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1916-2016

The Equilibrium between Ferrous and Ferric Oxides in Glasses JSGT_V22_T372-T389

The Equilibrium between Ferrous and Ferric Oxides in Glasses JSGT_V22_T372-T389

A soda-lime-silica glass of composition 75% SiO2, 10 CaO, 15 Na2O was melted in an electric furnace at 1400° for 22 hours. Subsequently, a series of 26 glasses containing from 0.002 to 12.5% of iron oxide expressed as Fe2O3 was prepared under precisely similar conditions. The maximum dissociation of the iron oxide occurred at a total iron oxide content of 0.04% Fe2O3, namely, 36%, and fell steadily until it reached a nearly constant value of about 9% for iron oxide concentrations between 3 and 12.5%. The corresponding colours depended both on the degree of dissociation and on the total iron concentration, blue-green predominating up to 0.1% Fe203, green from 0.2% to 0.9% and olive green from 1 to 2%, the colours being examined in rods 10 cm long. Melting the glass containing 0.075% total Fe2O3 for 100 in addition to 24 hours, and heating rods for 24 to 48 hours at 500° to 600° produced no appreciable change in the iron oxide equilibrium or the colour. In a series of seven soda-silica glasses containing from 15 to 45% Na2O, with total Fe203constant at 0.075%, all melted for 22 hours at 14:00°, the ratio of ferrous to total iron oxide progressively dropped from 37.7 to 13.3%, and the colour passed from blue-green to green. Using E. Preston's viscosity data for the soda-silica glass, it would appear that the greatest dissociation is present in the glass of highest viscosity, and vice versa. In a set of four soda-silica glasses containing from 20 to 35% Na2O and 1% total Fe2O3, the ratio of ferrous to total iron oxide was practically constant. Four potash-silica glasses containing from 20 to 35% K2O and 0.075% Fe2O3 reproduced results similar to the corresponding soda-silica glasses both in degree of dissociation and in change of colour. On the other hand, whilst in four lithia-silica glasses containing from 10 to 25% Li2O the ratio of ferrous to total ferric oxide fell with increase in alkaline content, only much more rapidly than in the case of the soda-silica and potash-silica glasses, the colour changes showed a complete reversal of those in the three other series investigated. The explanation of these phenomena is not yet apparent; but it seems that the chemical factors predominate over the physical in so far as dissociation of the ferric oxide is concerned, viscosity apparently being relatively unimportant, and that, in particular, the basic or acidic character of the glass is important. An observation of special interest related to the influence of composition on the colour in that in the three series of alkali-silica glasses a sharp change of colour was observable at glass compositions corresponding to lithium disilicate, sodium disilicate and potassium tetrasilicate, respectively.

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