Indigo - a dye

Indigofera tinctoria L. (Fabaceae)

Originally given its scientific name by Linnaeus in 1753, using plants from various sources, the species was said to come from India. The species epithet refers to its properties as a dye and the generic name from the Indus River, via the Portuguese word indigo, which was derived from the Greek Indikon — meaning from the River Indus.

Probably native to India, cultivation was widespread before the advent of synthetic dyes.

Life form: small shrub, often rather spindly.

Parts used: leaves used to produce a dye giving various shades of blue.

Ploidy level: diploid

Wild relatives

Pigments are colours which are applied to, but do not penetrate, a surface. They are applied as washes, or as oil paints. Dyes, on the other hand, differ from pigments in that when dissolved they penetrate and chemically bind to the material involved. They are used mostly on textiles or leather.

Many dyes do not chemically bind firmly to materials, so additives, called mordants, are used to promote absorption and chemical binding. Different mordants can produce strikingly different colour results using the same plant material. Dyes sued without mordant are called substantive dyes, while those that require a mordant to bind to fibres properly are known as adjective dyes. Mordants are usually naturally occurring materials or metal salts, one common mordant used both today and in ancient times is alum, potassium or ammonium aluminium sulphate. Common mordants used with particular dyestuffs are detailed in the Post Harvest section. Rather than treating all dye plants here, we have selected a few plants to highlight, showing the range of colours that can be achieved with natural products.

Blues

The famous blue dye indigo comes from the leaves of Indigofera tinctoria L., a shrub native to India, but now found worldwide in the tropics and subtropics. The genus Indigofera belongs to the pea family (Fabaceae) and has more than 500 species, found all over the world in tropical and subtropical regions. Other species of Indigofera used for dye include:

  • Indigofera arrecta Hochst. ex A. Rich. is a native of Ethiopia and commonly cultivated in tropical Africa; it has red-violet flowers and rounded pods. This species is often called 'Java indigo' as it was formerly extensively cultivated on Java.
  • Indigofera suffruticosa Miller is a native of tropical America and has been introduced into Asia and tropical West Africa; this species has yellowish flowers and straight pods.
  • Indigofera argentea Burm.f. is a native of Eygpt and north Africa and is extensively cultivated in that region. It is a very hairy plant with orange-yellow flowers.

The genus also contains several ornamental species that are cultivated for their attractive reddish purple flowers such as Indigofera heterantha Wall. ex Brandis in Britain.

Woad, or dyer's woad, comes from Isatis tinctoria L., a member of the mustard family, the Brassicaceae. Isatis contains some 30 species of biennial or perennial herbs, distributed from the Mediterranean to central Asia. Isatis tinctoria is native to Europe, north Africa and western Asia, and has been cultivated for many centuries. It is a biennial, forming an attractive rosette of bluish-grey leaves in the first year and a large inflorescence of yellow flowers followed by black fruits in the second. Dye is produced using the leaves, especially those of the first year's growth (the rosette).

Other blue (or bluish) dyes are derived from logwood (Haematoxylon campechianum L. of the pea family, the Fabaceae), a tree native to the forests of Central and northern South America, which produces a blue colour when mixed with certain mordants; and dyer's alkanet (Alkanna tinctoria (L.) Tausch of the borage or forget-me-not family, the Boraginaceae). Dyer's alkanet was the primary plant adulterant used with the deep purple dyes from shellfish (molluscs), and gives a purplish blue colour depending upon the mordant used.

Yellows

The roots of Reseda luteola L., a member of the mignonette family (Resedaceae), are the source of the yellow dye called weld. Reseda is a genus of approximately 55 species, native from the Mediterranean to central Asia. Reseda luteola is native to the lime-rich soils of the eastern Mediterranean basin and is also found in Macaronesia, but is now found naturalised throughout Europe, western Asia and North America. Another common name for Reseda luteola is dyer's rocket. Other species of Reseda are cultivated for their fragrant flowers (Reseda lutea L., Reseda odorata L.) and are called mignonette. Flowers of Reseda odorata are widely used in the perfume industry, although are odourless for some people.

