Red-brown ceramic pigments based on chromium doped ferrian armalcolite, effect of mineralizers

A new red-brown ceramic pigment based on chromium-doped ferrian armalcolite have been synthesized and characterized. (MgFe)(CrxTi3−xFe)O10 powders (x=0-0.3) fired at 1200 °C crystallize ferrian armalcolite as the only crystalline phase detected. Samples fired at 1000 °C show red-brown shades in glazes that darken and bluish (b* turns to negative values) at 1200 °C. The x=0.2 sample fired at 1000 °C shows the best red colour (L*a*b*=49.5/15.2/10.3). Assignment of bands in the UV-Vis-NIR spectra is difficult due to the overlapping of Cr3+, Cr4+ and Fe3+ absorptions in octahedral coordination. Analysis of UV-Vis-NIR spectra of powders shows that these spectra are dominated by the strong absorption associated to Fe3+ ions in octahedral sites. In contrast, an intense band at 520 nm dominates the UV-Vis-NIR spectra of glazed samples, which should be associated to Cr4+ in octahedral coordination. This absorption increases when the amount of chromium increases, indicating that chromium is the real chromophore of the system. Finally, the weak shoulder at 600 nm and the double weak band at 700 nm, detected more evidently when chromium amount in sample increases, indicate the progressive presence of Cr3+ in octahedral sites. The entrance of Cr4+ in x=0.1 sample shrinks the crystalline cell, but when chromium amount in the samples increases, both Cr4+ and Cr3+enter simultaneously and the unit cell remains practically stable. The microstructure of the powders analysed by SEM microscopy indicates aggregates of 6-10 fine particles of 200-400 nm of diameter. The addition of mineralizers (boric acid, sodium perborate, NaF and a mixture BaF2.4MgF2) does not modify significantly the reactivity of the system; at 1000 °C hematite and rutile remain as residual crystalline phases, except in NaF additions where the crystallization of NaFeTi3O8 is detected. SEM-EDX mapping analyses of pigment powders confirm in all cases a homogeneous distribution of ions in the particles.