A simple procedure for the preparation of Cr-doped tin sphene pigments in the absence of fluxes

A procedure for the preparation of Cr-doped tin sphene pink pigments based on the pyrolysis at 600°C of liquid aerosols generated from solutions of silicon tetraethoxide, calcium chloride, tin(IV) chloride and chromium(III) nitrate in a mixed solvent (ethanol/water) is reported. The as produced powders were amorphous and consisted of spherical particles of broad size distribution (<2 μm). On calcination at 1200–1300°C, these powders transformed into crystalline tin sphene (CaSnSiO5) irregular particles (<11 μm) developing the pink colour characteristic of the commercial pigments with similar composition. The main advantage of this method is that it does not require the addition of the fluxes used in the traditional ceramic procedure, which is highly desirable from the environmental point of view. Additional advantages are its simplicity (no grinding required) and continuous character. The effects of the chromium content on the colour properties of the pigments were also evaluated, finding that the optimal pigment (best colour with minimum Cr content) was obtained for a Cr/malayaite mole ratio of 4.4%.