Oxidation — Reduction Equilibrium in Glass Forming Melts

Abstract

The effect of temperature, oxygen fugacity, and melt composition on the oxidation-reduction equilibrium in glass-forming oxide melts has been discussed. The thermodynamics of mutual oxidation-reduction in glass, and the effect of heat treatment at lower temperatures has also been described.

Oxidation-reduction processes play a very important part in glass manufacture, particularly in the preparation of homogeneous glass free from bubbles and in making coloured glasses containing transition metal ions. Transition metals, due to their unique electronic structure, exhibit variable valence states and coordination geometries in glass forming oxide melts. When a transition metal oxide is introduced into an oxide melt, very often it distributes into different states of oxidation depending upon the time and temperature of melting, the melt composition, and the furnace atmosphere. In a given set of conditions, after sufficient time of melting, a melt comes to equilibrium with the partial pressure of oxygen in the ambient atmosphere and the relative concentrations of the different oxidation states reach equilibrium values; a typical example of iron (II) – iron (III) equilibrium is shown in Figure 1.