Cobalt Minimisation in Violet Co₃P₂O₈ Pigment

This study considers the limitations of cobalt violet orthophosphate, Co₃P₂O₈, in the ceramic industry due to its large amount of cobalt. Mg_xCo_3-xP₂O₈ (0 ≤ x ≤ 3) solid solutions with the stable Co₃P₂O₈ structure were synthesised via the chemical coprecipitation method. The formation of solid solutions between the isostructural Co₃P₂O₈ and Mg₃P₂O₈ compounds decreased the toxically large amount of cobalt in this inorganic pigment and increased the melting point to a temperature higher than 1200 °C when x ≥ 1.5. Co₃P₂O₈ melted at 1160 °C, and compositions with x ≥ 1.5 were stable between 800 and 1200 °C. The substitution of Co(II) with Mg(II) decreased the toxicity of these materials and decreased their price; hence, the interest of these materials for the ceramic industry is greater. An interesting purple colour with a* = 31.6 and b* = -24.2 was obtained from a powdered Mg_2.5Co_0.5P₂O₈ composition fired at 1200 °C. It considerably reduced the amount of cobalt, thus improving the colour of the Co₃P₂O₈ pigment (a* = 16.2 and b* = -20.1 at 1000 °C). Co₃P₂O₈ is classified as an inorganic pigment (DCMA-8-11-1), and the solid solutions prepared were also inorganic pigments when unglazed. When introducing 3% of the sample (pigment) together with enamel, spreading the mixture on a ceramic support and calcining the whole in an electric oven, a colour change from violet to blue was observed due to the change in the local environment of Co(II), which could be seen in the UVV spectra of the glazed samples with the displacement of the bands towards higher wavelengths and with the appearance of a new band assigned to tetrahedral Co(II). This blue colour was also obtained with Co₂SiO₄, MgCoSiO₄ or Co₃P₂O₈ pigments containing a greater amount of cobalt.