Effect of Quebracho Bark on the Properties of Opaque Glazes

In this work, the effect of quebracho bark (QB) on the physical and chemical properties of opaque glaze/glazes (OG) was investigated.  Various ratios (0.25-10 weight percent) of quebracho bark were incorporated into the glazes to obtain experimental glaze recipes and melting temperatures of the glaze compositions were calculated through the fluxing factors of the oxides present in the glazes. Thereafter, the glaze compositions were fired at the calculated temperature value of 1100°C. Viscosity, thixotropy and pH tests were conducted on OG and OG+QB glaze suspensions. Standard coloring analyses, crazing resistance, impact resistance, resistance to acid and alkalis, water absorption, firing strength, and surface hardness tests were conducted. After establishing the Seger formulas, surface tensions of the glazes were calculated by making use of the molar surface tension factors of the oxides already present in the glaze compositions at the same sintering temperature of 1100°C. QB additions decreased the viscosity and standard thixotropy values of the OG and had no significant effect on the pH of the OG slurry. On the other hand, QB additions caused an increase in the whiteness (opacity) of the OG. No crazing and impact fractures were observed on the glaze surfaces. Furthermore, the glaze surfaces showed good resistance to acid and alkalis. On QB incorporation, water absorption decreased in all the experimental glazes. Firing strength and surface hardness increased with increasing QB content. According to the experimental results, 5 weight percent (wt. %) QB addition was found to be the optimum amount.