In continuous casting of steel slabs, all the processing steps have as their objective the attainment of the best superficial quality. The selection of an optimized mold powder is an important goal in attaining the best operating conditions. Therefore, the study of the main thermal properties of this raw material, which regulates the heat transfer between the steel plate and the mold and the lubrication of this interface during slab casting, needs special attention, as well as the additional problems due to the absorption of non-metallic materials from the fused steel and from the corrosion of the refractory materials of the equipment. These impurities change the properties of the fluxing material and its performance. If not controlled, they can cause the loss of the quality of the steel produced, and even the interruption of the process.In this work samples of processed mold powder, taken directly from the top of the mold during continuous industrial casting of steel slabs, were analyzed by differential thermal analysis (DTA), to determine the changes in thermal properties at different processing times. There is an increase of 41 degrees C in both, the crystallization and melting temperatures after 140 min of processing. From DTA curves it can be seen that the original mold powder has only one crystallization peak and all the contaminated samples present two peaks. In spite of that, in both samples, pure and contaminated mold powders, cuspidine (Ca4Si2O7F2) and nepheline (NaAlSiO4) were identified by X-ray diffraction after thermal treatment that induced the crystallization.