Method for making molded glass articles having bulk homogeneity and optical quality surface

DETAILED DESCRIPTION We claim: 1.
A method of preparing an optical glass article comprising the steps of A.
preparing a base anhydrous glass comprising in mole percent, on an oxide basis, 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; B.
hydrating the base glass; C.
partially dehydrating the glass to a water content ranging from about 3 to 8 weight percent at a temperature above the softening point of the glass and, using an inert gas, at a total pressure above the vapor pressure of the water within the glass; and D.
preforming a sample of the glass of step (C) and then molding the preformed sample of the partially dehydrated glass against a die having a desired optical quality surface figure under conditions sufficient to impart to a surface of the glass the surface figure of the die, the molding being done under conditions sufficient to maintain the water content in the glass, to assure that at least one portion of the glass is capable of transmitting an amount of light about equal to the maximum amount theoretically possible, and to maintain or result in an optical quality surface on that portion of the glass surface to which the surface figure of the die is transferred.
2.
The method of claim 1 wherein the inert gas is nitrogen.
3.
The method of claim 1 wherein the optical quality surface of the molding portion of the die is such that over a distance of at least 0.
1 inch, on an arithmetic average basis, the Roughness Height is less than about 3.
0 microinches.
4.
The method of claim 3 wherein the Roughness Height is less than 1.
6 microinches.
5.
The method of claim 1 wherein the base glass includes up to 5 mole percent Al.
sub.
2 O.
sub.
3 and/or ZrO.
sub.
2.
6.
The method of claim 1 wherein the base glass includes up to 3 mole percent of a member selected from B.
sub.
2 O.
sub.
3, BaO, and MgO.
7.
The method of claim 1 wherein after the partial dehydration of step (C), the total water content of the glass consists of silanol groups and molecular water in such a proportion that the ratio of silanol groups to molecular water is equal to or greater than 1