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The fluorine in traditional mold fluxes could be harmful in steel-plant environments. Accordingly, fluoride-free mold fluxes have received great attention in recent years. In this work, a method to adjust the crystallization and radiative heat transfer of fluoride-free mold fluxes is proposed. MoO₃ and C mixtures (mass ratio: 4:1) were added into CaO−SiO₂−B₂O₃-based mold fluxes and produced MoB, Mo₂CB and Mo₂C foreign particles. The influences of foreign particles on the radiation of glassy CaO−SiO₂−B₂O₃-based mold fluxes were investigated by measuring the transmissivity of a glassy disk for light in the wavenumber range of 300 to 2500 nm. It was found that transmissivity in all wavenumber ranges were reduced and extinction coefficients were enhanced by the scattering of foreign particles. The effect of foreign particles on crystallization (devitrification) of bulk glassy CaO−SiO₂−B₂O₃-based mold fluxes was also investigated. The crystallization mechanism of glassy mold fluxes disks is mainly surface crystallization. The introduction of foreign particles induced heterogeneous nucleation and the crystallization mechanism of the bulk sample with MoO₃% = 2% changed into bulk crystallization.