Unveiling the science behind the tea bowls from the Jizhou kiln. Part I. Chemical compositions and the two-layer glazing technique

Abstract

The Jizhou kiln, a folk kiln of ancient China, is well-known for the production of black-glazed tea bowls. Polished cross sections showed a white opaque glazed surface covering a black lower layer and a light brown opaque layer existed between them. Analysis by X-ray fluorescence yielded two different calcia-alkali glaze compositions-a black ground glaze and a white cover glaze. Using multi-variable factor-loading analysis, the two-layer glazes were compared to those on white, greenish-white and white-rim black porcelains, in order to understand the variability. In addition, compositions of bodies were compared to the glaze compositions to determine if the same porcelain stone was used along with ash to form the glaze. Based on the physicochemical foundation of the glazes, the opacification of the white cover glazes was due to the phase separation between SiO2 and CaO. It is the combined contribution of melting, bubbling, flowing and diffusion between the cover and ground glazes, as well as the development of phase separation during the firing process, that bring about the “Yao Bian” tea bowls.