The effect of Al2O3/TiO2 ratio on viscosities of CaO–MgO–Al2O3–SiO2–TiO2 melts was investigated by the rotating cylinder method in this study. In addition, structural characterizations of these quenched vitreous samples were also studied by Raman spectroscopy. It was indicated from the experimental results that viscosity increases as gradually increasing Al2O3/TiO2 ratio while keeping the contents of other components constant. The Raman spectra analyses indicated that TiO2 mainly exists in the form of [TiO4] as a network former in composition range of 3–17 mol%. With increasing the Al2O3/TiO2 ratio, the [TiO4] content decreases and the degree of polymerization of the melt increases resulted from the increase of Al2O3 which behaviors as an acidic oxide and incorporates into the SiO2 network with the charge balance of CaO. Consequently, there will be an increase of viscosity with increasing Al2O3/TiO2 ratio.
