Solid-state NMR studies of ultramarine pigments discloration

Ultramarines are a family of pigments widely used as colorants in artists' paints, coatings, plastics, cosmetics, and various industrial materials. They are aluminosilicates characterized by a sodalite cage framework which enclathrate paramagnetic (S₃, S₂) and diamagnetic (S ₄ or S₃Cl) chromophores responsible for the color of these pigments. Solid-state ²⁷Al MAS NMR studies showed that the fading mechanisms in ultramarine pigments, both in acidic and alkaline environments, is initiated via de-alumination leading to framework destruction which in turn triggers the release of the chromophores. This results in color loss and in the emergence of extraframework aluminum. ²⁹Si MAS studies provide new insight into these processes and suggest that acidic attack leads to the formation of Q¹-Q³ silicates and possibly to the emergence of Si(3Al), Si(2Al), Si(1Al) and Si(0Al) fragments indicative of the formation of secondary pores through which the guest chomophores can leave the cage. These findings are important for the design of proper conservation treatments and preservation procedures for artwork containing ultramarine pigments.