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What role do calcium and magnesium have in the Betterton-Kroll process?

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A labeled periodic table element box for bismuth shows atomic number 83, symbol Bi, atomic weight 208.9804, electron configuration, and properties including solid state, rhombohedral crystal structure, and weakly acidic oxides. — bismuth Properties of bismuth.

In the Betterton-Kroll process, calcium and magnesium are added to a molten lead-bismuth bath to facilitate the removal of bismuth from lead.12

These metals react with bismuth to form compounds that possess higher melting points and lower densities than the surrounding molten lead. Because these resulting compounds are lighter than the lead, they rise to the surface of the bath, where they can be separated as a solid dross or skimmed off. Once removed, the bismuth can be recovered from the calcium and magnesium through treatment with chlorine.12