Formation mechanism of the “Green Above, Brown Below” phenomenon in Yaozhou Kiln Celadon

Yaozhou Kiln is a famous ancient center for celadon production in China, located in present-day Shaanxi Province. While analyzing its olive-green celadon produced during the Song Dynasty, a common occurrence of brownish base (foot and bottom) was observed. This phenomenon can also be found in porcelain produced at other kilns in China and Vietnam. However, previous research has not systematically explored the coloration mechanism behind it. Through different analytical methods, coupled with reproduction firing experiments, this paper concludes that the brownish base is attributed to the diffusion of iron from the body and sand cushion into the thinly applied glaze on the base, as well as the crystallization formed by the combination of the sand cushion and the surface glaze. Factors influencing the depth of the brownish color include: (1) the iron content of the body; (2) the thickness of the base glaze; and (3) the sand cushion material.