Ultramarine pigments

Ultramarine pigments are representatives of the inorganic blue pigments. They are characterized by intensive shades of blue and relatively weak violet and pink color tones. The composition of the pigments can be summarized by the formula Na7Al6Si6O24S3. The color of ultramarine is generated by incorporation of free polysulfide radicals in the structure of a sodalite-derived crystal lattice. Ultramarine pigments are synthesized using a series of complex solid-state reactions. The pigments are mainly used in plastics, paints and powder coatings and printing inks, but also in paper and paper coatings.

Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

The long debated nature of the red tetrasulfur chromophore in ultramarine pigments is clarified through computational studies.

From lapis lazuli to ultramarine blue: investigating Cennino Cennini’s recipe using sulfur K-edge XANES

As one of the most desired and expensive artists’ materials throughout history, there has long been interest in studying natural lapis lazuli. The traditional method of extracting the blue component, lazurite, from lapis lazuli, as outlined in Cennini’sIl Libro dell’Arte, involves a lengthy purification process: (1) finely grind the rock; (2) mix with pine rosin, gum mastic, and beeswax; (3) massage in water to collect the lazurite. Repeating the process produces several grades of the pigment, typically referred to as ultramarine blue. Here, we investigate the sulfur environment within the aluminosilicate framework of lazurite during its extraction from lapis lazuli. The sulfur XANES fingerprint from samples taken at the different stages in Cennini’s extraction method were examined. All spectra contain a strong absorption peak at 2483 eV, attributable to sulfate present in the lazurite structure. However, intensity variations appear in the broad envelope of peaks between 2470 and 2475 eV and the pre-peak at 2469.1 eV, indicating a variation in the content of trisulfur (S3−˙) radicals. By studying the effect of each step of Cennini’s process, this study elucidates the changes occurring during the extraction and the variability within different grades of the precious coloring material. The increasing application of XANES to the study of artist’s materials and works of art motivated extending the research to assess the possibility of X-ray induced damage. Direct comparison of micro-focused and unfocused beam experiments suggests an increase of the S3−˙ radicals with prolonged exposure. Analysis indicates that induced damage follows first-order kinetics, providing a first assessment on the acceptable amount of radiation exposure to define the optimal acquisition parameters to allow safe analyses of lapis lazuli and ultramarine pigments.