|MadSci Network: Chemistry|
Hi there. Your question about natural dyes is very interesting, but unfortunately there isn't an easy answer.
The first thing to say is that compared to man-made dyes natural dyes fade very easily - which at least means your experiment will be relatively short! Imagine trying to find out the light-fastness of high quality coloured curtains - it might take years to see an effect.
Secondly, the assessment of fading is very difficult and lots of factors can influence how the result looks.
This absorption is where the light-fastness comes in - if a molecule absorbs light it means it is continually taking in energy and has to have a mechanism for dealing with the energy. If the mechanism is not very good then the energy could be enough to break chemical bonds within the dye molecule and once you break bonds within dye molecules then they won't absorb light in the visible spectrum - the fabric they're fixed to will appear to lose colour i.e. fade.
Unfortunately, you can't predict the light fastness of a dye from its colour. To make any kind of prediction you would need to know the molecular structure of the dye molecules (and even then it could be very difficult). With natural dyes I think this could be complicated by the fact that colour comes from a mixture of molecules. So, I think it very unlikely that, for example, red natural dyes will fade more than orange or yellow at the same depth of shade - if they do it is coincidence.
However, it might be interesting to see if the same dyes fade differently on different fabrics e.g. cotton versus wool. Or, test out if what Iíve said about fading assessment is correct - carry out the assessment in different lighting conditions, or test strong and pale shades of the same dye, or assess the fading with a white background and then black and red backgrounds. You might find lots of ways in which our eyes can be fooled when looking at colours!
Finally, don't worry about making a hypothesis that might turn out to be wrong! I read in a book somewhere a quote like: "the phrase spoken before most often scientific discoveries is not 'Eureka' but 'that's funny...'" In other words you learn a lot more from spotting things that don't fit an expected pattern (and finding out why) than in proving yourself right.
Good luck. David
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