Compositional and chromatic properties of strontium hexaferrite as pigment for ceramic bodies and alternative synthesis from wiredrawing sludge.

This paper reports a new black pigment based on strontium M-type hexaferrite (SrM) obtained from pure reagents or by the inclusion of Fe-rich wiredrawing sludge. The fired powders show a clear predominance of SrFe12O19. Also, haematite (?-Fe2O3) and iron spinel were detected in variable amounts. These two phases are present in higher amounts (57 wt.%) in the sludge-based pigment, due to the interference of contaminants. The optical measurements illustrated that the prepared pigments displayed good colorimetric properties, close to those shown by a black commercial pigment. However, the addition of Fe-rich sludge slightly reduced this black colour by increasing the L* and b* colour coordinates. Since the variance is slight, further trials were carried out by minor addition of chromophores (Co, Mn and Ni). The chromatic measurements showed that the best pigment was obtained by adding cobalt. Nevertheless, in ceramic porcelainized bodies, the combination of Co and Mn assured the nearest hue to the commercial pigment. Nickel was not effective in the colouration because it was mostly assimilated by the spinel (trevorite) phase and was not inserted in the hexaferrite phase. Other trials with non-stoichiometric formulations showed a bleaching of the black colouration in stoneware supports, especially on sludge-containing formulations, along with an apparent increasing of the green hue.