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, CoxZn2−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 (Co0.5Zn1.5SiO4) to 2.5 mol% Co (Co0.05Zn1.95SiO4). 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).