The temperature characterizes a certain glass transition range from the elastic brittle (low temperature) state to the viscous (high temperature) state of glass. It is used for specifying cooling programmes. The transformation temperature is determined by measuring the change in length, related to the length at intial temperature, of a rod made from the glass under test with temperature. The relative change in length is plotted against the temperature (dilatometer curve). From this dilatometer curve the transformation temperature is determined by a graphic procedure as shown in a diagramm.