Mold Flux plays a critical role in continuous casting of steel. Along with many other functions, the mold flux in the gap between the solidifying steel shell and the mold serves as a medium for controlling heat transfer and as a barrier to prevent shell sticking to the mold. This manuscript introduces a novel method of monitoring the structural features of a mold flux film in real-time in a simulated mold gap. A 3-part stainless-steel mold was designed with a 2 mm, 4 mm and, 6 mm step profile to contain mold flux films of varying thickness. An Extrinsic Fabry-Perot Interferometer (EFPI) was installed at each of the three steps in the mold. Mold flux was melted in a graphite crucible at 1400 °C and poured into the instrumented step mold for analysis. Interferograms from the three EFPIs were acquired and processed in real-time to measure the air gap and thickness of each flux film during solidification. Measurements were performed on two different mold flux compositions. Results demonstrate that the proposed system successfully records structural features of the flux film in real-time during cooling. It has a large real-time impact on the process control of steel making and optimizing the quality of steel castings. In addition, the measurement method has potential to monitor crystal nucleation and growth in a variety of crystallizing glass systems.