Redox and sulphur reactions in glass melting processes

For about 95 wt.% of all industrially produced glasses, sulphate is added to the batch for several reasons, the most important task of sulphates is to remove small bubbles and seeds from the melt. The chemistry of the sulphates during heating of the batch and melting determines the foaming behaviour, the fining efficiency and the final colour of the glass product. Fluctuations in sulphate addition and redox state of the batch ingredients including recycled cullet, may lead to changes in glass colour even beyond the required specifications. The paper describes the chemistry of sulphur species during batch melting and glass melt fining. The impact of process conditions and the batch composition on the redox state and sulphur reactions in the batch blanket and molten glass will be shown. These reactions and the redox state of the batch will determine the efficiency of bubble removal and the final colour of the glass product.