Stretched exponential relaxation of glasses: Origin of the mixed-alkali effect

As nonequilibrium materials, lasses continuously relax toward the supercooled liquid metastable equilibrium state.(1.2) Although the myth of flowing glasses in cathedrals suggests otherwise,(3) the dramatic increase of glass viscosity with decreasing temperature renders relaxation effectively "frozen" at ambient temperature. However, specific glass compositions can surprisingly deform over time, even at low temperature. This phenomenon is known as the thermometer effect(1,4) and is usually attributed to the mixed-alkali effect, which occurs in oxide glasses comprising at least two alkali oxides, AO(2) and BO2, and manifests as a nonlinear evolution of properties with respect to the fraction A/(A + B).