Very fast fading photochromic glass

Claim 1

A photochromic glass composition consisting essentially, expressed in terms of weight percent on the oxide basis, with the levels of Ag, Br, Cl, and CuO representing values as analyzed in the glass, of ______________________________________ SiO.sub.2 56-60 Al.sub.2 O.sub.3 6-9 B.sub.2 O.sub.3 18-21 Li.sub.2 O >2.5-3.5 Na.sub.2 O 0.5-2.5 K.sub.2 O 5-7 ZrO.sub.2 3.75-5 PbO 0.1-0.15 Ag >0.15-0.25 Cl 0.2-0.35 Br 0.075-0.15 CuO 0.004-0.02 CeO.sub.2 0-0.5 ______________________________________ capable of being chemically strengthened to modulus of rupture values in excess of 35,000 psi with a depth of compression layer of at least 0.030 inch and demonstrating the following photochromic properties when present in cross section of about 1.5-3 mm; (a) a clear luminous transmittance of approximately 90% when free from added tint; (b) a darkened luminous transmittance at 40° C. below 35% when the glass is free from added tint and a fading rate at 40° C. such that after five minutes the luminous transmittance will have increased at least 40 percentage units; (b) a darkened luminous transmittance at 25° C. below 25% when the glass is free from added tint and a fading rate at 25° C. such that after five minutes the luminous transmittance will have increased at least 35 percentage units; and (c) a darkened luminous transmittance at 0° C. greater than 15%.