Walk into any antique show in the Southwestern United States, and you are likely to see at least one booth filled with colored glass, especially purple glass of varying shades, from almost clear to a deep purple that still maintains some transparency. Some buyers go immediately for the darkest shades; others prefer colors that are on the lighter side.
What is this colored glass?
The controversy
According to many experts and collectors of Early American Pattern Glass, purple glass should be considered damaged goods and is actually worth less than glass in clear, pristine condition.
The problem
Glass that turns purple in the sunlight, or in other words, glass that turns purple when exposed to ultraviolet rays, looks different than it did when the glass was originally manufactured. Since the agent that allows the glass to turn purple, manganese, was only used in manufacturing for a relatively short period of time, a "sun colored" purple piece can be dated more easily than can many other types of glass. In order to capitalize on the purple color, some dealers leave glass outside in the sun to turn the glass color faster than it would if the glass was displayed in or used inside a home. In addition, some dealers have used germicidal lamps that emit ultraviolet rays to speed up the process even more. This process cannot be reversed. Since there are fewer and fewer examples of clear manganese glass as time goes on, deliberately changing the glass color reduces the number of better examples that collectors can enjoy. To make the problem worse, there is no way to tell whether a piece has been altered artificially or is the result of a natural process.
Why does this glass turn purple?
Beginning in the 1860's, glass makers in the United States started using manganese to give glass clarity instead of lead due to lead shortages created by the Civil War. Glass containing manganese eventually turns a light purple when exposed to sunlight, a side effect that was unknown at the time that this practice started. By roughly 1915, most companies in the United States switched from manganese to selenium to maintain clearness in their glass. Thus, with a few exceptions, including Heisey, Fostoria and Imperial, companies that produced this type of glass likely manufactured it from the 1860's to the years around 1915.
How can I tell if clear glass has manganese in it?
You could put the glass out in the sun and see what happens, but this presumes that you own the piece and are willing to risk reducing the long-term value of it by "damaging" it. If you have a piece of glass that you think may contain manganese, take it into a dark room and flash a black light on it. The glass should glow yellow. However, black lights vary in wave lengths and filters, making it more difficult for a novice to interpret color ranges. The best advice is to ask an expert.
