Interaction between Cr-bearing pigments and transparent glaze: A transmission electron microscopy study

Spinel ZnAl2O4 and corundum Al2O3 doped with chromium are used as pink pigments in porcelain glazes. However, alteration of the color can occur during firing, and the resulting glaze appears brown and green respectively, instead of the expected pink. To investigate the mechanism of color change, electron transparent sections were prepared by Focused Ion Beam (FIB) at the interface between pigment and glaze. Observations under a transmission electron microscope (TEM) reveal a common mechanism for both systems. Al migration from the pigment toward the silicate melt results in alteration of the grain of pigments, and formation of a Cr enriched phase, with the same crystalline structure than the initial pigment. Furthermore, the formation of anorthite is reported, due to the increase of the local concentration of Al in the melt. The different morphologies of altered grains of pigment encountered in the glaze are discussed base on the TEM observations.