Discoloration of fired kaolinitic clays (study of Fe3+ coordination by Mössbauer and UV-ViS-NIR Spectroscopy)

Autores: Hanzlicek, T.|Niznansky, D.|Dedecek, J.|Steinerova, M.|Straka, P.|Triskova, J.
Fuente: J. Am. Ceram. Soc.
90 (9), 2843-2848
2007

The final color of a ceramic body is generally determined by the contents of Fe3+ ions. The Central European market accepts kaolinitic clays for the production of tableware, electro-insulators, and wall tiles only if the Fe2O3 in chemical analyses of the kaolin does not exceed 1.0 wt%. The chemical analyses calculate the content of all iron components as if they were in the form of Fe2O3 and then their quantity excludes even good-quality clay from the ceramic industry of white bodies. We found that oxidizing firing of the samples fired at over 1180°C changes the color in cases where the initial Fe3+ component is in the form of hydro ferric oxide [FeO(OH)] or if the ferric ions were added to the white kaolin samples in the form of ferric nitrate [Fe(NO3)3·9H2O]. The Mössbauer spectroscopy confirms the presence of only diluted Fe3+ ions. The UV-ViS-NIR spectroscopy confirms that even if the concentration of Fe2O3 is above 3.0 wt%, these ions of Fe3+ in the case of their initial hydro ferric oxide formed in clay are incorporated into the mullitic structure in tetrahedral coordination. The iron-coloring effect depends on the coordination of Fe3+ ions with the studied discoloring effect of fired bodies?the very small and well-distributed particles enter into the formatted mullitic structure.