Effect of preparation procedure on redox states of iron in soda-lime silicate glass

The redox state of iron in soda-lime silicate glass was determined by analysis of the optical absorption bands due to Fe2+ and Fe3+ states. When raw materials containing Fe2O3 were heated gradually to 1400 °C, the total-Fe content of the glass was 9.4% Fe2+, but rapid heating at 1400 °C increased the Fe2+ content to 11.7%. The oxygen activity (aO2) in the corresponding melts was measured using zirconia and Pt electrodes. The value increased with increasing temperature in the gradually heated sample and reached log(aO2) = 0.03 at 1400 °C, but was about 2.5 times lower in the rapidly heated sample at log(aO2) = −0.37. After SnO addition to the raw material, oxygen activity depended strongly on heating speed: log(aO2) at 1400 °C fell as low as −1.8 with rapid temperature increase but was about −0.2 or higher with gradual heating. The Fe2+ content of the cooled glass was consistent with the oxygen activity of the melts. The effect of heating speed was attributed to the formation of a melt layer on the surface of the raw material.