Why Does Sunlight Fade Colours ?

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Offline neilep

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Why Does Sunlight Fade Colours ?
« on: 04/03/2007 20:18:13 »
Ewe sit in the sun...you get sunburn !..well..I do...and I hate sunbathing anyway !

But...why do things like paintings, wood, even hair...fade in the sunlight ?

Me wants to know cos me wants to know ?

Can ewe (groan) shed some light on this ?  (double groan !..with apologies)........
Men are the same as women, just inside out !

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Offline eric l

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Why Does Sunlight Fade Colours ?
« Reply #1 on: 04/03/2007 20:24:39 »
It would take a load of organic chemistry top give a complete answer, so I stick to the headlines.
  • colour in pigments (for paints) is very often related to the presence of double bounds (mainly conjugated double bounds
  • sunlight gives enough energy to trigger a reaction by which these double bounds can form radicals in a first stage, after which the radicals react with each other to give structures without double bounds.
It is not a complete proof, but mineral pigments like red lead, or chrome yellow will not fade in sunlight.
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