Making an indigo vat from dried, ground woad leaves (Isatis tinctoria).

Use dried, ground whole Isatis tinctoria leaves for creating light natural-blue reduction vats and exploring one of Europe's oldest blue dye plants.

You will need: dried woad leaves, slaked lime, and fructose. pH strips are a great idea too.

We are used to make an indigo vat with fermented and extracted indigotin pigment from Indigofera plants (roughly 30% indigotin for good quality indigo powder), or woad leaves extract (around 2-3% indigotin and isatins).

The classic 1-2-3 recipe indigo vat by Michel Garcia, of
one part indigo, 2 parts lime and 3 parts fructose, is based on purified or concentrated indigo of 30% indigotin, so applying the same ratio directly to raw dried woad leaf powder does not make much sense.

Commercial natural indigo powder may contain around 30% indigotin, while dried ground woad leaves contain mostly plant material and only a relatively small amount of potential blue dye. For these experiments, I am using 3% indigotin-equivalent as a working assumption for the dried woad. This is not a laboratory analysis of the material; it is simply a practical starting point that allows us to formulate and test the vat.

For example, 4 g of an indigo powder containing approximately 30% indigotin provides about 1.2 g of actual indigotin. To obtain a similar theoretical amount from dried woad at an assumed 3%, we would need approximately 40 g of woad leaf powder.

This is where simply applying the 1-2-3 ratio to the weight of the woad becomes problematic. A classic 1-2-3 calculation based on 40 g of woad would give us:

40 g woad + 80 g lime + 120 g fructose

But the 40 g of woad is not 40 g of indigo. Most of it is plant material. There is therefore no good reason to calculate the amount of lime from the total weight of the leaves.

One of the reasons for developing an organic fructose vat is to create a useful reduction vat without unnecessarily extreme alkalinity. Adding 80 g of calcium hydroxide per liter simply because we have 40 g of plant material would work against that goal.

Instead, I am starting from a fructose-vat recipe that already works well with concentrated indigo, but works per volume of 1 liter:

4 g indigo + 4 g calcium hydroxide + 12 g fructose per liter.

For the first raw-w​​oad experiment, I am therefore replacing the estimated dye equivalent of the indigo rather than its weight:

40 g dried ground woad + 4 g calcium hydroxide + 12 g fructose per liter.

The calcium hydroxide should then be adjusted according to the actual pH and behavior of the vat, because raw plant material will affect the chemistry differently from purified indigo. (I can always add lime, but reducing alkalnity i much more tricky).

The aim is not to force raw woad into the numerical 1-2-3 formula. The aim is to determine how much alkalinity and reducing sugar this particular material actually needs to reduce its available indigo precursors successfully, while keeping the vat as mild and workable as possible.

The steps:

Add half a liter of boiling water to half a liter of cold water to create a nice hot bath.

Add and stir in 40 grams of ground woad leaves. Then stir in the 12 grams of fructose and the 4 grams of slaked lime. Make sure you stir in all ingredients until fully dissolved. Let this vat rest for one hour while keeping it reasonably warm.

Where I live it is naturally hot, but if you live in a cold area you may want to insulate your vat and cover it to keep the heat in.

Check the pH using pH strips. With fresh calcium hydroxide, you should reach the required alkalinity without difficulty, although older or poorly stored lime may require a little more. My vat measured approximately pH 11, which is a good starting point.

Because this vat contains a large amount of raw plant material, its chemistry will behave differently from a vat made with purified indigo. Plant material can contribute organic acids and other compounds that consume some of the available alkalinity, so the pH may gradually decrease as the vat develops and is used. Rather than automatically adding more lime, check the pH periodically and adjust only when necessary.

Observe your vat (if using a glass beaker): you will slowly see the solids compound at the bottom, a clear red to yellow liquid on top of it and a green/blue frothy scum at the top.

After about an hour, your mini vat is ready to get to work. I dropped three pieces of fabric into the vat: one silk, one wool, and one cotton. I left them in for four minutes.

When you first remove the fabrics from the vat, they look quite horrible, to be honest. But after oxidation and rinsing, the final result is exactly what I was looking for: that characteristic grayish woad blue, with a softer, more muted and lived-in quality than the blues I usually get from regular indigo vats.

Of course, multiple dips will create deeper shades.

I am loving this for my color palettes, and I already know I will be using it much more often.. Which one do you prefer? The deeper regular indigos or this woad blue?