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Synchrotron Radiation Induced X-Ray Fluorescence for the characterization of ancient gold objects

Fluorescence de rayons X induite par radiation de synchrotron pour la caractérisation d’objets anciens en or
Martin Radtke, Uwe Reinholz et Heinrich Riesemeier
p. 39-44

Résumés

Grâce à leur non-destructivité, les techniques analytiques qui utilisent les rayons X sont très répandues dans le domaine de l’archéométrie. L’utilisation de la fluorescence de rayons X induite par radiation de synchrotron (SR-XRF) est illustrée à partir de trois exemples : les dessins à pointe d’argent de Jan van Eyck, le trésor de Hiddensee et le disque céleste de Nebra. L’analyse d’objets, même de grandes tailles, à l’air permet d’obtenir des informations sur leur origine, leur homogénéité et leurs techniques de fabrication.

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1. Introduction

1Synchrotron radiation (SR) is one of the most versatile tools for the characterization of materials. It combines high spatial resolution with good detection limits, the possibility to work in normal atmosphere with the capacity to examine large objects, which do not fit into sample chambers. SR measurements on metals are non-destructive.

2Technically, a synchrotron is a machine which generates brilliant beams of light by moving electrons through a strong magnetic field. This light, called synchrotron radiation, ranges from the infrared to hard X-rays, with some extremely useful properties. Additionally, a synchrotron provides the infrastructure for a number of different measuring techniques, and the facilities for sample treatment and preparation. Last but not least, synchrotrons are located in government funded science centre, where measurements can be carried out for free, provided that a beam time proposal was evaluated successful by an independent beam time committee. One of the disadvantages is that, even if there are now around 40 synchrotron light sources worldwide and the number is still increasing significantly, not every country has a synchrotron. Due to limitations related to moving precious objects and their crossing national boundaries, access to this methodology is limited. In any case, because of its high flux, SR-based techniques can usually be performed on minute samples.

2. Methods

SR is useful for all X-ray techniques; however, for the analysis of ancient metal objects, X-ray fluorescence is the predominant method. For XRF, the use of SR presents especially the following advantages:

The radiation is linearly polarized, and therefore the scattering in the plane of polarization is minimized and detection limits are improved.

The excitation energy can be selected and adapted to the problem. It is possible to excite an element of interest just above the absorption edge in order to optimize excitation, or to go under an absorption edge to avoid excitation.

SR has high brilliance. This means that the photons are emitted in a small solid angle, and this allows focusing the radiation effectively in a small excitation spot.

3All examples referred to in this publication have been selected from measurements carried out with the SR-XRF setup at the BAMline at BESSY II, Berlin. At this beam line, a super conducting wavelength shifter with a maximum field of 7 Tesla is the X-ray source, with a useable energy range from 5 keV up to 80 keV. Therefore, nearly all elements can be detected by measurement of their K-shell fluorescence. A W/Si double-multilayer-monochromator (DMM) and/or a Si [111] double-crystal-monochromator (DCM) are used to select the desired excitation energy. In order to achieve higher fluxes, it is possible to focus the beam to a size of approximately (300x300) µm² by bending one multilayer and/or one crystal (Görner et al., 2006). If smaller beam sizes are necessary, compound refractive lenses (CRL) can be used to achieve spot sizes of less than (2x2) µm² (Strub et al., 2008). For the detection of the fluorescence signal, different types of solid state detectors (Si(Li) HPGe and SDD) are available. For data evaluation, various program packages are freely available. At the BAMline, QXAS from the IAEA, a Monte Carlo code, capable of simulating photon-matter interaction (Vincze et al., 1995) and pyMCA (Sole et al., 2007) are implemented. Details about the SR-XRF technique can be found elsewhere (Adams et al., 1998).

4SR-XRF is a non-destructive multi-element method. However, not all elements can be analyzed with the same sensibility. For gold, an excitation energy of around 33-38 keV, depending on the expected trace elements, has been found to be most useful. In this case, the minimum detection limits (MDL) of trace elements in gold are in the order of 1 ppm. The detection of platinum requires special procedures because of the closeness of these two elements in the periodic table. By a very sophisticated approach (Guerra et al., 2005; 2008), the MDL of Pt can be decreased as low as 10-20 ppm.

3. Examples

If an archaeologist or art historian comes to use a synchrotron, typically one of these questions is asked:

– What material was used to manufacture this object?

– How was it made?

