Crystallization behavior of a mold flux under conditions found in the mold gap of the continuous casting process was investigated. A laboratory technique that employs molten tin as the quench medium was utilized to “quench and age” molten flux samples to generate a time temperature transformation (TTT) diagram for nucleation and growth of crystallites from the molten mold flux. A commercial flux film sample of the same composition was extracted from a caster at cap-off. The phases, structure, and quantity of crystallites formed in the mold gap are compared to the crystal phases and structures observed in the “quench and age” experiments and with crystal structures formed in the lab by direct cooling and crystallization of the flux. The crystal phases and morphologies present in the extracted flux film compare favorably with the structures and phases observed in the “quench and age” experiments.