Spectroscopic study of thioindigo-mineral composite*

We report in this study structural changes taking place for the thioindigo-clay mixture in forming a pigment similar to Maya Blue. Different proportions of dye concentrations relative to that of the mineral, as well as different heating times, were applied in pigment synthesis and the outcomes were investigated using FT-IR and FT-Raman spectroscopy. For the pigment samples, the FT-IR peaks at 1627 cm$^{-1}$ are attributed to a downshifted C=O stretching mode of thioindigo due to dye-clay interaction. This interpretation is corroborated by FT-Raman C=O peaks with 14~cm$^{-1}$ shifts to lower frequency for the pigment relative to thioindigo alone. Additional Raman scattering between 550~cm$^{-1}$ and 650~cm$^{-1}$ also suggests dye-clay interaction through metal-oxygen bonding. We thus consider the possibility of hydrogen bonding between silanol and carbonyl dominating at lower dye concentration, with mostly metal-oxygen bonding at higher dye concentration.