Oxide melts demonstrate a deviation from Arrhenius-type viscosity behaviour in which the activation energy of viscosity changes from high value at low temperature to low value at high temperature. This change in activation energy with temperature can be used to classify network liquids as strong or fragile depending on the form of dependence of logarithm of viscosity vs reduced temperature. Activation energy changes only slightly with temperature for strong glass forming liquids which demonstrate nearly Arrhenius temperature dependence of viscosity. In contrast, activation energy of fragile glass forming liquids changes significantly with temperature & their viscosity deviates significantly from Arrhenius behaviour. Doremus suggested a criterion for oxide melt fragility utilising ratio of activation energy at high viscosity to that at low viscosity.
