Insight into framework destruction in ultramarine pigments

We report key evidence on the framework destruction in ultramarine pigments upon color fading. Experiments on faded pigments in a fresco painting environment reveal that the paramagnetic chromophores are set free via sodalite framework destruction and are subsequently degraded. Fading in acidic media produces similar results, although a larger number of β-cages appear to be destroyed, and H₂S is released. The findings are further supported by studies on natural and synthetic ultramarine pigments of various shades via solid-state resonance-Raman spectroscopy, colorimentry, and solid-state ²⁹Si and ²⁷Al NMR spectroscopy. NMR parameters are shown to correlate well with the intensities of Raman signals corresponding to the S₃^-. chromophores. A further correlation is established between the colorimetric parameters, L* (lightness) and C* (chroma), and the paramagnetic shift and paramagnetic linebroadening in NMR spectra for both ²⁷Al and ²⁹Si.