More Rhubarb Samples

For several years a collection of my naturally-dyed samples has been held at The Royal Institute for Cultural Heritage ( in Brussels, Belgium. This centre covers all aspects of Belgium’s artistic heritage and its laboratories carry out work on conservation techniques and materials, including the analysis of dyes on textiles from all periods and from all over the world. My samples are sometimes used in this analysis work, mainly for purposes of comparison.

Every now and then I receive a request for further samples and a few weeks ago I was asked for some more rhubarb samples, in addition to the ones they already have. The samples requested were to show the pinks and greys available from rhubarb root when the colour is extracted in an alkaline solution, using either washing soda or household ammonia.

As I couldn’t readily locate any suitable samples, I decided to produce some. I first soaked the chopped rhubarb root overnight in a solution of water and washing soda. By this time the liquid was deep red in colour and I added two alum-mordanted samples and two unmordanted samples. I left the samples to soak for about 24 hours, then I removed them. I dried one pair of samples away from the light and the other pair of samples was dried outdoors in direct sunlight. The samples exposed to the light dried to a greyish shade through a process known as photo-oxidisation.

The photo below shows the colours.









The upper two samples are alum-mordanted and the lower two are unmordanted. The pink samples were dried away from direct light and the greyish ones were dried outdoors in sunlight.

6 replies
  1. Mel
    Mel says:

    So do the pink samples gray after exposure to sunlight or does the photo-oxidation only work with direct samples?

  2. Elisabeth Beverley
    Elisabeth Beverley says:

    My experience is that woad pink – the pink derived from boiling up the leaves a second time – changes in the light, it’s pinker in sunlight, browner out of the light. It continues to be photosensitive for some time but if kept out in the light it eventually stabilises to the (to my mind the more attractive) pinker colour .

    • Jenny Dean
      Jenny Dean says:

      Yes, you are quite right about the way the pinks and tans from woad seem to change as they dry. I’ve noticed this too. I don’t think they ever actually completely change colour in the sense of becoming grey or blue, but it’s possible to control the shade to some extent by the way the wool is dried.

  3. Rachael B
    Rachael B says:

    Does this explains a problem I had with fennel dye this spring? I prepared a really strong bath (400g fennel/50g wool) of bronze fennel and did a preliminary dip with some alum mordanted wool while it was hot and the wool dyed a strong yellow. Then I left it outside (in the sun) to cool for a number of hours before putting the alum mordanted wool in and re heating it. I could get no colour out of the dye bath at all…..until I added sodium bicarb and then I got a pretty lemon yellow. Does the fennel dye not respond well to reheating or did it photo oxidise which was somehow reversed by the sodium bicarb – a mystery to me. Also Rita Buchannan reckons fennel doesn’t respond to pH changes but mine did!

    • Jenny Dean
      Jenny Dean says:

      I doubt whether photo-oxidisation played a part in what you describe, Rachael. Photo-oxidisation usually refers to changes in the dyed colour on fibres when dried in the sunlight. I really can’t offer an answer as I don’t use fennel for dyeing, but I can’t see how letting the dyebath cool, then reheating it, could be a problem. It seems more likely that your first dip exhausted the amount of yellow dye you’d extracted & the dyebath needed to be simmered again to extract more colour. Adding an alkali (such as sodium bicarb) will usually enhance the depth of colour from yellow dyes & I expect it assisted the extraction of the colour.
      Good wishes

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