Magnetic Studies on Glasses Containing Iron, in Relation to Their Colour and Constitution
JSGT 1951 V35 T058-T092
A study has been made of the magnetic properties of two series of glasses containing iron, with the object of identifying the state or states in which the iron was present. The glasses of Series I were soda–lime–silica glasses made under strongly reducing conditions so as to ensure, as far as possible, that the whole of the iron was present as ferrous iron. Series II consisted of potash–lead oxide–silica glasses melted under conditions designed to maintain the whole of the iron in the fully ferric state, sub-series II(a) being made on a base-composition: PbO 70%,
K2O 5%, SiO2 25%; and sub-series II(b) on a base-composition: PbO 70%, K2O 10%, SiO2 20%. From the magnetic susceptibilities of the glasses of Series I the mass susceptibility of the iron at 25° was determined as approximately 220×10-6, which corresponds very closely with the mass susceptibility of iron in the ferrous condition. The mass susceptibility of the iron in the glasses of Series II ranged from 210×10-6 to 350×10-6, the variation being associated with a marked increase in the intensity of the ferric colour. In one glass, the colour of which showed a large “grey” component, the mass susceptibility of the iron was determined as 369×10-6. These values indicate that the mass susceptibility of the colouring ferric iron is greater than 300 and probably greater than 350×10-6, and that the iron contributing the grey colour has a mass susceptibility at least as large as that of the colouring ferric iron, probably larger. They also suggest that the "colourless" iron has a mass susceptibility less than 200×10-6. The conclusions drawn are that iron can exist in glasses as: (1) ferrous iron, producing a blue colour; (2) ferric iron in the form of colloidal ferric oxide, giving a yellow or amber colour, (3) ferroso-ferric iron, in the form of colloidal ferroso-ferric oxide, which contributes a grey colour; and (4) iron which is linked into the glass structure in a form which contributes no colour. The low mass susceptibility of the colourless iron suggests that the iron in this form is linked into the structure by four bonds, probably as FeO4 tetrahedra. This cannot, however, be regarded as definitely established without additional experimental evidence.
H. Moore & S. Kumar
