Ty pe U [ ‰] (s)(a) A [ ‰] (s)(a) destructive. It is based on rubbing the jewellery article on the surface of a 333/932 CG 333.05 ± 0.16 (8) 332.96 ± 0.15 ( 5) smooth, slightly abrasive hard stone – 376/904 CG 375.86 ± 0.13 (8) 375.77 ± 0.10 ( 5) or touchstone – to leave a metallic 585.1403 CG 585.32 ± 0.13 (7) 585.27 ± 0.10 ( 5) streak of the item on the stone. 585/1430 CG 584.88 ± 0.21 (8) 585.00 ± 0.35 ( 5) Rubbings of known standard 585/1451 CG 587.74 ± 0.12 (8) 588.19 ± 0.23 ( 5) reference alloys (touch needles) are 751/1802 CG 750.98 ± 0.07 (7) 750.70 ± 0.16 ( 5) placed beside it, Figure 5a; all are 751/1810 CG 750.08 ± 0.36 (7) 749.68 ± 0.32 ( 5) then treated with some drops of 755/1885 CG 753.27 ± 0.12 (8) 753.43 ± 0.12 ( 5) special touch acids, which are based 750/1851 CG 751.90 ± 0.14 (8) 751.90 ± 0.15 ( 3) on nitric acid, Figure 5b. This reacts 750/1853 CG 747.00 ± 0.08 (8) 747.44 ± 0.07 ( 5) 585 Pd WG WG 584.88 ± 0.08 (8) 585.00 ± 0.00 ( 2) with the base metals and silver to 590 N i WG WG 590.30 ± 0.14 (7) 590.60 ± 0.35 ( 2) leave a coloured mark which, after 750 Pd WG WG 750.92 ± 0.13 (8) 750.50 ± 0.00 ( 2) drying, is compared to the colour of 760 N i WG WG 759.83 ± 0.12 (8) 758.90 ± 0.14 ( 2) the reference alloy rubbings, Figure 22/G old G CG 917.18 ± 0.11 (7) 917.30 ± 0.57 ( 2) 5c. A darker colour means more Au99/T11 CG 988.57 ± 0.14 (8) 988.60 ± 0.00 ( 2) chemical attack that, in turn, means Standard de viation of less gold. It is a subjective test, in all sin gle determina- s (12 3) = 0,134 s (48) = 0.215 which the colour of the sample after tions of Au (‰) attack is compared to that of the reference alloy rubbing. a maximum of 25%. Otherwise the respectively. Sample size is typically gold atoms w ill surround some of the about 250 mg, although silver atoms and prevent access by microcupellation techniques allow the nitric acid. In practice the gold for much smaller samples of about content can be higher than 25%, up 1 0m g w ith cons equent s horter to 34% (9). The remaining gold processing time (10). At such small cornet is pure and can be w ashed, sample sizes, ensuring that it is dried and weighed. representative of the bulk alloy If insoluble platinum group metals becomes more difficult. such as iridium and ruthenium are Typically, Fire Assay can achieve present in the jewellery, these remain an accuracy of about 2 parts per ten in the gold and so will lead to high thousand, as dem ons trated from the v alues of gold being recorded unless standard deviation values in Table 2, further steps are taken to remove which is taken from Brill’s detailed them. Their presence can often be review (10). Brill notes that the observed on the gold-silver bead efficiency of fire assay can be after cupellation (8). Nickel and described as excellent and its soluble platinum metals such as performance enables it to be palladium are more difficult to em ployed routinely in ass ay offices remove and modified procedures to for hallmarking w here up to 1000 ensure their removal must be taken samples per day are analysed. He (8). Thus, if this method is not done also remarks that, at such outputs, properly, there will be a tendency to fire assay (cupellation) has no overstate the gold content. It helps to competitors with regard to cost. Not know the approximate gold content bad for an ancient technique! of the sample so that the correct amount of silver can be added. This 2. Touchstone can be determined roughly and This technique is know n to date back quickly by touchstone or X-ray to the Greeks in the 6th century BC, fluorescence, as we shall see later. although there is some belief that it A typical Fire Assay takes about 2 originated in Egypt in the 12th hours but many samples (up to 100 at century BC (1). An ancient a time) can be assayed touchstone found in F rance is s how n simultaneously. Where nickel or in Figure 4. It dates back to the late Figure 4 - Touc hstone from Choisy-au- palladium are present, processing Bronze Age, about 8-7th century BC Bac, France (from reference 11) time is extended by 30 and 50 min. (11), showing that the use of

