The colour 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 colour 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 colouring 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 colouration 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, colourations such as olive green or antique green can be obtained. Tuning the total chromium content and the redox state are key issues in controlling these colours, and only slight variations may lead to dramatic variations in colouration 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 colour stability. Experimental results obtained int he 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.
