The foaming index was measured for CaO–SiO₂–FeO–Al₂O₃ and CaO–SiO₂–FeO–MgO_satd–X (X = Al₂O₃, MnO, P₂O₅, and CaF₂) slags to understand the foaming behavior. The foaming index of the CaO–SiO₂–FeO–10Al₂O₃ slags (C/S = 0.93 and 1.2) decreases with increasing content of FeO up to 20 % and is almost constant for FeO content through 20 to 40 %. The viscosity of slags could be considered as the major contributor to foaming behavior. The addition of Al₂O₃ into the silicate slags results in an increase of foaming index due to an increase of slag viscosity; this could be explained by the structural role of Al₂O₃ in aluminosilicateslags. In the MgO-saturated and Al₂O₃-containing slags, FeO behaves as an acidic oxide, because slag melts would be more basic than MgO-saturated and non-Al₂O₃slags, where FeO behaves as a basic oxide, due to Al₂O₃ enhances the dissolution of MgO into the slags. The addition of MnO into the MgO-saturated slags decreases foaming index, simply due to a decrease of slag viscosity. However, the addition of CaF₂ and P₂O₅ into the slags results in the complex foaming behavior of slags; this is probably due to the Marangoni effect. The relationships between foaming index and the physical properties of slags can be obtained from the dimensional analysis as follows:

Σ = 214(μ/√ρσ) (for the CaO-based slags)

Σ = 999(μ/√ρσ) (for the MgO-saturated slags)

The foam height is predicted as a function of decarburization rate from the molten iron and the contribution of slag foaming in EAF process was discussed as a function of decarburization rate.