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.
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.
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
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.
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.
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.
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.