Saffron, Crocus sativus L., is cultivated for its large 3-pronged stigmas, which are highly prized as both a dye and a condiment. The genus Crocus belongs to the iris family, Iridaceae, and contains about 80 species of corm-bearing herbs, often flowering in the spring, although some species, including saffron are autumn-flowering. Crocus sativus is a triploid species of hybrid origin, probably derived form the wild Greek species Crocus cartwrightianus Herbert, and is known only in cultivation. Other species of Crocus are important spring-flowering bulbs; e.g. Crocus flavus Weston, Crocus vernus (L.) Hill and Crocus chrysanthus (Herbert) Herbert. The autumn crocus belongs to a different genus, Colchicum — also a member of the Iridaceae.

Among other important yellow dyes are those derived from turmeric (Curcuma longa L. of the ginger family, the Zingiberaceae), which turned from red to yellow as cloth dried; and from the safflower (Carthamnus tinctoria L. of the daisy family, Asteraceae) of western Asia. Both these plants produced reddish dyes that changed to yellow.

Reds

Henna is produced from the leaves of Lawsonia inermis L., a spindly shrub to 5 m tall in the family Lythraceae (the loosetrife family). Henna is the only species in the genus Lawsonia and is found naturally in Eurasia from Iran to western India and along the Mediterranean coast of Africa, it has become naturalized in the West Indies, where it is known as West Indian mignonette, possibly due to its fragrant flowers. Despite its species name, which means 'spineless', adult plants of Lawsonia can be extremely spiny. The common name henna comes from the Farsi language of Iran (Persia) while in Egypt the common name is khenna, in Arabic al khenna, in Indian it is know as mendee, whilst in Britain it is sometimes known as Egyptian privet. Other members of the family Lythraceae include the crape myrtle (also sometimes known as Pride of India), Lagerstroemia indica L. and the purple loosestrife, Lythrum salicaria L., in which Charles Darwin studied sex expression in flowers. The dye from henna was usually used to tint hair or skin rather than cotton or wool fabrics.

Several species of the genus Rubia in the coffee family, Rubiaceae, are the source of bright red dyes. Rubia is a genus of some 60 species found from the Mediterranean through Africa and Asia, and the Americas. The genus is related to the familiar bedstraws (Galium spp.), and resembles them in its whorled leaves and small white flowers. Madder, also known as dyer's madder, is obtained from the roots of Rubia tinctorum L. a native of southern Europe and Asia Minor; this species is also used for the production of rose madder and madder brown paints. Munjeet or Indian madder, Rubia cordifolia L., is native to the mountainous regions of tropical and temperate Asia, and is also found in Africa. Unlike Rubia tinctorum, the entire plant is used to produce the dye. Rubia cordifolia is still used extensively in Nepal, where interest in native and natural dyestuffs is popular. Species of Rubia are also used in medicine, such as Rubia peregrina L. from Europe and the Mediterranean, which was also used locally in the same way as were the other species of Rubia. Other members of the family Rubiaceae include; coffee (Coffea arabica L.); Psychotria ipecacuanha (Brot.) Stokes, the source of the purgative ipecac; and Cinchona, the source of quinine.

Another important early source of reds was brazil wood — named not for the country, but for the colour of red hot coals. The original brazil wood was Caesalpinia sappan L. (Fabaceae) from the Malay Peninsula and Indonesia; as the name implies, it was the wood that was used to produce the dye.

Blacks

Oaks (the genus Quercus of the family Fagaceae) often produce galls as a response to attack by insects and mites. Inside each of these galls is usually a developing larva whose interaction with the plant chemistry causes growth and production of chemicals. The galls are a valuable source of tannin. The Aleppo oak, Quercus infectoria Oliv., is a frequently used source of galls in Mediterranean Europe. Aleppo tannin is also used in making ink.

Leaves and fruits from several species of walnuts, Juglans spp. (Juglandaceae) were used to produce a black dye. Anyone who has prepared walnuts from the fruits will know of the staining properties of walnut sap — it turns the fingers dark black for several weeks. The genus Juglans contains more than 20 species most are native to eastern Asia and North America, but several species also occur in the Andes and others in Europe. Juglans nigra is a species from the central and eastern United States, and is also used for wood and its edible seeds. Juglans regia L. is the English walnut, highly prized for its edible seeds.

Origins of cultivation

Indigofera tinctoria was originally domesticated in India, where it is mentioned in manuscripts dating from the fourth century BC. It was wrongly believed to be a mineral pigment by Dioscorides (c. 40-90 AD) or Pliny (23-79 AD), neither of whom appreciated the extensive cultivation in the Levant and Egypt.