– Is it possible to discriminate different compositions within one object or between different members of a group of objects?

Examples of questions that are more difficult to answer are the following:

– Where does it come from?

– When was it made?

– Is the object authentic or a forgery?

5Unfortunately, there is no possibility to measure the age of a gold object with SR. Therefore, the last two questions cannot be answered with our methods in a straightforward manner. In the following, examples of answers to the other categories of questions are presented. At first, the apparently trivial task of the identification of the material is illustrated by the non-destructive analysis of silver point drawings of Renaissance artists. For this purpose, extremely sensitive synchrotron – or accelerator-based techniques (PIXE – Particle Induced X-Ray Analysis) are necessary, because only small quantities of metal were deposited on the paper (Reiche et al., 2004a). On Van Eyck’s drawing, the ‘Portrait of Cardinal Albergati’, at least three different metal points have been identified (Fig 1). Two different types of silver marks have been found in the examined portrait. Silver containing gold marks has been detected in the inscriptions and over-subscriptions. This discovery has represented the first evidence of the use of gold points in metal point drawings from the Middle Ages. The fact that a gold stylus different from the other silver points was used represents therefore an important result. An explanation of the exceptional use of a gold stylus could be the difference in mechanical properties of a gold point, which is much smoother than pure or especially Cu-containing silverpoints. Thus, it seems easier to write with a gold point than with a silver one. Furthermore, it is possible to obtain different colour aspects of the marks when using another metal than silver. A gold stylus leaves a dark grey mark, whereas a silver mark becomes brownish quite rapidly. Thus, it can be postulated that Van Eyck consciously used diverse metal points in order to make the most of the changing properties of different metal styli, and to achieve special graphic effects, such as the particular three-dimensionality or profundity of this portrait.

6It is thus possible to draw a conclusion regarding the genesis of the drawing. Van Eyck probably used first the pure silver stylus for drawing the face, the robe, and the hair of the figure. The cross line was also made with this stylus. The hatchings on the left and on the right of the head were made with another copper-containing silver point, which emphasizes the plasticity of the portrait. Then, the eyes and other parts of the portrait were intensified using the gold stylus. With the same stylus, the letters on the face were inscribed (Reiche et al., 2004b).

7The next example provides an insight into the fabrication techniques of Viking gold objects. The so-called ‘Hiddensee Gold’ objects (Fig 2) are said to have been washed ashore after two great storm tides in 1872 and 1874, and saved by fishermen from Neuendorf, Germany. From there, they came to their present whereabouts, the Cultural and Historical Museum of Stralsund, under somewhat adventurous circumstances. The 16 objects, with a total weight of 600 g, have been manufactured around 1000 AD in a Viking workshop. The first results showed a great homogeneity of the different gold alloy of all sixteen objects, except for the brooch, the neck ring, and one of the cross-shaped pendants (no. 12). This finding was not surprising for the neck ring, which was processed with the help of a different technique (twisted wire instead of soldered sheet metal, wire, and granules), and not even for the brooch, which is a unique piece among the filigree decorated objects. However, there is no answer yet to the question of why the cross-shaped pendant no. 12 has a significantly lower gold content than the other nine cross-shaped pendants. Perhaps it was produced separately and later added to the set? Another important result concerns the composition of the solder. Remnants of metallic solder distributed on the sheets could clearly be seen under the microscope. For the solder, gold had been alloyed with other types of metal so that the melting point was significantly lower than that of the gold alloy of sheets and decorations. Upon heating, the sheets and decorations were connected by the solder without melting themselves. Normally, the melting point of pure gold is 1084 °C. By adding silver or copper to the gold, the melting point can be reduced to a temperature between 850 °C and 975 °C, depending on the exact proportions of the different metals. Our investigations of pendant no. 13, for example, confirm the presence of a high amount of copper in the solder (Armbruster et al., 2004).

Figure 2: (See colour plate) The Hiddensee Gold treasure (Kulturhistorisches Museum Stralsund, Germany, inv. no. 1873: 499a et cetera).
Figure 2 : (Voir planche couleur) Le trésor de Hiddensee (Kulturhistorisches Museum Stralsund, Allemagne, inv n° 1873 : 499a).

Figure 2: (See colour plate) The Hiddensee Gold treasure (Kulturhistorisches Museum Stralsund, Germany, inv. no. 1873: 499a et cetera). Figure 2 : (Voir planche couleur) Le trésor de Hiddensee (Kulturhistorisches Museum Stralsund, Allemagne, inv n° 1873 : 499a).

