Use of rice husk ash for quartz substitution in stoneware glazes

455-460Two types of rice husk ashes (RHAs) with different silica contents were used for quartz substitution in a standard opaque glaze and a crystalline glaze. One RHA (RHA1) was a waste from a brick firing process and was used as received. RHA1 (particle size, 36.55±0.26 μm) contained only 71.41% SiO₂. Other RHA (RHA2) was prepared in laboratory under a well controlled process to obtain similar silica content (95.95%) and average particle size (15.83 ±0.18 μm) to quartz. Coefficients of expansion (COE) values of opaque glazes were: RHA1 glaze >RHA2 glaze >quartz glaze. RHA1 glaze, which exhibited highest COE value, crazed with finest craze pattern. All glazes melted at approx. the same temperature and exhibited similar phase changes upon firing. When RHAs were used to replace quartz in a crystalline glaze recipe, RHA1 glaze exhibited a high number of crystals with finer size, while crystals found in the quartz were larger in size with less density