A Validated Reference Database for Twentieth-Century Cd-Based Pigments: Integrated Structural and Compositional Characterization.

Abstract

Cadmium sulfur selenide and cadmium zinc sulfide pigments have been widely employed by artists of the twentieth century. A deep knowledge of these artworks, also in view of their conservation and/or restoration, relies on comprehensive and robust databases of reference materials of the pigments to easily detect their composition and their tendency and routes to deterioration. In this work, we initiate the construction of such a database by applying exclusively non-invasive techniques, X-ray fluorescence (XRF) and fiber optics reflectance spectroscopy (FORS) in the visible to shortwave (Vis-SWIR) range, to two sets of historical pigments: a group of pure commercial tube paints and a mixed palette representative of artistic practice. To strengthen the interpretative framework, mock-up samples were prepared using modern Cd-based pigments. These reference materials were first analysed using the same non-invasive protocols to ensure full data compatibility. In addition, they were further characterized through synchrotron-based techniques, namely high-hesolution X-ray powder diffraction (HR-XRPD) and X-ray absorption spectroscopy (XAS), providing additional structural and phase information not accessible through laboratory non-invasive methods alone. The integration of non-invasive and synchrotron-based data on the modern reference materials enables a more rigorous interpretation of the spectra collected on historical pigments. This combined approach enhances phase discrimination, compositional assessment, and the understanding of pigment variability, thereby increasing the reliability and internal consistency of the historical database. The resulting dataset establishes a solid foundation for future non-invasive studies of twentieth-century Cd-based pigments in complex stratigraphic and binding media contexts.