Over 30 white and coloured titanium enamels of different origin were tested with a derivatograph. Thermoceram crucibles were used, which enabled the tests to be interrupted when a change was indicated by the DTA curve. The crucible was removed from the derivatograph, and the sample pulverized and studied by X-ray diffraction method. A major difference compared to previous observations was the fact that the exothermic peak indicating the anatase-rutile transformation was also found on the DTA curves of certain anatase-type enamels. With these enamels the temperature relating to the start of the transformation peak was regarded as the critical temperature, as at a temperature below this the enamel was stable, but above it unstable.
The mathematical theory is developed for heat transfer in the cells of DTA instruments with no temperature gradient. Equations are derived for the various portions of the differential curve. The concepts of real and hypothetical base lines are introduced and rules are formulated for plotting them. Three methods are proposed for processing the curves, with and without the introduction of base lines. The physical meanings of the geometrical elements of the thermal curves and their relationship with the experimentally determined thermal quantities are elucidated. Equations in differential and integral forms are derived for the base line of the cell and the base line of the reaction.