Indigo dye is an
organic compound with a distinctive blue
color (see
indigo). Historically, indigo was
extracted from plants, and this process was important economically
because blue dyes were once rare. Nearly all indigo produced today
- several thousand tons each year - is
synthetic. It is the blue of
blue jeans.
Uses

Indigo dye
The primary use for indigo is as a dye for cotton yarn, which is
mainly for the production of
denim cloth for blue
jeans. On average,
a pair of blue jean trousers requires 3 – 12 g of indigo. Small
amounts are used for dying wool and silk.
As a colorant for
food, indigo is called "indigotine", and is listed in the USA
as FD&C Blue No. 2, and in the
European Union as
E
Number: E132. Approximately 20M kilograms are produced
annually, again mainly for blue jeans.
Natural indigo
Relevant plant sources
A variety of plants have provided indigo throughout history, but
most natural indigo was obtained from those in the genus
Indigofera, which are native to
the
tropics. In temperate climates indigo
can also be obtained from
woad (
Isatis
tinctoria) and
dyer's
knotweed (
Polygonum tinctorum), although the
Indigofera species yield more dye. The primary commercial
indigo species in
Asia was true indigo
(
Indigofera tinctoria,
also known as
Indigofera sumatrana). In
Central and
South
America the two species
Indigofera suffruticosa (Anil)
and
Indigofera arrecta (Natal indigo) were the most
important.
Extraction
The precursor to indigo is
indican, a
colorless, water-soluble derivative of the amino acid tryptophan.
Indican readily
hydrolyzes to release
β-D-
glucose and
indoxyl.
Oxidation by
exposure to air converts indoxyl to indigo. Indican was obtained
from the processing of the plant's leaves, which contain as much as
0.2 – 0.8 % of this compound. The leaves were soaked in water and
fermented in order to
convert the
glycoside indican present in
the plant to the blue dye idigotin. The precipitate from the
fermented leaf solution was mixed with a strong base such as
lye, pressed into cakes, dried, and
powdered. The powder was then mixed with various other substances
to produce different shades of blue and purple.
Cultivation
The demand for indigo in the 19th Century is indicated by the fact
that in 1897, 7000 square kilometers were dedicated to the
cultivation of indican-producing plants, mainly in India.
For
calibration, the country of Luxembourg
consists of 2,586 square kilometers.
In literature, the play
Nildarpan by
Dinabandhu Mitra is based on the indigo
slavery and forceful cultivation of indigo in India.
It played an essential
part in the Bengali
indigo
revolt of 1858 called Nilbidraha.
History of natural indigo
Indigo was
used in India
, which was
also the earliest major center for its production and
processing.Kriger & Connah, page 120 The
Indigofera
tinctoria variety of Indigo was domesticated in India. Indigo,
used as a dye, made its way to the
Greeks and the
Romans, where it was valued as a luxury
product.
Indigo is among the oldest dyes to be used for textile dyeing and
printing.
Many Asian countries, such as India
, China
, Japan
and South East Asian nations have used indigo as
a dye (particularly silk dye) for
centuries. The dye was also known to ancient
civilizations in Mesopotamia, Egypt, Greece,
Rome, Britain, Mesoamerica, Peru
, Iran
, and
Africa.
India is believed to be the oldest center of indigo dyeing in the
Old World. It was a primary supplier of indigo to Europe as early
as the Greco-Roman era. The association of India with indigo is
reflected in the Greek word for the dye,
indikón (ινδικόν,
indian). The Romans
latinized the term to
indicum, which passed into Italian dialect and eventually
into English as the word
indigo.
In Mesopotamia, a Neo-Babylonian
cuneiform tablet of the 7th century BC
gives a recipe for the dyeing of wool, where
lapis-colored wool (uqnatu) is produced by
repeated immersion and airing of the cloth. Indigo was most
probably imported from India. The Romans used indigo as a pigment
for painting and for medicinal and cosmetic purposes. It was a
luxury item imported to the Mediterranean from India by Arab
merchants.
Indigo remained a rare commodity in Europe throughout the Middle
Ages.
Woad, a chemically identical dye derived
from the plant
Isatis tinctoria
(
Brassicaceae), was used instead.
In the
late fifteenth century, the Portuguese
explorer Vasco da Gama
discovered a sea route to India. This led to the
establishment of direct trade with India, the Spice Islands
, China, and Japan. Importers could now
avoid the heavy duties imposed by Persia
, Levantine, and Greek middlemen and the lengthy and
dangerous land routes which had previously been used.
Consequently, the importation and use of indigo in Europe rose
significantly.
Much European indigo from Asia arrived
through ports in Portugal, the Netherlands
, and England. Spain
imported the
dye from its colonies in South America. Many indigo
plantations were established by European powers in tropical
climates; it was a major crop in Jamaica
and South Carolina
, with much or all of the labor performed by
enslaved Africans and African-Americans. Indigo plantations
also thrived in the
Virgin Islands.
However,
France
and Germany
outlawed imported indigo in the 1500s to protect
the local woad dye industry.
Indigo was the foundation of centuries-old textile traditions
throughout
West Africa.
From the Tuareg nomads of the Sahara to
Cameroon
, clothes dyed with indigo signified wealth.
Women
dyed the cloth in most areas, with the Yoruba of Nigeria
and the Manding of Mali
particularly
well known for their expertise. Among the Hausa male dyers working at communal dye pits
were the basis of the wealth of the ancient city of Kano
, and can
still be seen plying their trade today at the same
pits.
In Japan, indigo became especially important in the
Edo period when it was forbidden to use silk, so
the Japanese began to import and plant cotton. It was difficult to
dye the cotton fiber except with indigo. Even today indigo is very
much appreciated as a color for the summer
Kimono Yukata, as this traditional clothing recalls
Nature and the blue sea. In colonial North America there were three
commercially important species: the native
Indigofera caroliniana, and the
introduced
Indigofera tinctoria and
Indigofera
suffruticosa.
Era of synthetic indigo
By 1897, 19,000 tons were produced from plant sources. Largely due
to advances in
organic chemistry,
production by natural sources dropped to 1000 tons by 1914 and
continued to contract.
These advances can be traced to 1865 when
the German
chemist
Adolf von Baeyer began working with
indigo. His work culminated in the first synthesis of indigo
in 1878 (from istatine), a second synthesis in 1880 (from
2-nitrobenzaldehyde. The production of
o-nitrobenzaldehyde was too complicated for a commercial product,
so the search for alternative starting materials was crucial for
BASF and Hoechst. The synthesis of N-(2-carboxyphenyl)glycine from
the easy to obtain
anthracene provided a
new and economically attractive route.
BASF
developed a
commercially feasible manufacturing process that was in use by
1897. In , 17,000 tons of synthetic indigo were produced
worldwide.
Developments in dyeing technology