23 ancient man realised that measurement of density could be used as an indication of debasement of the gold. The philosopher, Archimedes, who lived about 287 – 212 BC, discovered the principle known by all schoolboys that when a solid is immersed in a liquid, its mass is reduced by an amount equal to the mass of liquid displaced. This allows calculation of density by weighing the solid in air and then again when immersed in a liquid. It is reported (1) that Archimedes himself used the technique to check whether the craftsman who made a crown for King Hiero had added any base metals to the gold. How ever, the use of the density method as a quantitative assaying Figure 5 - Touchstone testing (from reference 12) technique is not recorded until much (a) Rubbings from test piece (left) and touch needle later in the 6th century AD, followed (b) Applying touch acids to rubbings (c) Visual observation of rubbings by several later manuscripts dating (d) Rubbings of alloys containing (from left) 700, 720 and 750 parts per thousand gold from 10th - 13th century AD, one of w hich refers to ass aying of gold -silv er The accuracy of the method alloys. Figure 7 shows its use in depends, in part, on knowing the London in the 17th century. silver : copper ratio. Oddy claims an With modern analytical balances, it accuracy of 1-2 wt % at best from is possible to weigh very accurately historical reports. This accords well so, in theory, determination of with the modern estimates of density can be made to a high distinguishing between alloys of accuracy. The technique measures slightly different gold contents, if the whole piece of jewellery and is done by skilled hands, Figure 5d. non-destructive. There are no Clearly, it is not nearly accurate sampling errors in this method! In practice, however, there are a enough for determination of a definitive ass ay v alue for hallmark ing number of problems in using this Figure 6 - Equipment for touchstone method. Firstly, how do we relate the testing (from reference 12) purpos es but it is a quick tes t and useful as a sorting test and as a preliminary test prior to fire assay. It is still used in Assay Offices for this latter purpose. It is not so good for the soft high carat and the white golds (12). The equipment is simple and cheap, as seen in Figure 6. Full details of testing by this method, including details of the touch acids, are given in the paper by W alchli & V uilleum ier in Gold Technology no. 3(12).

3. Density Method Gold is one of the heaviest metals with a density of 19.32 g/cm 3, w hilst the alloying metals in carat golds are much lighter. Silver has a density of only 10.5 g/cm 3 and copper is Figure 7 - Density measurement even lighter at 8.9 g/cm3. (foreground) in London, 17th century. Therefore, as caratage is lowered, Fire assay can be seen behind (from the alloy density – or specific gravity reference 1) – reduces . Thus

24 of its limitations is given by Nordt et only to the nearest 1-2 carats (4-8 al (14). wt.%). It is not consistent and nowhere near accurate (15) and so 4. Parting (False Fire Assay) should only be used as a quick I include this assaying technique to sorting test to differentiate caratages. illustrate some of the ignorance still found today in the industry around the world. Not infrequently, I have found manufacturers and so-called assayers claiming to do fire assay when all that they do is to place the s am ple of the gold alloy in nit ric acid – the parting stage of the fire assay procedure. The assumption is that Figure 8 - Alloy densities in gold-silver- nitric acid w ill dissolve all base copper alloy system (from reference 13) m et als and s ilv er and leav e pu re gold untouched. This is not true. For a start, not all the silver will be measured density to gold content? dissolved away as some will be For binary alloys such as gold-silver masked by the gold, unless the gold or gold-copper, this is content is 25% or less. There is also straightforward. Real jew ellery alloys no guarantee that all other base are rarely simple binary alloys but metals will be removed completely. contain 3 or even more metals – Certainly, any non-metallic gold -silver-copper with, perhaps, inclusions, oxides, etc are unlikely to additions of zinc, nickel, palladium, be removed. Thus, this method will grain refiners and deoxidisers. Thus, always give false high results, so that for a given gold content, the actual a gold item that is actually density of the alloy will depend on undercarated can appear to be over the relative amounts of the alloying the minimum gold content and metals. For 18 carat golds, typical within caratage. A method not to be densities range from 15.15 for red recommended! golds to 16.1 for white golds. It must be remarked that a density 5. Electronic Gold ‘Pen’ figure for one alloy m ay be identical It is possible to buy cheap portable to that of another with a differing electronic