Woad Fermentation Vat




I mentioned in the previous post that I’d tried a fermentation vat from Helen Melvin’s booklet on indigo and the above photos show the initial stages of the vat. (I think the difference in colour between the vats in the photos is because one photo was taken indoors and the other photo outside.) The recipe is for indigo powder but I used woad powder instead. I have to confess that indigo and woad fermentation vats present a challenge to me, as I’m not a naturally patient person and I tend to become over-impatient with vats that can take several days to come into order and require frequent tending. This particular vat uses yeast and molasses to induce fermentation and remove the oxygen from the vat, and washing soda as the source of alkali. The pH level needs to be checked regularly, as the fermentation tends to make the vat increasingly acidic. When this happens, more washing soda has to be added. The vat also has to be kept warm over several days and this can require ingenuity as, even in the hottest UK weather, the temperature can drop considerably at night. As keeping the vat on a hotplate seems an unnecessary use of electricity and the weather was fairly hot anyway, this time I placed the vat (in a lidded plastic container) inside the tray of a  plant propagator, before putting on the lid. At night, I brought the vat into the conservatory and kept it overnight in a container of hot water. (On reflection, it might have been better to have left the vat in the propagator and packed it round with polystyrene or straw to keep it warm, as the water became cool during the night.) After about two days the vat was usable and I managed to dye a skein of wool a reasonable shade of blue. (As I wasn’t sure how well the recipe would work and I didn’t want to waste my precious woad powder, I suspect I was rather mean with it and that meant the colour produced was not very deep.) I kept the vat active for another couple of days but by this time my patience was running low and the odour of the vat was beginning to be rather unpleasant, even to my less than delicate nostrils. So I decided it was time to pour the vat away. 

IMG_2308                                                                                       This shows some of the dyed skeins

So why bother with fermentation vats? This is a question I ask myself each time I make a fermentation vat. I think the answer is that it does me good to be presented with a challenge and to have the chance to appreciate how skilled the dyers of the past must have been. I have convenient pH papers, with which to check the pH value of the vat, but the dyers of the past had to rely on experience, appearance and taste. They knew exactly how and when to “feed” the vat to keep it active over a long period of time and when to allow it to rest. I think I regularly try out fermentation vats so that I can feel entitled to call myself a dyer.

8 replies
  1. Debbie Bamford
    Debbie Bamford says:

    Thanks for showing the vat at work Jenny! It does make you think about how they worked – they may not have known the chemistry, but they knew what was happening and how to bring the vat into order – there’s a lovely recipe in John Edmonds Medieval Dyeing book (from Dominique Cardon I think) that describes the setting of the vat – even to the time of day to start so the water is at the correct temperature!

  2. Kathy Jolman
    Kathy Jolman says:

    Your information is so wonderful for the natural dying community, thank you.
    I have a question I hope you can help me with, I’m going to Window Rock, Arizona next week for a 3 day natural dying workshop with a Native American women, Rose (from the blogsite Weaving in beauty). I’m a quilter and am bringing my wool fabric along to use with the dyes. I have premordanted the fabric with alum, will that be a problem if they add alum to the dye pot? Should I bring some unmordanted fabric along also? Thanks for all your help.

    • Jenny
      Jenny says:

      Thanks for your comment, Kathy. Lucky you, going to the Native American dye workshop! I think it’s a good idea to take pre-mordanted fabric with you, just in case they have done their mordanting in advance. It won’t make any difference if they add the alum directly to the dyebath, as I think there is a limit to the amount of alum fabric will absorb anyway. However, I’d take some unmordanted fabric as well as this should give you a wider range of shades. Some of the dyes will probably fix without a mordant.
      Have a wonderful time!
      Good wishes

    • Jenny
      Jenny says:

      I think a pH level of 9 to 10 is best. With a fermentation vat you have to check regularly & add washing soda (or alternative source of alkali such as wood ash water) if the pH drops too much. Much will also depend on the recipe you are following.

      Good wishes

  3. Alison Daykin
    Alison Daykin says:

    I marvel at the skill of the dyes of the past and the dyers in many of the so called Third World countries. I too periodically go back to fermentation vats, but find they rather tricky without an Aga or Rayburn and only ever get a very pale blue before I can’t stand the smell any more!
    Thanks for sharing.

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