An Investigation into the Variation with Temperature of the Spectral Transmission of Glasses. The Transmission in the Visual Spectral Region at Temperatures from 20° to the Softening Point

JSGT 1941 V25 T164-T220

Determinations of the transmission the visual spectral have been made at temperatures of 20°, 100°, 200°, 300°, 400°, or near, the approximate temperature in the ease of 27 glasses, save that for a copper glass the upper was restricted because of permanent colour and transmission change occurring on heating at higher temperatures. For one heat-resisting glass the measurements were carried to 800°. The determinations were made for rising and falling temperatures and the original 20° measurements finally re-checked to ensure absence of permanent change on heating. Of the 27 glasses used in the form of optically polished discs 19mm in diameter, a soda-lime-silica and a soda-boric oxide-silica glass were melted in platinum from pure materials and served as a basis of reference, the 25 others being commercial glasses covering a wide range, including chemical and heat-resisting, plate and decolourised bottle, lead-containing glasses for domestic coloured glasses for bottles, for heat-protecting purposes colour filters. The complete composition of glasses is set out, including all the minor constituents likely to exercise selective absorption. Two standards of thickness, 2mm or 10mm, were employed, according as the glass was colourless (or nearly so) or coloured. The range of wavelength was from 4400 to 7200A, measurements being carried out at each intermediate 100A interval. Thorough tests showed that the whole of the light incident on the glass specimen could be accounted for in the experiments, and, in particular, it was demonstrated that no change in the loss by reflection with rise or fall of temperature took place other than that dependent on the variation with temperature of the refractive index of the glass. As a general rule, reduction in the transmission and a shift of the maximum transmission towards the long wavelength end of the occurred progressively with rise of temperature. This was the case with colourless and only faintly coloured glass, although the effect of temperature rise affected the transmission of such glasses differently at different wavelengths, and particularly for the extreme ends of the visual spectrum. The least effect was observed for the colourless standard glasses free from any but traces of selective absorbing agents (Fe₂O₃ and Pt), the soda-lime-silica glass having its overall transmission for illuminant C reduced from 91 to 86 per cent between 20° and 600°, the corresponding values for a soda-boric oxide-silica glass being 90 and 88 per cent, respectively. In the case of glasses coloured by Fe₂O₃, FeO, CoO, NiO, Cr₂O₃, Mn₂O₃, “Di₂O₃,” S, Se, Cu and Au, the following observations are especially noteworthy:

(1) The dominant effects of certain oxides, particularly Cr₂O₃ and CoO even in extremely small amounts, whether in coloured or faintly coloured glasses

(2) the sharper "cut-off" in the violet end produced by relatively small amounts of sulphur (with carbon and low iron oxide) than by considerable amounts (1-2%) each of Fe₂O₃ and MnO

(3) The presence of points of intersection in the temperature-transmission curves for glasses containing Cr₂O₃, Fe₂O₃-MnO, and CoO, whereby in certain wavelength regions transmission fell with rise of temperature and showed inverse behaviour in others

(4) the presence of points of intersection in quite a number of glasses in the graphs obtained by plotting temperature against wavelength for a specific transmission, these points to the transformation temperature (T_g) and others to additional points of the glass. A potash-lead oxide (31%)-silica glass alone of the colourless glasses showed this phenomenon. For the coloured and three colourless glasses the effect of temperature on the colour was calculated by the method of A. C. Hardy for the C.I.E. coefficients, x, y, and z, and the results tabulated and also set out graphically.

A. J. Holland, M.Sc., Ph.D., A.R.C.S., D.I.C., & Professor W. E. S. Turner