Due to the rapid cooling of the slag infiltrated into the mold-strand gap, it is expected that the rate of devitrification of the glass formed plays a key role in determining the proportion of crystalline phases influencing mold heat transfer. In this investigation, a novel procedure to calculate Time-Temperature-Transformation (TTT) curves of mold slags is presented. It is based on the integration under isothermal conditions of a Kissinger-type rate equation using rate parameters determined from differential thermal analysis (DTA). The estimated TTT curves agree very well with transformations observed metallographically by inspecting glass disks, 3 mm thick, quenched after isothermal treatment. In the latter tests a transverse face of a glass disk, insulated laterally, was seated onto an inductively heated metallic plate subjected to prescribed thermal cycles. Both procedures were found suitable for evaluating and comparing the rate of devitrification exhibited by different mold fluxes.