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I was recently sorting some Mardi Gras beads. These are cheap plastic beads on strings, coated in cheap metallic paint. While sorting the beads into different colors, I noticed that the purple beads seem to have suffered more fading than the other colors. This is an effect I have noticed in the past: if the beads sit in the sun for a day or two, the colors fade rapidly. I'm guessing that the cause of that is that the UV radiation from the sun is breaking the molecules used for the pigments in the paints.

My first thought was that I had a pile of beads exposed to sunlight, and the purple beads in that pile had been at the top of the pile. But I noticed this happening in a few different piles that I had stored in different places and for different lengths of time. I have also noticed that the red beads and the purple/pink beads show some fading, though not as much as the purple beads. The yellow, silver, and green beads show almost no fading.

My current hypothesis is that the manufacturers use a pigment in the red and purple beads that is more susceptible to UV damage than the other pigments. So my question is: what is the chemical in these purple and red paints that is breaking down? (I guess there's also an implicit question here: is my thinking here chemically plausible?)

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  • $\begingroup$ My first guess would be that they use organic dyes, instead of inorganic pigments. Dyes are in general less stable and may decompose via UV decomposition, chemically (UV assisted oxidation) or thermally. Without knowing the exact dye used, it is difficult to sketch a reaction, but in principle I fully agree with your reasoning. $\endgroup$ – ssavec Nov 24 '14 at 7:05
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I would also agree with you and the comment from ssavec. The only point that I would make is that it shouldn't be because of the colour of the pigment only the stability of the particular dye it contains (almost certainly organic). In fact the colour of the paint should make it less susceptible to fading as it is reflecting colours at the violet end of the spectrum (subject to your definition of purple) rather than absorbing them.

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