An arc spectroscopic method was developed on the analogy of thermogravimetry. Using spectrographic recording, line intensity vs. arcing time relations were determined and transformed to weight loss vs. time curves for the components evaporated from a carbon electrode cavity. The temperature of a spot 3 mm below the bottom of the cavity was followed with a thermocouple and temperature of the bottom was calculated. As a first attempt the decomposition of tricalcium phosphate was investigated. It was estimated that in the reductive surroundings of the cavity the temperature at which the rate of decomposition was maximum, was about 2000°.