Method of making photochromic glasses suitable for simultaneous heat treatment and shaping

A method for forming glass sheet from potentially photochromic glass and thereafter simultaneously shaping articles from said glass sheet and developing photochromic properties therein which comprises the steps of:

• (a) melting a batch for a glass consisting essentially, in weight percent on the oxide basis, of about 57.1-65.3% SiO.sub.2, 9.6-13.9% Al.sub.2 O.sub.3, 12-22% B.sub.2 O.sub.3, 1-3.5% Li.sub.2 O, 3.7-12% Na.sub.2 O, 0-5.8% K.sub.2 O, 6-15% total of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, the molar ratio Li.sub.2 O:Na.sub.2 O +K.sub.2 +K.sub.2 O not exceeding 2:3, 0.15-0.7% PbO, 0.12-0.18% Ag, 0.2-1% Cl, 0.06-0.13% Br, 0-2.5% F, and 0.002-0.02% CuO, the Ag, Cl, and Br ranges representing values as analyzed in the glass;
• (b) adjusting the temperature of at least one region of the glass melt to provide a viscosity therein of about 10.sup.4 -10.sup.6 poises;
• (c) drawing the glass melt at a viscosity of about 10.sup.4 -10.sup.6 poises directly past platinum or platinum-clad refractory forming means to produce potentially photochromic drawn glass sheet of optical quality, said glass exhibiting good resistance to surface crystal growth in contact with platinum such that over a contact period of 30 days the growth of a crystalline layer will not exceed 10 microns in thickness;
• (d) cooling the glass sheet below the softening point of the glass and cutting articles of desired geometries therefrom;
• (e) edge supporting said articles on alveolated molds;
• and then
• (f) heating said articles at a temperature between about 610.degree.-660.degree. C. for a period of time sufficient to simultaneously sag the glass to a desired curvature into the concave portions of the alveolated molds, but not into contact with the inner surface of said molds, and develop photochromic properties in the glass.