Why does glass turn purple?

A year or so ago, my mother - a frequent visitor to the American Southwest in the course of her life - asked me if I knew why glass would turn purple if left in the sun, particularly in the intense sun of the Southwest.

It wasn't something that I'd heard of before, but I'll admit that Mom's spent more time in the Arizona sun than I have, so I went a'researching.

In material science we discuss additions to glass - to lower the melting point, change the thermal expansion, widen the softening range, further dull the electrical conductivity, improve the sparkle, change the color, improve the strength. These additives each have their own historical profiles due to research, economics, politics, environmental concerns.

Here I'm going to quote liberally from the Corning Museum's article on 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, 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.)

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.

The article also mentions that selenium and cerium oxides can also solarize in glass but to an amber or light brown color.

Dumpdiggers (an antique blogger - or rather a blogger about antiques, I don't know that he's an actual antique) writes about a different process through which this solarization can be recreated on a shorter time scale...

Sometimes called desert glass, or sun-colored amethyst glass, these pretty purple bottles are fake; their color is artificially produced by gamma radiation in a lead lined chamber by an unscrupulous merchant with one motive – profit.

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When exposed to the radioactive isotopes Cobalt-60 and Cesium-137, most manganese glass will turn amethyst, while glass made with selenium will become either straw, wheat, or honey colored.

Dumpdiggers then links to a fairly scientific article on the National Insulator Association's website addressing these concerns about artificially inducing colors in glass insulators, including a practice of heat treating solarized (naturally or artificially) glass insulators...

Exposure to high levels of heat will reverse the sun’s ultra-violet purpling effects on glass. This procedure is often referred to by collectors as “cooking”. During the thermal reversal or “cooking” process, the manganese is once again the key stimulant. In most cases, when a sun “purpled” insulator is heated to high temperatures, generally a step below melting, it will revert back to a shade in close proximity to its original manufactured color.

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Our final category of alterations involves exposing irradiated insulators to high levels of heat. In some instances this creates a secondary altered color, much different in appearance to the primary irradiated color. During one of our numerous experimental procedures, one particular pony insulator was first altered from its original color of light blue aqua to a burnt olive brown. This same insulator was then subjected to extreme thermal exposure, transforming to a medium shade of cornflower blue.   We know that some of the unscrupulous new era “nukers” are using this same process to dial in colors that very closely mimic authentic insulator colors, and with considerable accuracy. Even though such color duplications look authentic, they are, of course, outright frauds. These “chameleons” can be very difficult to identify.
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We are currently looking at a possible method for identifying irradiated insulators that fall within a specific color range. As we know, natural purple or sun colored purple glass insulators contain manganese. Generally, the darker the purple the more manganese content in the glass. Manganese is very sensitive under a black light, providing a yellow to greenish yellow glow. The glowing intensity of authentic purple insulators is fairly easy to measure with the human eye, particularly after viewing several examples with a long wave black light in a completely dark room. We have found that most irradiated purple insulators display a diminished glow when compared to authentic purples. We have also noted that some of our insulator samples exhibited good black light glowing characteristics prior to radiation exposure, then deadened under a black light after radiation exposure. We are continuing our testing with this method and hope to have more validated and expanded information in the near future.

I find all of this absolutely fascinating and a wonderful application of the multivalent colors of manganese. For collectors, though, this is clearly a hugely decisive issue.

I kind of want to buy a few cheap glass insulators to line my classroom's windows now...and I kinda want some two-tone ones...