Influence of alumina characteristics on glaze properties

Aluminium oxide is a synthetic raw material manufactured from bauxite by the Bayer process, whose Al2O3 content typically exceeds 99%. Four main types of alumina can be defined, depending on the processing used: hydrargillite Al(OH)3, boehmite AlOOH, transition alumina (calcined at low temperatures, 1000 degree centigrade, with an intermediary crystallographic structure between hydrates and alpha alumina), and a-Al2O3 (calcined at high temperatures, >1100 degree centigrade). In glaze manufacturing, α-Al2O3 is the main type of alumina used. This raw material acts as a matting agent: the matt effect depends on alumina particle size and content in the glaze. This study examines the effect of the degree of alumina calcination on glaze technical and aesthetic properties. For this purpose, alumina with different degrees of calcination were added to a glaze formulated with a transparent frit and kaolin, in order to simplify the system to be studied. The results show that, depending on the degree of calcination, alumina particles can react with the glaze components (SiO2, CaO, and ZnO) to form new crystalline phases (anorthite and gahnite). Both crystallisation extract CaO and ZnO from the glassy phase, increasing glassy phase viscosity. The variation in crystalline phases and glassy phase viscosity yields glazes with different technical and aesthetic properties. (Author 17 refs.