In this study, a hot stage microscope, thermogravimetry–differential scanning calorimetry (TG-DSC), and a scanning electron microscope were used to study the role of carbonaceous materials in mold powder and their influence on the melting process. It was found that carbon black has a good isolation effect in mold powder. It delayed the solid-state reaction between the raw materials and increased the softening and hemispherical temperatures of the mold powder. Moreover, the steric hindrance effect of carbon black led to the formation of irregular 2CaO·SiO2 crystals in the solid-state reaction and a significant increase in the flow temperature that exceeded the 30 ℃ error limit of the current standard. The TG-DSC thermal analysis shows that during the heating process of mold powder with 15% graphite, the exothermic combustion of graphite accelerated the decomposition reaction of Na2CO3 and Li2CO3, promoted the melting of mold powder, and decreased the softening and hemispherical temperatures. However, the decrease was smaller than 30 ℃, and there was no need to decarbonize the carbon before the melting range test. [ABSTRACT FROM AUTHOR]