IMPACT OF REDOX IN INDUSTRIAL GLASS MELTING AND IMPORTANCE OF REDOX CONTROL

The color of a glass is mainly determined by the valence of the multivalent ions it contains. Controlling the oxidation state of the melt is therefore crucial in terms of color stability. The redox state of the melt also greatly influences the fining processes and thus the final quality of the product. In addition, the redox state of the coloring species in the melt affect the radiative heat transfers from the combustion system to the molten glass, and can therefore significantly impact the energy consumption of the furnace. Amongst the most largely produced type of glass, amber glasses owe their specific coloration to the presence of the amber chromophore, a complex between a ferric iron and sulfides. When chromium is added to glasses containing an amber component, colorations such as olive green or antique green can he obtained. Tuning the total chromium content and the redox state are key issues in controlling these colors, and only slight variations may lead to dramatic variations in coloration and to process instabilities, hence to substantial production losses. In this paper, the key parameters governing redox and stability of industrial glasses are presented, with emphasis on amber-containing reduced glasses. Special attention is given to the role of chromium on the color stability. Experimental results obtained in the lab are correlated to theoretical calculations and practical observations in industrial melting tanks. The results and observations presented highlight the crucial role played by redox in industrial glass making, and the importance of its control.