In order to estimate surface tension for multicomponent silicate melts, expanded approximation rules was proposed in view of practical use, on the basis of the regular solution approximation rules of excess molar quantities for binary system melts. Necessary parameters for the above rules have been derived using the previous data in binaries and ternaries, etc.. Using these parameters, it was found that near the melting point, surface tensions of pure oxides and halides ⁰σ_i were related to (T_i)_mp/⁰V_i^2/3, where (T_i)_mp and ⁰V_i are the melting point and molar volume of pure oxides and halides, respectively.
From the collation with the data of multicomponent silicate melts, it was confirmed that the above expanded approximation rules were applicable to predict the surface tension of melts over the wide range of composition and temperature discussed.