A homogeneous melt can be obtained at the melting point, where viscosity equals 2.0 dPas. The working point is the viscosity at which a melt is delivered to a forming device, typically around 4.0 dPas. The softening point is the minimum viscosity that can prevent the glass from deforming under its own weight on a typical glass working time scale, which corresponds to 7.6 dPas. The temperature range between the working point and the softening point is defined as the working range. After the glass is formed, the internal stresses which result from the glass forming process need to be released by annealing. The annealing point (13.0 dPas) is defined as the temperature where stress is substantially relieved within a few minutes, whereas at the strain point (14.5 dPas) the stress release is achieved within several hours.
A typical viscosity temperature curve for a soda lime glass, containing the characteristic viscosity points, is shown in the Figure above.
In practice, no single analytical method is capable of measuring the full range of viscosities and/or the entire temperature range. To fully cover this range three methods are available (as illustrated below):