Photoreduction of iron by a synchrotron X-ray beam in low iron content soda-lime silicate glasses

The redox of alkaline-earth silicates with iron concentrations ranging from 150 to 5000 ppm was measured using X-ray absorption spectroscopy at the K-edge of iron, in fluorescence mode. These measurements were carried out at three beam lines with different characteristics: FAME (BM30) and DUBBLE (BM26) at ESRF (Grenoble, France) and XAFS2 at LNLS (Campinas, Brazil). In those conducted at BM26, we observed X-ray photoreduction of iron in soda-lime silicate glasses, and we studied it systematically as a function of the iron content of the samples. The photoreduction due to the X-ray beam has a critical impact on the redox measurements at room temperature for glasses with iron contents below 1000 ppm. Results show that a constant amount of Fe3 + seems to be reduced by the beam, regardless of the total iron content within the sample. A tentative explanation for this phenomenon is proposed. The photoreduction effect is reversible and can be prevented by making the measurements at a temperature of 450–500 °C, instead of room temperature. Using this new measurement protocol, the redox state at equilibrium with air of a family of glasses with iron contents ranging from 150 to 5000 ppm has been analysed and compared to values obtained by wet chemistry analyses and additional results by Densem and Turner (1938). Wet chemical analysis, commonly used nowadays, does not show any particular variation of the redox with iron content. XANES results indicate a more complex behaviour where the redox increases as the amount of iron decreases, which is in closer agreement with the early results by Densem and Turner (1938) obtained with a different wet chemical analysis protocol no longer used.