Heat treatment of flat photochromic sheet glass

A method for heat treating a sheet of potentially photochromic glass to effect the in situ growth of silver halide crystals which impart photochromic properties to the glass, wherein the heat treated sheet of glass exhibiting photochromic properties will be essentially free from thermal deformation and surface defects, and the photochromic properties will be essentially uniform throughout the area of the sheet, which comprises the steps of:

• (a) placing a sheet of potentially photochromic glass having a thickness of about 0.5-3 mm atop a carrier sheet of silica-based refractory glass having a thickness of about 1-5 mm and exhibiting a softening point at least 50.degree. C. higher than the temperature at which said potentially photochromic glass is to be heat treated to effect in situ growth of silver halide crystals;
• (b) exposing the stacked glass sheets to a temperature and for a period of time sufficient to effect adequate in situ growth of silver halide crystals in said potentially photochromic glass sheet to impart photochromic properties to the glass;
• and then
• (c) removing said photochromic glass sheet from atop said carrier sheet to provide said heat treated sheet of glass exhibiting photochromic properties which are uniform throught the entire sheet, with the heat treated glass being essentially free from thermal deformation and surface defects.