Microstructure and formation conditions of the reddish hi-iro marking on traditional Japanese ceramics

The microstructure and conditions for the formation of the reddish color referred to as hi-iro on Japanese ceramics were investigated through model experiments. The hi-iro marking was found to be formed by the reaction between clay and alkali ions supplied from the glaze. When a mixture of porcelain clay and 20 wt%KCl was heated in air at 1250°C and then cooled slowly to 800°C, hi-iro clearly appeared on the sample surface. In hi-iro, corundum [α-(Al,Fe)₂O₃], hematite (α-Fe_2O3) and a liquid phase are formed. Upon heating until 1250°C, corundum is firstly precipitated as hexagonal plate-like crystals. During the cooling process, hematite precipitates on the edges of the corundum crystals to form specific composite particles. When rapidly cooled from 1250°C to room temperature, a brownish red color appeared on the sample surface, which was found to be caused by the formation of approximately 50μm Al-substituted hematite (Fe1.9Al0.1O3) particles. The relationship between the microstructure of the phases formed and the color is discussed.