Vacuum differential thermal analysis has been used to study the coking properties of a number of bituminous coals. Samples of 200 mgs were run from room temperature to 600 degrees C at a heating rate of 10 degrees /min.The thermograms show two peaks that can be related to the plastic properties: (1) a strong exothermic peak beginning between 340-440 degrees C and reaching a maximum height between 440-520 degrees C; (2) a small, sharp exothermic peak superimposed on the limb of the larger peak and occurring between 430-500 degrees C. The beginning of No. 1 peak represents the softening temperature of the coal while its summit corresponds to the solidification temperature. No. 2 peak occurs in the temperature zone of maximum fluidity and maximum rate of swelling. The sharpness of this peak appears to be influenced by the fluidity of the coal.The changes that occur in the plastic properties of a coal through blending with other coals can be detected on the thermograms and in the small coke buttons formed in the thermocouple well. For example, by blending a high-volatile A coal with increasing amounts of a low-volatile coal, No. 2 peak occurs at progressively higher temperatures, diminishes in intensity and finally disappears. The coke buttons show changes in their cellular structure and degree of swelling.The thermograms and coke buttons can also be modified by the addition of diluents such as fusain, silica gel, quartz, clay, sulfur, charcoal, various inorganic salts and phenanthrene. The degree to which the thermal curves are affected depends on the type and amount of diluent, as well as the properties of the original coal.