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.