The effect of tin on some physical properties of the bottom surface of float glass and the origin of bloom

The surface composition of the tin side of float glass has been investigated by XPS and found to contain a large quantity of tin in the near surface region resulting in an increase in refractive index and viscosity. Transmission Mossbauer spectroscopy was used to examine tinted glass and the oxidation of Sn 2+ to Sn 4+ was found to take place on heat treatment as in clear glass. Conversion electron Mossbauer spectroscopy was applied to both clear and tinted glasses and showed that tinted glass had a greater proportion of Sn 4+ and that both clear and tinted glass showed faster conversion of Sn 2+ to Sn 4+ in the near surface region on heat treatment. Bloom was only seen on glasses for which this conversion occurred at a temperature where the surface viscosity was sufficiently low for wrinkling to occur. Tinted glasses were found to be less prone to bloom than clear glasses. This was attributed to their greater proportion of Sn 4+ in the near surface region before heat treatment. This may result in a smaller change in thermal expansion and in a higher viscosity making wrinkling less likely.