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 NCl solution and subsequently heat treated in vacuum at 525 degre es C. The surface was characterised wit secondary ion mass spectrometr y (SIMS) and the leachate solution was analysed by spectrochemical met hods. The leaching kinetics of Pb were found to follow a t(1/2) depend ence for short times, indicating an interdiffusion mechanism between P b2+ and H3O+ ions. For longer times the leaching rate of Pb showed a l inear 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. H eat treatments of the leached glass in vacuum resulted in preferential segregation of alkali into the leached surface layer. However, the he at treated surface was resistant to further leaching of lead indicatin g densification of the SiO2 gel layer.