Differentiation between anonymous paintings of the 17th and the early 18th century by composition of clay-based grounds

Abstract Numerous anonymous paintings are preserved in depositories of European galleries. The reasons why they are not being shown to the public differ, but one of them is their doubtful attribution and relative dating. There is a large amount of unsigned paintings of varying artistic quality and also lots of copies of unknown provenance. We have found out that mineralogical analysis based on X-ray powder micro-diffraction measurements is able to provide exact parameters for the classification of earthy ground layers of paintings into well-defined types. In the period of the 17th and the early 18th century, these types were regionally specific, because various coloured clay-based materials became popular thanks to significant changes in the painting technology. They gradually substituted previously used white chalk and/or gypsum (gesso). Within comparative research performed on paintings from Czech collections, we were able to distinguish those of Italian and those of Central-European provenance. We have defined five types of grounds that differ in clay minerals' structures (presence of expandable and interstratified structures, various crystallinity indices etc.) and their relative contents, and, in addition, in characteristic elemental ratios and admixtures, either natural or artificial.