The Effect of Small Amounts of Certain Colouring Oxides on the Colour of a Soda-Lime-Silica Glass
JSGT 1941 V25 T005 T020
The extent to which the colouring oxides of chromium, vanadium, cobalt, nickel, manganese, copper and titania can be tolerated in a standard soda-lime-silica glass of composition 73.5 SiO2, 10 CaO and 16.5 per cent Na2O was determined by trial melts in platinum at 1400° in an electric furnace under slightly oxidising conditions. The glass test-pieces employed for colour estimation and spectrophotometric measurements were ground and optically polished discs of 12·5 mm thickness and 25 mm diameter. Decolourising tests using selenium and cobalt oxide were carried out on the chromium oxide-containing glasses, and the spectral transmissions measured. Glasses containing more than 0.002 per cent. Cr2O3 could not be decolourised satisfactorily. When a glass containing 0.001 per cent Cr2O3 was decolourised, further heating at the melting temperature, causing loss of selenium, resulted in the re-development of the original colour more quickly than in the parallel case of a glass in which the original colour was due to iron oxide. Titania up to 0.5 per cent had no noticeable colouring effect on the glass or on the colour produced by 0.05 per cent Fe2O3, and the decolourising of the iron oxide was not affected. Vanadium was only one-fiftieth as effective as chromium in producing a tint, and continued heating tended to remove that tint. The amounts of the different oxides which can be tolerated in a soda-lime-silica glass without development of appreciable tint in test pieces 12.5 mm thick and 25 mm diameter were found to be: TiO2 (not less than) 0.5, Cr2O3 0.001, V2O5 0.1, MnO2 0.01, CuO 0.01, NiO 0.0005 and CoO 0.0005 per cent. The paper discusses the variation of these limits with circumstances.
Eric Preston, Ph.D., D.Sc., F.I.C., F.Inst.P., and Professor W. E. S. Turner
