Based on observed η vs T relationships for silicate melts and CaF₂-based slag melts, the R_min is raised to discriminate between long slag and short slag by quoting the mathematical concept of curvature radius. The R_min values of the smallest curvature radius of η vs T plot implicate the apparent activation energy E_η,hemispherical softening point temperature T₀ and viscosity η_R at the character temperature T_R. Using CaF₂ and SiO₂ as reference standard of short slag and long slag respectively, the classification of slags has been made. Calculation of R_min for flux or slag is of significance in ESR, protective casting and welding.