Photochromic sheet glass compositions and articles

Claim 1

A chemically-strengthened photochromic drawn sheet glass article formed of a glass having a composition consisting essentially, in weight percent as calculated from the batch, of about 54-66% SiO 2 , 7-15% Al 2 O 3 , 10-25% B 2 O 3 , 0.5-4.0% 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, 0-3.0% PbO, 0.1-1.0% Ag, 0.1-1.0% Cl, 0-3.0% Br, 0-2.5% F, 0.008-0.16% CuO, 0-1.0% total of transition metal oxide colorants, and 0-5.0% total of rare earth metal oxide colorants, said composition providing a glass having a viscosity at the liquidus temperature thereof of at least about 10 4 poises, said article having excellent chemical durability, an unabraded modulus of rupture strength of at least about 45,000 psi, and a compressively-stressed surface layer at least about 3.5 mils in thickness, and said glass being haze-free and having, in a thickness not exceeding 1.7 millimeters, a darkened luminous transmittance not exceeding about 25%, a clear luminous transmittance of at least about 60%, and a fading rate such that the glass exhibits a faded luminous transmittance at least about 1.5 times the darkened luminous transmittance after a 5-minute fading interval from the darkened state.