Viscosity of alumina doped soda lime silicate glasses – observation of anomaly in the linear increase as Al2O3 replaces SiO2

•An anomaly in the viscosity trend as Al2O3 replaces SiO2 is observed.•The anomaly is suggested to be caused by Ca2+ inhibiting viscous flow.•Angell-Rao and Waterton-MYEGA give a good fit to the viscosity data.•The thermodynamic data shows linear increase with the Al2O3 content.•The Doremus fragility index show a deviation as a function of the Al2O3 content. Adding alumina to the conventional soda lime silicate glass composition improves many properties, however, also increases the melting temperature. In the current paper, alumina doping of soda lime silicate glasses and its implications to high temperature viscosity are investigated in order to verify the linearity when replacing SiO2 for Al2O3. An anomaly in the linearly increasing viscosity was found when the ratio CaO/Al2O3 is