Glass containing certain heavy metal compounds can under exposure to ultra-violet radiation undergo an electronic modification of the metal ions. On subsequent heat treatment this leads to a change in the colour or the transparency of the exposed areas. In such glass it is therefore possible to produce photographic images. The first photo-sensitive glass of this kind was announced in 1947 in America.
There are three main kinds of photochemical action in glass: metallic image formation, non-metallic crystal formation and differential solubility.
Photo-sensitive glass for metallic image formation contains compounds of gold, silver, copper or palladium. It may also contain optical sensitizers such as cerium to extend the spectral absorption range; copper and silver act as their own sensitizers in this sense.
The glass is exposed through a normal black-and-white negative to ultra-violet rays between 3,000 and 3,500 A in wavelength, usually to a high intensity mercury vapour lamp or carbon arc. The sensitivity is very low, requiring exposures of the order of ten minutes. The image is formed throughout the depth of the glass with ultra violet rays of longer wavelength, but only in the surface if very shortwave radiations are used.
After exposure the image is developed by heating the glass to around 600°C. The image colour depends on the exposure and may vary from blue to brown, according to the metallic compound incorporated. Development takes about half an hour to one hour. The image itself is due to the conversion of the metal ions into sub-microscopic metal particles. Such images are virtually grainless and of very high resolving power. Cooling of the glass fixes the image which is then virtually permanent unless the glass is heated to its softening point again.
Photo-sensitive glass of this type is used for making graticules, photographic reproductions and photographically decorated glass ware.
In certain types of glass the metal particles produced by the exposure can act as nuclei for the growth of non-metallic crystals. The image then is an area of greater scattering power or opacity than the original glass itself.
Photo-sensitive glass with differential solubility yields a milky opalescent image after exposure and heat treatment. This image is preferentially soluble in glass etching solvents like hydrofluoric acid. Exposed and developed areas of the glass etch up to fifty times faster than unexposed areas, leading to direct glass etching in depth. Such images are much sharper and more detailed than indirectly photo etched glass with a resist produced by conventional photographic methods.
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