Development of a black ceramic pigment from non stoichiometric hydrotalcites

Non stoichiometric layered double hydroxides (LDHs) with the hydrotalcite-like structure and containing Co2+ and Cr3+ cations in the brucite-like layers have been prepared by coprecipitation at room temperature from aqueous solutions of the corresponding chlorides. Five samples have been prepared, with Co/Cr molar ratios ranging from 2/1 to 1/2. The solids have been characterised by element chemical analysis for cations, powder X-ray diffraction, differential thermal analysis, thermogravimetric analysis, temperature-programmed reduction, Fourier transform-infrared and visible–ultraviolet spectroscopies, and particle size and specific surface area determination. The colorimetric parameters have been also measured. Their calcination at high temperature to yield homogeneously mixed metal oxides has been carried out. Both the original and the calcined solids have been dispersed in two enamels and the colorimetric parameters have been determined too. The optimum composition to yield a black pigment (bulk or dispersed in colourless or white enamels) has been determined.