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1. IntroductionX-ray diffraction is the first exact method for the study of the structure of matter. In archaeology, this analysis is used successfully to study stone constructions, ceramics, pigments, etc.The present study of gold and silver items outlines the first steps in a new field – analyzing the line profile of objects in order to obtain information about the microstructural properties of materials: sizes of coherently diffracting domains in crystals (crystallite sizes) and micro-strains in the lattice. These parameters depend on the forming conditions of the material and on various influences (mechanical and thermal) the material was exposed to.The present work is aimed at determining:the sizes of crystallites making up gold and silver objects of natural and artificial origin;the variability of crystallite sizes (D) due to the origin of the material and technological processing of the object;the applicability of such an approach to the characterization and study of archaeological artefacts.The main objects of the study are Thracian gold and silver jewellery items, dated to the first millennium BC, and belonging to the collection of the National Institute of Archaeology with Museum of the Bulgarian Academy of Sciences. Gold and silver objects belonging to different ages and regions were also studied for purposes of comparison. Natural gold and silver samples, both exposed and not to thermal and mechanic influences, were studied as well.The studied alloys belong to the systems Au-Ag (Ag-Au) and Ag-Au-Cu (Au-Ag-Cu). They represent cubic solid solutions with no decay and decomposition. Every item was submitted to 2÷8 XRD analyses. A total of 49 items were submitted to 136 XRD analyses.2. MethodologyTheoretical Basis – crystallite sizes and their relation with profile line broadeningEvery grain of the microstructure of a crystal in an object is, in fact, a mosaic of small blocks showing angular discordance ranging from 20’ to 1-5°. The dimensions of these blocks are comparable with those of the so-called Areas of Coherent Scattering (ACS). That is why small angular discordances cause a widening of the diffraction angle (q) with some extra value (±∆a) (Howard and Preston, 1989).This effect is observed as a broadening of the diffraction peak width.Crystallite size determination entails a study of the nanostructure of the object. For comparison:size of Au atom = 0.142 nm;size of Ag atom = 0.143 nm;Au unit cell parameter a = 0.4078 nm;Ag unit cell parameter a = 0.4086 nm (Strunz, 2001);crystallites measured in the present work: D(Au) = 11.2 ÷ 31.8 nm, D(Ag) = 8.7 ÷ 28.1 nm.Apart from crystallite sizes, peak broadening is also influenced by the instrumental broadening caused by the equipment, and by micro-strains (Balzar, 1993).The instrumental broadening measured in this case was 0.05 (FMHM ~ 30° 2q, CuKa), which is in line with the values quoted in the literature for this type of equipment (Balzar, 1992).In the present research, the authors’ interest was focused exclusively on studying crystallite sizes.MeasurementsX-ray diffraction data were collected by means of a TUR M 62 diffractometer (Germany) with a standard two-circle goniometer in Bragg-Brentano geometry with secondary graphite monochromator. The surface of the studied object was perpendicular to the goniometer plane. We used CuKa radiation (l = 0.15418 nm) under the following measurement conditions: tube voltage = 32 kV; tube current = 15 mA; step-scan mode with step size = 0.02° 2q; and counting time = 2 s per step for standard researches for phase analysis, and 0.01°/15-30 s for researches in the interval 35-41° 2q for experiments performed in order to study the broadening. The gold items’ indexed diffractogram in this mode is shown in Figure 1. The instrumental broadening was determined through standard Si and La B6 powder.Data AnalysisThe diffractometer was controlled by a computer and all measurements were stored on the hard disk. Data were transferred to a personal computer for processing.We used WinFit (version 1.2.1-1997) freeware (Krumm, 1994) to fit profiles and calculate crystallite sizes. WinFit calculated crystallite sizes according to the Warrren-Averbach method. The Pearson VII function was used to fit profiles (Mittemeijer and Scardi, 2004; Uvarov and Popov, 2007).Profile AnalysisThe diffraction peak width (DPW) is sensitive to the mean value of crystallite thickness (size) in a direction normal to the diffracting crystal planes. DPW increases when crystallites diminish and conversely becomes narrower when the nanostructure is coarser. The crystallite sizes were determined by the one-order (single line) method.The profile of the reflex with indexes 111 at Bragg angle 2q ~ 38.3°, corrected for instrumental broadening, was used for the determination of crystallite sizes (Scardi et al., 1994; Balzar and Popovic 1996; Balzar et al., 2004). The profile fitting of the 111 reflection of an Auitem is shownin Figure 2 as an example.3. ResultsBase group– Thracian jewellery from the Collection of the National Institute of Archaeology with Museum.1st group of objects: 2nd century BC – 2nd century AD:The four silver bracelets from Rouzhintsi (Belogradchik region), Nos. 2858, 2859, 2860 and 2861, are decorated with snakes’ heads. The rings of the bracelets are cast. Nos. 2858 and 2859 have almost identical D (16.4 nm; 16.7 nm), i.e., they can be considered as a pair. The other two samples, Nos. 2860 and 2861, possess similar crystallite characteristics, with some insignificant diversion of D (D = 16.0 nm; 16.8 nm), which makes us accept them as products of the same atelier. The D values measured in the areas of the snakes’ heads diminish, an aspect which can be interpreted as a result of forging (D = 15.4 nm). A different technology was used for modelling the snakes’ heads.Figure 1: