High aluminum containing steels react vigorously with silica-based mold fluxes to form alumina. The change in alumina content increases the viscosity and thus silica-based mold fluxes are compensated with significant amounts of Na₂O, CaF₂, and Li₂O to ensure both lubrication and heat transfer control. Detailed viscosity studies using the rotating spindle method showed that additions of CaF₂ up to 8 wt% in the CaO–SiO₂–12wt%Na₂O system at a constant CaO/SiO₂ ratio of 0.8 decreases the viscosity by breaking the network structure of molten fluxes, but is negligible above 8 wt%. A similar modification of the network was observed for Li₂O up to 2 wt%. The viscosity data was correlated with the XPS analysis and verified CaF₂ and Li₂O as effective in modifying complex silicate structures into simpler silicate structures depending on the availability of complex silicates and thus was limited to below certain concentrations.