Naturally irradiated fluorite as a historic violet pigment: Raman spectroscopic and X‐ray diffraction study

Abstract

Naturally irradiated violet fluorite, a cubic CaF2 mineral, is a rare historic pigment. Its documented usage in Europe stretches from ca. 1450 to ca. 1550. The intensely coloured violetish black naturally irradiated fluorite is commonly called antozonite, which is only vaguely defined based on its dark colour and specific odour emanated during grinding. In the published literature, there have been some discrepancies about its Raman spectrum. Therefore, sixteen samples of antozonite were analysed by Raman (micro‐)spectroscopy using five different excitation laser wavelengths (445, 532, 633, 780 and 1064 nm), which revealed specific bands located below 500 cm−1 probably related to radiation‐caused defects. Their intensity increased with increasing violet colour saturation, thus providing a specification for antozonite's definition. Spectra excited at 445 and 780 nm contained also numerous broad bands above 500 cm−1, which seem to be caused by the presence of rare earth elements. The structural damage of antozonite samples has been assessed by X‐ray diffraction and related to their lightness using analysis of image histograms. The obtained results have been applied in the analysis of micro‐samples of a Late Gothic altarpiece located in an Italian Court in UNESCO city Kutná Hora, Czech Republic, which contained exceptionally large grains of deep violet fluorite identified as antozonite. Copyright © 2015 John Wiley & Sons, Ltd.

Raman spectroscopy of minerals and mineral pigments in archaeometry

“…Raman spectra of fragment of a crystal from naturally irradiated fluorite (photograph in the inset), indicating the increase of the bands below 500 cm À1 with increasing violet colour saturation (colourless-spectrum at the bottom, very deep violet-spectrum at the top): (a) 532 nm laser, (b) 780 nm laser. The bands above 500 cm À1 most probably relate to the different REE contents in different positions (by Čermáková et al [612] ).…”

“…Dáňová et al, [611] in a Raman study of the late Gothic polychromy of the Nativity Relief from The Corona Sanctae Mariae Monastery in Třebařov, Moravia, reported the use of purple fluorite in the underpainting beneath the strong azurite layer on the reverse of the cloak of the Virgin Mary. Two samples of purple fluorite from an altarpiece of the Triptych with St James the Lesser and St Philip (dated 1497) located in the chapel of St Wenceslas and Ladislaus in Italian Court, Kutná Hora, Czech Republic, were compared with 16 samples of mineral antozonite by Čermáková [612] using five different Raman excitation wavelengths (Fig. 15).…”

“…Two samples of purple fluorite from an altarpiece of the Triptych with St James the Lesser and St Philip (dated 1497) located in the chapel of St Wenceslas and Ladislaus in Italian Court, Kutná Hora, Czech Republic, were compared with 16 samples of mineral antozonite by Čermáková using five different Raman excitation wavelengths (Fig. ).…”

Irradiation effects in CaF2 probed by Raman scattering

“…Before irradiation, only the single T 2g (Γ 25 ′) Raman active band at 320 cm −1 can be observed. The spectra of the irradiated samples show several characteristic broad bands at both lower and higher wavenumbers than that of the T 2g mode, in agreement with the bands observed at 142, 222, 284, 335 and ~420 cm −1 for naturally irradiated samples . The intensities of these bands become larger with increasing fluence.…”

“…Alternatively, colloids can be generated by additive coloration where samples are quenched after thermal treatments in a metallic gaseous atmosphere . Natural fluorites are also found colored when bearing high‐level radioactive elements …”