Borosilicate and aluminosilicate are called hard glasses because they soften or fuse at relatively higher temperatures. The oldest of the glasses is soda-lime, which was known some 4,000 years ago. Today, many glass products are made from composites, made up of several glasses of differing composition. Light-sensitive glasses, although not considered a basic type, are available in three grades. Photochromic glass darkens when exposed to ultraviolet radiation and fades when the ultraviolet stimulus is removed or when the glass is heated.
Some photochromic compositions remain darkened for a week or longer.
Others fade within a few minutes after ultraviolet is removed. A chief use for the faster-fading compositions is in eyeglass lenses that automatically darken and fade when exposed to or removed from sunlight. Photosensitive glass also responds to light, but in a different manner from photochromic glass. When exposed to ultraviolet energy and then heated, photosensitive glass changes from clear to opal. When the UV exposure is made through a mask, the pattern of the mask is reproduced in the glass.