Occupancy of the Sodalite Cages in the Blue Ultramarine Pigments

A quantitative EPR study of blue ultramarine pigments has been performed in order to determine the concentration of the S3- chromophore. Copper sulfate CuSO4·5H2O has been used as a standard, while a ruby crystal was used as an inner standard to take into account the changes of the quality factor of the cavity. These experiments show that, in the most-colored pigments, less than half of the sodalite cages are occupied by a S3- radical. In other experiments, it has been shown that the blue ultramarine pigments can be significantly modified by heating under a dynamic vacuum. The concentrations of S3- and S2-, as deduced from EPR and Raman experiments, are increased after this type of treatment. These changes imply that sulfur species are transformed into S3- or S2- during this treatment. It is discussed that these sulfur species could be S2-.