The case of precious metals

Silver and gold occur in rocks and ores in many ways but, unlike silver, gold can easily be found free in nature either as veins in auriferous quartz or as pellets and nuggets in alluvial deposits. Gold found free in nature is a more or less rich alloy of gold, silver and copper. Refining techniques, which have changed over time, separate the precious metals from the other elements present in the natural alloys. In the past, refining was achieved by cupellation and parting:

cupellation means separating gold and silver from the base elements, whilst parting means separating gold from silver.

Precious alloys are produced by addition of other metals to gold or to silver

— in general addition of copper and silver to gold to produce gold alloys and of copper to silver to produce silver alloys — according to the necessary mechanical properties (mainly hardness and tensile strength) that can also be changed by annealing, quenching and hammering. We must, however, remind the reader of the importance of the colour of alloys: the addition of copper to gold produces a reddish alloy; the addition of silver to gold produces a whitish–greenish alloy (see, e.g., Ref. [5.6]). In addition to this, the importance of the quality of the alloy (its fineness for a precious metallic alloy) should still be considered. In the case of coins, the quantity of precious metal in the alloy — silver in silver coins and gold in gold coins — is officially established by an authority, together with the weight of the coins, enabling payment by counting coins rather than by weighing.

Debasement can be carried out either by decreasing the quantity of gold or silver in the alloy or by reducing the weight of the coins. The same situation can in more recent times be found for jewellery: for example, an 18 carat gold alloy corresponds to an alloy containing 75% of gold and 25% of one or more metals whose quantities are chosen according to the required mechanical properties and aesthetic requirements of the item. In the past, the other metals were silver and copper.

The production of objects made with precious metals varies from very simple constructions to very complex constructions, the latter comprising a large number of parts joined together. Simple constructions are, for example, small cast pieces obtained by lost-wax casting followed by polishing or burnishing of the surface of coins, which are obtained by pouring the alloy into a mould or by lamination and cutting of a plaque before striking. Complex objects are produced by hammering and/or casting many different small parts that are mounted together to produce the final object. These parts can sometimes be decorated either by chasing, engraving, stamping, etc., or by addition of the same or other materials. The addition of materials can in general be attributed either to the production of polychrome objects, for example, precious stones or any other coloured material setting and niello inlaying, or to the production of low priced objects by deposition of a precious metal on the surface of a poor quality metal or alloy by silvering, gilding, depletion gilding, etc. [5.7, 5.8]. Complex objects should also be considered which are decorated by addition of small elements produced with the same alloys, such as filigree and granulation.

The fragility, rarity, value and small dimensions of objects made with silver and gold (jewellery, coins, art objects, etc.) can increase the difficulties associated with their scientific study. The study of such small objects, which sometimes consist of several tiny elements, involves the use of microanalytical techniques.

Their preciousness and rarity constantly require the use of totally non-destructive techniques, but it must be kept in mind that it is the same preciousness and rarity that increases the recycling and reuse of these materials; hence, this type of transformation attains here its highest levels.

Associated with religion and power, rare and precious metals are strongly connected to the political, social and economic histories of civilizations. The study of these objects as a whole, by using science based techniques that bring together complementary information, allows tackling all the different aspects covering their production, from the exploitation of the raw material to the finishing of their surfaces, and their circulation. The identification of the decoration, joining, mounting and surface finishing techniques requires accurate observation under light of different wavelengths and different types of radiation in order to obtain information on the surface morphology and also to reveal invisible details. Much information can be recovered using techniques such as

CHAPTER 5. AUTHENTICITY VERIFICATION

photography, macrophotography, optical microscopy, scanning electron microscopy (SEM) and X ray radiography. Low and high magnification images of the surface morphology of a gold or silver item are in general obtained by optical microscopy and by SEM. Provided an analytical system is integrated [5.9]

and the object fits into the sample chamber, SEM has the advantage of combining imaging with elemental analysis. The use of high resolution surface analysis provides quantification of the tool marks on the surface of the objects related to the goldsmith decoration techniques using mechanical working (engraving, chasing, repoussé, stamping, etc.).

If many techniques of production of gold work can be described by examination, others can only be established by analytical techniques. For example, the use of fire gilding can be identified by the presence of mercury on the surface of the objects so treated. Similarly, queries related to gold provenancing and circulation can in most cases only be answered after chemical analysis. If several science based techniques are available for the study of metals, ancient gold items require the use of non-invasive techniques. Metallurgical and isotopic analyses — the first providing information on the shaping processes and the second on the provenance of gold — requiring a sample, are in many cases excluded. However, it should be mentioned that isotopic analysis was successfully applied to a few cases of silver alloys (e.g., in Ref. [5.10]) but, in the case of gold, this technique gives poor results.

Elemental analysis is the most established method for gold jewellery studies [5.11]. It can be totally non-destructive, it can give access to microanalysis, it can measure a large number of elements sometimes simultaneously even when present at low concentrations.

In spite of the poor detection limits in the case of metals, the most popular technique of elemental analysis is energy dispersive X ray fluorescence (EDXRF), a non-invasive technique using portable equipment convenient for analysis of items which cannot be moved from collections [5.12, 5.13]. With the same level of detection limits, SEM-EDX, providing imaging and elemental analysis of tiny details accurately selected on the surface of an object [5.14], is certainly the most attractive technique for the study of small gold and silver objects entering the chamber (Fig. 5.1).

For provenancing metal objects, trace element determination [5.15]

frequently requires the use of large facilities: particle accelerators [5.16] and synchrotrons. Techniques such as particle induction X ray analysis (PIXE) (see, e.g., Refs [5.17, 5.18]) and synchrotron radiation X ray fluorescence analysis (SR-XRF) (see, e.g., Refs [5.5, 5.19]) based on heavy and expensive equipment are non-invasive and include microbeam and mapping facilities. In general, it is the type of incident radiation that determines the detection limits [5.20, 5.21].

In order to reach detection limits allowing the determination of ultra-trace elements, methods based on inductively coupled plasmas (ICPs) are necessary.

According to the acquisition mode — solid or liquid — ICP-MS (ICP mass spectrometry) is basically destructive, but it is the only technique providing the sensitivity for trace and ultra-trace elements in gold [5.22, 5.23]. Inductively coupled plasma atomic emission spectrometry (ICP-AES), frequently applied to the elemental analysis of ancient metals, is seldom used for the study of gold items (this technique was used by Hall et al. [5.24] to study Samartian gold objects). It should be mentioned that both techniques require the removal of a sample, which is totally consumed during analysis; the sample required by ICP-AES being larger than the sample required by ICP-MS [5.23].

5.3. STUDIES ON MANUFACTURING TECHNIQUES OF JEWELLERY