Woad Fermentation Vat No. One: Day Seven.

In fact a summary of days 2–7. In pictures and words. I really, REALLY wish I could share the smell as well, as I think that’s significant, but I daresay many of you wouldn’t enjoy the, er, pungency. It’s living in my office/spare bedroom, and I’m just hoping the smell won’t linger once the vat is finished. I had intended to keep it in the garage, but it wouldn’t be fair to inflict it on my husband.

According to the thermometer on the vat the brewing belt is running slightly hot, I think, nearer 28 than 23°C, so I’ve wedged a pencil between the belt and the vat to reduce the heat input.

pH papers from vat testing over the week:


The vat on Day Three, 12 May. pH 8-9
The organic matter is waterlogged and has sunk to the bottom. There are a few bubbles, some patches of scum, the vat smells of rotting vegetation. The pH is low (acid): I sprinkled 1/2tsp of garden lime on the surface and stirred it briefly and gently.


Over the next three days the vat liquid became darker, the smell of rotting vegetation intensified, the greyish scum became thicker. Daily pH tests gave readings of 7–9, lowered by the acids produced by the rotting woad and bran; I sprinkled another 1/2 tsp of lime on the vat on Days 3–7. When in doubt, go alkaline as indigo will not convert to the soluble form in an acid environment. The smell became more complex. I can’t think of a good way to describe it, but no longer ordinary rotting vegetation. Richer, darker.

Day Seven. I suspect fermentation has peaked, as after yesterday’s lime the pH remains high – it’s well over 9, the maximum on my packet scale. The vat liquid is dark, there are now scattered patches of a thin shiny, almost metallic scum on the surface. The smell is richer, more complex, and not particularly pleasant (I much prefer the odour of a fermenting urine vat!).


The combination of changed odour and high pH made me think it might be worth testing for blue: the strings are tied to some scraps of scoured muslin that I’ll leave in overnight, and for multiples of 12 hours. I have a strong suspicion that 100g of woad leaves won’t produce a strong blue in any case.


1 thought on “Woad Fermentation Vat No. One: Day Seven.

  1. Beth H

    So… what happened?!? You did such a wonderful job of recording your woad couching a fermentation vat process (with really useful details, like pictures and pH records). But… did it work? Did you get blue? Or is this actually a cautionary tale? We have made our woad balls, and plan to use them soon, so we are looking for experienced-based information like yours. Too many sources only recount the industrial methods (as seen by an outsider)- not super helpful!

    Thanks for any help!



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