In the 1860s the formula for making pressed glassware changed. Manganese was substituted for lead to act as a stabilizer and to make the glass brighter and clearer. Nearly every major American glass manufacturer used manganese dioxide for such a purpose until 1915, when industrial chemists realized that selenium made for a better stabilizing/clarifying agent.
Because of the nature of manganese, the glassware manufactured during the late nineteenth century has some unique properties. Original manganese glassware glows brightly under a blacklight (although vaseline glass, glass tinted yellow with uranium does the same thing). When exposed to the sun over the years, as in a bright kitchen or a window, manganese glass takes on a slight amethyst purple cast. Glass objects with a faint hint of sun purple betray their provenance, but they also lose some value–since antiques dealers regard the effect as “discoloration”.
However, some people became obsessed by the sun purple effect and put their antique glassware outside for months in order for it to fully turn cloudy purple in the sunlight. This “solarized” glassware could then be sold to novice antiques collectors (often with a little card explaining “sun-purpling”). Dealers realized that the causative factor behind the color change was ultraviolet radiation, and so instead of putting glass outside they exposed it to radiation from UV sterilizers (a common anti-microbial tool in bio labs and hospitals). As you read this, somewhere out there is a room full of ornate glass pitchers, sugar dishes, and goblets being irradiated with blistering ultraviolet waves!
Seasoned glass dealers are aghast at the practice, which leaves everything a murky washed-out pale purple. Additionally there is glass currently being deliberately manufactured to resemble the manganese purple solarized glass. To confuse the issue even further, there is also glass manufactured in a robust shade of purple (for people who like purple) which is named “amethyst” glass.