Having shown experimentally that the birefringence in glass after annealing depends mainly on its structural gradient, we conclude that annealing must be carried out in such a way that the glass is practically in structural equilibrium at any time; moreover, the final cooling must be started with a glass free from thermal/structural gradient. Studying thermal treatment of several glasses & working over small temp. ranges & taking care that the glass was completely stabilised at each stage, we have ascertained that in this way we got complete stabilisation in a time definitely shorter than that needed when stabilising either as a single temp. chosen in the lower part of the transformation range or by steps as pointed out by Winter-Klein. This process has been used for very dissimilar glasses & for thickness from 2-20cm, always with the same final result, i.e. the structural heterogeneity was not higher than one unit of the fifth decimal of the refractive index.//We have also found that when a glass undergoes partial heat treatment, a high degree of stabilisation may be obtained with a final birefringence much smaller than that obtained by classical annealing in the same time.
