The complete crystallization process of mold slag in the crucible (inner diameter: 6.2 mm) was successfully recorded in situ using a digital optical microscope and image processing software in an infrared furnace (IR-MOP). The crystallization behavior of non-volatile mold slag tested with the new method is consistent with the findings of differential scanning calorimetry (DSC) analysis. Although the volatilization ratios of volatile mold slag during both DSC and SHTT experiments are greater than 8%, it is less than 1.2% during IR-MOP experiments equipped with an isolated observation system, which explains why the difference in the initial crystallization temperature measured by the three methods was very large. Therefore, IR-MOP equipped with an isolated observation system can record and accurately measure the crystallization behavior of volatile mold slag.