A blue dye, probably from woad as indigo was not yet recorded as being used, was found in Egyptian cloth dated from the 2000 BC and hieroglyphics dating from 1500 BC refer to the sue of blue dyes. There is some disagreement as to whether this blue dye in early Egypt was woad or indigo, but woad has also been found in early Neolithic sites in France. Woad was extensively used in Eurasia and the Mediterranean basin by 1000 BC. It was cultivated in Asia and elsewhere before the introduction of indigo from India. The Celtic tribes of the British Isles painted their bodies with woad, giving rise to the name Britain – from brith, the Celtic word for paint (Julius Caesar in 52 BC referred to them as the Pictae in Latin). The blue colouring extracted from woad is chemically similar to that found in indigo and is treated in the same way.

Henna is one of the oldest plants used for cosmetic purposes, but whether it was cultivated or collected from the wild is unclear. The earliest written evidence of its use is in 1000 BC; it was clearly used before this time as it has been found as a colorant of the fingernails, finger tips, palms and soles of the feet of Egyptian mummies. There is some evidence that as early as 3200 BC, indigo and henna were being used together to dye hair black. By the time the Jewish people were captives in Egypt in about 1200 BC the custom of dying hair and skin with henna was becoming popular among them as well as among the locals, but the practice was frowned upon by the Orthodox.

Madder was cultivated extensively cultivated all over southwest Asia and Europe until the nineteenth century. It was largely propagated vegetatively from rhizomes, but the exact date of its original cultivation is impossible to determine. Chemicals identifiable as coming from madder have been extracted from red-dyed flax textiles from the Egyptian 18th Dynasty (1370 BC), and various Greek, Roman and Hebrew sources from about 1000 indicate that madder was extensively cultivated from Persia (now Iran) to the Mediterranean basin. Cultivation and domestication must have occurred before then. Munjeet, Rubia cordifolia, has not been cultivated outside its native region, nor has Rubia peregrina. This latter is the species that was used to the dying of Coptic textiles of approximately 500 AD, combined with indigo to produce a brilliant purple.

Dyer's rocket or weld has been cultivated since at least 1000 BC, where evidence of its use has been found in lake sites (Neolithic) in Switzerland. It was the preferred dye for the robes of the Roman vestal virgins, and was clearly cultivated throughout the Roman Empire at the time.

Saffron is unknown outside cultivation, and it probably originated in the Eastern Mediterranean/Near East region, then spread towards Asia. The use of saffron as a dye was mentioned in the Song of Solomon in the Bible and Cleopatra used it as a cosmetic. Saffron plants are featured in Minoan paintings in Crete (before 1000 BC). It was also apparently cultivated in ancient Persia (now Iran), but dates are not known.

Oak galls cannot be 'cultivated'; the gall is an outgrowth of the plant in response to attack by an insect that, unlike mistletoe, another parasite of oak trees, is impossible to encourage.

Walnut leaves and fruits were certainly gathered from the wild rather than being cultivated as were more herbaceous plants such as woad or madder. Their use, to prevent hair turning white, is recorded by the Roman historian and naturalist Pliny in the first century AD.

Early uses

The dyeing of linen and leather in Mesopotamia and northern Syria and Egypt was being carried out as early as 2000 BC in temple workshops, where the sacred vestments for gods and priests were dyed. In Ancient Greece, dyeing was a state monopoly, but private dyeing took place, after the purchase of a dyeing licence. In Palestine, as in Syria, dyers were concentrated in certain towns. Complete information on the processes of dyeing is not available, although recipe books for dyers did exist in Ancient Egypt. The oldest known book on the subject is from the third century; this includes notes of an alchemist describing dyeing cloth with alkanet, safflower, saffron, kermes, madder and woad. Woad and indigo were used by the ancient Egyptians: dyes have been found on cloth of about 2500 BC and on later mummy wrappings, though they were apparently not in common or extremely use until 300 BC. In Hellenic times, vat dyeing of wool with indigo was commonplace. It was Caesar who described the native tribes of Britain as fierce warriors painted blue with woad; Britain takes its name from Celtic word brith meaning paint. The Greek herbalist Dioscorides also described the leaves as being astringent, which helped to stem the flow of blood. The detergents used in the dyeing process were contained in the roots of Saponaria officinalis L. (soapwort of the carnation family, Caryophyllaceae), a well known plant in ancient times. Natron and the root of asphodel were also used. The use of urine as a mordant for fixing woad and indigo is mentioned in about 200 BC describing the foul smelling hands of the dyer. The urine-vat is rarely used now, whilst the fermentation method is common in the East.

