Surface layer formation due to leaching and heat treatment of alkali lead silicate glass

The surface layer formed after leaching and subsequent heat treatment of an alkali lead silicate glass was studied. The glass was leached in 1 N HCl solution and subsequently heat treated in vacuum at 525°C. The surface was characterised with secondary ion mass spectrometry (SIMS) and the leachate solution was analysed by spectrochemical methods. The leaching kinetics of Pb were found to follow a t^1/2 dependence for short times, indicating an interdiffusion mechanism between Pb²⁺ and H₃O⁺ ions. For longer times the leaching rate of Pb showed a linear dependence with time. This was attributed to the formation of a hydrated gel like SiO₂ rich layer at the surface of the glass in which ions have a very high mobility similar to that of ions in solution. Heat treatments of the leached glass in vacuum resulted in preferential segregation of alkali into the leached surface layer. However, the heat treated surface was resistant to further leaching of lead indicating densification of the SiO₂ gel layer.