Certain types of colorless, transparent glasses, when exposed to sunlight for extended periods of time, develop a pink or pale purplish color. Bottles, insulators, and other objects having their color changed in this way are often called "desert glass," but the scientist prefers the term "solarized glass." Other well-known examples are the famous purplish windows on Beacon Street in Boston and the little circular glass disks sometimes found in sidewalks of older parts of cities. Occasional examples of solarized glass are also found from the ancient world.
Color from Impurities
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, most sands contain iron as an impurity, and this imparts a greenish tint to glass. (In ancient times, glassmakers used very impure sands, with iron contents higher than those of sands used today, so most ancient glasses have a pronounced greenish color.)
Decolorizers (Manganese Dioxide)
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.
Manganese dioxide is believed to have been first used as a decolorizer as early as about the second century B.C. It was probably introduced as the mineral pyrolusite. From Roman times onward, glasses often contain about 0.5% to 1.0% manganese oxide (MnO). Later on, manganese dioxide (MnO2) was sometimes called "glassmakers' soap."
If pieces of decolorized glass containing reduced manganese are exposed to ultraviolet light for long periods of time, the manganese may become photo-oxidized. This converts the manganese 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 Decolorizers (Selenium and Cerium)
Other chemical elements that are subject to photo-oxidation can also undergo color changes in glasses when exposed to ultraviolet light. Since about the turn of the century, some of these, such as selenium and cerium, were occasionally used as decolorizer and therefore can produce solarization colors, just as manganese does. The colors developed by these two elements are said to range from yellow to amber.