Woad was more commonly used in Western Europe as it grew widely in the region. The young leaves were milled to a pulp, made into balls and dried for a few weeks after which the mass was powdered and left to ferment with water until a dark colour was produced, after which it was put into barrels for storage or to be sold. When used, it was re-fermented in a heated vat and the cloth dyed. Because colour production from woad is laborious compared to production from indigo, indigo began to replace woad for production of in the sixteenth century.

The most highly valued, and noblest, colour of ancient times was purple. In general however, purple dyes were not of plant origin, but instead derived from molluscs such as Murex. But to obtain the prized purple colour, wool could be dyed blue by the Romans with indigo and then dyed again with some other dye that added a red colour, such as madder. There are also recipes for this process in early Egyptian papyri. After dying with indigo (or woad, there is some disagreement as to which blue was being used), ashes were scattered over the blue-dyed wool and trodden in by foot. The wool was rinsed and clay and pickled in alum (an important mordant). Finally the madder was dissolved, together with flour of beans (to soften the water) and the treated wool dyed for the second time.

The Egyptian papyri also mention that by using different mordants colour shades of blue produced by the use of indigo or woad could be varied.

Henna was a symbol of immortality and resurrection in Egyptian and Greek mythology and has been used as a cosmetic since ancient times. The ancient Egyptians powdered the leaves and made them into a paste that was then used by women (and men) to dye their finger and toenails. It was also used to dye hair and by men to dye their beards and moustaches, even for dying the manes and tails of horses. The dying of both toe and fingernails was a common practice, which has continued for a long time - as the nails of mummies have been found to have been dyed this way. The Egyptians are also thought to have extracted oil and made an ointment from the flowers for making limbs supple. The ancient Hebrews and Indians and people in northern Africa also used henna to dye the soles of their feet and palms of their hands. In Persia, men also used henna to dye their beards red, and then dyeing them black with a solution of indigo. Even children had their hair dyed red and it is said that Mohammed dyed his beard red with henna. Even today in Yemen, men who have made the pilgrimage to Mecca have red beards that have been dyed with henna. It is said that henna helps cool the skin, which may be why it came into use as a dye for the skin of palms of the hands in the way it did. The 'camphire' mentioned in the Song of Solomon in the Judeo-Christian Bible is probably henna, which was thought to have been introduced to Jewish women by Pharaoh's daughter when she became Solomon's wife. The fragrance of henna flowers was said in the Song of Solomon to be like that of roses.

Dyeing with madder or munjeet is a long process, and must take place slowly. The wool was worked for five hours in stages of alternate immersion and removal until the correct shade was obtained. In ancient Palestine, dyeing was a respected craft. Wool thread was dyed before being woven. Yarn was laid out to dry after dyeing and the surplus liquid expressed. Sometimes a bran mash was used as a mordant. Purple-dyed wool was stored in bags or wrapped in skins. The re-dying of indigo-dyed wool was also common in Palestine. Dyes were expensive: the Hebrew word for spices, which were also expensive, was the same as the word for dyes. In ancient and medieval times madder was also used for medicinal purposes, especially for amenorrhea (loss of menstruation).

Madder produces a variety of shades of red depending upon the mordant used. A very strong, non-fading red was developed in India and spread, from where it became know as 'Turkey red'. Its production involves some twenty separate processes using blood, oil and rancid fat, along with charcoal, animal dung of various sorts and the liquid contents of animal stomachs. It is not surprising that villages in which the industry was concentrated were inhabited solely by dyers and their families! The technique was imported into Europe in the eighteenth century (Turkish carpets were, before 1750, unique in having Turkey red in their make-up). In northern Europe madder production was a speciality of the Low Countries (Holland). In the eighteenth century in England madder was used for soldier's uniforms and for the classical pink coats worn in fox hunting.