Indigo white (leuco-indigo)

Yarn dyed with indigo dye
Indigo white
Indigo is a challenging dye because it is not
soluble in
water. To be
dissolved, it must undergo a chemical change (
reduction). Reduction converts indigo into "white
indigo" (leuco-indigo). When a submerged fabric is removed from the
dyebath, the white indigo quickly combines with
oxygen in the air and reverts to the insoluble,
intensely colored indigo. When it first became widely available in
Europe in the sixteenth century, European dyers and printers
struggled with indigo because of this distinctive property. It also
required several chemical manipulations, some involving toxic
materials, and had many opportunities to injure workers. In the
19th century, English poet William Wordsworth referred to the
plight of indigo dye workers of his hometown of Cockermouth in his
autobiographical poem "The Prelude". Speaking of their dire working
conditions and the empathy that he feels for them, he wrote,
Doubtless, I should have then made common
cause
With some who perished; haply perished
too
A poor mistaken and bewildered offering
Unknown to those bare souls of miller
blue
A preindustrial process for production of indigo white, used in
Europe, was to dissolve the indigo in stale urine. More convenient
reducing agent include zinc.
Another preindustrial method, used in
Japan
, was to dissolve the indigo in a heated vat in
which a culture of thermophilic,
anaerobic bacteria was
maintained. Some species of such bacteria generate
hydrogen as a metabolic product, which convert
insoluble indigo into soluble indigo white. Cloth dyed in such a
vat was decorated with the techniques of
shibori (tie-dye),
kasuri,
katazome, and
tsutsugaki. Examples of clothing and banners dyed
with these techniques can be seen in the works of
Hokusai and other artists.
Direct printing
Two different methods for the direct application of indigo were
developed in England in the eighteenth century and remained in use
well into the nineteenth century. The first method, known as
pencil blue because it was most often applied by pencil or
brush, could be used to achieve dark hues.
Arsenic trisulfide and a thickener were
added to the indigo vat. The arsenic compound delayed the oxidation
of the indigo long enough to paint the dye onto fabrics.

