Glass photochromic lenses were first offered in eyeglasses in a process invented by Corning Glass Works in the 1960s. Corning infused glass with a silver halide (generally silver chloride), and then, after casting, precisely heat-treated the lens blanks. Silver chloride molecules were transparent after the heat treatment, as was the lens, but exposure to ultraviolet (UV) light caused the silver chloride to decompose into its constituent elements. In bright sunlight, the lens filled with microscopic particles of elemental silver, darkening in the process. Remove the UV light, and the silver would again react with the adjacent chloride still trapped in the glass, and become transparent once again. Varying the percentages of silver chloride and glass created lenses of differing darkening characteristics.