In this study, an α-Fe2O3 (hematite) coated porcelain plate was sintered in a temperature range from 1100 oC to 1250 oC using ferrous sulfate. The specimens were investigated by X-ray diffractometer (XRD), scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and UV-visible spectrophotometer. It was confirmed that α-Fe2O3 (hematite) was densely fused to the surface at several tens of μm, the α-Fe2O3 (hematite) was in the form of thin platelet and polyhedra, and no other compounds appeared in the sintering process. In the specimen coated with α-Fe2O3 (hematite), the reflectance spectra show a red absorption band of 560-650 nm. The L* value decreased from 53.18 to 46.94 with the firing temperature. The values of a* and b* were at 19.03 and 15.25 at 1100 oC and gradually decreased with increasing temperature; these values decreased rapidly at 1250 oC to 11.54 and 7.98, respectively. It is considered that the new phases are formed by the phase transition of the porcelain plate (clay), and thus the a* and b* values are greatly influenced.

In this study, a cobalt sulfate ceramic coating was sintered on various clays at 1250˚C. The specimen characteristics were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), TG-DTA, UV-vis spectrophotometer, and HRDPM. The ceramic coating had a constant thickness of thousands μm, and the surface was confirmed to be densely fused. Other new compounds were produced by the cobalt sulfate sintering process and reactions. These compounds were a CoAl2O4 phase, Co2SiO4 phase, anorthite(CaAl2Si2O8) phase, and FeAl2O4 phase, respectively. UV properties of the coated specimen were investigated, celadon clay specimen in 530-550 nm band is showing a dark gray color. The white clay and white mix clay specimen in 460-500 nm band is showing a blue color. The cobalt-aluminate(CoAl2O4) spinel and the cobalt-silicate olivine(Co2SiO4) were the strongest of the ceramic pigments, producing a very pure, navy blue color.