Photochromic glasses

A photochromic glass, comprising:

• a glass matrix that is at least substantially free of rare earth elements, comprising in weight percent (wt %) based on oxides:
• 48 wt %≦SiO2≦58 wt %;
• 15 wt %≦B2O3≦21 wt %;
• 5 wt %≦Al2O3≦9 wt %;
• 2.5 wt %≦ZrO2≦6.5 wt %;
• 2 wt %≦Li2O≦4 wt %;
• 0 wt %≦Na2O≦3 wt %;
• 3 wt %≦K2O≦10 wt %;
• 0 wt %≦MgO≦2 wt %;
• 0 wt %≦CaO≦2 wt %;
• 0 wt %≦SrO≦2 wt %;
• 0 wt %≦BaO≦2 wt %;
• 0 wt %≦TiO2≦2.5 wt %;
• 2 wt %≦Nb2O5≦4.5 wt %;
• and
• a plurality of photochromic agents, comprising in weight percent (wt %) with respect to the glass matrix:
• 0.100 wt %≦Ag≦0.250 wt %;
• 0.200 wt %≦Cl≦0.500 wt %;
• 0.0100 wt %≦Br≦0.300 wt %;
• and
• 0.0050 wt %≦CuO≦0.0110 wt %