The reduction of the firing temperature obtained for wollastonite bodies is explained by a higher amount of glass phase according to CaO-Al2O3-SiO2 diagram, compared to feldspar body. The wollastonite flux improved the viscosity of the glass phase by increasing the amount of calcium oxide in it, which allowed a larger firing range. This is a consequence of a higher level of CaO (and a higher distribution of CaO in the microstructure) which increases the surface tension of glass phase. A higher homogeneity of CaO level in microstructure is a result of two sources of CaO in raw material batch: bone ash and wollastonite.