1. Field of the Invention
This invention relates to glass articles having walls whose composition in the surface layers is different from the interior. In particular this invention relates to a method of making such articles.
2. Brief Description of the Prior Art
U.S. Pat. Nos. 2,106,744 (1938); 2,215,039 (1940) and 2,355,746 (1944) granted to Messrs. H. E. Hood and M. E. Nordberg describe a method of producing glasses having almost pure silica in the surface layers and containing alkali-boro-silicate compositions in the interior. According to this method, an alkali-boro-silicate glass of phase-separable composition is melted in the usual manner and is then fabricated in the usual way into desired shapes.
The preformed article is then subjected to a heat-treatment. The glass article is then subjected to a leaching treatment in acid solutions whereby one of the phases is leached out up to a definite depth leaving the interior portion of the glass unaffected. The prior art process then employed a water wash. The article is then dried and consolidated at a high temperature giving rise to a desired composition profile across the walls of the glass article.
According to Hood and Nordberg, in articles produced by their processes, "there is a tendency to break during the dehydration and vitrification steps."