Weld was described by the Greek herbalist Dioscorides as a classic yellow dye-plant. In the first century AD, the Roman historian Pliny said that weld was used exclusively for colouring women's garments. The use of other yellow dyes was discouraged in the thirteenth century, partly because it was thought to impart a permanent colour. In medieval times Jewish men were compelled to wear yellow caps through much of Europe, these were dyed with weld. Weld was used to make blue cloth green and when added to madder could produce an orange dye. It was used as a dye for cotton, silk, linen and woollen fabrics as well as calico-printers, colour-makers and wallpaper manufacturers. Gradually, the use of weld was superseded by use of black oak (Quercus velutina Lam.) from North America, which weight or weight produced a stronger yellow. Dyeing cloth first with weld (yellow) and then woad (blue) produced green, but boiling fabrics in a solution with verdigris and alum could also produce a green.

In medieval times, ink was produced from crushed oak galls, which contain much gallic acid and tannin and ferrous sulphate. Also used was carbon made from scorched vine-shoots, though getting the exact consistency and density was difficult. Medieval craftsmen had recipe books for making dyes, some of the information having been collected from Alexandria about AD 600 and translated into Latin 200 years later. Certain terms for dyestuffs are Arabic and Persian. There is an early medieval craftsman's manuscript from about 1130 written in Catalonia (near Barcelona in southern Spain) that contains many recipes for pigments, dyestuffs, inks, varnishes and glues. By medieval times many pigments and dyes were not made from plant products; for example green from copper plates packed in fermenting grape-skins, or but other greens still came from plant products.

The most common black dye in medieval times was produced by adding extract of galls to iron sulphate or to superimpose several dark layers of shades, one over the other, so that the density deepened layer on layer.

Any mixture of iron salts in alum gives darker shades, the preparation of pure alum became important, and hence the alum trade was very important from the earliest classical times until the end of the nineteenth century. alum is important historically as it is the first substance prepared in what was a pure state. This was not due to chemical knowledge but the resulted from the dependence of alum for its use on its freedom from irons salts making it valued as a mordant.

Transfer and spread

Indigo was recognised as a valuable blue dye by most early explorers of the Indian region. The Venetian explorer Marco Polo described in detail the Indian indigo industry and by the eleventh century, Arab traders had introduced indigo to the Mediterranean region, where it became more popular than the local blue dye (woad). Indigo was brought to Britain in Elizabethan times (1500-1600), but its use was banned there and in other European countries due to protests from woad growers, whose business was being undercut. Today, indigo is still used to dye jeans –the irregular attachment of the dye causes the bleaching and mottling effect. Indigo has become naturalized in the southern United States.

Woad is a native plant from the Mediterranean basin to central Asia and was probably taken into cultivation in a wide variety of areas. Its use in ancient times was widespread, from Egypt to Britain. Cultivation of woad was continuous in France from the 1400s to 1700s. The early centre of woad cultivation in Britain was the county of Somerset, where the stench of its production (involving fermentation) was such that Elizabeth I (1533-1603) forbade its production within 7 km (5 miles) of any her palaces.

Henna grows naturally from the Mediterranean to central Asia and is widely cultivated throughout the region. It was taken to the West Indies by European settlers, where it has become firmly established and naturalized.

Use of madder for red dye spread from India to the Middle East and thence to Europe by the Middle Ages. By medieval times, madder cultivation was important in France and Germany, but by the sixteenth century the Dutch maintained a monopoly on cultivation and production. The production of Turkey red was a closely guarded secret in France until the mid-eighteenth century, when industrial spies obtained the secret recipes and modified them for use in Holland and England. By 1784 a cleaned-up method of producing Turkey red was being employed in the textile mills of Manchester. Both cultivation and use of madder declined precipitously after the invention of synthetic dyes in 1826. Munjeet, Rubia cordifolia, has become widely cultivated throughout tropical Asia, but has not been taken further afield.

Weld is now known in Europe, and grows as far north as Scotland; dyer's rocket has been extensively naturalized around the world in temperate areas. Grown easily from seed, it has been taken by European settlers all over the world.

The black walnut was taken from North America to Europe in the sixteenth or seventeenth century as a timber tree; the wood is prized for cabinet making.

Agriculture

Most species of Indigofera grow in tropical or subtropical climates and cultivation, such as it is today, occurs usually in small plantations of shrubs.

