Fostoria Century, TALL STEM drinking glasses, set of 4, Purple, irradiated For Sale

4 Vintage Fostoria CENTURY, TALL STEM, purple water glasses. There are no chips or cracks.
They are approximately 6 inches tall, 2.5 inch base, 3.5 inches across the top.
Purple/Amethyst Glass and the Irradiation Process:
The beautiful purple color of this glass is caused by the presence of manganese, which was used in early glass production as a de-colorizer to turn the glass clear. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history. Around 1915 most glass companies stopped using manganese, substituting selenium as the clearing agent. Glass that contains manganese will turn amethyst/purple when exposed to ultraviolet rays of the sun or other sources of UV rays. When the process of the sun is accelerated by using UV light exposure, it is called “irradiation”. It is a permanent process which reproduces the effects of many years of exposure to natural sunlight, but much faster. A glass item manufactured after this era will not turn purple with UV light exposure, so it is a true indicator of the items age.