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X-ray Diffraction method for determination of crystallite sizes of gold and silver items – New opportunities for archaeology and for protection against forgery

Méthode de diffraction de rayons X pour la détermination des tailles de cristallites  des objets d’or et d’argent – Nouvelles opportunités pour l’archéologie et pour la protection contre la production de faux
Denka Yanakieva, Milena Tonkova, Ernst Spiridonov, Zlatil Vergilov et Petia Penkova
p. 45-50

Résumés

Ce travail présente les résultats d’une étude des rayons-X de la nanostructure d’objets en or et en argent ; les données obtenues sont utilisées pour l’identification et la comparaison. Cette étude concerne des objets archéologiques en or et en argent ainsi que des pépites d’or et argent (n’ayant subi aucune action thermique ou mécanique). Les objets ont été étudiés au moyen de l’analyse DRX, l’accent ayant été mis sur l’analyse des profils de lignes, de façon à obtenir des informations sur les propriétés microstructurales : taille des domaines diffractant de façon cohérente (taille des cristaux). La taille des cristallites, caractéristique numérique constante d’un objet, peut être utilisée avec succès pour : vérifier l’authenticité d’un objet et ainsi fournir protection contre la contrefaçon ; étudier des questions pertinentes liées au traitement de métaux utilisés par les faussaires dans le passé, aux débuts de leur utilisation par l’homme ; fournir des corrélations exactes de temps et localisation (inclues dans des recherches complexes sur les objets archéologiques et combinés à d’autres méthodes), etc.
Ce travail fait partie d’une étude détaillée des rayons X de l’or et de l’argent, ayant pour but de spécifier les caractéristiques nano-structurelles des objets : tailles de cristallites, micro-tensions au niveau de cellules d’unité, paramètres de la cellule d’unité. Ce travail se base sur les caractéristiques cristallites et offre les premiers exemples démontrant le potentiel de la méthode ainsi que suggérant des actions futures possibles permettant d’évaluer ce potentiel.

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Texte intégral

Texte intégral en libre accès disponible depuis le 21 mars 2011.

1. Introduction

1X-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.

2The 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. Methodology

Theoretical Basis – crystallite sizes and their relation with profile line broadening

3Every 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.

4Crystallite 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.

5Apart 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.

Measurements

6X-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 Analysis

7The diffractometer was controlled by a computer and all measurements were stored on the hard disk. Data were transferred to a personal computer for processing.

8We 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 Analysis

9The 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.

10The 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. Results

Base group– Thracian jewellery from the Collection of the National Institute of Archaeology with Museum.

1st group of objects: 2nd century BC – 2nd century AD:

11The 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.

Figure 1: Indexed diffractogram of gold items.Figure 1 : Diffractogramme indexé des objects d’or.

12The 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:

13The 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.

14Another 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.

Figure 2: Fitted diffraction line of studied peak 111.Figure 2 : Ligne de diffraction ajustée au pic 111.

15A 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.

16A 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.

17The 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 Sizes

First Group – Gold and Silver Nuggets:

1. Gold Nuggets

a) 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 Nuggets

a) 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 discussion

18Multiple 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 research

191. 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.

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.
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Bibliographie

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2. Crystallite sizes can be used to study issues relevant to the treatment of forgeable metals in the ancient ages when man started using them (the processes of forging, heating under the melting point and melting only).

3. Crystallite sizes can provide information useful for distinguishing the different ateliers (metal workshops), since the thermal treatment of an item is a determinant feature for crystallite sizes.

4. Using XRD analysis in complex researches of a given archaeological site and particularly in researches allowing wider correlations, made according to data bases, creates opportunities for a more accurate interpretation of the studied archaeological material.

5. We are confident that working together with archaeologists on genuine items and in the framework of specific projects will contribute to both the clarification of issues relevant to cultural heritage and to the development of the method, rendering it more precise.

The present work represents only a first stage of the application of the method for studying gold and silver archaeological items and an illustration of its possibilities. Detailed studies of the regularities that the objects in the specified groups show are a subject of another work.

We believe that further precise researches of diffractogram profiles, comparing crystallite sizes and measured micro-strains, as well as the correlation of the results obtained through different software, will provide new results, to be used successfully within archaeological sciences.

