Abstract The Clusters model, previously developed to describe the viscous sintering kinetics of polydispersed powders undergoing simultaneous surface crystallization, was tested in isothermal sintering experiments of devitrifying spherical glass particle compacts. The experiments were conducted at different temperatures to vary the relative kinetics of sintering and crystallization. The viscosity, glass–vapor surface energy, particle size distribution, average number of neighbors around each particle, particle to sample size ratio, and the effect of pre-existing surface crystals on the sintering kinetics were determined and taken into account in the calculations. The model describes the experimental sintering kinetics for a series of temperatures for two different size distributions and predicts the saturation of densification due to pre-existing surface crystals and surface crystallization. The calculations and experimental results confirmed that higher temperatures benefit sintering over crystallization.