This curve is a graphic depiction of the correlation between ambient temperature and viscosity of glasses.
Viscosity is usually displayed logarithmically.
The arithmetical correlation is described by the Vogel-Fulcher-Tamann (VFT) equation in literature. The coefficients of the VFT equation for a glass can be calculated from the measured values of the transformation temperature, the Littleton point and the working point. The VFT curve can then be displayed graphically.