Landman, A: Ultramarine Pigments from Fly Ash

Ultramarine pigments have sulphur-based chromophoresin an aluminosilicate framework. This book describesthe synthesis of ultramarine blue from fly ash (apredominantly aluminosilicate waste product of coalcombustion). Microscopy shows that sulphur has astructure-directing effect and facilitates theformation of the zeolite-like structures ofultramarine pigments. The identities of thechromophores are a source of debate and this workcontributes to the debate, by modelling severalcandidate species for the yellow, blue and redspecies in ultramarine pigments. For sulphur chainswith two and three sulphur atoms, the singly chargedspecies are the most stable, supporting thehypothesis that the exothermic transition from greento blue ultramarine is the transformation of thedoubly charged species to the singly charged species.[A specotroscopic study leads to thecounter-conclusion that the yellow to blue transitionis the disulphide to the trisulphide transition.] Forthe single charged chain with three sulphur atoms theopen chain is more stable than the closed triangle.The best candidate for the red ultramarinechromophore is the cis isomer of four sulphur atomsin a chain.