Walk into any antique show in the Southwestern United States, and you are likely to see at least one booth filled with pretty purple glass of varying shades. The colors run 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 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.