The Colour of Soda-Lime-Silica Glasses Containing Iron Oxide and Manganese Oxide
JSGT 1940 V24 T073-T092
A series of twenty six soda-lime-silica glasses containing iron oxide and manganese oxide singly and in combination has been prepared by melting in platinum in an electric furnace with slightly oxidising conditions and grinding the cast discs to a known thickness, the standard of reference being 2 mm. The light transmission in the visible spectrum was then measured spectrophotometrically. In the glass containing iron oxide only, that oxide was added as Fe2O3 in amounts ranging from 2.92 to 9.3%, and the proportions of FeO and Fe2O3 in the glass showed a slight falling off, namely, from 9.93 to 7.80% of the total iron as FeO with rise in total concentration, and the successive transmission curves showed relative increase in transmission at the long wave-length end and a falling off in overall transmission from 61.6 to 6.2% for the change of concentration stated. The manganese oxide was added as MnO2 and analyses were not made to ascertain the respective proportions of MnO2 and other oxides resulting from liberation of oxygen. For glasses in the preparation of which MnO2 had been added in amounts equivalent to 3.1 to 9.62%, the curves all possessed the same form showing strong absorption in the green and blue-green, the overall transmission ranging from 5.14 to 0.02%. Several series of glasses were prepared in which Fe2O3 and MnO2 were used jointly in total amounts ranging from 3.1 to 17.54%, but in varying ratios. Marked increase in transmission took place when iron oxide was added to a glass containing manganese oxide. Thus a glass containing 3% of manganese oxide had an overall transmission of 6% but addition of 3% of iron oxide increased it to 65%. Iron oxide-manganese oxide glasses in which the iron oxide content was equal to the manganese oxide content did not become opaque until a concentration of 10% iron oxide + 10% manganese oxide was reached. For a given concentration of combined iron oxide and manganese oxide, the glass with the highest overall transmission was the one which contained iron and manganese oxides in the ratio of 2:3 approximately. Glasses in which the ratio of iron oxide to manganese oxide was 1:2 and 1:1 also had high overall transmissions. In applying the results to commercial practice, the importance of the conditions of oxide concentration, time of melting, temperature, batch composition and furnace atmosphere was emphasised. In the manufacture of commercial amber coloured soda-lime-silica glasses using iron oxide and manganese oxide as colouring agents, the equilibrium conditions as defined and aimed at in preparing the specimens for the investigation described in this paper are rarely attained. The resulting colours in commercial practice are unlikely to be repeated in different factories; or, indeed, in any one factory, when the same batch is used for glass melted in gas-fired furnaces, in the one case in a pot in the other in a tank.
A. Lawton, B.Sc.Tech., A. J. Holland, M.Sc., Ph.D., D.I.C., and Professor W. E. S. Turner
