Solarized Glass

Certain types of colorless, transparent glasses, when exposed to sunlight for extended periods, develop a pink or pale purplish color. Bottles, insulators, and other objects having their color are often called “desert glass,” but the scientist prefers the term “solarized glass.”

The major constituent of most glasses is silica, which is usually introduced as a raw material in the form of sand. Although silica itself is colorless in glass form, most sands contain iron as an impurity, and this imparts a greenish tint to glass. By adding certain other ingredients to a molten glass, it is possible to offset the greenish color and produce colorless glasses. Such ingredients are known as decolorizers, and one of the most common is manganese dioxide (MnO₂). In chemical terms, the manganese acts as an oxidizing agent and converts the iron from its reduced state (which is a strong greenish blue colorant) to an oxidized state (which has a yellowish, but much less intense, color). In the course of the chemical reaction, the manganese goes into a chemically reduced state, which is virtually colorless.

If pieces of decolorized glass containing reduced manganese are exposed to ultraviolet radiation for long periods of time, the manganese may become photo-oxidized. This converts it back into an oxidized form, which, even in rather low concentrations, imparts a pink or purplish color to glass. The ultraviolet rays of the sun can promote this process over a matter of a few years or decades, thus accounting for the color of desert glass. The effect has been reproduced in the laboratory.

Other chemical elements that are subject to photo-oxidation can also undergo color changes in glasses when exposed to ultraviolet light. Some of these elements, such as selenium and cerium, have occasionally been used as a decolorizer and can produce solarization colors, just as manganese does. The colors developed by these two elements are said to range from yellow to amber.⁵

Solarization becomes more than an aesthetic problem when it occurs on glass components of an optical system. The solarization causes deterioration of the UV performance and eventual failure of the glass and the system.

Polymicro Technologies, has developed solarization resistant optical fiber. It resists darkening as a result of high UV radiation levels. More about solarization resistant optical fiber characteristics in the Fiber Optics & Optical Fiber chapter.

5 Education Department, The Corning Museum of Glass, One Museum Way, Corning, NY 14830-2253 U.S.A.

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