Process for producing optical glass product

A process for producing an optical glass product having a predetermined distribution of refractive index in the interior of the product, said process comprising the steps of:

• (a) mixing together a silica compound with a silicate selected from the group consisting of rubidium silicate, thallium silicate and cesium silicate, and thereby preparing an aqueous silicate solution containing from about 0.1 to about 0.6 mol/liter of Si, as SiO.sub.2, said solution comprising silicic acid and at least one element selected from the group consisting of rubidium, thallium and cesium;
• (b) mixing the silicate solution with an acidic aqueous solution of dilute acetic acid, dilute H.sub.2 SO.sub.4, H.sub.3 BO.sub.3, H.sub.3 PO.sub.4, Al.sub.2 (SO.sub.4).sub.3 or HCl to produce a reaction mixture having a pH of about 6.3 when HCl is used or a pH of about 4.4 when acetic acid, H.sub.2 SO.sub.4, H.sub.3 BO.sub.3, H.sub.3 PO.sub.4 or Al.sub.2 (SO.sub.4) is used, and allowing the reaction mixture to react for a period of time sufficient for polymerization to produce a completely gelled porous product wherein the silica particles of said product are agglomerated;
• (c) leaching the gelled product with a liquid capable of dissolving Rb.sub.2 O, Cs.sub.2 O or Tl.sub.2 O, said liquid being selected from the group consisting of (i) water, (ii) a weakly acidic aqueous solution, (iii) an aqueous solution having an alkali metal salt dissolved therein, and (iv) an aqueous solution of organic matter or (v) a mixture of any two of (i) to (iv), said leaching being continued until the concentration of at least one compound selected from the group consisting of Tl.sub.2 O, Rb.sub.2 O and Cs.sub.2 O parabolically decreases from the center of said gelled product outward;
• and
• (d) drying the leached product in a predetermined atmosphere, and further heating the partially leached gelled product at a temperature sufficiently high to eliminate the particulate material but lower than the melting point of the silica, whereby the micropores in the leached product are caused to collapse.