Abstract: According to the invention, a) at least the first and second compositions of this material in the fluid phase are prepared, b) a sheet (61) of the first composition is poured continuously onto a flat support (4), c) a sheet (62) of the second composition is poured continuously onto sheet (61) of the first composition, and d) the thermal interdiffusion of the materials of the two sheets is ensured so as to obtain the desired gradient of composition in the cooled bar.

Abstract: The present invention relates to a method of making a rectangular bar have a gradient composition. In optical or ophthalmic applications, the gradient of composition ensures formation of a corresponding gradient of optical property, such as refractive index or optical density. The glass material is manufactured using a device containing several reservoirs, all designed to contain glass in the fluid state. According to the invention, the composition of the glass varies from one reservoir to the other so that the refractive indices, for example, of the three compositions are different, with an incremental progression. The fluid glass is deposited one composition on top of the next and then heated, creating a certain degree of interdiffusion between the layers. The glass is then heat-treated to create the desired property profile.

Abstract: A glass ceramic or glass-ceramic sheet is formed by progressively reducing the thickness of the sheet by passing the sheet between successive pairs of counter-rotating rollers. The average viscosity of the sheet is maintained between 5.times.10.sup.4 and 10.sup.9 P during the sheet forming operation.

Abstract: A glass ceramic or glass-ceramic sheet is formed by progressively reducing the thickness of the sheet by passing the sheet between successive pairs of counterrotating rollers. The average viscosity of the sheet is maintained between 5.times.10.sup.4 and 10.sup.9 P during the sheet forming operation.