Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Ion Beam Analysis for the provenance attribution of lapis lazuli used in glyptic art: The case of the “Collezione Medicea”
Introduction
Lapis lazuli is a blue semi-precious stone that requires peculiar conditions to be formed, and for this reason it occurs in few places in the world [1]. Even so, it has been widely used in glyptic since the antiquity and by different civilizations. Although the Badakhshan mines in Afghanistan (the most famous being Sar-e-Sang) are now widely considered as the only source of lapis lazuli in ancient times, other sources have been taken into account [2], [3]. A systematic and exhaustive provenance study of the raw material employed for artworks is still lacking, even if over the last few years different approaches, all exploiting non-invasive techniques, have been used to discriminate among different geological sources [4], [5], [6].
In previous works we have already analysed samples of rocks of known provenances by means of focalized ion beam techniques, in particular μ-PIXE (Particle-Induced X-ray Emission) and μ-IL (Iono-Luminescence). These techniques were chosen because they allow to analyse single crystals of different minerals, a fundamental aspect in a heterogeneous material as lapis lazuli, and also because of their non-invasivity, being often impossible to take samples from artworks or to work in vacuum. Some markers were found and proposed to distinguish among the four different provenances: Afghanistan (Badakhshan), Tajikistan (Pamir Mountains), Siberia (near Lake Baikal) and Chile (Ovalle) [7], [8], [9], [10].
In this work we focused on two among the several mineral phases of lapis lazuli proposed as markers: diopside (CaMgSi2O6) and wollastonite (CaSiO3). Twelve samples were already analysed in previous works [8], [9]. Chilean rocks can be easily distinguished from the others: in fact, as reported in previous studies [7], [11], [12], [13], we confirmed the high contents of wollastonite in all the analyzed Chilean samples (absent in all other analyzed samples). About diopside, in many Afghan samples we observed a high quantity of one or more of the following trace elements not observed in the other Asian samples: Ti (up to about 5000 ppm), V (up to about 600 ppm) and Cr (up to about 900 ppm). However, due to the limited number of characterized samples, the validity of the markers had to be confirmed increasing the amount of analysed rocks prior to use them to attribute the provenance of lapis lazuli in artworks. Therefore, in this paper, a total of 28 samples from 4 different provenances were characterised (8 from Afghanistan, 4 from Tajikistan, 12 from Siberia and 4 from Chile). They belong to different collections: the Mineralogy and Lithology section of the Museum of Natural History (University of Firenze), the Mineralogical, Petrographical and Geological section of the Regional Museum of Natural Sciences of Torino and some were bought by the University of Torino at International Mineral Shows since 2010.
Simultaneously a campaign on artworks and archaeological finds was started to verify the applicability of the method on objects. In this work six precious artworks of the “Collezione Medicea” made of lapis lazuli were analysed to find some indication about the provenance of the raw material. This collection includes both rock samples and artworks, made of precious and semi-precious stones, belonged to the famous Medici family in Firenze and now conserved at the Museum of Natural History (University of Firenze). For many of these objects there is not precise information, especially about the provenance of the raw material used for their manufacturing. In the original catalogue, for some of the analysed artworks an historical provenance such as “Levant” or “Persia” is reported, thus suggesting a generic Asian origin of the raw material, not sufficient to distinguish among the different possible locations.
A first chemical characterisation by means of invasive techniques (XRD and SEM), mainly focused on lazurite, was performed in the past [12] on the rock samples of the collection, but the provenance of the raw material used for artworks has not been investigated yet.
Section snippets
The artworks
The Museum of Natural History (University of Firenze) hosts the “Collezione Medicea di pietre lavorate”: a collection of great historical value composed by precious stones both polished and carved, collected from the members of the Medici family since the 15th century.
In Firenze, the passion of the Medici for importing precious stones led to Ferdinando I de’ Medici’s foundation, in 1588, of the Grand-ducal workshops “Gallerie dei Lavori”, a court manufactory, located in a wing of the Uffizi
Instrumentation and operating conditions
About lapis lazuli rock samples for which there are no problems in performing invasive analysis, IBA measurements on graphite-covered thick petrographic sections (ca. 100 μm) were carried out at the in-vacuum microbeam line [14] at the AN2000 accelerator of the INFN-LNL Laboratory in Legnaro (PD). A 2 MeV proton beam was used, with an ion current of about 200 pA and a spot dimension of about 5÷10 μm; the analysis chamber is shown in Fig. 2. From SRIM 2013 [15] simulations, the penetration depth in
Results and discussion
The first step of this work was related to the extension of the database of trace and minor elements concentration in diopside by means of μ-PIXE measurements and of luminescence spectra in diopside and wollastonite by means of μ-IL measurements. A total of 28 rocks from 4 different provenances were characterised. Analysis were carried out on 1 1 1 crystals (99 diopside and 12 wollastonite).
As already stated, the selection of points of interest was done on the basis of cold-CL maps, looking for
Conclusions
In this work a full characterisation of wollastonite and diopside crystals inside lapis lazuli from four different geological provenances has been carried out. Thanks to μ-PIXE and μ-IL some markers to distinguish among samples coming from Chile, Afghanistan, Tajikistan and Siberia have been found. The widespread presence of wollastonite as a clear indication of the Chilean provenance was confirmed, quickly distinguishable even only by means of luminescence. The presence of high content of
Acknowledgements
This work has been carried out and financially supported in the framework of the INFN experiment “FARE” and partially supported by the project TEMART of Regione Toscana. Thanks are due to Dr. Luisa Poggi for her support in the loan of samples and to Dr. Lorenzo Mariano Gallo for the availability of one Siberian sample. The authors warmly thank Prof. Luca Martire for the availability of his cold CL apparatus and Elisabetta Colombo, Elisa Maupas and Ettore Vittone for their contribution during
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