Ultramarine Pigments from Fly Ash: Synthesis, Microscopy, and Modelling

Ultramarine pigments have sulphur-based chromophores in an aluminosilicate framework. This book describes the synthesis of ultramarine blue from fly ash (a predominantly aluminosilicate waste product of coal combustion). Microscopy shows that sulphur has a structure-directing effect and facilitates the formation of the zeolite-like structures of ultramarine pigments. The identities of the chromophores are a source of debate and this work contributes to the debate, by modelling several candidate species for the yellow, blue and red species in ultramarine pigments. For sulphur chains with two and three sulphur atoms, the singly charged species are the most stable, supporting the hypothesis that the exothermic transition from green to blue ultramarine is the transformation of the doubly charged species to the singly charged species. [A specotroscopic study leads to the counter-conclusion that the yellow to blue transition is the disulphide to the trisulphide transition.] For the single charged chain with three sulphur atoms the open chain is more stable than the closed triangle. The best candidate for the red ultramarine chromophore is the cis isomer of four sulphur atoms in a chain.