Colors on Bizen Stoneware Fired in a Firewood Kiln Reaction between the Clay and Potassium

R. Maki, R. Oketani, S. Naka, A. Shiroma, M. Fukuhara, Y. Kusano

Reddish, purplish red, brownish, and lustrous gold colors can appear on the surfaces of Bizen stoneware after firing in a firewood kiln. The reddish color results from the formation of an Fe-substituted spinel [(Mg,Al,Fe)3O4] and from composite particles consisting of corundum (α-Al2O3) and hematite (α-Fe2O3). The brownish and lustrous gold colors were found to be caused by the generation of dendritic luogufengite (ɛ-Fe2O3) and 100-nm-thick hematite particles, respectively. These oxides are formed by reactions between the Bizen clay and K
supplied by the firewood used as fuel. The present study reproduced these colors by heating Bizen clay pellets with K2CO3 instead of firewood in an electric furnace. Specifically, a reddish sample was obtained by heating at 1230 ◦ C for 5 h in air. A brownish color appeared after annealing pellets at 1200 ◦C for 2 h in air after heating at 1230 ◦C with K2CO3 under an Ar:CO (90:10, v:v) mixture for 5 h. A lustrous gold color was produced by annealing at 900 ◦C for 2 h in air after heating in the same Ar:CO mixture. The color and chemical composition of the stoneware surfaces were found to be largely determined by the temperature at which the specimen was heated in air.

Journal of the American Ceramic Society

https://doi.org/10.1111/jace.70219