Chapter 33
Natural Colours and Dyes
INTRODUCTION
To understand the concepts of natural dyes and dye-yielding plants, there are three basic questions to be addressed: Why only certain plants are able to yield dyes? How does the plant benefit by producing dyes? What is the evolutionary explanation for production of dyes? Answers to the first two questions can be substantiated with two further questions, i.e. ‘Why do plants have so many different colours?’ and ‘What purpose might they serve for the plant?’ Green in most leaves is surely the most ubiquitous plant colour. The green pigment chlorophyll in leaves helps capture the sun’s energy and converts it to chemical energy, which is then stored and used as food for the plant. Colours in flowers are adaptations that attract insects and other animals that in turn pollinate and help the plants reproduce. Some plants have colourful fruits that attract animals to eat them, thus inadvertently spreading the plant’s seeds as they do so. Scientists believe that other pigments may help protect plants from diseases. Despite what we know about the role of a few of the thousands of plant pigments, the role of most colours in plants remains a mystery to us till date. Although plants exhibit a wide range of colours, not all of these pigments can be used as dyes. Some do not dissolve in water, some cannot be adsorbed on to fibres, whereas others fade when washed or exposed to air or sunlight. It remains a mystery: Why plants reward us with vibrant dyes? India has a rich biodiversity and it is not only one of the world’s 12 mega-diversity countries, but also one of the eight major centres of origin and diversification of domesticated taxa. It has approximately 490,000 plant species of which about 17,500 are angiosperms; more than 400 are domesticated crop species and almost an equal number their wild relatives. Thus, India harbours a wealth of useful germ plasm resources and there is no doubt that the plant kingdom is a treasure house of diverse natural products. One such product from nature is the dye. Natural dyes are environTo understand the concepts of natural dyes and dye-yielding plants, there are three basic questions to be addressed: Why only certain plants are able to yield dyes? How does the plant benefit by producing dyes? What is the evolutionary explanation for production of dyes? Answers to the first two questions can be substantiated with two further questions, i.e. ‘Why do plants have so many different colours?’ and ‘What purpose might they serve for the plant?’ Green in most leaves is surely the most ubiquitous plant colour. The green pigment chlorophyll in leaves helps capture the sun’s energy and converts it to chemical energy, which is then stored and used as food for the plant. Colours in flowers are adaptations that attract insects and other animals that in turn pollinate and help the plants reproduce. Some plants have colourful fruits that attract animals to eat them, thus inadvertently spreading the plant’s seeds as they do so. Scientists believe that other pigments may help protect plants from diseases. Despite what we know about the role of a few of the thousands of plant pigments, the role of most colours in plants remains a mystery to us till date. Although plants exhibit a wide range of colours, not all of these pigments can be used as dyes. Some do not dissolve in water, some cannot be adsorbed on to fibres, whereas others fade when washed or exposed to air or sunlight. It remains a mystery: Why plants reward us with vibrant dyes? India has a rich biodiversity and it is not only one of the world’s 12 mega-diversity countries, but also one of the eight major centres of origin and diversification of domesticated taxa. It has approximately 490,000 plant species of which about 17,500 are angiosperms; more than 400 are domesticated crop species and almost an equal number their wild relatives. Thus, India harbours a wealth of useful germ plasm resources and there is no doubt that the plant kingdom is a treasure house of diverse natural products. One such product from nature is the dye. Natural dyes are environmentally friendly for example, turmeric—the brightest of naturally occurring yellow dyes—is a powerful antiseptic which revitalizes the skin, while indigo gives a cooling sensation.
After the accidental synthesis of mauveine by Perkin in Germany in 1856, and its subsequent commercialization, coal tar dyes began to compete with natural dyes. The advent of synthetic dyes caused rapid decline in the use of natural dyes, which were completely replaced by the former within a century.
However, research has shown that synthetic dyes are suspected to release harmful chemicals that are allergic, carcinogenic and detrimental to human health. Ironically, in 1996, Germany became the first country to ban certain azo dyes.
In this chapter, we review the origin of natural dyes, plants and animals yielding dyes, chemical nature of these dyes, their advantages with limitation, technology involved with natural dyes production and present status of these dyes.
HISTORY
Natural dyes, dyestuff and dyeing are as old as textiles themselves. Man has always been interested in colours; the art of dyeing has a long past and many of the dyes go back into prehistory. It was practised during the Bronze Age in Europe. The earliest written record of the use of natural dyes was found in China dated 2600 B.C. Dyeing was known as early as in the Indus Valley period (2500 B.C.); this knowledge has been substantiated by findings of coloured garments of cloth and traces of madder dye in the ruins of the Indus Valley Civilization at Mohenjodaro and Harappa (3500 B.C.). Natural matter was used to stain hides, decorate shells and feathers, and in cave paintings. Scientists have been able to date the black, white, yellow and reddish pigments made from ochre used by primitive man in cave paintings. In Egypt, mummies have been found wrapped in dyed cloth. Chemical tests of red fabrics found in the tomb of King Tutankhamen in Egypt show the presence of alizarin—a pigment extracted from madder. In more modern times, Alexander the Great mentioned having found purple robes dating to 541 B.C. in the royal treasury when he conquered Susa, the Persian capital. Kermes (from the Kermes insect) is identified in the Book of Exodus in the Bible, where references are made to scarlet coloured linen. By the 4th century A.D., dyes such as woad, madder, weld, Brazilwood, indigo and a dark reddish-purple were known. Brazil was named after the woad found there.
