Influence of engobe and glaze layers on the evolution of porosity and permeability of single-fired porcelain tiles

Despite the well-known effects of mismatching between degassing of the porcelain tile body and melting of the glaze on the surface quality of single-fired porcelain tiles, there is still a knowledge gap regarding how the permeable microstructure of the finished product is affected by the tile body-engobe-glaze layers. In this study, industrial formulations with a single tile body, tile body with engobe, or tile body with engobe and glazes of varying gresification performances were produced. The linear shrinkage, bulk density, porosity, water absorption and gas permeability of samples single-fired up to 1200 °C were experimentally assessed. Results demonstrate unequivocally that higher porosity and gas permeability levels are induced in supports containing covering layers compared with single tile bodies. The overpressure caused by the hampered degassing counterbalanced the gresification process, promoting the formation of a fraction of larger and less tortuous interconnected pores of 0.8 to 3.0 μm in the tile body layer.