Change in colour of CoO-Cr2O3-Al2O3 system pigments

A wide range of contents was investigated for CoO-Cr2O3-Al2O3 oxide system of commercial purity to determine variations in colour. It was found that ceramic pigments acquired the most stable and clear colour after two stage heat treatment. The first firing of CoO-Cr2O3 mixture was followed by grinding of the cake obtained, mixing with Al2O3 and repeated firing. X-ray diffraction studies showed formation of continuous series of solid solutions of Cr2O3, CoAl2O4 and CoCr2O4 colourings with A2O3 as a free constituent. Various combinations of colouring contents resulted in dark green, blue and blue-green colours.