Fig. 2  Changing of volume of a melt when cooling,
(a) crystallization
(b) glass formation

In order to differentiate glasses from crystalline materials, the changing of volume is described when the respective substances are cooled from melting temperature to ambient temperature (cf. Fig. 2). Here such systems are compared which (a) solidify by crystallization and (b) by formation of glass. One detects that crystalline materials go through a enormous variation in volume with the melting temperature T_S, they crystallize (red line). In contrast to crystalline materials glasses condense over a wide temperature range (blue curve) also beyond T_S. In the area of the transformation temperature T_g they change from the plastic state to the brittle state when cooled slowly (blue dotted line). Rapid cooling leads to an undercooled liquid (blue line). The changing of viscosity described here is fundamental for the for the processing of glasses and depends only on the chemical composition of the respective glasses.