Colour of Ca(CoxMg1-x)Si2O6 pyroxenes and their technological behaviour as ceramic colorants

Cobalt-based pigments were synthesized using clinopyroxene as crystalline host. The pigments are a Ca(CoxMg1-x)Si2O6 solid solution, with x = 0.05, 0.2, 0.4, 0.5, 0.6, 0.8 Co apfu, and were synthesized from melt. The pigments were characterized by single-crystal X-ray diffraction, SEM-EDS, optical microscopy, colorimetry and optical spectroscopy. The pigments were then tested in glazes and glassy coatings for ceramic tiles to determine their colouring performance. The synthesized pigments showed a pink colour with different shades, due to the absorption of Co2+ in octahedral coordination. The colour disappeared in the Ca(Co0.05Mg0.95)Si2O6 pigment after grinding. The Ca(CoxMg1-x)Si2O6 pigments partially dissolved in glazes, acting like dyes, and bestowed a blue shade on coatings, with a colour efficiency similar to industrially used blue colorants (cobalt aluminate spinet and cobalt olivine). In the glassy coatings, pigment dissolution occurred at a lower extent, and a colour intermediate between the pyroxenes and the pigment-bearing glazes was obtained. The Co content in pyroxene dyes is lower than conventional pigments, being down to 5%wt.