Besides Raman and infrared spectroscopy, DSC/DTA/TGA, dilatometry, XRD, and SEM techniques were used to characterize phase transitions and microstructural evolution in porcelain stoneware. An experimental porcelain composition has been formed by slip casting and sintered in an electric furnace. Thermal evolution of raw materials shows dehydroxylation of kaolinite, transformation to spinel and mullite, and transformation of α-quartz into β-quartz evidenced by Raman spectroscopy. Spinel and granular (primary) mullite is seen to form in clay relicts which may subsequently become seeds for topotactic grown of acicular reinforcing mullite. As vitrification progresses the bridging silicon oxygen varies and can lead to connectivity changes in the silica network. The development of glassy matrix was followed by infrared spectroscopy at the porcelain intermediate sintering stage. Si-O-Si bands at 780 and 1070 cm−1 for symmetric and asymmetric stretching vibrations in the spectra have been analyzed using a polymerization index (PI). PI indicates the degree of depolymerization of the vitreous matrix through densification. PI was found to correlate well with thermal expansion, bulk density and Young's modulus. This correlation of glass structure and properties may suggest that changes in the IR spectra could be used to predict physical properties.

Keywords: Microstructure, Infrared Spectroscopy, Raman, Sanitaryware, Glass ceramics, Polymerization Index.