Spectrophotometric and Magnetic Studies of Glasses Containing Iron in Relation to Their Structure. Part II. The Forms in which Iron Exists in Glass
JSGT 1954 V38 T146-T196
The effect of the presence of iron in the form provisionally considered to be the same as in an alkali ferrate on the results obtained from the colour analysis of a number of the high-alkali glasses has been studied, and it has been found that it causes the calculated values of the brown ferric and of the colourless iron to be slightly higher than the true values. The effects of the composition of the base glass, the form in which iron was introduced, the concentration of iron, the conditions of melting and the stage of melting at which the iron was introduced, on the proportions of the different forms of iron present in a glass were studied by colour and magnetic measurements, and the results obtained indicated that: (1) In alkali-silica glasses there is a “critical” composition which permits the maximum development of ferrous iron and the minimum development of colourless iron. The molecular alkali content corresponding with the “critical composition” is highest in glasses containing lithia and lowest in glasses containing potash, the all-soda glasses being intermediate. (2) The introduction of sodium nitrate into the batch increased the proportions of brown ferric and colourless iron and decreased the proportions of ferrous and ferroso–ferric oxides, as was to be expected. When sodium nitrate was added to the glass which had been melted and the mixture was then re-melted, the proportion of colourless iron was much greater than when the nitrate was included in the original batch. (3) The introduction of part of the soda into the batch as sodium oxalate, sulphate, fluoride or chloride increased the proportions of ferrous and ferroso–ferric oxides, but decreased the proportions of brown ferric and colourless iron, when the glasses were melted at 1350°C. When part of the soda was introduced as sodium sulphate and the glass was melted at 1450°C, the proportions of ferrous and ferroso–ferric oxides decreased and the proportions of brown ferric and colourless iron increased. (4) The introduction of iron in a ferrous form as ferrous oxalate or ferrous sulphate increased the proportions of ferrous and ferroso–ferric oxides and decreased the proportions of brown ferric and colourless iron. (5) Increase in the iron concentration increased the proportions of ferroso–ferric and brown ferric oxides, but decreased the proportion of colourless iron; the proportion of ferrous iron increased with the increase of the iron concentration up to a certain value, beyond which any further increase in the iron concentration caused a decrease in the proportion of ferrous iron; the iron concentration at which this change occurred depended on the temperature of melting and on the composition of the glass. (6) Raising of the temperature of melting increased the total ferrous content at a very slow rate in the range between 1200 and 1300°C, at a moderate, but increasing rate, between 1300 and 1400°C and at a muoh higher rate at temperatures higher than 1400°C. The temperature T at which the rate of reduction of the ferric oxide changed comparatively rapidly could be determined approximately, and was found to depend upon the alkali present in the glass. (7) The effect of prolonged founding on the proportions in which the different forms of iron existed in the glass depended on the glass composition and on the temperature of melting. (8) The observed mass susceptibilities agreed with the calculated values based on the colour analysis except in: (a) the high-alkali glasses, in which an appreciable proportion of the iron is present in the form provisionally regarded as in a ferrate. (b) the glasses containing ferroso–ferric oxide in amounts of the order of 1–2% of the glass, where the observed mass susceptibilities were higher than the calculated values; and (c) the glasses containing iron equivalent to more than 4% Fe2O3, where part of the iron was ferro-magnetic, although no discrete particles could be detected when the glass was examined under the microscope.
Abd-El-Moneim Abou-El-Azm
