Cast iron is a material with a unique combination of design flexibility, mechanical properties, wear resistance, recyclability, low life cycle energy consumption and low cost. The optimization of current production and the development of new material variants with improved properties rely on the control of the melt throughout the process, from charge selection to casting. Thermal analysis just prior to pouring the molds has been developed over the years for melt control, e.g., for composition verification or for graphite shape prediction. Since minor additions of tellurium prevent graphite growth, thermal analysis with Te-coated cups is commonly used for compositional control of slightly hypereutectic alloys. The present study was designed to test the feasibility of using Te-coated cups to monitor the magnesium content of melts after a full or partial spheroidizing treatment leading to spheroidal or compact graphite, respectively. In the course of this work, it was realized that thermal analysis could give unreliable results when the microstructure of the casting is not homogeneous, i.e., it exhibits a mixture of metastable and stable eutectics. Tellurium has been shown to have an ambivalent effect on solidification of Mg-bearing melts.