Woad

Isatis tinctoria

Woad is an ancient source of blue dye - Indigo. It was used for centuries in Europe before cheaper indigo (from Indigofera species) was imported from the Far East. The blue colour is obtained by an elaborate process involving boiling, alkali, oxidation and reduction, but the beautiful shade which results is well worth the effort.

Woad is a perennial plant which is usually grown as a biennial. The leaves produced during the plant's first year are used for dyeing. In the second year, the plant grows a tall, branching flower spike.

Woad likes fertile soil that remains moist throughout the growing season.

Cultivation

Isatis belongs to the brassicae family, and like cabbages will benefit from lime being added to the soil. Woad seed should be planted early in the year, as soon as the soil is workable. Sow about one inch deep, spacing the rows 18 inches apart. Thin out the plants to about 10 inches apart in the row.

The woad plant is a heavy feeder. Apply a general fertilizer or compost when the plants are established. After each harvest, top-dress with compost or high-nitrogen fertiliser (or apply liquid feed) to boost leaf production. Water well during dry spells of weather.

Woad plants in full flower

Harvest, Preparation and Preservation

Pick the leaves between June and October. Several harvests should be possible, depending on the weather, and in a mild autumn it may be possible to continue harvesting later than October. Process the leaves as soon as possible, because the dye content will be lost if the leaves are stored.

The leaves contain Indoxyl and Isatan B, two chemicals which are converted to indigo during the preparation process. To extract these "precursors", place the leaves in boiling water for about five minutes. There is no need to chop the leaves first, but you may have to crush them so that you can get a sufficient quantity into your container.

After about five minutes, strain the liquid from the leaves, and cool it as quickly as you can. When the temperature is below 40 degrees Celsius, add an alkali, such as sodium carbonate or ammonia, to make the solution alkaline (pH 9 - 10). The brownish-red liquid will become dark green.

Next, oxygen must be introduced into the liquid to enable the formation of indigo: pouring the liquid several times from one container to another is effective. The foam on top of the liquid will become blue as indigo forms. Leave the liquid for about five hours for the process to complete. The insoluble indigo will settle to the bottom of the container.

Siphon off the excess liquid. I usually add cold water to the container to dilute the liquid, as otherwise it can be difficult to see the indigo sediment. After siphoning, I add water again, let it settle and siphon a second time. This "washing" helps to remove impurities (there are other dye chemicals in woad, beside indigo) which can produce purple or green tones.

When you have mostly sediment left in the container, you can use this to dye with, or you can filter it through a close-woven cotton cloth then dry the indigo for storage. If you're lucky, you may get 8 grams of solid dye from a kilogram of woad.

Dyeing

To use the extracted indigo, it must be converted to a soluble form with a "reducing" agent - a chemical that removes oxygen - such as sodium dithionite. Prior to the discovery of chemical reducing agents, the oxygen was removed by bacterial fermentation - a very smelly process!

If you have dried the indigo, it should be crushed to a fine powder. Heat the water in your dyepot to just over 50 degrees celsius. Add the indigo and stir gently to disperse it. Stir in a tablespoon per gallon (15g. to 5 litres) of sodium carbonate, to make the water alkaline. Next, sprinkle sodium dithionite on the surface of the dyebath - a level teaspoon per gallon, 5g. per 5 litres - and let it stand while the oxygen is removed. The dyebath will become greenish-yellow as the indigo converts to soluble "indigo white". If there is blue insoluble indigo still visible in the dyebath after about 15 minutes, add a little more dithionite.

The dyebath is now ready for dyeing the wool. Be sure to stir very gently during the dyeing process, so that you introduce as little oxygen as posible. Carefully add the prepared wool. Let the wool absorb the dye for a few minutes, then lift it gently out of the dyebath. Catch the drips from the wool in a bucket, rather than allowing the liquid to drip back into the dyebath. As the wool comes out of the dyebath, oxygen from the air will combine with the dye, turning it blue and insoluble. You can build up the depth of colour by repeated dipping, letting the wool "air" for several minutes between dips.

Overdye yellow (such as weld) with woad to obtain a good green which is quite difficult to achieve any other way using natural dyes. Magenta red (from cochineal) can be overdyed with woad to give purple shades.