instruments for measuring gold content. This is clearly shown gold content, Figure 9. A probe or for ternary gold-silver-copper alloys ‘pen’ is placed on the surface of the in Figure 8, taken from the work of jew ellery in contact with a Kraut and Stern (13). conductive gel and the gold content The second practical problem is read off a meter attached to it. This t hat of meas u ring d ens ity accu rat ely. type of ins trum ent us ually works on If the jewellery contains porosity or the capacitance decay principle of inclusions, the measured apparent the surface and relating it to gold density will be lowered. It is also content. Whilst quick and non- difficult to ensure that no air bubbles destructive, it is dependent on are trapped when weighing the item surface composition. It measures in a liquid. Lastly, a piece of real jew ellery will also contain solders of different compositions (and hence densities) w hich w ill complicate things further. Thus, the density method is a simple, cheap and quick non-des tructive m ethod but it cannot give an accurate assay. At best, it can only give a semi-quantitative indication of caratage, unless the item is a know n binary alloy without any solders. Hence its continued use in examining old coins. A full description of the method and some Figure 9 - Electronic Pen

25 6. X-ray Fluorescence em iss ion of X-rays , Figure 10 , hence Spectroscopy (‘XRF’) the term ‘fluorescence’. These are Commonly known as ‘XRF’, this is an collected and measured in a increasingly popular technique, spectrometer. The important point is finding service in both jew ellery that the X-rays excited and emitted manufacturing and in assaying from the surface will have a defined laboratories. In this technique, the energy w ith a wavelength s u rface of t he jew ellery is bom bard ed characteristic of the atom from which with radiation, usually from an X-ray it is generated. Thus, different metals tube source. This radiation impacts em it X-rays of different w avelengths. the atoms and causes electrons to The amount of energy emitted will m ove lev els ( orbit al shells ) w it h t he depend on the relative number of atoms of that metal present. If we count that energy, we can determine the amount of that metal present. Figure 11 shows a typical spectrum from a 14 carat gold. This technique is non-destructive but it only measures a small sam ple at the surface of the jewellery, typically a layer of 10 – 50 microns thick, depending on alloy composition and the metal being measured. Thus, ensuring the sample is typical of the bu lk alloy is critical. Elect roplat ing or chemical treatments, such as bombing and acid pickling which change the surface composition, will distort the analysis. Measurements Figure 10 - X-ray Fluorescence: the photoelectric effect (from reference 16) can be made on a single spot, on an average of several spots or line or an area by rastering the beam over the surface. In the latter mode, an elemental distribution map can be produced. For optimum measurement, the surface should be clean, flat and polished and at least 3mm diameter. Curved surfaces reduce accuracy as the emitted X-rays are scattered. Typically, measurements take about 2 – 3 min. with the instrument computing the results automatically. Accuracy improves with increasing measuring time. The physics of the technique are complex and it is not appropriate to go into much detail here, as Kloos has recently review ed this topic (16). Suffice to say that there are matrix interaction effects betw een metals and so the raw data collected must be corrected to yield an accurate analysis. As you w ill note in Figure 11, there can be overlap of adjacent wavelengths and these must be mathematically deconvoluted to separate the individual metals. Cheaper instruments may not be able to quantitatively separate metals with overlapping curves due to their Figure 11 - X-ray spectrum from a 14 carat yellow gold (from reference 16) poorer resolution. Zinc is a typical

