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Cobalt-based pigments were synthesized using clinopyroxene as crystalline host. The pigments are a Ca(Co x Mg 1- x )Si 2 O 6 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 Co 2+ in octahedral coordination. The colour disappeared in the Ca(Co 0.05 Mg 0.95 )Si 2 O 6 pigment after grinding. The Ca(Co x Mg 1- x )Si 2 O 6 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 spinel 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.

A reappraisal of crystal structure, optical properties and technological behaviour of black spinels in the Co—Cr—Fe—Mn system was carried out to define the best compositions and disclose the role of crystal chemistry in colouring performance and pigment-glaze interactions. Twenty ternary and quaternary spinel formulations were designed using crystallochemical criteria and prepared by simulating the industrial synthesis. Powder samples were characterized through X-ray diffraction (Rietveld), optical, Raman and Mossbauer spectroscopies, and technological testing in several glazes and glassy coatings. Black spinels fall in a relatively narrow field of unit cell and inversion parameters in between “chromites” and “magnetites”. Their cation distribution is governed by Co2+ occupancy of tetrahedrally-coordinated site T and Cr3+ occupancy of octahedrally-coordinated site M, with Mn3+ hosted at M, while Mn2+ and Fe3+ are distributed over both sites. Nevertheless, Raman and Mossbauer spectra indicate a growing disorder in cation partitioning going towards the iron-rich terms. The pigment technological behaviour depends to a large extent on crystal chemistry of spinels, with no effect by grain or crystallite size. The best colouring performance corresponds to recommended compositions able to withstand corrosion and change in crystal chemistry in contact with melted glaze. Three different pathways can give rise to excellent black pigments: (a) strongly disordered spinels having an inversion parameter i∼0.2; (b) moderately disordered spinels, free of Mn3+, with i