BS 7034-7:1988

Lays down a method of determining the annealing and strain point of a glass by beam bending. These values have been found useful for specifying the cooling programme in the production of glassware. The annealing and strain point include a well-defined temperature decrease during the measurement. At temperatures corresponding to the annealing and strain points, the viscosity of glass is highly time dependent. Any viscosities that might be derived from measurements cannot be assumed to represent equilibrium structural conditions. Therefore, the insertion of the strain point into VFT-equation is always impossible. The insertion of the annealing point causes in some cases marked failures.

This part of ISO 7884 specifies a method of determining the annealing point and the strain point of a glass by beam bending. These values have been found useful for specifying the cooling programme in the production of glassware. The annealing point and strain point include a well-defined temperature decrease during the measurement.

At temperatures corresponding to the annealing and strain points, the viscosity of glass is highly time dependent. Hence, any viscosities that might be derived or inferred from measurements carried out according to this part of ISO 7884 cannot be assumed to represent equilibrium structural conditions. Therefore, the insertion of the strain point into the VFT-equation (see ISO 7884-1) is always impossible. The insertion of the annealing point causes in some cases marked failures.

NOTE The annealing and strain points by beam bending can also be determined using devices as specified in ISO 7884-4, but these devices are more expensive and the procedures lead to some viscosity-temperature and viscosity-time relationships besides the fixed points. In this part of ISO 7884, however, the device and procedure are restricted to the fixed-point determination.