During cooling of reduced soda-lime-silica glass melts, doped with iron oxides and sulphur species, ferrous iron (Fe2+) reacts with sulphite (SO32-), and the Fe3+ (ferric iron) and also the S2- (sulphide) concentration will increase. The sulphite concentration in the amber glass melt after fining is sufficient to produce ferric iron and sulphide, required for amber chromophore formation during the cooling process. The product of the concentrations of Fe3+ and sulphide will strongly increase during cooling below about 1000 K. This mechanism implies that a necessary condition for amber formation is the availability of sufficiently large iron concentrations in the presence of a sufficient sulphite content in the high-temperature glass melt. The formation of a chromophore, based on Fe3+-S2 - 3O2- - nNa+ complexes in the silicate glass will lead to amber colouring. A strong dependency of the amber intensity of the glass on the maximum temperature of melting, the alkali concentration of the glass, the total iron concentration and on the oxidation-state of the melt has been found experimentally. The charge of the network-modifying alkali ions stabilizes the ferric iron-sulphide-oxide-alkali chromophores. In strongly reduced melts, hardly any sulphite can be formed and during cooling, the amber chromophore formation is limited. Experiments show a decrease in ferric iron concentrations and in the amber intensity when over-reducing glasses. A decrease in the amber intensity of amber glasses with increasing water content is probably caused by lower sulphite retention after fining of water-rich melts.
|Journal||Glass Science and Technology|
|Publication status||Published - 2003|