In order to clarify a guideline for designing of composition of a flux which can achieve both minimizing a refractory corrosion by the flux and maximizing of solubility of Mn oxides into the flux, corrosion tests for refractory was conducted in the atmosphere. The basic composition of flux is Na₂O-B₂O₃ and the refractory is Mullite (3Al₂O₃・2SiO₂), assuming a process of melting of a copper alloy containing Mn as easily oxidized elements. Although the corrosion ratio of refractory became larger with increasing of mole fraction of Na₂O in flux, the concentration of refractory's constituents in the flux have different tendency predicted by the results of corrosion ratio. Through the corrosion test, the Na₂O-B₂O₃ based flux has penetrated inside the refractory with Mn, and a part of that Mn has reacted with Al₂O₃ to form MnAl₂O₄. However, in the refractory/flux interface no clear formation of the compound layer could be confirmed due to the reaction between the refractory's constituents and the flux. In addition, the relationship between the corrosion ratio and the equilibrium solubility of 3Al₂O₃・2SiO₂ for Na₂O-B₂O₃ flux calculated by thermodynamic database was investigated. The result shows that there is not a clear relationship between them. The cause of this can be explained by the affection of corrosion inside the refractory by the penetration of the flux through the pores in the refractory. Furthermore, it was shown that the amount of Mn oxide dissolved in the flux was strongly affected by the viscosity of the flux by calculation.

Consequently, in order to design a proper composition of flux in this study, it became clear that the thermodynamic approach alone was not enough and more detailed examinations such as the wettability between the flux and refractory, properties of flux, especially penetration phenomena were also important.