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Woad and Indigo

Photo by Adobe Stock/goodmanphoto

Blue is among the most coveted of natural dye colors, in large part because true blue pigments are rare in nature. Many blue flowers are actually purple-tinged, and while animals, such as birds, fish, and lizards, often make use of refractive scales and feathers to appear blue, that’s caused by a trick of the light rather than imbedded pigments. Of the plants that do produce blue pigments, indigo and woad are by far the most well-known, and both are rich sources of indigotin, a crystalline compound that’s the main constituent of indigo dye. This article is the second of a two-part series on natural dye plants that create primary colors; we discussed plants that produce red and yellow dyes in “Madder and Weld,” Heirloom Gardener, Winter 2019.

Growing Woad

Woad (Isatis tinctoria) is a cruciferous plant — a member of the cabbage family, like broccoli and mustard — native to Turkey and the Mediterranean, though it’s now naturalized throughout Europe and much of North America. It may be most famous for its use in the skin paint Celtic warriors applied before battle. In addition to its striking blue color, it has mild antiseptic properties. Woad will readily spread in rich, high-nitrogen soils, but it can tolerate poor soils provided it’s in full sun and well-watered. The biennial plants make low clumps of oval leaves in their first year, and send up tall flowering stalks with masses of bright-yellow blooms the following spring.

Most dyers use first-year leaves for color, and allow only a few plants to bloom and set seed; each flowering stalk will produce huge quantities of seeds for saving. Because of their tendency to reseed, woad is considered invasive in the western United States, so check with your local extension office before you make plans to grow it. Direct-sow the seeds about 1⁄2  inch deep in spring, and water them well. The seed coats have a germination inhibitor that must be washed away before they’ll sprout. Reports vary on how far to space plants; denser planting means less weeding, but more competition for water and nutrients, so weigh your options.

Harvesting Woad

The indigotin content in woad leaves depends on the soil’s nitrogen content and the plant’s access to heat and sunlight. Harvesting after a hot, sunny period will yield far more blue dye than harvesting in cloudy weather.

Photo by Adobe Stock/Erik

Cut the leaves right at the base, leaving the stalks, and allow the plant to keep about one-third of its youngest leaves so it’ll regrow quickly. You may be able to get four harvests of leaves from a single plant, depending on your climate.

Growing Indigo

Persicaria tinctoria, or Japanese indigo, is the plant most people are referring to when they talk about “indigo.” It’s a tropical perennial; in Zone 9 and warmer, it can be grown directly in the ground, but aspiring dyers in cooler climates can grow it in greenhouses or containers. 

Soak the seeds overnight, and then sow them in 3-inch or larger plug trays 4 to 6 weeks before the last frost in your area, or direct-sow into warm, moist soil in tropical areas. Indigo doesn’t like having its roots disturbed, so try to start the seeds in the biggest pots that are practical, and repot infrequently. Indigo is a heavy feeder, so fertilize often, and keep the seedlings well-watered and in a humid environment. In many places, you’ll be able to put the pots outside for summer, but be sure to pull them into a protected shed or garage before the first frost. Keep overwintering plants watered, but don’t fertilize them until warm weather returns.

You can pinch the growing tips of the plants to encourage them to send out side shoots, which will offer more leaves for use in dyeing. Avoid stressing the plants, because they respond to stress by flowering instead of producing leaves, and the leaves are where the indigotin is.

Harvesting Indigo

Harvest depends on how you plan to dye with your indigo. For fresh-leaf dyeing, cut the stems and strip the leaves from them immediately; you’ll need the leaves to be as fresh as possible for the process to work.

Photo by Adobe Stock/bennnn

For extracted-pigment dyeing, monitor your plants for signs of flowering, and plan to harvest when you see the first flower shoots appear. To keep your plants growing, cut them 4 to 6 inches above the ground. See “Extracting Indigo Pigment” for the next step in the process.

Dyeing with Fresh Leaves

Materials dipped in a fresh indigo or woad leaf bath tend to look more turquoise than the deep-blue possible with extracted pigment baths (also known as “reduction vats”). However, fresh-leaf dyeing is the quicker and easier of the two dyeing methods.

Place your fresh woad or indigo leaves in a blender, add ice cubes and just enough water to help the mixture liquefy, and blend thoroughly. Pour the resulting slurry through a strainer into a bowl or pot; this will be your dyebath, so make sure it’s big enough and full of enough slurry to submerge whatever material you plan to dye.

