Oxidation-reduction Effects in Glasses of Different Composition

JSGT 1949 V33 T267-T277

Consideration is given to the conditions which determine the state of oxidation of metallic oxides introduced as colouring agents into glass. It is pointed out that, in general, reduction is to be expected on account of the high temperatures employed in melting, and also because the ions of the alkali metals present in the glass will favour the dissociation of oxides which, owing to the high temperature, are approaching instability. The relative activities of sodium and potassium ions in this respect are compared, and it is deduced that reduction will occur much more rapidly in soda glasses than in potash glasses of corresponding molecular composition. It is also deduced that reduction should occur to a greater extent in glasses containing higher proportions of the alkalis, i.e. in more basic glasses, but to a smaller extent in glasses containing boric oxide replacing soda. Evidence obtained from the analysis of the colours of experimental glasses is given and discussed. This bears out the deductions drawn from the theoretical considerations insofar as the colour differences are concerned, and also as regards the increased reduction obtainIOed in glasses containing higher proportions of alkali; the colours obtained in glasses containing boric oxide replacing soda do not, however, fall into line with the theory put forward. It is suggested that to explain the colour effects produced in glasses containing boric oxide, structural as well as chemical considerations will need to be taken into account, and reference is made to some preliminary observations supporting this view.

H. Moore