Influence of crystallinity on chromatic parameters of enamels coloured with malayaite pink pigments

Chromium-doped malayaite ceramic pigments, CaSnSiO5:Cr were obtained by solid state reaction using CaCO3, SnO2, SiO2 and PbCrO4 as precursors. The synthesis process was studied by means of thermal analysis and mass spectroscopy, the results evidencing the volatilization of carbon dioxide and lead in a large temperature range. The major aim of the study was to evaluate the dependence of the pigment colour and crystallinity from the soaking time at a maximum synthesis temperature fixed at 1400 °C. X-ray diffraction data indicate a nearly linear tendency of crystallinity increase with increasing soaking time. The colour of glazes prepared with the pigments, which was monitored by measuring their chromatic coordinates, presents a clear tendency of improvement with increasing crystallinity. This was interpreted in terms of decreasing solubility of the pigments as their crystallite size is increased. SEM and EDX studies confirm that hypothesis, evidencing very small dissolution of the highly crystallized pigments in the glaze.