This project is intended as an in-depth study on the origin of the precious red pigment cinnabar by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). The importance of this pigment is partly due to its low availability. When the mineral was used in the Roman Age, the district of Almadén, Spain, was probably the most exploited region. However, it is still not well defined if further ores were chosen for supply. It has been recently proved that a few localities show remarkable structural or chemical variations that can be associated to a specific provenance. In the present project, cinnabar samples with known provenance (Spain, Slovenia, Italy, Russia, China and minor European deposits) and already collected from Italian Mineralogical and Earth Science Museums will be analysed by LA-ICP-MS. The project aims at integrating data previously collected by XRD and ICP-MS. The latter was used as a previous screening to define the list of chemical elements to be investigated and then preliminary attest chemical variability. It showed that Chinese samples have already shown to be characterized by selenium. Other elements were found has indicative of a specific exploitation site, namely Li, Si, Fe, Co, Cu, As, Se, Rb, Sr, Y, Zr, Mo, Cd, Sb, Te, La, Ce, Pr, Nd, Eu, Sm, Gd, W, Tl, Pb, Bi, Th and U. LA-ICP-MS is hereby proposed to avoid the difficult sample preparation required for cinnabar by ICP-MS. The aim is to systematically measure trace elements concentration and highlight chemical markers to be linked with provenance. Finally, data treatment by principal component analysis (PCA) is also planned. LA-ICP-MS is intended as a promising in-depth step to extend knowledge on cinnabar trades and provenance in ancient times using a single, micro-destructive, fast and innovative technique. Defining provenance criteria may be also fundamental for Conservations Scientists to detect forgeries.
LA-ICP-MS is constantly gaining importance in archaeometric research, due to its versatility of application, sensitivity and non-destructive or micro-invasive features. In the last decades, conservation scientists have started to use it on different kinds of artworks: glasses [1], metals [2], pottery [3], stones [4] and finally pigments [5]. The candidate believes that this is the best time to test its efficiency as main technique in the present project.
The candidate intends to pursue this aim with a low-invasive, high-innovative technique and to use it independently, thanks to her previous working experience. In fact, the candidate is experienced in archaeometric protocols, having recently tested the efficiency of X-ray diffraction to highlight structural variations induced by a specific origin in the same kind of pigment. Cinnabar has proved to have specific structural features when coming from China [6]. Specifically, selenium substitutions are frequent in the Chinese samples and they have been also assessed by µ-Raman analysis. Selenium occurrence is thus a remarkable and easily recognizable feature, even with fast and non-destructive techniques. Its determination may be very useful in the detection of both forgeries and modern retouching on paintings where a Chinese supply is historically excluded. Some other elemental markers were provided during the candidate¿s PhD: arsenic, zircon, strontium, molybdenum, rhodium, thallium and lead have shown to have variations in their concentration that may depend on a specific exploitation site. However, the application of ICP-MS to measure this elemental variability proved to be a complex procedure, because accessory minerals in several samples led to a difficult sample preparation. In fact, impurities were retained even after strong digestion, which is the standard procedure reported in references for cinnabar [7]. This factor led to a sensible reduction of the analysed samples from 44 to 11. Thus, with the present project, the candidate aims at improving ICP-MS efficiency by its coupling to laser ablation. This methodology will lead to the elimination of difficulties in sample preparation. LA-ICP-MS does not require sample preparation or, at best, the embedding in epoxy resin may be chosen, which is faster and not sample-dependant. The application of this technique will lead to the acquisition of a complete set of chemical data. This data-set is crucial both for archaeometric and geochemical investigations. That is why the candidate also aims at sharing the results in a public open-source online database. The latter step represents an innovative aspect in the study of pigments.
Hence, the project derives from the candidate¿s expertise in the archaeometric study of pigments. It also represents an integration of her PhD research, where it was show by XRD that the genetic environment may cause variations in cinnabar unit cell parameters [6]. A fundamental step to solve provenance tasks may be the matching of those structural features with chemical variations. This is the very first, innovative attempt to systematically apply such a provenance protocol on cinnabar and on pigments in general, where both chemical and structural data are contemporary considered. As shown in the state of the art, cinnabar provenance has been scarcely investigated, and mainly by isotope ratios-based studies. This will be the first chance to systematically collect chemical data and match them to the structural ones for provenance assessment purposes.
[1] S. Cagno, M. Mendera, T. Jeffries, K. Janssens, J. Archaeol. Sci. 2010, 37, 3030. [2] L. Dussubieux, P. Robertshaw, M. D. Glascock, Int. J. Mass Spectrom. 2009, 284, 152. [3] A. Aldrabee, A. H. Wriekat, Microchem. J. 2011, 99, 289. [4] Poretti, M. Brilli, C. De Vito, A. M. Conte, A. Borghi, D. Günther, A. Zanetti, J. Cult. Herit. 2017, 28, 16. [5] R. L. Green, R. J. Watling, J. Forensic Sci. 2007, 52, 851. [6] A. Maras, M. Botticelli, P. Ballirano, Int. J. Conserv. Sci. 2013, 4, 685. [7] G. A. Mazzocchin, P. Baraldi, C. Barbante, Talanta 2008, 74, 690.