Cobalt minimisation in willemite (CoxZn2-xSiO4) ceramic pigments

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).