Fired microstructures of standard porcelain stoneware tile and tile made from mixes containing waste glass as part of the flux system were studied by XRD, SEM, and TEM. The standard porcelain stoneware microstructure consists of 100-1000 mum long mullite needles, feldspar relics, and partially dissolved a-quartz embedded in a glassy matrix. The use of soda-lime-silica (SLS) glass in the flux system led to crystallization of plagioclase, wollastonite, and sodium silicates. CaO-rich areas adjacent to quartz particles, as a result of interactions between SLS glass and silica from the quartz, and eutectic morphologies, revealed that SLS glass accelerated liquid formation and thus sintering and densification. Formation of these additional phases led to lower levels of quartz, mullite, and Na-feldspar in the microstructure although lower firing temperatures could be used to achieve full density due to generation of more fluid liquid. Use of PbO-containing waste glasses had little effect on the microstructure compared with standard composition while use of mixed PbO-containing and SLS glasses led to microstructures containing plagioclase but to lower extent than in tile with higher levels of SLS.