Woad is an exhaustive crop and needs to be moved to ensure that crop rotation works well and soil fertility is maintained. The leaves can be harvested 4-6 times a year and are then immediately milled and ground.

Henna grows as a shrub or small tree throughout its range, but is not intensively cultivated. Because it is perennial, trees can be harvested over many years.

Plants of madder were propagated vegetatively by rhizomes, and can also be grown from seed. It prefers well-drained, lime-rich soil and can survive on almost pure chalk. The stems are brittle and must be supported to encourage growth. Madder should be grown for at least three years before harvesting the roots. Similar time scales are necessary for munjeet.

When the flowers seeds and stalks of Reseda luteola turn yellow the plants are ready for harvesting. The plants can be hung upside down to dry for processing later. Weld is generally propagated by seed and is grown as an annual or biennial crop. Species of Reseda often grow in disturbed places, and weld has been introduced to suitable areas all over the world.

Many Juglans species are cultivated as street trees in the United States and Europe, and plantations are established for nut production. Its use as a dyestuff is now largely confined to the speciality craft market.

Modern context

The cultivation and extraction of dyestuffs from Indigofera tinctoria was an important industry in India from 1750 up to the beginning of the 20th century. Synthetic dyes, developed since, have almost completely replaced indigo (and most other naturally occurring dyes as well).

The indigo dye was known in ancient Asia, Egypt, Greece, Rome, Britain and Peru. It was the dye used to colour denim for work clothes, now a worldwide fashion item as denim blue jeans, and also (darker) navy, especially navy woollens.

Woad was once grown primarily as a source of blue dye but now mostly cultivated for its clusters of small yellow flowers. The blue of woad is brighter but not as durable as that derived from indigo, and woad eventually became a source of fixing and reinforcing a colour rather than providing it. The last two woad mills in Lincolnshire stopped production in the 1930s, but cultivation of woad in France and Belgium has continued but with a focus on its production as a reinforcer and improver of indigo dyes.

The introduction of synthetic aniline dyes in the 1880s completely changed the scene for dyes. Woad's medicinal uses were for the treatment of St. Anthony's Fire and for plasters and ointments for ulcers and inflammation were of minor importance to that as a dye but today woad is mainly grown by those interested in its history and those who research into its ancient craft use.

Henna has been widely used in Europe since 1890 for tinting hair, usually applied in the form of a shampoo. When henna is mixed with other plants, such as indigo, a variety of yellow-red and black dyes can be produced. Chemically altered henna is now the basis of many modern hair colorants.

The active chemical component of madder, alizarin, reddens the bones of animals fed on madder. This property was used in the 19th century by physiologists as a way of tracing bone development during the study of the function of the various cells in growing bone. In the 1860s alizarin could be synthesized and the use of madder as the sole source of alizarin has almost completely ceased.

In England, from approximately the Middle Ages, weld in combination with dyer's greenweed (Genista tinctoria L., Fabaceae), indigo and Woad, was used to produce the colour known as Lincoln or Kendal green. The shade is referred to in Act II Scene IV of Shakespeare's play King Henry IV, part 1, where Falstaff laments 'But, as the devil would have it, three misbegotten knaves in Kendal green came at my back...'

Post-harvest

Most ancient dyes needed a mordant to fix them to the fibres being dyed. alum and various iron salts, earths and organic matter were used. Many dyes were prepared involving a method of fermentation, in some cases plant materials were left for many years. It is recorded that in the making of the famous Bulgarian Gobelin tapestries walnut husk were covered with water and left to ferment for two years before being used to dye the wool. In some cases various different plants were used to produce the necessary acids for dissolving some dyes. Dilute acids have the same effect as fermentation, and an earlier process involved the use of urine to dissolve various dyes. Conservationists repairing and cleaning old woollen tapestries can become well aware that urine was used as a mordant.

