The progress of sintering of pyrrhotite cinder during the process of roasting was studied by measuring the resistance of cinder to the penetration of a slender rod. The results showed that pyrrhotite cinder, in general, began to sinter at 250-350℃ with a very high resistance to the penetration of a rod. The sulfate formed below 550℃, and the oxide formed above 550℃, both played a main part in the sintering process of cinder. In the range from the temperature at which the sintering began to about 400℃ and in temperatures above 550℃, the resistance increased with the rise of temperature, whereas in the range from 400 to 550℃ it decreased with the rise of temperature by the progress of the reaction of oxide formation due to "the first decreasing change in weight". It was also found that an addition of over 30 per cent of pyrite to pyrrhotite would enable to complete the roasting almost without the sintering of cinder
