Cobalt minimisation in willemite (CoxZn2−xSiO4) ceramic pigments

Abstract

Industrially available blue ceramic pigments, with the exception of vanadium–zircon blue, (Zr,V)SiO₄, contain Co. To minimise the use of toxic and scarce cobalt, while also maintaining an intense blue colour, the performance as blue ceramic pigments of both Co²⁺–Zn₂SiO₄ (willemite) and Zn²⁺–Co₂SiO₄ (olivine) solid solutions was studied. The addition of cobalt in the willemite lattice was minimised. A wide range of compositions, Co_xZn_2−xSiO₄ (x = 2, 1.95, 1.5, 1, 0.9, 0.5, 0.05 and 0), were prepared and characterised by X-ray diffraction, lattice parameter measurements, UV–VIS–NIR spectroscopy, CIE-L*a*b* colour measurements and electronic microscopy (SEM/EDAX). The results show that an intense and more blue ceramic pigment is obtained based on the willemite lattice, with an optimised, low Co content, ranging from 25 mol% Co (Co_0.5Zn_1.5SiO₄) to 2.5 mol% Co (Co_0.05Zn_1.95SiO₄). This last composition (with only 2.5 mol% Co) exhibits the best blue hue once enamelled (b* = −39), though it is also considerably lighter (L* = 57).