The chemical substances responsible for the blue colours in indigo and woad are exactly the same, but arise from slightly different precursors. In indigo, the naturally occurring precursor is a colourless water-soluble compound of indoxyl. Indoxyl is oxidized with oxygen from the air to produce indigotin, which is blue and insoluble. The blue of woad is also a result of indigotin, but there a precursor isatan-B hydrolyses to form the indoxyl leuco-indigo, two molecules of that combine to form indigotin. Both plants also contain indirubin and flavonols derived from kaempferol as well. Both woad and indigo are substantive dyes and do not need mordants to effect chemical binding. In 1883 the German chemist Adolf Bayer discovered the chemical structure of indigo which then made it possible to produce the dye synthetically in the 1890s. His method is still used - this synthesises the indoxyl by fusion with sodium phenylglycinate in a mixture of caustic soda and sodamide. Indigo can be converted into other similar compounds but the only commercially important chemical reaction is the reduction of the soluble yellow indoxyl leuco-indigo which is the form in which it is applied to textile fibres from where it then reoxidizes to indigo and turns blue.

Woad does not produce as much indigotin as indigo, though the chemical composition is the same. In ancient times the leaves of woad were pulped between immense wooden rollers turned by horses each led by a man. The woad was heaped, drained, kneaded and rolled into balls, two handfuls of the pulp making a ball. These balls were then dried, powdered, wetted again and fermented and left for 9 weeks during which time they were turned and sprinkled with water. The result was a dark clayey substance which as the dye. It was used as the basis for many 'sad colours' as the dark blue could be reduced to different shades and used as a fixer of other colours.

The chemical compounds responsible for the reds of madder and munjeet are the three-ring compounds the anthraquinones, the most active of which is alizarin. The pigments in madder are alizarin, rubiadin, purpurin, pseudopurpurin and mungistin, while munjeet contains many of these but also a unique pigment known as munjistin. Madder and munjeet both require a mordant to produce the deep red, common mordants used are alum (potassium or ammonium sulphate), chrome (chromium dichromate) and copper (copper sulphate). Depending upon the mordant used, the colour produced from madder can vary from apricot orange to deep red.

Lawsone (a napthaquinone substantive dye) in the leaves of henna reacts with the keratin in skin and human hair allowing it to hold an orange-red pigment. Other chemicals active in henna include the napthaquinone juglone (also found in walnuts) several flavonoids and tannins. The leaves and young twigs are pulverized into a fine powder, sometimes with various other plan additives. This is then made into a paste with hot water and then spread or painted into designs on the area of skin to be dyed, usually be left overnight. The action of the active components when mixed with hot water and applied to hair seals the oils and tightens the hair cuticle giving a rich, healthy shine. Modern henna hair colorants are applied for 1-2 hours, and wash out within 2-3 months. Distilled water from the sweet-smelling flowers can be made into a perfume, or used as a cosmetic. Henna used as a substantive dye gives a light tan, while the use of mordants such as alum, chrome or copper gives different shades of brown. Tin (stannous chloride) and iron (ferrous sulphate) used as mordants with henna give more orangish shades.

Medically henna has been used internally and externally for the treatment of jaundice, leprosy, smallpox and the fruit is thought to have properties that regulate menstruation.

The yellows of both weld and saffron are due to long-chain chemicals called carotenoids; Vitamin A is also a carotenoid. The main carotenoids in weld are luteolin and apigenin, luteolin is not found in other species of the genus Reseda. The principal pigment in saffron is crocin. Both these yellow dyes are adjective and are used principally with mordants, raw wool takes up very little colour. Weld used with alum or tin produces the brightest yellows, other mordants tend to produce greens or olive-greens. Use of tin (stannous chloride) as a mordant with saffron also produces a brilliant yellow, but in contrast to weld, the dye is readily taken up by unmordanted wool. Turmeric contains the carotenoid curcumin, also known in the food industry as the permitted food colouring E100.

The principal component in Juglans species is the napthaquinone juglone, whose action and ultimate colour is modified by the presence of acidic tannins also found in the leaves and husks. Mordants used with walnuts include copper, iron and alum – often in combination.

Since dyes were synthesized more than a century ago, vegetable dyes are rarely used commercially. But plant dyes produce more mellow tones as can be seen in 'village' or 'country' Oriental carpets, pre-industrial tapestries or ancient leather used in, say, the bindings of books. It is difficult to replicate these effects with synthetic dyes and the use of natural dyes is enjoying a renaissance. That said, however, it must be pointed out that the chemicals in these plant products are every bit as chemical as those of synthetic dyes and can have serious environmental effects if used without due care.


From: Seeds of trade (http://www.nhm.ac.uk/seeds)

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