Glass Manufacturing)..Glass solarization )

Glass solarization is a phenomenon which occurs when certain types of colourless, transparent glass are exposed continuously to the sunlight. This exposed glass develops a pink or violet color. When bottles and fragments have this color they are usually referred to as "desert glass". However scientists and experts prefer the term solarized glass.

The main ingredient in the majority of glass types is silica, usually introduced as a raw material in the form of sand at the beginning of the glassmaking process. Although silica itself is colorless, most sand contains iron as an impurity which produces a greenish color in the glass. By adding various ingredients to molten glass, it is possible to offset this and produce water-white glass. The dioxide manganese (pyrolusite) is used to "decolorize" this glass containing impurities.  Solarization of glass occurs when pieces of colorless glass containing manganese oxide are exposed to ultraviolet light for long periods of time, causing the manganese to become photo-oxidized. This converts the substance back into an oxidized form which, even in rather low concentrations, imparts a pink or violet 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.  Due however to improvements in the industry, nowdays most clear glass uses much purer raw materials that do not react to solar rays. article source  http://www.glassonweb.com