Towards building a Cathodoluminescence (CL) database for pigments: characterization of white pigments

Abstract

Paintings and painted surfaces are considered to be extremely complex due to their multitude of materials and thus form the basis for particularly intricate Cultural Heritage studies. The combination of Scanning Electron Microscopy (SEM) with Cathodoluminescence (CL) can serve as a powerful tool for the identification of individual pigments. SEM/CL has the potential of identifying both organic and inorganic pigments and can focus on a micrometer or even nanometer scale. The combination with Energy Dispersive Spectrometry (EDS) allows for robust, cross-checked, elemental and mineralogical characterization of pigments. In order to apply SEM/CL in a routine-based way for the identification of pigments, it is necessary to have a robust, open-access database of characteristic CL spectra of pigments. A large project has been undertaken to create such a database, focusing primarily at the pigments, both organic and inorganic, which were most commonly used from antiquity until today. In the present paper, the CL characterization of common white pigments is presented. White pigments were selected, due to their significance and frequency of use, since they were also present on the ground layers or mixed with other pigments in most of the painting layers. More specifically, the CL spectra of samples in pure form of calcite, kaolinite, lead white, zinc oxide, barium sulfate, lithopone and titanium white are presented. In all cases, the CL spectra present characteristic bands, which allow for a secure identification of the pigments. In order to facilitate comparison with other databases, EDS and RAMAN spectra are also presented. Additionally, the effect of weathering on the CL spectra was evaluated, by comparison to naturally and artificially aged samples and to pigments identified on areas of two paintings, of the 19th and 20th c., respectively. Finally, the effect of binding media was also studied, using combination of pigments with four common media: egg yolk, linseed, walnut and poppy oil. Overall, both weathering and binding media appear to cause minor differences in the occurring spectra, without preventing the identification of pigments.