Indexed diffractogram of gold items.Figure 1 : Diffractogramme indexé des objects d’or.The silver torque from Zelenigrad (No. 2996) is formed in a similar manner as the four bracelets from Rouzhintsi – each end is shaped as an onion with a small granule. Archaeologists consider them as synchronous, dating to a period between the 2nd century BC and the 2nd century AD. The D (10.54 nm) of the torque, if compared to those of the four bracelets, appears to be quite different, which suggests a different origin and/or technology.2nd group of objects – 2nd half of the 4th century BC, items from western Bulgaria:The pair of fibulae from Penkovtsi (Tran region), Nos. 3015a and 3015b, have exactly identical D (15.9 nm) as a result of one and the same technology being employed in their production. One of the fibulae is restored, but this did not influence its D.Another pair of fibulae from Garbino (Kyustendil region), Nos. 3008a and 3008b, is of a similar morphological type as the above-mentioned ones. The fibulae of this pair are characterized by equal D (17.5 nm), differing however from the value of the Penkovtsi fibulae. These pairs are obviously products of different technologies and presumably different ateliers.Figure 2: Fitted diffraction line of studied peak 111.Figure 2 : Ligne de diffraction ajustée au pic 111.A single silver fibula (No. 630) of unknown origin was analyzed in seven spots of the as-cast part and in one of the forged zone. All seven spot analyses show an exactly equal D (22.7 nm), while a single forged area shows some small difference (D = 21.7 nm). The D value of this fibula differs drastically from the above-mentioned two pairs, but it is quite similar to the D of the following objects.A pair of silver bracelets from Granitovo (Belogradchik region), Nos. 4036a and 4036b, show absolutely equal D (20.0 nm) (composition: Ag – 97.5%; Cu – 2.5% for No. 4036a, and Ag – 99%, Au – 1% for No. 4036b) to the above-mentioned item, which may be a sign of a common origin.The analyses of the cast part and forged tip of the fragmented silver torque from Stolat (Sevlievo region), No. 3013, show similar sizes of crystals (D = 19.1 nm; 18.5 nm).3rd group of objects – 5 fragments of gold earrings, Krun (Bulgaria), No. 2682:All measured D values are identical (D = 17.8 nm), which confirms their common origin.Objects for Comparison of Crystallite SizesFirst Group – Gold and Silver Nuggets:1. Gold Nuggetsa) Nugget – Samples of vein gold from the following regions were studied: Etropole, Bulgaria (D = 22.7 nm), Central Rhodope Mountains, Bulgaria (D = 19.8 nm), as well from the large deposits of Bestube (D = 20.0 nm), Aksu (D = 26.0 nm), and Zhana-Tyube (D = 31.8 nm) in Kazakhstan. Alluvial gold from the Rhodope Mountains rivers (D = 23.6 nm) and the Iskar River (D = 28.8 nm) was also studied.b) Nugget, exposed to thermal and mechanical influences. Samples from Bestube (northern Kazakhstan) were exposed to heating under and over the melting point (D = 19.8 nm and 16.2 nm, respectively), and to mechanical actions (forging) (D = 19.8 nm).2. Silver Nuggetsa) Nugget – Samples of the following deposits were studied: Chiprovtsi, Bulgaria (D = 28.1 nm), Sassa, Macedonia (D = 22.3 nm), Kongsberg, Norway (D = 22.4 nm), and Dzhezkazgan, Kazakhstan (D = 28.0 nm).b) Nugget, exposed to thermal and mechanical influences. Samples from Chiprovtsi and Dzhezkazgan were exposed to heating under (D = 28.1nm; D = 28.0 nm), and over (D = 17.7 nm; D = 17.6 nm) the melting point, and to mechanical actions (forging) (D = 28.0 nm; D = 27.7 nm).Second Group – Gold and Silver Artefacts, Differing Sharply in Terms of Chronology and Regions: 1. Gold items – 13th century gold appliqués from Preslav (D = 11.2 nm) and 18th century Turkish gold coins (D = 15.1 nm).2. Silver items – The following objects were studied: 13th century silver belt appliqués from the regions of Shumen (D = 14.1 nm), Pliska (D = 15.4 nm) and Novi Pazar (D = 14.7 nm), Bulgaria; 18th century tobacco pipe cleaning set (D = 8.7 nm), hair decoration (D = 11.1 nm) and massive fork (D = 10.1 nm), Mongolia. The last item was also studied after a standard restoration process (D = 10.1 nm). Size splitting in groups is shown in Figure 3.4. Results and discussionMultiple measurements of crystallize sizes of a given item (in one and the same point and in different points) always show the same results.Crystallite sizes of Au and Ag nuggets are larger than those of artefacts (gold and silver nuggets are formed at temperatures varying between 160 and 200 °C) (Spiridonov and Pletnev, 2002).Thermal treatment of Au and Ag has the following impacts: heating up to/over melting temperature – crystallite sizes decrease sharply (30% D); heating under the melting point – no effect on crystallite sizes.Mechanical treatment (forging) either does not show any size decrease or shows a slight size decrease (0÷5% D).Surface chemical treatment (restoration) has no impact on crystallite sizes.Items from different cultures (regions) differ noticeably in terms of crystallite sizes.The objects with different chronology show differences in terms of crystallite sizes.5. Conclusions and guidelines for further research1. Crystallite sizes, as a constant numerical feature of an item, can be used successfully as a ‘passport’ characteristic of the object. This value can be used confidently to prove an item’s authenticity, and it can also be used with certainty as an absolute protection against forging. While characteristics such as exterior features and chemical composition can be reproduced, imitating the nanostructure of the material is absolutely impossible, since it depends on the thermal history of the item.Figure 3 (See color plate): Distribution of crystallite sizes as per groups of studied items.Figure 3 (Voir planche couleur) : Distribution des tailles de crystallites selon les groupes des objets étudiés.