Natural
indigo was the only source of the blue color until about 1900. The raw
materials used in the natural production of indigo are leaves from a specific
plant species containing only a small amount of the dye (about 2–4%).
Therefore, a large amount of plant material is required to produce a
significant quantity of dye. To ensure a controlled supply, indigo was planted
in many parts of the world. There are three types of plants with about 300
species that make indigo.
Leguminosae (pea family)
The most
famous indigo bearing plant is I.
tinctoria, commonly known as indigo. It is a shrub originally grown in the
tropics, particularly in India, Southeast Asia and the Middle East.
Cruciferae (cabbage
family)
This plant
is Is. tinctoria, commonly known as
woad. The plant was grown in the Mediterranean and Western Asia. Woad is also
grown in North America and in Europe.
Polygonaccae (dock family)
Dock is the
name applied to a group of broad-leaved wayside woads. Rhubarb comes from the
same family. This species is commonly called Japanese or Chinese indigo. The most
important indigo species is I. tinctoria. The species is also known as Indigofera
sumatrana. A common alternative used in the relatively colder subtropical
locations such as Japan’s Ryukyu Islands and Taiwan is Strobilanthes cusia. In
Central and South America, the two species I. suffruticosa (Añil) and
Indigofera arrecta (natal indigo) were the most important. In temperate climates,
indigo can also be obtained from woad (Is. tinctoria) and dyer’s knotweed
(Polygonum tinctorum)
Indigo extraction
The
traditional methods of extraction of indigo from the plants I. tinctoria and
woad (Is. tinctoria) differ from each other.
Extraction from I.
tinctoria
Indian method: Cultivation of indigo for
extraction of dye is an age-old practice in India. Although there are several
variations, traditionally the cut plant is tied into bundles, packed into the
fermenting vat and covered with clear water. The vats, which are usually made
of brick lined with cement, have an area of about 400 square feet and are 3 feet
deep, arranged in two rows over each other. The top vat is known as the
fermenting vat and the bottom as the beating vat. The indigo plant can steep up
to 10–15h, during which natural fermentation sets in. The liquor, which varies
from a pale straw color to a golden yellow, is then run into the beating vat,
where it is agitated either manually or mechanically. The color of the liquid
becomes green, then blue, and finally indigo separates out as flakes and is
precipitated to the bottom of the beating vat. The indigo can thoroughly
settle, when the supernatant liquid is drawn off. The pulpy mass of indigo is
then boiled with water for a few hours to remove impurities, filtered through
thick woolen or coarse canvas bags, then pressed to remove as much of the moisture
as possible, after which it is cut into cubes and finally air dried.
Japanese method: Historically, the Japanese have
used another method, which involves extracting indigo from the Polygonum plant.
In this process the plant is mixed with wheat husk powder, limestone powder and
lye ash. The mixture can ferment for about one week to form the dye pigment,
which is called Sukumo.
Extraction from Is.
tinctoria
In the
traditional method of producing indigo dye (also called woad) from woad, the leaves
were crushed to a pulp, which was kneaded into balls, which were then allowed to
dry for several weeks. These dried balls could then be stored. The balls needed
to be couched. The couching meant crushing the balls into powder and wetting it
and allowing the material to ferment for several weeks. After couching, the
woad was dark clay like material that was dried and packed tightly before use.
The dye
from woad was very impure and it gave only light colors, whereas the indigo
from the tropics was of better quality and could be used to produce darker blues.
This was the reason why the exotic indigo from the Indigofera species could overtake
woad so completely.
The modern
extraction method of indigo from woad uses the water solubility of the indigo
precursors in steeping the leaves in hot water. The precursors are broken down to
indoxyl and sugar moieties by enzymes in the plant. Subsequent aeration
produces indigo by oxidation of indoxyl.
Coloring component in
plant species
Indigo as
such does not exist in the leaves of indigo producing plants. Instead, there are
its precursors, indican in Indigofera and P. tinctorium species, and isatan B
in addition to indican in Is. tinctoria. Indican in fresh green leaves is
stable, as it is attached to glucose, forming a stable indicant glucoside.
However, when the leaves are fermented, indican is hydrolyzed (cleavage of
sugar residue) by an enzyme glucosidase present in the leaves to yield indoxyl,
which transforms rapidly into indigo by oxidative dimerisation.
Surprisingly,
indican is also biosynthesized in the human body from the amino acid tryptophan.
Part of the indican is degraded by intestinal bacteria to the smelly indole. Some
of the colorless and water soluble indican is also eliminated via the kidneys. In
rare cases certain people are unable to metabolize the indican properly, and
they excrete traces of the intensely blue indigo in their urine. This medical
condition is also known as PUB, ‘purple urine bag syndrome’.
Natural indigo purity
The purity
of plant-derived indigo even with the modern extraction method is somewhat low
when compared to synthetic indigo. Natural indigo contains impurities such as
indirubin, indigo-brown, indigo gluten and mineral matter. The indigo purity
for woad has been reported to be 20–40%, and for Indigofera indigo from 50% up
to 77%. There is also the question of the efficiency of the extraction; the theoretical
yield of indigo formation from indoxyl molecules has been discovered to be
approximately 60%. So, 40% of the indoxyl is lost during the process to
impurities such as isatin and indirubin and other by-products of the reaction.
Revival of natural indigo
Since the commercialization
of synthetic indigo, the use of natural indigo has almost become extinct. In
recent years, the demand for natural dyes has been increasing in many
countries, because of health and pollution effects and a revival of interest in
the relationship between dyes and culture. In the present time, indigo is still
cultivated for dyeing on a small scale in India and in some parts of Africa and
Central America. It is frequently grown as a secondary crop.
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