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.
...
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.
...
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.
...
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...
