An XPS study into sulphur speciation in blue and green ultramarine

A number of ultramarine paint samples were analysed by Raman spectroscopy and X-ray photoelectron spectroscopy to determine the binding energies of the S3.− (blue) and S2.− (yellow) radical anions responsible for the pigment colour. The two sulphur radicals (chromophores) have well assigned Raman signals, however, no previous XPS study has been able to observe both radicals in natural or synthesised blue ultramarine samples and assign binding energies. This study focused on the analysis of synthetic ultramarine samples, which have a higher chromophore concentration than natural pigments, and on the comparison of blue and green shades, as green pigments have a higher quantity of the yellow chromophore. XPS spectra from six blue and three green paints were processed and found to contain sulphide, sulphite, sulphate and two unassigned signals at 162.9 and 163.8 eV. The ratio of these two signals varied between blue and green samples. Raman spectra of the samples were measured and the ratio of the blue versus yellow signals at 549 cm−1 and 580 cm−1 calculated. Comparison of the Raman and XPS data sets found good agreement between signal ratios when the 162.9 eV XPS signal was assigned to the yellow and the 163.8 eV signal was assigned to the blue chromophore.