Process for making gradient photochromic glass articles

Claim 1

A process for producing a gradient photochromic glass article by the selective over-nucleation of a limited portion of the glass article which essentially comprises the steps of: (a) subjecting a selected limited portion of a potentially photochromic glass article containing silver and a halogen selected from the group consisting of chlorine, bromine and iodine to a nucleation heat treatment at a temperature above the minimum temperature necessary for the development of nuclei for silver halide growth in the glass but below the minimum temperature required for the growth of photochromic silver halide particles in the glass, for a time at least sufficient to reduce the photochromic development potential of said selected limited portion, in order to obtain the selective over-nucleation of said portion, while maintaining the remainder of the glass article at a temperature below the minimum temperature necessary for the development of nuclei for silver halide growth therein; and (b) subjecting the entire potentially photochromic glass article to a photochromic development heat treatment at a temperature above the minimum temperature sufficient to permit the growth of silver halide crystallites in the glass for a time at least sufficient to develop photochromic properties in the remaining portion of the glass article not subjected to selective over-nucleation.