26 example of this problem in gold a llo y s The accuracy of the technique depends, therefore, on the correction models used in the software as well as the physical conditions of measurement. Therefore, the instrument needs to know what metals are present and to measure them all, if accurate values for gold are to be obtained. The s pectrom eter commonly used is the energy Figure 13 - Typical XRF instruments (a) Spectro Analytical Inc (b) X-Tester installed in retail dispersive type, which collects and shop in India measures the whole spectrum of X- radiation simultaneously, which different areas to determine if samples or samples in solution. The allows all metals present to be undercarated solders or components solid sample is sputtered as the analys ed. A more accurate type is the have been used, for example, Figure cathode and the resulting vapour wavelength dispersive spectrometer, 12, or if the item has been gold measured by the atomic absorption which measures only one metal at a plated or the solders contain technique. Solution AAS, using time but it is more expensive. cadm iu m. This can help t o select samples dissolved in aqua regia, is Accuracy is significantly improved which areas of jewellery are sampled another approach but solutions of by calibrating the XRF using standard for fire assay. It is a quick and very gold standards are not stable, alloys of known composition and flexible technique and can analyse according t o Brill (10) w ho conclu des clos e to that of the unknow n sam ple. for all metals pres ent in the that AAS is not as econom ical as ICP In some instruments, such calibration jewellery. spectrometry. is done by the XRF manufacturer and It also finds increasing application MicroAAS uses only a 10 mg stored in the instrument, so that it can in jew ellery manufacturing as a sample but Brill claims t hat t he resu lts be used without the need for further quality control tool, not only for gold are not as good as I CP spectrom etry. calibration during use. According to content but for complete analysis of On this basis, AAS will not be Brill (10), XRF can measure gold to jewellery alloys and scraps. In India, discussed further here. Accuracy of 0.75‰ (parts per thousand) under one major upmarket jewellery chain the technique is about 0.1 % at best, optimum conditions but for has installed XRF machines in each of making it u nsu itable as a rou tine t est microXRF, typical of many gold its shops, Figure 13b, so that for fineness measurement for ass aying ins trum ents, an accuracy of customers can check that their new hallmarking. 0.2 – 0.3% can be achieved with a jewellery is on caratage. A certificate measurement time of 3 min. is printed out as a guarantee within 3 Marrucco and Stankiewicz (17) minutes. demonstrated an accuracy of 0.12% There are many manufacturers of was attainable in their research XRF instruments on the market, many studies of the technique. with models tailored for assaying of Thus, XRF is not quite accurate gold alloys and jew ellery. Tw o enough for Hallmarking purposes, examples are shown in Figure 13. but is close and improving every The price and performance of these year. It finds considerable application v aries . For example, cheaper models in Assaying Offices as an alternative may be fitted with a proportional to touchstone for preliminary testing counter which only has a resolution of jew ellery. It can be used to of 0.5%, whereas more expensive examine jew ellery by scanning models are fitted with solid state counters with a higher resolution of 50 ppm. However, these latter cost a factor of around 6 times more. A typical cos t of su itable inst rum ents is US$20,000 – $35,000, depending on performance and additional features fitted. Figure 14 - Atomic Absorption 7. Atomic Absorption Spectrometer (from reference 10) Spectrometry Atomic absorption spectroscopy (AAS) using a multi-element Figure 12 - XRF analysis of ball links in a spectrometer, Figure 14, is another coloured gold chain (from reference 10) possible technique for assaying of gold jew ellery, using either solid 27 8. Ind uct ivel y Co upl ed Plas ma that it measures only a thin surface Spectroscopy (‘ICP’) layer that may not be typical of the Inductively coupled plasma solution bulk alloy composition. spectrometry (ICP) is a serious For a rough preliminary contender to fire assay for ass aying of assessment, touchstone or XRF may carat golds and it is finding be used. XRF has advantages in being considerable use in many precious able to quickly scan items to metal analytical laboratories. For determine any potential problem white golds containing nickel or Figure 16 - Inductively Coupled Plasma areas and to assist in selecting where palladium, ICP has an adv antage due Spectroscopy machine (from reference 18) samples are taken. to its greater element selectivity (10). I CP has a num ber of adv antages The technique allows for a full 2. Quality control in production over fire assay: analysis of t he gold s am ple, w hich is Fire assay