Synthetic mother slag (SiO₂-CaO-Al₂O₃ ternary system) of acid range was prepared, and the influence of Na₂O, Na₂SiF₆, MnO and Fe₂O₃ on the viscosity of molten slag was studied. Also, the viscosity of the acid slag obtained from the operating cupola was measured and was compared with synthetic slags.
  The viscosity of molten slag decreased by rise in the temperature. Increasing the CaO content in the slag, decreased the viscosity of molten slag. By adding Na₂O and Na₂SiF₆, the viscosity decreased in the same way as by the addition of CaO. Especially, by the addition of Na₂SiF₆, there was a remarkable decrease in the viscosity of the molten slag. This effect is due to the considerable break-down of slag structure by the action of F^- ion. By the addition of MnO and Fe₂O₃, the viscosity of molten slag decreased and the influence of both oxides resembled each other. The viscosity of molten slag obtained from the operating cupola was approximate to that of the molten synthetic slags which contained the same amount of the network modifier (CaO, Na₂O, MnO, Fe₂O₃, etc.) as cupola slag.