Beause they made purple bottles [emoji6]
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Actually it wasnt always purple, in fact I would keep it near a window that see's lots of sunlight, it may get darker. Here's some info I copy and pasted from another site.. Nice find!
Upon exposure to sunlight, this glass will turn a light pink or lavender to moderately dark amethyst or purple depending on the amount of manganese and amount of ultraviolet (UV) light. This is called “sun-purpled” or “sun colored amethyst ” (SCA) glass.
This bottle began its life as colorless glass and has “turned” a much darker than average color of amethyst due to the exposure to (likely artificial) UV light . The light lavender tint produced by manganese offsets the green tint of the iron impurities in sand creating a largely colorless glass.
The Venetians apparently discovered by the 15th century that manganese could be used to decolorize glass. Manganese became known as “glassmakers soap” due to the ability to “cleanse” or neutralize the effects of other impurities in the sand, particularly iron (Hunter 1950). Manganese dioxide induced colorless glass was, however, by far most commonly used from the 1880s to about the end of World War 1. At that time manganese dioxide use was greatly reduced for a variety of reasons, although largely because it did not work as well as other chemical decolorizers ( in the open continuous glass tanks used by the increasingly dominant bottle making machines – both semi-automatic and automatic.
It is often noted in the literature that the reason for the switch from manganese dioxide to other decolorants was due to the cut-off of German imports to the U. S. during WW1. Although all imports from Germany (and Europe in general) were greatly constricted during this time, Germany was not a significant source of manganese for the U. S., providing only 2% of the imported manganese supplies in 1910 just prior to the war (Kendrick 1964; Lockhart 2006b). It should be noted that occasional manganese dioxide decolorized bottles may date as early as the 1820s or as late as the 1930s, although the large majority of bottles made with manganese decolorized glass were made between about 1890 and 1920.