Ageing induced changes in the refractive indices of seven Russian optical glasses that represent the main types of silicate glasses for periods up to 50 years are measured using the standard reference samples and the problem of how to calculate reliably such changes is solved for alkali silicate-containing glasses. The calculations were shown to require, for glass samples finely annealed with quite low cooling rates, that the period of artificial ageing of a glass in the course of annealing be introduced. A method for calculating the ageing induced changes in the refractive indices of commercial optical silicate glasses (both crudely & finely annealed) for their age up to 50 years is proposed. Borosilicate glasses lacking alkali oxides are shown, by measurements for ages up to 13-16 years, to behave as practically non-ageing ones.