Photo by FlickrTim Green

Wet the material you’re dyeing, and wring it out so it’s no longer dripping. Add it to the dyebath, and gently stir it around to get it thoroughly soaked with slurry. You can leave it in the bath as long as you wish; fresh indigo dyeing is an exception to the usual rule about color depth, and longer soaking will produce deeper color.

The material may appear green in the bath because it hasn’t yet oxidized. Remove the dyed material when it’s about the shade you want; it’ll darken slightly as it oxidizes, but not as dramatically as it would if dyed in an extracted pigment bath. Rinse the dyed material in cool water, and let it dry. Indigo- and woad-dyed materials are light- and wash-fast, though they may fade slightly as excess dye washes out.

Dyeing with a Chemical Woad Vat

In my experience, woad tends to produce a slightly greener blue than indigo, but has the same potential range of sky-blue to near-black depending on how many times you re-dip your material. These instructions are for a chemical vat, which can be disposed of through a city water system, or by pouring into a nonfood garden when you’re finished.

Roughly chop or tear the fresh leaves, and pack them into a pot or bucket. Pour boiling water over them until they’re fully submerged, and then let them steep for 30 to 45 minutes. Place a colander over a second pot or bucket of the same size, and pour the “woad tea” through the colander into the second pot. Discard the leaves. The steeping liquid should be a dark red-brown at this point.

Next, add ammonia or dissolved soda ash to the steeping liquid until it reaches a pH of 9.0 or slightly higher. The liquid will turn dark, murky green.

Aerate the dyebath by pouring the liquid from one container to the other for 5 to 10 minutes, until it’s very frothy and blue-green. This step must be continuous and rapid to fully aerate the dyebath. This will convert the indigo precursor molecules into nearly dye-ready molecules — but they’ll still be in insoluble form. 

Photo by Getty Images/c11yg

When your dyebath is the right color, add thiourea dioxide (also known as “thiox”), dissolved in a small amount of water, at a rate of 1 tablespoon per gallon of dyebath. This will reduce the dyebath, converting the indigo molecules to soluble form. After 20 to 45 minutes, depending on the size of your dyebath, the liquid should be a translucent yellow-green, with a thick, oily, rainbow sheen on top. If it’s not translucent (indigo white), or if it’s still dark-green, wait another 10 to 15 minutes, and then add another teaspoon of dissolved thiourea dioxide. Stir gently to avoid reoxidizing the dyebath.

Wet the materials you want to dye, and squeeze them out so they’re no longer dripping. Add them slowly to the dyebath, to avoid introducing oxygen, and stir them gently to distribute the dye evenly across their surfaces. Remove them after about five minutes, and hang them up or spread them out to allow the indigo to oxidize. The materials will turn from bright yellow-green to blue. You can deepen the color by dipping the material as many times as you wish, although you’ll eventually use up all the indigo in the bath.

When you’re done dyeing all your materials, stir the bath vigorously to get the thiourea dioxide to react with the remaining indigo. This will make the thiourea dioxide safe to dispose of outside or down a household drain. If you want to be extra careful about disposing of used dye water, make sure that the pH is 7.0 or neutral.

Extracting Indigo Pigment

Traditional indigo extraction involves several fermentation periods alternated with drying periods, and requires hundreds of pounds of indigo leaves to work properly. Few home growers have that kind of space (or spare labor), but you can still extract and refine pigment to store for later. The process is very similar to creating a woad dye bath.

Submerge packed indigo leaves in a bucket or tub, weighted with bricks or clean rocks to keep them underwater, and allow them to ferment for 3 to 5 days. After the fermentation, remove and compost the leaves. Use some of the remaining water to dissolve soda ash, or add ammonia directly to the tub, until the bath reaches a pH of 9.0 or slightly higher. 

Next, aerate the tub vigorously, until the liquid turns dark red-brown. You may even see particles precipitating and falling to the bottom of the tub at this point. The larger the container you’re using, the longer you’ll need to agitate; a canning jar may only take 5 to 10 minutes, while a large tub or livestock tank may require 30 minutes of focused work.

Photo by Adobe Stock/Brent Hofacker

You’ll now start concentrating the pigment: Let the container sit for hours or days, until you can see a layer of sediment on the bottom. Gently dip or pour the liquid off the pigment, stopping every time you see the pigment starting to lift from the bottom. Allow the sediment to resettle. As you remove liquid from the top of the container, neutralize it by adding dissolved citric acid before discarding it down a household drain or outside. Continue settling and removing liquid until you can’t pour or siphon off liquid without disturbing the sediment. If you’re working with a large container, transfer the solution to smaller buckets or jars as you go to make pouring off the excess liquid easier.

