Molded glass articles having bulk homogeneity and optical quality surface

Abstract

An optical glass article having a desired surface figure and demonstrating maximum theoretical light transmission can be prepared by forming a base anhydrous glass of defined compositional ranges, hydrating the glass, partially dehydrating the glass at a temperature above the softening point and, using an inert gas, at a pressure above the vapor pressure of water contained therein, and then molding a preformed blank of the glass against a die having a desired optical quality surface figure, the molding being under conditions sufficient to transfer the surface figure of the die to the glass. The base glass comprises, in mole percent, about 70 to 82% SiO.sub.2, 10 to 17% Na.sub.2 O and/or K.sub.2 O, and 5 to 15% of an oxide or oxides selected from ZnO and PbO. The water content of the partially dehydrated glass ranges from about 3 to 8 wt. percent and the molded glass has a substantially uniform refractive index throughout its bulk. Homogeneous distribution of the water throughout the glass bulk appears to be assured by limiting the total water content to a range at which the diffusion coefficient of the water species (H.sub.2 O, H.sup.+, and OH.sup.-) exhibits the greatest dependence on the total water concentration (i.e. the range at which the coefficient is a quasi-step function).