Interaction of Co–ZnO pigments with ceramic frits: A combined study by XRD, FTIR and UV–visible

The development of blue colour when mixed oxides of the system CoxZn1−xO are incorporated, to industrial frits is reported. The Co–ZnO pigments were prepared by solid state reaction. A complete series of Co(II) substituting Zn(II) in wurtzite lattice was studied, from the structural point of view, by XRD and FTIR. In order to test these materials as ceramic pigments, 5 and 10 wt% compositions of them were enamelled with three different transparent glazes, representative of single and double firing industrial procedures, and then submitted to thermal treatment between 750 and 1000 °C. The frits, of known composition, were fully characterized by FTIR spectra, prior to the mixing with pigments. The change of colour in the glazes was interpreted by analyzing the dependence of the UV–visible electronic spectra (reflectance) of the samples on the heating temperatures. The phases developed in each stage of the thermal treatment as a result of pigment–frit interaction were controlled by XRD data, complemented by vibrational FTIR analysis. The blue colour obtained, once the glazes were formed is attributed mainly to the formation of crystalline Zn2SiO4 with Co(II) ions in tetrahedral coordination which remain dissolved, over 1000 °C in the glassy matrix. The colorimetric parameters measured are in good agreement with the structural and spectroscopic analysis. Their values are within the range of typical silicate and aluminate Co(II) pigments. This study demonstrates that ZnO of the pigment is capable of forming its own Zn-based crystals in conventional ceramic frits giving rise to deep blue cobalt glazes.