The article represents a thermodynamic analysis of phase interaction during cast iron melting in cupola and electric furnaces at foundry. The author has studied the influence of the components’ activities on the cast iron melting in various compositions of the interacting phases corresponding to the melting conditions in cupola, induction and arc furnaces. The influence of each factor on the phase interaction has been studied in the presence of temperature fluctuations. The effect of the components’ activities on the Gibbs free energy in both the equilibrium conditions and the presence of temperature fluctuations have been determined. This is particularly essential when contacting with the molten metal, slag and carbon. The influence of temperature fluctuation on the interaction of the metal with the gas phase, slag and carbonic solid materials has been revealed. The conditions of cast iron melting can be characterized by the presence of temperature fluctuations disturbing the equilibrium of the process. Temperature macrofluctuations with a significant relaxation time can be observed in arc furnaces. Such macrofluctuations can be considered as the spatial temporal heterogeneity of the temperature field. There are also temperature fluctuations in gas flame. Temperature fluctuations occur upon contact of metal drops with hot surface of coke lumps. Based on the research, a fundamentally new design of gas cupola furnace with heterogeneous fire-resistant bed charge has been developed, patented and introduced. © 2017, National University of Science and Technology MISIS. All rights reserved.