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Apr . 08, 2024 14:58 Back to list

Background on Indigo Leaf

Natural indigo is extracted from the leaves of indigofera tinctoria, a short shrub that is a member of the legume family. Indigo is now cultivated world-wide, but is probably native to South Asia (Glowacki et al. 2012:542). It has been the most important source of blue dye for much of recorded history, and there is archaeological evidence that indigo was being used to dye fabric up to 4,000 years ago in India. Natural indigo was a major global commodity in the 18th and 19th centuries, and industries sprang up all across the world. It was one of the major exports of the antebellum U.S. South, especially around low-country South Carolina and Georgia, which have subtropical climates ideal for growing indigo. However, a German chemist identified the chemical structure of indigotin and discovered how to synthesize it in 1883 (ibid:542). This lead to the collapse of the indigo-growing industry. Today indigo is still the most important blue dye for clothing and jeans, but almost all of it is synthetically produced indigo, not indigo extracted from a plant.

Indigo is unique among all natural dyes in how it attaches to fiber. Boiling it in hot water will have no effect, because the blue coloring compound indigotin is insoluble in normal water. The traditional method of applying it involves fermenting the leaves, extracting the dyestuff, dissolving it in a reduction vat, and then dipping fabric in the vat. When fabric is removed from the vat, the dissolved indigotin oxidizes and comes back out of solution, bonding immediately to the fiber. However, it is possible to short-circuit this laborious process by taking advantage of the chemistry of indigo leaves.

Indigo does not exist in the plant in the form of blue indigotin—otherwise the plant itself would be blue. Instead, it exists in the form of two precursors, beta-glucosidase and indican. If the leaf is damaged by grazing herbivores or gnawed by insects, the precursors are mixed together and blue indigotin forms. This may have evolved as a defense mechanism against predation, although the evidence is unclear (Daykin 2011:5). If high-quality indigo leaves are harvested very carefully, the two precursors are preserved even after the leaves are dried. They can later be blended with ice water to mix the precursor compounds and begin the chemical reaction leading to blue indigotin. As the cold water warms up, the beta-glucosidase cleaves the indican into a molecule of indoxyl, and the indoxyl reacts with oxygen dissolved in the water to form blue indigotin. Any wool or silk soaking in the water while this reaction occurs will be dyed blue, too. Using fresh indigo leaves it is possible to achieve beautiful shades of turquoise and ice blue, without needing to build a reduction vat.

Nature’s Strongest Blue

2. Safety Precautions

 

3. Recommended Supplies

 

  • Dye pot. Use a dye pot large enough to hold all your fibers, with plenty of room for them to move around and for the liquid to circulate freely.

  • Metal tongs. A pair of tongs is useful for stirring and taking fabric out. Use tongs dedicated to dyeing, and not for food preparation.

  • Rubber gloves. Wear rubber gloves while removing fibers from the indigo solution—when it is nearing completion, it can stain your hands blue.

  • Scale. Use a scale to weigh out fiber and dyestuff.

  • Blender. The indigo leaves will have to be blended with water; if possible, we recommend using a blender that is reserved for dyeing and not for food preparation.

  • Strainer. The ground indigo leaves will need to be strained out of the dyeing solution. We strain once through a metal strainer, then again through a coffee filter; cheesecloth would also probably work.

4. Preparation: Scour (Clean) the Fiber

Indigo bonds to fiber at the molecular level and does not require any mordanting. However, this is one dye where you want to be absolutely certain your fibers are scoured clean before you begin. With hot-dyeing, sometimes you can get away with not scouring your fibers completely because the heat of the dyebath will disperse minor bits of grime. Since this is a cold-dyeing method, however, any dirt on the fiber—including oily fingerprints, sweat stains, and lanolin—will become hugely obvious on the finished product.

 
 

For protein fibers (wool, silk, alpaca, etc.): Scour with a PH-neutral detergent or a wool scour.

  1. Fill a dye pot 3/4 full with warm water. Add a teaspoon of PH-neutral detergent like Synthrapol. Alternately, use a product sold specifically as a wool scour or silk degummer. Mix well until fully dissolved.

  2. Add the fibers to be scoured.

  3. Raise the heat to 160F (for silk) or 180F (for wool or alpaca). Hold for 30 minutes, stirring regularly but gently (you do not want to felt your wool).

  4. After half an hour, lift the fiber out with tongs and rinse in hot water that is the same temperature as the scour bath. This will prevent thermal shocks that damage the fiber. We usually have a second dye pot full of hot water, heated to the same temperature as the scour bath, to plunge the fibers into.

  5. Let the fibers cool to room temperature, then squeeze out excess water while wearing rubber gloves (you do not want to the oil in your fingerprints to attach to the newly cleaned fiber).

  6. Proceed with dyeing.

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