Photochromic glasses suitable for simultaneous heat treatment and shaping

Claim 1

A glass composition consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 54-66% SiO 2 , 7-15% Alihd 2O 3 , 10-25% B 2 O 3 , 0.5-4% Li 2 O, 3.5-15% Na 2 O, 0-10% K 2 O, 6-16% total of Li 2 O + Na 2 O + K 2 O, 0-1.25% PbO, 0.1-0.3% Ag, 0.2-1% Cl, 0-0.3% Br, 0.002-0.02% CuO, and 0-2.5% F, having a viscosity at the liquidus of at least 10 4 poises, long term stability against devitrification in contact with platinum at temperatures corresponding to glass viscosities in the range of 10 4 -10 6 poises, excellent chemical durability, and being chemically strengthenable to modulus of rupture values in excess of 45,000 psi with a depth of compression layer between about 0.0035-0.004 inch, said glass, in bodies of about 1.3-1.7 mm cross section, exhibiting the following photochromic properties: (a) at about 25°-30° C. will darken to a luminous transmittance below 30% in the presence of actinic radiation; will fade to a luminous transmittance at least 1.75 times the darkened transmittance after 5 minutes' removal from the actinic radiation; and will fade to a luminous transmittance in excess of 80% of its clear luminous transmittance in no more than one hour after being removed from the actinic radiation; (b) at about 40° C. will darken to a luminous transmittance below 50% in the presence of actinic radiation and will fade to a luminous transmittance in excess of 80% of its clear luminous transmittance in no more than one hour after being removed from the actinic radiation; (c) at about -18° C. will not darken to a luminous transmittance below 5% in the presence of actinic radiation; and (d) in the undarkened state will exhibit a luminous transmittance of at least 60%.