Density and microstructural evolution of porcelains containing 0-25 wt% soda-lime-silica (SLS) waste glass fired over a range of temperatures from 600° to 1400° C have been investigated. After firing 3 h at 1100° C, batches containing 6.25 wt% SLS glass and 18.75 wt% nepheline syenite flux system attained open-pore closure and a bulk density of 2.40 g/cm(3), comparable to results from commercial porcelain after firing at 1200° C. SLS glass softens and melts, conferring early densification and overfiring on porcelains fired at normal commercial firing temperatures. The microstructural evolution examined using XRD, SEM/energy dispersive spectroscopy (EDS), and TEM/EDS revealed formation of a variable composition plagioclase, rounded wollastonite particles, sodium silicates, and tridymite in batches containing SLS glass, in addition to primary and secondary mullites, partially dissolved quartz, and a glassy matrix as found in the waste-free batch. Ca2+ and Na+ from the SLS glass migrate to regions containing the products of clay decomposition to form plagioclase, limiting the extent of mullite crystallization. The presence of a solution rim surrounding quartz and different glass compositions around wollastonite crystals indicate that the system is not in equilibrium, although phases predicted by the Na2O-CaO-SiO2 equilibrium diagram were formed.