Mossbauer. spectroscopy of tin and iron has been used to probe silicate glasses. The research was motivated by the need to understand the structure and behaviour of float glass when tin diffuses into the surface during manufacture. Iron is also present in small quantities as it is an impurity in the sand from which the glass is made, and it is known to affect the tin uptake. Because float glass is a complex mixture of,silica and several. modifier oxides, Mossbauer spectroscopy on simpler systems containing silica and a single alkali or alkaline earth modifier has been carried out first to facilitate the interpretation of the data. The spectra have broad lines, reflecting the large number of,different sites in an atomically disordered material. Several conclusions were reached from a review of the literature on silicate glasses. The isomer shifts and quadrupole splittings identify the ionic states of the Mossbauer atoms, and the changes when the composition is varied by adding modifier atoms are due to small changes in the local structure and atomic volume. It appears that Sn2+ and Fe3+ act as conditional glass formers in silicate glasses, while Sn4+ and Fe2+ are modifiers. From measurements of the relative areas of Sn2+ and Sn4+ in the Mossbauer spectrum of sectioned specimens, the depth profiles of each oxidation state were determined separately. By monitoring the oxidation of Sn2+, to Sn4+, produced by heat treatment in air, the diffusion coefficient of oxygen was determined. Measurements on tinted float glass containing several per cent of iron show that the iron and tin react with each other, the Fe3+, oxidizing the Sn2+ to Sn4+. By combining these data with the depth profiles, a flow diagram of the chemical reactions in the float-glass process has been proposed.
