From lapis lazuli to synthetic ultramarines: a u-Raman spectroscopy investigation on the history and development of the Most Perfect Color

Abstract

Natural ultramarine has been one of the most precious blue pigments employed in the past in the artifacts. It is typically obtained by crushing and grinding the lapis lazuli rock and selectively extracting the blue mineral lazurite. Since the early 19 th century, when the synthetic version was produced, the use of this much less expensive material became widespread, and synthetic ultramarine blue replaced the natural one in painting palettes. The present study is conducted as a preliminary μ-Raman investigation for creating a comprehensive and detailed database of the ultramarine pigments, both natural and synthetic, employed over the centuries until today.

The geological heritage of the historical collections of the University of Messina

“…The typical blue color of the lapis lazuli may be due to the presence of S 3sulfur in the crystalline lattice of the lazurite mineral [23]. The darkness or lightness of the blue color of the mineral may depend on the number of lazurite grains per given area of the white wollastonite matrix [22].…”

“…The μ-Raman spectra of the authentic lapis lazuli gemstone (blue line, sample 198B) [23] and the simulant sample of lapis lazuli (brown line, sample 199B) were reported in the range between 100 and 2500 cm -1 (Figure 20).…”

“…The analysed pale blue mineral authentic lapis lazuli sample showed a spectral profile typical of lazurite with peaks at ~ 338 cm -1 (S 4anions), 545 cm -1 (S 3chromophore), 634 cm -1 (SO4 2anions), and 972 cm -1 (SO4 2anions) (Figure 20) [23]- [24]. In addition, broad luminescence bands appeared in the highfrequency range (1000-2000 cm -1 ), ascribable to transition elements contained in the crystalline structure of accessory phases (Figure 20).…”

X-ray fluorescence analysis of bronze sculptures by Giuseppe Renda

“…As in the case of the "Scugnizzo", the bronze "Non mi toccare" also lacks information about the commissioner, place and period of execution. In this study, the elemental compositional analysis of the three aforementioned bronze sculptures was performed, for the first time, on representative fragments taken during restoration operations, by means of portable X-ray fluorescence (XRF) spectroscopy [3][4][5][6][7][8][9][10][11][12][13], with the aim: (i) of knowing the constituent elements of the bronze alloy of the three works now preserved in Polistena, in order to verify the hypothesis that the bronzes came from a single foundry, probably the same one that made "La Fortuna" in the second half of the 20 th century; (ii) of identifying the most appropriate methodologies to carry out the cleaning during the restoration of the three artifacts, particularly of the bronze sculpture of "La Fortuna", which is composed of several metal elements welded together [14][15][16][17][18][19]. Indeed, evident are the chromatic and material differences in the bronze that constitutes the allegory of "La Fortuna", represented as a naked woman, compared to the wheel that provides its base, executed by making use of an alloy with a decidedly darker tone.…”