Feldspar and firing cycle effects on the evolution of sanitary-ware vitreous body Review Article

This work is focused on the study of macroscopic and microscopic properties of traditional sanitary-ware vitreous bodies as a function of feldspar flux and firing time-temperature profile, using a fixed slip formulation (50 wt.% clay, 25 wt.% quartz and 25 wt.% feldspar). Two flux particle sizes (45 and 75 μm), three flux compositions (Na-based feldspar, K-based feldspar and a mix of them) and three firing cycles with the same soaking temperature (i.e. 1240 °C) have been combined to evaluate their effects on the relevant industrial properties of water absorption and thermal expansion. The micro-scale observables, phase composition and micro-morphology, have also been investigated. Despite a general similarity exhibited by the ceramic samples, qualitative and quantitative differences in terms of feldspar dissociation temperature, phase-composition and densification trends have been observed. In particular, for a fixed firing cycle, the combination of the sodium based feldspar with the smallest flux particle size leads systematically to a water absorption value that is below the 0.5 target value and to a glass amount that approaches 70 wt.%. Thermal expansion coefficients below the quartz α–β transition are found in the 6.2–6.9×10−6 °C−1 range; the highest values seem to be favoured by incorporation of potassium based.