Chemistry of sulphur in soda-lime-silica glass melts

The amber colour intensity, characterised by the absorption/extinction of the visible light at a wavelength of 410 nm has been measured for soda-lime-silica glasses dependent on the redox state of the melt, maximum fining temperature and the contents of iron oxide and sodium oxide in the glass. The amber colour (strong absorption of the glass at a wavelength of about 410 nm) is stable for oxygen activities (pO2) of the melt in the range of 10-5-10-9 bar, measured at 1400°C. The amber colour intensity vanishes above these activity levels and will also decrease below a pO2 level of 10-9 bar (1400°C). At similar oxidation states, the extinction of the glass at a wavelength of 410 nm decreases as sodium oxide concentration of the glass is decreased from 16 down to 13.8 wt%. The slope of the curve of the logarithmic value (log pO2) measured at 1400°C as a function of the logarithmic value of the optical extinction of the glass at 410 nm (10logE410) is about -0.65: the amber intensity or extinction at 410 nm at room temperature is almost proportional to the pO2-213 at melting temperatures for all investigated glass compositions. The Fe3+-S2- compound (stabilised by alkali ions in the glass and acting as basis for the amber colour chromophore) is formed during cooling of the glass melt. In very strongly reduced iron oxide sulphur containing glass melts, the ferric iron content and the sulphite level in the glass will be rather low and during cooling of the melt hardly any ferric iron can be formed. The concentration of ferric iron (Fe3+) sulphide complex in the glass structure will decrease for melting conditions with pO2 levels (1400°C) below 10-9 bar and this weakens the amber colour.