Balzar, D., 1993.X-ray Diffraction Line Broadening: Modeling and Applications to High-Tc Superconductors.Journal of Research of the NIST98: 321-353.
DOI : 10.6028/jres.098.026

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Balzar, D., 1992. Profile Fitting of X-ray Diffraction Lines and Fourier Analysis of Broadening.Journal of Applied Crystallography 25: 559-570.
DOI : 10.1107/S0021889892004084

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Balzar, D. and Popovic, S., 1996.Reliability of the Simplified Integral-Breadth Methods in Diffraction Line-Broadening Analysis.Journal of Applied Crystallography 29: 16-23.
DOI : 10.1107/S0021889895008478

Balzar, D., Audebrand, N., Daymond, M.R., Fitch, A., Hewat, A., Langford, J.I., Le Bail, A., Louër, D., Masson, O., McCowan, C.N., Popa, N.C., Stephens, P.W. and Toby, B.H., 2004. Size-Strain Line-Broadening Analysis of the Ceria Round-Robin Sample.Journal of Applied Crystallography 37: 911-924.

Howard, S.A. and Preston, K.D., 1989. Profile fitting of powder diffraction patterns, in D.L. Bish, J.E. Post (eds.), Mineralogical Society of America, Modern Powder Diffraction, Reviews in Mineralogy 20: 1-17.

Krumm, S., 1994. WINFIT 1.0 – A computer program for X- ray diffraction line profile analysis. XIII Conference on Clay Mineralogy and Petrology,Acta Universitatis Carolinae, Geologica 38: 253-256.

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Scardi, P., Leoni, M. and Delhez, R., 1994.Line broadening analysis using integral breadth methods: a critical review. Journal of Applied Crystallography, 27: 345-357.
DOI : 10.1107/S0021889804004583

Spiridonov, E. and Pletnev, P., 2002.Deposit of the cuprian gold Zolotaya Gora. Moscow, Scientific World (in Russian).

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Mittemeijer, E.J. and Scardi, P., 2004.Applicabilities of the Warren-Averbach analysis and an alternative analysis for separation of size and strain broadening. In Diffraction Analysis of the Microstructure of Materials. Springer Series in Materials Science, Vol. 68, XXV, 550.

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Uvarov, V. and Popov, I., 2007.Metrological characterization of X-ray diffraction methods for determination of crystallite size in nano-scale materials. Materials characterization 58: 883-891.
DOI : 10.1016/j.matchar.2006.09.002

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Table des illustrations

Titre Figure 1: Indexed diffractogram of gold items.Figure 1 : Diffractogramme indexé des objects d’or.
URL http://archeosciences.revues.org/docannexe/image/1985/img-1.png
Fichier image/png, 211k
Titre Figure 2: Fitted diffraction line of studied peak 111.Figure 2 : Ligne de diffraction ajustée au pic 111.
URL http://archeosciences.revues.org/docannexe/image/1985/img-2.png
Fichier image/png, 23k
Titre 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.
URL http://archeosciences.revues.org/docannexe/image/1985/img-3.png
Fichier image/png, 28k
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Référence papier

Denka Yanakieva, Milena Tonkova, Ernst Spiridonov, Zlatil Vergilov et Petia Penkova, « X-ray Diffraction method for determination of crystallite sizes of gold and silver items – New opportunities for archaeology and for protection against forgery », ArcheoSciences, 33 | 2009, 45-50.

Référence électronique

Denka Yanakieva, Milena Tonkova, Ernst Spiridonov, Zlatil Vergilov et Petia Penkova, « X-ray Diffraction method for determination of crystallite sizes of gold and silver items – New opportunities for archaeology and for protection against forgery », ArcheoSciences [En ligne], 33 | 2009, mis en ligne le 21 mars 2011, consulté le 21 septembre 2014. URL : http://archeosciences.revues.org/1985

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Auteurs

Denka Yanakieva

Earth and Man National Museum – 4 Cherni Vrach Blvd., Sofia 1421, Bulgaria. (denkay@abv.bg)

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Milena Tonkova

 Institute of Archaeology with Museum, Bulgarian Academy of Sciences – 2 Saborna Street, Sofia 1000, Bulgaria. (milenatonkova@hotmail.com)

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Ernst Spiridonov

Department of Mineralogy, Lomonosov State University of Moscow – Vorobiovy Gory, 1192334, Russia. (mineral@geol.msu.ru)

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Zlatil Vergilov

St. Kliment Ohridski Sofia University, Department of Archaeology, Petrology and Mineral Resources – 15 Tsar Osvoboditel Blvd., Sofia 1504. (zver@gea.uni-sofia.bg)

Petia Penkova

Institute of Archaeology with Museum, Bulgarian Academy of Sciences – 2 Saborna Street, Sofia 1000, Bulgaria. (petiapenkova@yahoo.com)

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Droits d’auteur

Article L.111-1 du Code de la propriété intellectuelle.

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