MadSci Network: Chemistry

Re: What kinds of dyes fade more in sunlight?

Date: Mon May 30 14:32:53 2005
Posted By: David Akerman, Staff, R&D Scientist, Madison Filter Ltd.
Area of science: Chemistry
ID: 1116991862.Ch

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.

  1. Always use natural light (go outside, or use a window-sill) when assessing fading - light from bulbs and fluorescent strips is slightly different and will distort the results.
  2. Use the same undyed fabric for all samples to minimise possible variation in the fabric.
  3. Try to dye each sample to the same "depth of shade" - in other words try to get the same quantity of colour (even if the actual colour is different) on to each sample. This is important because changes to pale shades are seen much more easily than changes to strong shades. If one of you dyes gives a much more intense colour than the others then use less of it when dyeing the cotton, or shorten the dyeing process.
  4. Assessment of fading is a comparison (you compare a before and an after), so cut samples in half and fade one half while keeping the other somewhere dry and dark. Assess the fading by placing the faded and unfaded halves on a white background (overlapping then slightly) and give it a rating from, say, 1 to 5 (1 = lots of fading, 5 = no fading at all). For more accurate results, get a group of people to assess the fading (without knowing anyone else's score) and average the numbers. This minimises subjective effects or variations in people's vision and colour perception, giving a more reliable result.
Thirdly, the cause of colour fading is down to the chemical structure of the dye molecule. To appear coloured a molecule has to absorb some of the light that reaches it and allow some light to reflect back - it is the reflected light which gives it a colour that you can see. So for example, when sunlight shines on a red t-shirt the dye molecules are absorbing all the light except the light at the red end of the spectrum. If a molecule absorbs light at the other end of the colour spectrum then it takes in the red light and reflects back the blue light - the dye will colour fabrics blue.

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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