Abstract: This invention is directed to a method for heat treating a sheet of a potentially photochromic glass to cause the growth of silver halide crystals therein which impart photochromic properties thereto, wherein the heat treated sheet will be essentially free from thermal deformation and surface defects, and the photochromic properties exhibited therein will be essentially uniform across the area of the sheet. The method involves first placing a sheet of potentially photochromic glass on top of a sheet of carrier glass that exhibits a softening point at least 50.degree. C. higher than the temperature at which the potentially photochromic glass will be heat treated, and thereafter subjecting the stacked sheets to the heat treatment temperature desired for growing silver halide crystals in the glass.