Differential thermal analysis (DTA) is a thermal analysis technique that measures the difference in temperature between a sample and an inert reference material, when they both are heated under the same controlled temperature conditions.
If during the test the sample under analysis undergoes physical changes or chemical reactions, its temperature will change, while the temperature of the reference material remains the same. This is due to the fact that phase changes and chemical reactions usually involve changes in enthalpy, the heat content of the material.
The reactions associated to heat absorption (i.e. melting, vaporization, sublimation, loss of crystallization water) are endothermic and are characterized by negative peaks. The reactions associated to heat development (i.e. oxidations and recrystallizations) are called exothermic and are characterized by positive peaks.