remains a preferred an added advantage over fire assay. • Speed of assaying is similar to technique for accurate gold assaying, The use of ICP for gold determination fire ass ay but XRF is finding increased in jew ellery has been discussed by • Speed is faster than fire assay for application, with a sufficient accuracy Brill (18) in some detail. nickel- and palladium-white golds for most purposes. A fast technique In this technique, a weighed • Smaller sample weights are that allows a complete analysis of the buffered aqueous s olution of the gold required alloying metals. sample is nebulised and injected into • There is no interference due to a plasma flame, Figure 15, and the platinum, rhodium and iridium 3. As a quick sorting test to spectral emission analysed in multi- • All alloying additions analysed determine approximate element spectrometers, with simultaneously, including trace caratage simultaneous measurement of elements Touchstone remains useful for this Y ttrium as an internal standard. The • There are no emissions of toxic purpose; it is quick and cheap and intensity of the emission is related to lead fumes as in fire assay portable. XRF is just as quick and the amount of metal present. more accurate, but more costly. Both • The process can be automated Calibration against solutions of alloy are non-destructible. The Electronic standards is required. Micro-ICP (19) The major disadvantage of ICP pen is also applicable to this sector is its cost, typically about $150,000. allows use of samples of about 10 – but it does not give consistent Thus, it finds use only in the 20 mg. Accuracy is 1 part per results. thousand for gold. laboratories of the major Assay Offices and large precious metal Summary and jew ellery companies, Figure 16. A summary of the techniques is given Comparison o f techniques in Table 3. The equipment costs are As stated earlier, Fire Assay only indicative. The cost of fire assay (Cupellation) is the standard will depend on the number of reference technique against which all samples tested per day. Where this is other techniques may be compared. low , capital costs will be lower than Despite its age, it remains the most those quoted. Brill also gives an accurate technique. In his review, indication of annual operating costs Brill (10) gives much data on such in his review (10), based on carrying comparisons in the assaying of gold out hundreds of assays per day. in jew ellery alloys. This clearly demonstrates the superiority of fire Future developments assay over all others. As we have seen, the most ancient The question of which technique is technique of fire assay continues to selected for assaying of gold dominate today. It has a number of jewellery turns on the specific advantages including accuracy, needs: although it can only measure gold content. The continuing 1. Fineness determination for improvements to the XRF technique Hallmarking purposes. hav e already been mentioned and it is Fire assay has the best accuracy. ICP reas onable to cons ider that it may yet Spectrometry is close and has a become a technique accurate enough number of advantages, including an for fineness determination. This will ability to prov ide a com plete analys is. depend on improved detectors and Its major dis advantage is its cos t. XRF software. Progress in solid state is not quite good enough, although Figure 15 - Plasma flame in ICP detectors is being made steadily. Spectrometry (from reference 10) improving, in accuracy terms but has Wider use of I CP spect rom et ry remains an advantage in being non- unlikely unless the capital cost of the d est ruct iv e. It s m ajor short com ing is instruments is reduced substantially. 28 Chem ical Compos it ion”, Go ld Table 3. Comparison of Assaying Techniques Bulletin, 33(2), 2000, 52-55. Equi pment Technique Versatility Sa mple si ze Accuracy Li mitat ions 14 Nordt, P.N., Hill, W.C. & Rohr, Cost K.L ., “P ract ical Applicat ions of Modifications Moderate Fire Assay Only gold ~ 250 mg 0.02% Specific Gravity Determination for Ni and Pd $50,000 to Platinum Jew elry Complete High ICP ~20 mg 0.1% – Manufacturing – The Uses and analysis $150,000 Limitations of Archimedes Complete Non- Surface layer, Moderate XRF 0.1 – 0.5% Principle”, in Plat inu m analysis destructive flat samples $25,000+ Manufacturing Process Vol. III, Unsuitable for Touc hsto ne Only gold Almost non- 1-2% high carat and Low 1997, 38-47, Publ. PGI, USA. destructive white go lds $100 15 Johnston, P.V.A., Proc. 16th Internat ional