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Because the viscosity of the CaO–SiO2–CaF2 system is very important in primary and secondary refining as well as continuous casting processes, several researchers measured the viscosity of this slag system. Shiraishi and Saito found that the addition of small amounts of CaF2 (2 to 10 mass%) reduces the viscosity of the CaO–SiO2 slags (basicity, B ( (mass%CaO)/(mass%SiO2) 0.5–1.3), especially in the relatively acidic compositions by employing the rotating cylinder method at temperatures from 1 673 to 1 873 K. Yasukouchi et al. reported the similar tendency for CaF2 (5 to 14 mass%) effect on the viscosity (B 1.0) using the rotating crucible method at temperatures from 1 763 to 1 838 K, while the viscosity values are lower than those by Shiraishi and Saito. Shahbazian et al. reported the similar conclusions in the basicity range 0.6–0.9 and the CaF2 concentration range 13–41 mass% at temperatures from 1 534 to 1 762 K using the rotating cylinder method. More recently, Park et al. had an attempt to interrelate the viscosity with the structural analysis by applying the rotating cylinder method for molten slags at temperatures from 1 473 to 1 823 K. The characterization of structural analysis was operated by the Fourier-transform infra-red (FT-IR) spectra for quenched samples. The authors observed that a decrease of the viscosity as well as the activation energy for viscous flow of the slags (B 1.0 and 1.3) in a Newtonian flow region is originated from a decrease in the degree of polymerization due to a network modifying role of F ions in silicate melts. Although the role of F ions in the (de)polymerization reaction of silicate melts is still in a room for further discussion based on various analytical methods such as FT-IR, Raman, X-ray photoelectron spectroscopy, and F nuclear magnetic resonance spectroscopy, it is simply believed that the existence of F ions decreases the viscosity of silicate melts, as noted by Sasaki et al. Here, the latest papers using respective techniques are referred for the sake of convenience and the details will not be treated in the present discussion. The iso-viscosity contours in the CaO–SiO2–CaF2 slag at 1 873 K estimated by Sasaki et al. are reproduced in Fig. 1, where the concentration scale is added by the present authors for eye-guidance. They expected these contours based on an idea that the viscosity of the slags at a fixed basicity should gradually decrease with increasing CaF2 concentration since the addition of CaF2 reduces the relative amount of Q (i 0 to 4) units in the melts although the relative distribution does not change. Furthermore, they discarded the viscosity data reported by Herty et al. These were very reasonable discussion and theoretical expectations, resulting in a good iso-viscosity diagram as shown in Fig. 1. However, the iso-viscosity contours shown in Fig. 1 might be drawn based on the restricted experimental data and some minor misunderstanding on the dicalcium silicate saturation region at highly basic compositions. For the CaO–SiO2–CaF2 system, the present authors suggest the iso-viscosity contours at 1 873 K as shown in Fig. 2 based on the experimental data reported by Kozakevitch and Bockris and Lowe, and Shiraishi and Saito for the CaO–SiO2 binary system and by Shiraishi and Saito 1) and Yasukouchi et al., and Park et al. for the CaO–SiO2– CaF2 ternary system. The viscosity values at different ba-

Discussion on "The Estimation of the Iso-viscosity Lines in Molten CaF2-CaO-SiO2 System"