Ultraviolet absorbing photochromic glass of low silver content

Claim 1

A method for producing a photochromic glass article that, in cross section of < 2 mm but >1.3 mm, will transmit less than 0.2% of radiations having wavelengths between 290-315 nm, can be chemically strengthened to pass the strength standards mandated by the FDA, and at 20°-25° C. will exhibit a darkened luminous transmittance below 35% and a fading rate such that after five minutes the luminous transmittance will be at least 1.75 times that of the darkened transmittance, which method comprises the steps of: (a) melting a batch for a glass consisting essentially, expressed in terms of weight percent on the oxide basis, of ______________________________________ SiO.sub.2 55-61 CeO.sub.2 0.1-1 B.sub.2 O.sub.3 18-21 Ag >0.1-<0.2 Al.sub.2 O.sub.3 5-11 CuO 0.003-0.015 Li.sub.2 O 1.5-3 Cl 0.2-0.6 Na.sub.2 O 2-5 Br 0.05-0.2 K.sub.2 O 4.5-8 Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3 0.1-1.5 ______________________________________ wherein the amount of Sb 2 O 3 and/or As 2 O 3 is at least 75% of the amount of CeO 2 present; (b) cooling said melt and simultaneously forming a glass article of a desired geometry therefrom; and thereafter (c) heating said glass article to a temperature between about 650°-675° C. for a sufficient period of time to cause the growth of silver halide crystallites therein whereby said photochromic glass article is provided which, at 20°-25° C., will exhibit a darkened luminous transmittance below 35% and a fading rate such that after five minutes the luminous transmittance will be at least 1.75 times that of the darkened transmittance.

Claim 2

A method for producing a photochromic glass article that, in cross section of < 2 mm but >1.3 mm, will transmit less than 0.2% of radiations having wavelengths between 290-315 nm, can be chemically strengthened to pass the strength standards mandated by the FDA, and at 20°-25° C. will exhibit a darkened luminous transmittance below 35% and a fading rate such that after five minutes the luminous transmittance will be at least 1.75 times that of the darkened transmittance, which method comprises the steps of: (a) melting a batch for a glass consisting essentially, expressed in terms of weight percent on the oxide basis, of ______________________________________ SiO.sub.2 55-61 CeO.sub.2 0.1-1 B.sub.2 O.sub.3 18-21 Ag >0.1-<0.2 Al.sub.2 O.sub.3 5-11 CuO 0.003-0.015 Li.sub.2 O 1.5-3 Cl 0.2-0.6 Na.sub.2 O 2-5 Br 0.05-0.2 K.sub.2 O 4.5-8 Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3 0.1-1.5 ______________________________________ wherein the amount of Sb 2 O 3 and/or As 2 O 3 is at least 75% of the amount of CeO 2 present; (b) cooling said melt and simultaneously forming a glass article of a desired geometry therefrom; and thereafter (c) heating said glass article to a temperature between about 650°-675° C. for a sufficient period of time to cause the growth of silver halide crystallites therein whereby said photochromic glass article is provided which, at 20°-25° C., will exhibit a darkened luminous transmittance below 35% and a fading rate such that after five minutes the luminous transmittance will be at least 1.75 times that of the darkened transmittance.