Redox behavior of selenium in industrial soda-lime-silica glasses

A thermodynamic model for the calculation of the redox distributions of polyvalent elements which is based on the balance of chemically bonded oxygen in glass melts has been applied to the redox reactions of selenium. A set of reliable thermodynamic data of the selenium oxidation reactions has been derived using results of electrochemical, optical and wet-chemical investigations. Calculation results showed that selenium is reduced during cooling by iron as well as by sulfur. Due to the small selenium/sulfur ratio in selenium-decolorized flint glasses under industrial conditions selenium is completely reduced to the selenide state. Obviously, in industrially melted flint glasses selenium decolorization is not provided by the pink color of elemental selenium as assumed so far but by the amber color of iron selenide, which is supported by the fact that selenium decolorization always causes an additional yellow tint besides the required red tint and a considerable shift of the UV edge to larger wavelengths.