Viscosity of CaO–SiO2–R2O system at elevated temperature has been systematically evaluated with a rotating-crucible viscometer over the wide range of temperature, which includes the region of solid–liquid coexistence, so called “multi-phase”. It was found that the viscosities of CaO–SiO2–R2O melts increased with temperature decrease, and were described with Arrehnius type temperature dependence between 20–100°C range of temperature below the liquidus. The rheological characterization of CaO–SiO2–R2O melts had a transition from Newtonian to non-Newtonian fluid at a given temperature, which was classified as Bingham fluid according to the relationship between the shear rate and the shear stress calculated based on the experimental condition and the viscosity data. It was unveiled that the crystallization behavior controlled the changes in the Bingham yield stress τB of CaO–SiO2–K2O multi-phase fluxes with temperature decrease. The gradual increase of τB was attributed to the crystallization of the super-cooled melt with the dispersed fine grains. The devitrification with the course dentritic crystals caused the steep increase of τB.