Freeze-dried indigo dye pot
The second method was known as
china blue due to its
resemblance to Chinese blue-and-white porcelain. Instead of using
an indigo solution directly, the process involved printing the
insoluble form of indigo onto the fabric. The indigo was then
reduced in a sequence of baths of
iron
sulfate, with air-oxidation between each immersion. The china
blue process could make sharp designs, but it could not produce the
dark hues possible with the pencil blue method.
Around 1880 the
glucose process was developed. It finally
enabled the direct printing of indigo onto fabric and could produce
inexpensive dark indigo prints unattainable with the china blue
method.
Since 2004
freeze-dried indigo, or
instant indigo, has become available. In this method the indigo has
already been reduced, and then freeze-dried into a crystal. The
crystals are added to warm water to create the dye pot. As in a
standard indigo dye pot, care has to be taken to avoid mixing in
oxygen. Freeze-dried indigo is simple to use, and the crystals can
be stored indefinitely as long as they are not exposed to
moisture.
Chemical properties

Indigo
Indigo is a dark blue crystalline powder that
sublimes at 390–392 °C. It is insoluble in
water,
alcohol, or
ether but soluble in
chloroform,
nitrobenzene, and concentrated
sulfuric acid. The
chemical formula of indigo is
C16H10N2O2.
The molecule absorbs light in the orange part of the spectrum
(λ
max = 602 nm). The compound owes its deep colour to
the conjugation of the
double bonds,
i.e. the double bonds within the molecule are adjacent and the
molecule is planar.In indigo white, the conjugation is interrupted
because the molecule is nonplanar.
Chemical synthesis
Given its once economic significance, indigo has been prepared by
many methods. The
Baeyer-Drewson indigo
synthesis dates back to 1882 but was impractical. The first
practical route is credited to Pfleger in 1901. In this process,
N-phenylglycine is treated with a molten mixture of
sodium hydroxide,
potassium hydroxide, and
sodamide. This highly sensitive melt produces
indoxyl, which is subsequently oxidised in
air to form indigo. Variations of this method are still in use
today. An alternative and also viable route to indigo is credited
to
Heumann in 1897. It involves heating
N-(2-carboxyphenyl)glycine to 200 °C in an inert
atmosphere with
sodium hydroxide.
The process is easier than the the Pfleger method but the
precursors are more expensive. Indoxyl-2-carboxylic acid is
generated. This material readily decarboxylates to give indoxyl,
which oxidises in air to form indigo.

centre

centre
Indigo derivatives
The benzene rings in indigo can be modified to give a variety of
related dyestuffs. Thioindigo CAS# [522-75-8], where the two NH
groups are replaced by S atoms, is deep red.
Tyrian purple is a dull purple dye that is
secreted by a common Mediterranean Sea
snail. It was highly prized in antiquity. In
1909 its structure was shown to be 6,6'-dibromoindigo. It has never
been produced on a commercial basis. The related Ciba blue
(5,7,5′,7′-tetrabromoindigo) (CAS# 2475-31-2) is, however, of
commercial value.

Structure of indigo carmine
Treatment with sulfuric acid converts indigo into a blue-green
derivative called sulfonated indigo (CAS#860-22-0). It became
available in the mid-1700s. Sulfonated indigo is also referred to
as
Saxon blue or
indigo
carmine. It is used as a colorant for food,
pharmaceuticals, and cosmetics. specification Indigotindisulfonate.
Safety and the environment
Indigo has a low oral toxicity, with an
LD50 of
5000 mg/kg in mammals.
In 2009, large spills of blue dyes had been
reported downstream of a blue jeans manufacturer in Lesotho
.[8107]
References
- Elmar Steingruber “Indigo and Indigo Colorants” Ullmann's
Encyclopedia of Industrial Chemistry 2004, Wiley-VCH,
Weinheim.
- Quoted in the Oxford English Dictionary, second
edition, 1989
- Kriger, Colleen E. & Connah, Graham (2006). Cloth in
West African History. Rowman Altamira. ISBN 0759104220.
Further reading
External links