Pink ceramic pigments based on chromium doped M(Al2-xCrx)O-4, M = Mg, Zn, normal spinel

In this paper M(Al2−xCrx)O4, MMg, Zn and x from 0.05 to 2 compositions, have been prepared by solid state reaction in order to analyse their reactivity, structure, limit of solid solution, stability and pigmenting properties in conventional ceramic glazes. Solid solutions are obtained at 1200 °C in the Zn case (gahnite) and at 1400 °C in the Mg case (magnesium spinel). In both cases powders become pink coloured up to x=0.4 and then progressively greenish. Likewise powders glazed in a conventional ceramic glaze matrix of the CaOZnOSiO2 system for double firing stoneware (1050 °C) produce glazes firstly orange coloured up to x=0.4, pink up to x=1.2 and finally green at higher x. The colour evolution is associated to the entrance at low doping levels of relatively big Cr3+ (0.755 Å) substituting Al3+ (0.675 Å) in VI coordinated sites of both spinel lattices; this produces an enhancement of the crystal field strength under the VI coordinated Cr3+ that shifts the light absorbances to higher energy and an orange-pink colour can be observed. But when the entrance of Cr3+ progresses, the crystal field strength under the ion relaxes and absorption bands shift to higher wavelength when x increases, producing green colours. The UV–vis–NIR spectra of low x pink M(Al2−xCrx)O4 compositions are similar to those of ruby (Cr0.1Al1.9)O3, indicating analogous mechanism of colour