An unannealed glass is characterised by (a) tensions causing birefringence, (b) heterogeneity and (c) aging. The removal of tensions is not a sufficient criterion of good annealing. he establishment of good annealing procedures presupposes the knowledge of variations with temperature and rates of such variations and of properties such as index of refraction, coefficient of expansion and birefringence. The variation of the index of prisms measured in a furnace by two special methods gives quantitative evidence of a range of transformation in glass and of the increase of rates of transformation with temperature in this range. The measurements of expansion and birefringence confirm the evidence. Moreover, the increase of birefringence on heating in the transformation range proves that tension can be established on heating and that tension caused by structural changes is superimposed on the mechanical tension of the classical concept. The problem of annealing therefore is complex. An exact and fast annealing curve stressing index homogeneity is derived from the nature of the curves obtained by plotting index change as constant temperature vs. time, considering values measured in quenched samples. The curve of a borosilicate crown glass (n=1.5224) is given as an example.
