Oxidation state of equilibrated and non-equilibrated glass melts

Abstract

The oxidation state of a glass melt is governed by the interdependence of the oxygen partial pressure of the furnace atmosphere, the basicity of the melt, and the concentration and type of polyvalent species present in the melt. The overall result of this interdependence is reflected in the oxygen activity of the glass melt. However, in the past the oxidation states of glass melts were not characterized by the activity, but instead by exploiting the ratio of valence states of a redox pair as an indicator. Such an indirect method is based on the assumption that the ratio of valence states at glass melt temperatures is preserved in quenched samples. Moreover, the relationship between the oxidation state and valence state ratio seems to fail completely when more than one redox pair is present, leading to ratios of valence states which become independent of oxygen partial pressure in non-equilibrated melts and thus are no longer indicative of the oxygen activity of the glass melt. Therefore, the possibility of directly determining the oxygen activity in glass melts is highly desirable. Such an in situ measurement has become feasible by utilizing a newly developed oxygen sensor based on a zirconia solid electrolyte. Data on the behavior of soda-lime-silica glass melts will be given, demonstrating the usefulness of the sensor in characterizing glass melts with respect to their oxidation state. In particular the action of additions of arsenic, sodium sulfate and carbon on the oxygen activity will be discussed.