Precious Metals Electronic Pen Only gold Non- 4-8% Not cons isten t Low $200 Con ference, 1992, 205, publ destructive IPMI, USA. 16 Kloos, D., “EDXRF Analysis of Density Only gold Non- Poor Only for binary Low destructive alloy s $500 Gold Karat Alloys using Proportional Counter based Techno logy No 26, July 1999, Micro-EDXRF”, Proc. 24th The use of lasers to put hallmarks 16-23. Internat ional Precious Metals on jew elle ry is a recent dev elopment 4 Rushforth, R.W.E., “International Con ference, June 2000, publ. (20, 21). It is possible to envisage that Hallmarking – Pipedream or IPMI, USA. the metal vapour generated from the Possibility”, Pro c . S ant a F e 17 Marucco, A., & Stankiewicz, W., laser engraving could be fed into an Symposium, 1999, 461-485, publ. “Development of an XRF atomic emission spectrometer for Met-Chem Research Inc.; also S pectrom etry A nalytical Method direct analys is , as Brill has suggested Gold Techno logy , No 27, for G old D et erm inat ion in G old (10) in his review. If this should November 1999, 21-29. Jew ellery Alloys”, Gold materialise, then it is possible to 5 Corti, C.W., “Quality in Jew ellery Technology, No 24, September conceive that the ancient methods for Manufacture – Beyond 2000”, 1998, 14-22. gold assaying will finally be replaced Proc. Santa Fe Symposium, 18 Brill, M. and Wiedemann, K-H., with the modern ones. 1998, 1-31, Met-Chem Research “Determination of Gold in Gold Inc. See also ‘Editor’s Jew ellery Alloys by ICP Conclusions Comments’, Go ld Technology, S pectrom etry”, Go ld Bulletin, This review has s how n that w hilst the No 29, Summer 2000, 30-31. 25(1), 1992, 13-26. m od ern ins t rum ental t echniqu es for 6 Shepherd, O.C. and Dietrich, 19 Brill, M., “Robotised ICP- assaying gold jewellery have much to W.F., “Fire Assaying”, 1940, Laboratory for the Analysis of offer, the ancient techniques of Fire reprinted by Met-Chem Res earch Precious Metals”, Go ld Bulletin, A ss ay and Touchs tone continue to Inc, USA, 1989. 28(2), 1995, 38-53. play a major role because of their 7 Smith, E.A., “The Sampling and 20 Anon, “Breakthrough in Laser advantageous characteristics, with Assaying of the Precious Metals”, Marking for Precious Metals”, accuracy and cost being of 2nd ed., 1947, reprinted Met- Gold Techno logy, No 24, significance in the cas e of fire as say. Chem Res earch Inc, USA , 19 87. September 1998, 11-12. However, XRF continues to progress 8 “Assaying and Refining of Gold 21 “Relief-Lasering makes a Fine and is finding increasing application. Jew ellery”, publ. 1997, World Impression”, The Anchor The assaying technique selected Gold Council, London. (Newsletter of the Birmingham will depend on the particular need 9 I. Bayley, Birmingham Assay Assay Office), Spring 2000, and cost of the equipm ent. For the O ffice, P rivate communication, page 1. latter reason, ICP Spectrometry will 2001 only find limited application. 10 Brill, M., “Analysis of Carat Gold”, Gold Technology, No 22, References July 1997, 10-25. 1 Oddy, A., “Assaying in Antiquity”, 11 Eluere, C., “A Prehistoric Gold Bulletin, 16(2), 1983, 52- Touchs tone from France”, Gold 59. Bulletin, 19(2), 1986, 58-61. 2 Jacobson, D.M., “Corinthian 12 Walchi, W. & Vuilleumier, P., Bronze and the Gold of the “Touchstone Testing of Precious Alchemists”, Go ld Bu llet in , Metals”, Gold Technology, No 3, 33(2), 2000, 60-66. January 1991, 9-18. 3 Grimw ade, M.F ., “The Surface 13 Kraut, J.C. and Stern, W.B., “The Enrichment of Carat Gold alloys Density of Gold-Silver-Copper – Depletion Gilding”, Gold Alloys and its Calculation from

29 Acknowledg ements and Peter Raw for their WGC seminar This review is adapted from a paper materials, which have served as a presented at the Santa Fe Symposium basis for this presentation and Ian on jewelry manufacturing techno- Bayley of the Birmingham Assay logy, held in Albuquerque, N.M., Office. I have drawn heavily on the USA in May 2001. papers of Andrew Oddy, British My thanks and appreciation to the Museum, and Dr Manfrid Brill many people and companies who (formerly W.C.Heraeus GmbH). have provided advice and Many of the figures hav e been taken information. These include Don from the referenced publications. Kloos, CMI International, and Figure 9 is courtesy of Rio Grande, Michael Haschke, Rötgenanalytik USA and Figure 13 is courtesy of Messt echnik GmbH for their help and Spectro and Titan Industries, India. inform at ion on XRF , Mark G rimw ade

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