Understanding mold flux crystallization is essential for evaluating heat transfer during steel casting. In casting, the mold gap's complexity raises questions about the ideal testing method and nucleation type—heterogeneous or homogeneous. This study examines how crucible materials influence mold flux crystallization, focusing on the wetting behaviors of platinum and graphite with mold flux. Confocal laser scanning microscopy (CLSM) was employed to observe in-situ nucleation, and differential scanning calorimetry (DSC) was used under non-isothermal conditions across cooling rates (5–30°C/min) for the flux sample to determine the crystallization temperatures. Results showed significantly lower crystallization temperatures in graphite crucibles versus platinum, with similar trends for synthetic slag and Li2SO4, validated through experimental DSC data and Factsage® simulations. X-ray diffraction (XRD) and scanning electron microscopy- energy dispersive X-ray spectroscopy (SEM-EDS) were utilized to identify crystal phases and interactions between the flux and graphite wall. Graphite’s non-wetting characteristics hindered nucleation, requiring greater undercooling, while platinum’s superior wetting properties facilitated nucleation by lowering surface energy and the free energy barriers. These findings highlight the influence of crucible materials on nucleation behavior, providing valuable insights for refining mold flux testing methodologies and, ultimately, advancing the understanding of nucleation mechanisms during solidification in the casting process.