The scatter plot (a) shows the different compositions of the brooch, the neck-ring, and pendant no. 12. In the plot (b), the increased copper content of the solder can be seen.
Le graphique ci-contre (a) montre les différentes compositions de la broche, du ras de cou et du pendentif n° 12. Le graphique ci-dessus (b) met en évidence l’augmentation de la teneur en cuivre pour la soudure.

8The provenance of gold objects based on elemental analysis is problematic, due to the frequent recycling of this precious material. However, for very old objects, it can be assumed that no recycling took place, and that the fingerprint of trace elements in the gold object is similar to that of the original gold. The third example presented in this publication, the ‘Sky Disc of Nebra’ (Fig 3), was manufactured around 2000 BC and is featuring the earliest record of Central European astronomy known to date. The question that needed to be answered with SR-XRF was whether the composition of the different gold parts was the same, or if it had changed during different periods of completion according to the purpose of the disc. The analysis of minor components such as silver and tin provided the basis for a cluster analysis resulting in the separation of three distinctly different groups:

1. sun, moon and the majority of the stars;

2. horizon and one single-shifted star;

3. the so-called bark.

9Using these results, we were able to support the assumption of the archaeologists, that the disc was manufactured in several phases, representing progress in knowledge and a change of religion and purpose. Additionally, we were able to identify the Carpathian basin as the most likely source of the gold. Using copper from Austria, and a ‘technology transfer’ from Mycenae (Greece), the manufacturing of the Nebra Sky Disk can be considered as a very early endeavour based on a joint European experience, possibly with contributions indebted to the symbolism of Egypt or the Levant (Pernicka et al., 2003).

4. Summary

10The special requirements for the use of a synchrotron and the advantages of synchrotron radiation have been discussed above. SR-XRF made it possible to characterize small amounts of metal, as in the case of the silverpoint drawings, analyse big objects such as the Sky Disc of Nebra, and to measure the composition of objects on a small scale, as in the case of the solder used for the Hiddensee gold.

Figure 3: (See colour plate) The Sky Disk of Nebra (Landesmuseum für Vorgeschichte, Halle, Germany, inv. no. HK2002:1649a).
Figure 3 : (Voir planche couleur) Le disque céleste de Nebra (Landesmuseum für Vorgeschichte, Halle, Allemagne, inv. no. HK2002 : 1649a).

Figure 3: (See colour plate) The Sky Disk of Nebra (Landesmuseum für Vorgeschichte, Halle, Germany, inv. no. HK2002:1649a). Figure 3 : (Voir planche couleur) Le disque céleste de Nebra (Landesmuseum für Vorgeschichte, Halle, Allemagne, inv. no. HK2002 : 1649a).

The measured silver and tin contents of the gold applications are shown in the scatter plot. The three groups are clearly recognisable: (a) bark; (b) star no. 23 and horizon; (c) the remaining stars with moon and sun.Les teneurs en argent et en étain mesurées pour les applications en or sont indiquées sur le graphique. Les trois groupes sont clairement reconnaissables : (a) écorce; (b) étoile n° 23 et horizon; (c) les autres étoiles avec la lune et le soleil.

11It has been shown that knowledge about the composition of the material can provide significant information about the origin and insights into the manufacturing process of an object. While the examples presented above are covering the main topics related to the analysis of ancient gold with SR-XRF, it should be mentioned that there are also other techniques able to offer useful information. For example, X-Ray Diffraction (XRD) was used to investigate the weathering of gilding decorations (Lluveras et al., 2008). X-ray Absorption Spectroscopy (XANES and EXAFS) and Small Angle X-ray Scattering (SAXS) are useful tools for characterizing nano particles in gold ruby glass (Eichelbaum  et al., 2007).

12In the future, the use of SR-XRF to characterize the Berlin Gold Hat (Museum für Vor- und Frühgeschichte, Staatliche Museen zu Berlin, Germany) and the Skythian Gold Treasures from Vettersfelde and Maikop (Antikensammlung, Staatliche Museen zu Berlin, Germany) will bring exciting new insights into the past.

The authors would like to thank our colleagues I. Reiche, M.F. Guerra, B. Armbruster, H. Eilbracht, A. Grüger, C.H. Wunderlich and E. Pernicka for the good collaboration. A special compliment for all the art historians and archaeologists involved, for the confidence to bring these unique objects to our beam line. The BESSY team is acknowledged for their good cooperation.

