Structure and colour properties in the Egyptian Blue Family, M1−xM′xCuSi4O10, as a function of M, M′ where M, M′ = Ca, Sr and Ba

Solid solutions in the Egyptian Blue Family, M1−xM′xCuSi4O10, where M, M′ are the alkali-earth cations Ca, Sr and Ba, have been successfully synthesised from stoichiometric mixtures of silica, alkali-earth metal carbonates and copper carbonate at high temperature using solid state reactions. The structure of the blue products has been determined by Rietveld analysis using high quality powder X-ray diffraction data. As the average size of the cations on the alkali-earth metal site increases, the cell parameters also increase, although the variation occurs in an anisotropic manner due to the flexibility of the structure. UV–vis spectra and colour coordinate data were collected on the materials to correlate the changes in the structure with the observed colour properties. A concomitant reduction in lightness indicating potential weakening of the tinting capability is observed as the cell parameters lengthen. Prolonged exposure to natural light has a deleterious effect on the colour of these synthetic materials in a similar way to the natural minerals.