why does crystal turn purple in the sun?

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Offline Boo

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why does crystal turn purple in the sun?
« on: 16/08/2006 17:30:35 »
so on many road trips going through many small shops i have seen purple glass or crystal ( it happens to both) and when i ask why its purple they say it has sat in the sun (purposly) until it turns purple, and some people collect this type of purple glass... but why does this happen? why purple? and is this the same effect that makes pictures fade and plastic covers (like sometimes headlight covers, or those side blinker covers) on cars turn yellowish? hmmm [8)]

becca :)
becca :)


Offline eric l

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Re: why does crystal turn purple in the sun?
« Reply #1 on: 18/08/2006 13:46:42 »
You do have some tough questions, Becca !
From what I remember, manganese and/or antimony were added to glass to make it clearer, but an exces of manganese makes the glass sensible to turning purple when subjected to sunlight or uv light.  As coal- or woodfired ovens were replaced by gasfired ovens, the importance of clarifying agents decreased.
I know that sometimes "modern" glass objects are put under uv-lamps to make them look older. And I suspect that sometimes mangenese is added, not to make it clearer but to make it sensible to growing purple.   And I don't think you will ever see modern window panes turning purple in the sunlight.  And why purple ?  That's a lot farther than my knowledge will stretch.
As for the fading of pictures or the discolouration of plastics, it is again the uv in action, by changing mainly conjugated double bounds (if you are familiar with those) that are responsible for colour.  And black-and-white photographs turning sepia-coloured is still some other effect.
"Wonder is no wonder" (Simon Stevin 1548-1620)