Glass-ceramic and process therefor

Claim 2

A process of making an article of glass-ceramic which comprises 1. forming an article from a thermally crystallizable glass containing at least one ingredient that combines with alumina and silica to provide bulk crystallization by a heat treatment, a nucleant present in sufficient concenration for the bulk crystallization, soda and a compatible inorganic oxide in a concentration that inhibits the rate of said bulk crystallization without preventing surface crystallization of said glass, 2. ion exchanging a surface layer of the glass article to replace at least part of the sodium ions with lithium ions, 3. heat treating the article of glass at an elevated temperature sufficient to provide crystallization of a surface layer of the article while the main body portion remains a glass, and 4. heat treating the surface crystallized article at a higher elevated temperature to convert the main body portion by bulk crystallization to a glass-ceramic.

Claim 3

A process of making an article of glass-ceramic which comprises 1. forming an article from a thermally crystallizable glass containing at least one ingredient that is lithia and that combines with alumina and silica to provide bulk crystallization by a heat treatement, a nucleant present in sufficient concentration for the bulk crystallization, and sodium oxide present in a concentration that inhibits the rate of said bulk crystallization without preventing surface crystallization of said glass, 2. ion-exchanging the surface layer of the glass article to replace at least part of the sodium ions with lithium ions, 3. heat treating the article of glass at an elevated temperature sufficient to provide crystallization of a surface layer of the article while the main body portion remains a glass, and 4. heat treating the surface crystallized article at a higher elevated temperature to convert the main body portion by bulk crystallization to a glass-ceramic, said lithia being present in a sufficient concentration in the thermally crystallizable glass to provide, in absence of said rate-inhibiting sodium oxide, bulk crystallization uniformly throughout the article, and said surface layer having a lower coefficient of linear expansion than that of the main body portion of the article of glass-ceramic, the final product containing both the main body portion and the surface layer as glass-ceramic in which the crystalline phase contains at least one material selected from the group consisting of beta-eucryptite and beta-spodumene.

Claim 4

A process of making an article of glass-ceramic which comprises 1. forming an article from a thermally crystallizable glass containing at least one ingredient that is lithia and that combines with alumina and silica to provide bulk crystallization by a heat treatement, a nucleant present in sufficient concentration for the bulk crystallization, and sodium oxide present in a concentration that inhibits the rate of said bulk crystallization without preventing surface crystallization of said glass, 2. ion-exchanging the surface layer of the glass article to replace at least part of the sodium ions with lithium ions, 3. heat treating the article of glass at an elevated temperature sufficient to provide crystallization of a surface layer of the article while the main body portion remains a glass, and 4. heat treating the surface crystallized article at a higher elevated temperature to convert the main body portion by bulk crystallization to a glass-ceramic, said lithia being present in a sufficient concentration in the thermally crystallizable glass to provide, in absence of said rate-inhibiting sodium oxide, bulk crystallization uniformly throughout the article, and said surface layer having a lower coefficient of linear expansion than that of the main body portion of the article of glass-ceramic, the final product containing both the main body portion and the surface layer as glass-ceramic in which the crystalline phase contains at least one material selected from the group consisting of beta-eucryptite and beta-spodumene.