When you have a super-concentrated slurry, strain it through a fine sieve to remove any remaining leaf bits. Remove leaf bits from the sieve, and then carefully rinse the pigment bits back into your slurry. You can store the resulting slurry in glass jars for up to a year. To dry the pigment for indefinite storage, set up a shallow tray with a screen over the top and a layer of fine silk cloth over the screen to catch the pigment. Gently pour the slurry over this, and allow the pigment to dehydrate in the sun. The dried pigment will easily peel off the silk, and can be stored in chunks or as a powder in a cool, dark, dry place.

Dyeing with a Henna-Reduced Indigo Vat

To transform extracted indigo into a dyebath, you’d normally need to make a stock solution and then add it to the dyebath. With henna (Lawsonia inermis) as the reducing agent, you can proceed straight to creating a dyebath.

Michel Garcia, a French chemist and dyer, developed this popular alternative to chemically reduced indigo dyebaths. Simply put, you’ll dissolve powdered extracted indigo; an alkalinizing agent (such as lime); and a powdered reducing agent (such as henna) in water in separate containers, mix them all in a pot with plenty more water, and keep the solution warm.

The trick to remember for this type of indigo dyebath is that it’s called a “1-2-3 vat,” after the proportions of ingredients in the order listed above. The total water volume of the dyebath matters only in terms of how much room you’ll need for the material you’re dyeing to move around and fully submerge, preferably without resting against the sediment at the bottom of the bath.

Photo by Adobe Stock/hepjam

For example, to make a medium-strength dyebath big enough for a shirt or a few skeins of yarn, use 4 tablespoons of indigo powder, 8 tablespoons of lime, and 12 tablespoons of henna powder. Mix the powders in separate containers, using about 1 cup of water for each. Indigo powder is hydrophobic; to make wetting it easier, you can fill a jar about three-quarters full of glass marbles, add the powder and the water, seal the jar, and shake it for 10 to 15 minutes. Avoid pouring the marbles into the vat with the wetted indigo, however.

Fill a 16-quart pot most of the way with hot water, and keep it between 90 and 130 degrees Fahrenheit. Add the three solutions you made in order, and mix thoroughly. You’ll need to scrape the bottom of the pot to keep the sediment suspended while you’re blending everything, but avoid introducing air, which will oxidize the indigo prematurely and turn the lime into insoluble calcium carbonate. Let the dyebath sit for a few hours to allow the sediment to settle again, and in the meantime, wet the materials you want to dye.

The dyebath is ready to use when it’s a clear green-brown, with a coppery iridescent film on the surface. Squeeze out the materials you’re dyeing so they’re not dripping wet, and gently place them in the dyepot, introducing as little air as possible. Let them soak for up to 15 minutes, and then gently remove them, squeezing them out as you go to avoid splashing. Let them oxidize fully, and then, if you want a darker shade, you can dip them again.

To neutralize the dyebath, stir vigorously, introducing oxygen until it’s fully blue. The lime will precipitate out as calcium carbonate as well. The dyebath can then be poured down a household drain or outside.

Entire books have been written about dyeing with indigo and woad; there’s far more to learn about creating, maintaining, and dyeing with vats of either plant than can be covered here. However, this article will get you started on a journey to creating these uniquely beautiful blues.

Seed and Powder Sources

This Color Won’t Run

Extracted pigment, whether from indigo or woad plants, requires specific chemical conditions to dissolve in water. Indigotin is only soluble in alkaline, low-oxygen solutions. Because these baths are almost clear, with only a slight green tinge, they’re called “indigo white” baths. Under these conditions, indigotin will be available in the dyebath to attach to your material. Removing the dyed material from the bath exposes it to oxygen and transforms the chemical compound into “indigo blue,” which is no longer water-soluble.

Adding oxygen to the prepared dyebath, whether by stirring vigorously or plunging material in too quickly, converts the soluble indigo white to insoluble indigo blue, which can no longer attach to your fiber materials.

Deeper color is achieved by layering indigo on the material you’re trying to dye, unlike other dyebaths, where color is deepened by spending more time in the bath, or by adding more pigment. If you want a darker blue than you achieved with your first dip, let your material oxidize fully, and then dip it in the vat again.


Caitlin Wilson is a Mother Earth Gardener editor, and lifelong textile enthusiast. You can find her projects, successes, and failures here.

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Published on Feb 10, 2020
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