Method of producing a photochromic glass and resulting article

Claim 1

THE METHOD OF FORMING A SURFACE LAYER EXHIBITING EXCELLENT PHOTOCHROMIC PROPERTIES ON A GLASS BODY WHICH COMPRISES: (A) PROVIDING A GLASS BODY CONSISTING ESSENTIALLY, AS ANALYZED IN WEIGHT PERCENT ON THE OXIDE BASIS, OF ABOUT 50-65% SIO2, 7-14% AL2O3, 15-22% B2O3, 5-15%R2O, WHERE R2O CONSISTS OF THE ALKALI METAL OXIDES 3-5% LI2O, O-5% K2O, AND K-15% NA2O, AND AT LEAST ONE HALIDE IN THE INDICATED PROPORTION SELECTED FROM THE GROUP CONSISTING OF 0.2-3% CL, 0.1-3% BR, AND 0.1-4% I, THE TOTAL AMOUNT OF SAID HALIDES NOT EXCEEDING ABOUT 5%, THE SUM OF THE SIO2, AL2O3, B2O3, R23, AND HALIDE COMPONENTS CONSTITUTING AT LEAST 95% BY WEIGHT OF THE GLASS; (B) CONTACTING A SURFACE OF SAID GLASS BODY WITH AT LEAST ONE SILVER-CONTAINING MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILVER AND STABLE SILVER COMPOUNDS AT A TEMPERATURE RANGING FROM ABOUT 100*C. BELOW THE STRAIN POINT OF THE GLASS TO ABOUT 100*C. ABOVE THE SOFTENING POINT OF THE GLASS; AND (C) MAINTAINING THIS TEMPERATURE FOR A SUFFICIENT LENGTH OF TIME TO EFFECT AN EXCHANGE OF SILVER IONS FOR ALKALI METAL IONS IN AT LEAST THE SURFACE OF THE GLASS AND TO CAUSE A REACTION OF THE SILVER IONS WITH THE HALIDE IONS PRESENT IN THE GLASS TO PRECIPITATE SILVER HALIDE CRYSTALS.