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Bibliographie

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Table des illustrations

Titre Figure 1: The drawing “Portrait of Cardinal Albergati” by Jan van Eyck (Kupferstichkabinett, Dresden, Germany, inv. no. C775). The plot shows a comparison of synchrotron-induced X-ray fluorescence spectra of a silver mark (stylus 1), a gold mark (stylus 2), and the paper with preparation backing measured with an excitation energy of 30 keV.Figure 1 : Le dessin Portrait du Cardinal Albergati par Jan van Eyck (Kupferstichkabinett, Dresde, Allemagne, inv. no. C775). Le graphique montre la comparaison des spectres obtenus par fluorescence de rayons X induite par radiation de synchrotron de marques en argent (stylus 1) et en or (stylus 2) et du papier avec la préparation d’entoilage mesurés avec une énergie d’excitation de 30 keV.
URL http://archeosciences.revues.org/docannexe/image/1969/img-1.jpg
Fichier image/jpeg, 368k
URL http://archeosciences.revues.org/docannexe/image/1969/img-2.png
Fichier image/png, 101k
Titre Figure 2: (See colour plate) The Hiddensee Gold treasure (Kulturhistorisches Museum Stralsund, Germany, inv. no. 1873: 499a et cetera). Figure 2 : (Voir planche couleur) Le trésor de Hiddensee (Kulturhistorisches Museum Stralsund, Allemagne, inv n° 1873 : 499a).
URL http://archeosciences.revues.org/docannexe/image/1969/img-3.jpg
Fichier image/jpeg, 620k
URL http://archeosciences.revues.org/docannexe/image/1969/img-4.jpg
Fichier image/jpeg, 396k
Légende The scatter plot (a) shows the different compositions of the brooch, the neck-ring, and pendant no. 12. In the plot (b), the increased copper content of the solder can be seen.Le graphique ci-contre (a) montre les différentes compositions de la broche, du ras de cou et du pendentif n° 12. Le graphique ci-dessus (b) met en évidence l’augmentation de la teneur en cuivre pour la soudure.
URL http://archeosciences.revues.org/docannexe/image/1969/img-5.png
Fichier image/png, 97k
Titre Figure 3: (See colour plate) The Sky Disk of Nebra (Landesmuseum für Vorgeschichte, Halle, Germany, inv. no. HK2002:1649a). Figure 3 : (Voir planche couleur) Le disque céleste de Nebra (Landesmuseum für Vorgeschichte, Halle, Allemagne, inv. no. HK2002 : 1649a).
Légende The measured silver and tin contents of the gold applications are shown in the scatter plot. The three groups are clearly recognisable: (a) bark; (b) star no. 23 and horizon; (c) the remaining stars with moon and sun.Les teneurs en argent et en étain mesurées pour les applications en or sont indiquées sur le graphique. Les trois groupes sont clairement reconnaissables : (a) écorce; (b) étoile n° 23 et horizon; (c) les autres étoiles avec la lune et le soleil.
URL http://archeosciences.revues.org/docannexe/image/1969/img-6.png
Fichier image/png, 584k
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Référence papier

Martin Radtke, Uwe Reinholz et Heinrich Riesemeier, « Synchrotron Radiation Induced X-Ray Fluorescence for the characterization of ancient gold objects », ArcheoSciences, 33 | 2009, 39-44.

Référence électronique

Martin Radtke, Uwe Reinholz et Heinrich Riesemeier, « Synchrotron Radiation Induced X-Ray Fluorescence for the characterization of ancient gold objects », ArcheoSciences [En ligne], 33 | 2009, mis en ligne le 21 mars 2011, consulté le 02 septembre 2014. URL : http://archeosciences.revues.org/1969

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Auteurs

Martin Radtke

BAM Federal Institute for Materials Research and Testing – Richard-Willstätter-Straße 11, 12489 Berlin, Germany. (martin.radtke@bam.de)

Articles du même auteur

Uwe Reinholz

BAM Federal Institute for Materials Research and Testing – Richard-Willstätter-Straße 11, 12489 Berlin, (uwe.reinholz@bam.de)

Heinrich Riesemeier

BAM Federal Institute for Materials Research and Testing – Richard-Willstätter-Straße 11, 12489 Berlin, Germany. (heinrich.riesemeier@bam.de)

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Droits d’auteur

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

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