The role of counterions (Mo, Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments Part 2. Colour and technological properties

Industrial rutile pigments are manufactured using several chromophores: Cr (giving an orange hue), Mn (tan), Ni (yellow) and V (gray); a second element, the so-called counterion (i.e. Mo, Sb, Nb or W) is always added in order to achieve the desired coloration and/or improve the technological properties (e.g. chemico-physical resistance in ceramic bodies and glazes). The colour of these pigments is determined by both metal–ligand charge transfer (Ti4+<->O2-) and crystal field effects (transition metals substituting Ti4+ in octahedral coordination). Though the absorbance bands are broad and frequently overlapped, the UV–vis–NIR spectra suggest the occurrence of Cr3+, Mn2+, Mn3+, Ni2+, V3+, and V4+ as chromophores. Rutile pigments are suitable for through-body (up to 1250 °C) and glaze applications (up to 1100 °C). The best coloration of porcelain stoneware bodies is achieved with Sb orWas counterions, though the higher stability is ensured by Sb, but in the Ti– Ni–W system. The best glaze colours are accomplished by W-bearing pigments, which however are less stable than Nb- or Sb-containing ones, except than for the V + W coupling. This latter represents a new and very interesting Co-free and Cr-free black pigment for low temperature applications.