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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.
Research papers (academic journals)
