Speaker
Description
Ultramarine has been for centuries one of the most highly prized pigments of all traditional artists’ materials, due to its durability, excellent color, and its intrinsic value. For the production of the blue pigment Ultramarine, the rare semiprecious stone Lapis Lazuli is used, which has been mined since ancient times in mines northeast of Afghanistan, making it difficult to transport to the Mediterranean region. This accounts for the pigment being named ultramarine blue, i.e. a blue pigment coming from beyond the sea. The production of a synthetic version by Guimet in 1828, which was obtained from the calcination of a mixture of metakaolin, sulfur, sodium carbonate, and a reducing agent, followed by an oxidation step, introduced an important change in artists' habits, in that a less expensive pigment was available for their palettes [1]. The verification of the natural or synthetic origin of the pigment is particularly important in a project, e.g. in authentication cases.
In this research work, ten samples of natural and synthetic Ultramarine pigments were used to investigate the possibility of their discrimination using characterization methods. In the first stage of the research, all samples were studied by Fourier transform infrared (FTIR) spectroscopy. The distinction between natural and synthetic ultramarine using FTIR -as proposed in previous studies- is difficult, as the FTIR bands of interest coincide with the peaks of atmospheric $CO_2$ ($2400-2300 cm^{-1}$) [2,3]. For this reason, X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM-EDS), and UV-Vis spectrophotometry were used for all samples to support the study [4,5]. The combination of elemental composition, morphological analysis, phase identification, chemical state, color calculation, and optical band gap estimation is studied, to propose an analytical protocol for the discrimination between samples of artificial and natural origin.
Acknowledgements The corresponding author acknowledges the Research Committee of Aristotle University of Thessaloniki (RC-AUTH) for the financial support of the participation in BPU11.
References
1. J. Plesters, in Artists' pigments: a handbook of their history and characteristics, vol. 2, edited by R. Ashok (Oxford University Press, Washington, 1993).
2. V. Desnica et al., e-PS. 1, 15 (2004).
3. C. Miliani et al., Chem. Phys. Lett. 466, 148 (2008).
4. M. González-Cabrera et al., Dyes Pigm. 178, 108349 (2020).
5. A. Škvarlová et al., Micropor. Mesopor. Mater. 284, 283 (2019).
