Spectroscopic characterization of commercial pigments for pictorial retouching

A set of 27 commercially available pigments by Kremer Pigmente (Aichstetten, Germany), including traditional and synthetic compounds, has been characterized in detail from a chemical and mineralogical point of view in the frame of a wide research project about pigments and colorants involving different spectroscopic techniques such as reflectance spectroscopy (in both visible and near-infrared intervals), Fourier-transform infrared spectroscopy (in both attenuated total reflection and reflection modes), and Raman spectroscopy. These pigments are currently employed for pictorial retouch purposes of both ancient and contemporary works of art, and a deep comprehension of their composition is fundamental in order to better understand their long-term behavior after application. This work presents Raman results found analyzing the set of pigments in order to identify existing phases and comparing them with datasheets available by Kremer Pigmente: some inconsistencies were found, especially concerning correspondences with Chemical Abstracts Service (CAS) numbers, pigment codes, and color indexes, together with the absence of some compounds in the records. Most Raman signals were attributed to specific vibrational modes, even if in some cases, only related chemical species were identified, whereas information regarding individual vibrational modes was not found in the literature. The whole volume of data generated in this project will represent a useful tool for the study of artworks permitting both the interpretation of spectral information obtainable mostly from portable instruments and for the evaluation of materials to be used for pictorial retouching.