Interaction between lead-free multicomponent alkali borosilicate glass frits and hematite in red overglaze enamels

Japanese traditional red overglaze enamel decorations on porcelain known as akae are produced using lead-containing frits, leading to a problem of lead release. Although many lead-free frits have been developed, a well-colored akae has not yet been realized, mainly because of no information on the reaction between lead-free frits and hematite during the heating process in akae production. To reveal the effect of such interaction between the frits and hematite in akae on the color tone, various akae test pieces were prepared and investigated. On using low-melting-point lead-free alkali borosilicate glass system, which contain zinc oxide, the formation of zinc ferrite proceeds simultaneously with the dissolution and particle growth of hematite in the frit melt. The particle growth rate due to dissolution and precipitation in the frit melt is faster than that due to sintering of particles caused by heat treatment of hematite powder in dry process. The dissolving capacity and promoting effect of particle growth depended on frit compositions. We also showed that the composition of akae glass layer varies because of ions diffusing from the glaze layer. Compositional changes in the akae glass could strongly affect the reactivity between the frits and hematite, resulting in variation in the color tone, the crystal phases formed, and their segregated states.