Colors on Bizen stoneware fired in a firewood kiln: Reaction between the clay and potassium

Abstract 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) 3 O 4 ] and from composite particles consisting of corundum (α‐Al 2 O 3 ) and hematite (α‐Fe 2 O 3 ). The brownish and lustrous gold colors were found to be caused by the generation of dendritic luogufengite ( ɛ ‐Fe 2 O 3 ) 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 K 2 CO 3 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 K 2 CO 3 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.