**2.3.1 Thermal properties**

Thermal characteristics of a glass, such as glass transition temperature Tg and crystallization temperature Tc, are determined using Differential Scanning Calorimetry (DSC). Fig. 6 represents the thermogram heat flow versus temperature for one given glass undergoing crystallization phenomenon (exothermic peak). Glasses which are stable against devitrification do not present any crystallization peak.

The glass transition temperature, Tg, is the main characteristic of a glass. Before Tg, the viscosity is infinite (solid state), at Tg, the viscosity is equal to 1013 poises (1012 Pa.s) and after Tg, the viscosity decreases as the temperature increases, therefore, the material can be easily shaped. The crystallization phenomenon is characterized by the rearrangement of atoms in organized lattice due to the change of viscosity. Crystallization is at the origin of the loss of the viscoplastic properties as well as the optical transparency. The stability against devitrification is associated with the difference Tc-Tg. The higher is the difference between Tc and Tg and better is the stability against devitrification.

Fig. 6. Typical DSC curve for a glass.