Henna was used even before 2500 B.C., while saffron is mentioned in the Bible. The first use of the blue dye, woad, by the ancient Britons may have originated in Palestine, where it was found growing wild. The most famous and highly prized colour through the ages was Tyrian purple (noted in the Bible)—a dye obtained from the spiny dyemurex shellfish. The Phoenicians prepared it until the seventh century, when Arab conquerors destroyed their dyeing installations in the Levant. In the prehistoric times man used to crush berries to colour mud for his cave paintings. Primitive men used plant dyestuff for colouring animal skin and to their own skin during religious festivals as well as during wars. They believed that the colour would give them magical powers, protect them from evil spirits and help them to achieve victory in war.
Dyes might have been discovered accidentally, but their use has become so much a part of man’s customs that it is difficult to imagine a modern world without dyes. The art of dyeing spread widely as civilization advanced.
Primitive dyeing techniques included sticking plants to fabric or rubbing crushed pigments into cloth. The methods became more sophisticated with time and techniques using natural dyes from crushed fruits, berries and other plants, which were boiled into the fabric and which gave light and water fastness (resistance), were developed. Some of the well-known ancient dyes include madder, a red dye made from the roots of the Rubia tinctorum L., blue indigo from the leaves of Indigofera tinctoria L., yellow from the stigmas of the saffron plant (Crocus sativus L.) and from turmeric (Curcuma longa L.). Today, dyeing is a complex and specialized science. Nearly all dyestuffs are now produced from synthetic compounds. This means that costs have been greatly reduced and certain application and wear characteristics have been greatly enhanced. However, practitioners of the craft of natural dying (i.e., using naturally occurring sources of dye) maintain that natural dyes have a far superior aesthetic quality, which is much more pleasing to the eye. On the other hand, many commercial practitioners feel that natural dyes are non-viable on grounds of both quality and economics. In the West, natural dyeing is now practised only as a handcraft, while synthetic dyes are being used in all commercial applications. Some craft spinners, weavers and knitters use natural dyes as a particular feature of their work.
TYPES OF NATURAL DYES AND MORDANTS
Natural dyes can be sorted into three categories: natural dyes obtained from plants, animals and minerals. Although some fabrics such as silk and wool can be coloured simply by being dipped in the dye, others such as cotton require a mordant.
Mordant
Dyes do not interact directly with the materials they are intended to colour. Natural dyes are substantive and require a mordant to fix to the fabric, and prevent the colour from either fading with exposure to light or washing out. These compounds bind the natural dyes to the fabrics. A mordant is an element which aids the chemical reaction that takes place between the dye and the fibre so that the dye is absorbed. Containers used for dying must be non-reactive (enamel, stainless steel). Brass, copper or iron pots will do their own mordanting.
Not all dyes need mordants to help them adhere to fabric. If they need no mordants, such as lichens and walnut hulls, they are called substantive dyes. If they need a mordant, they are called adjective dyes. Common mordants are alum (usually used with cream of tartar, which helps evenness and brightens slightly); iron (or copper) (which saddens or darken colours, bringing out green shades); tin (usually used with cream of tartar, which blooms or brightens colours, especially reds, oranges and yellows), and blue vitriol (which saddens colours and brings out greens shades).
There are three types of mordant:
- Metallic mordants: Metal salts of aluminium, chromium, iron, copper and tin are used.
- Tannins: Myrobalan and sumach are commonly used in the textile industry.
- Oil mordants: These are mainly used in dyeing turkey red colour from madder. The main function of the oil mordant is to form a complex with alum used as the main mordent.
NATURAL DYES OBTAINED FROM PLANTS
Some important dye-yielding plant habitats, their distribution and coloring pigments. The increasing market demand for dyes and the dwindling number of dye-yielding plants forced the emergence of synthetic dyes like aniline and coal tar, which threatened total replacement of natural dyes. Even today, some dyes continue to be derived from natural sources, for example, dyes for lipstick are still obtained from Bixa orellana L. and Lithospermum erythroidine Sieb and Zucc., and those for eye shadow from indigo. The content or amount of dye present in the plants varies greatly depending on the season as well as age of the plants. There are also several factors which influence the content of the dye in each dye-yielding plant. In some cases, the dye content has not been thoroughly studied so far.
Medicinal Properties of Natural Dyes
Many of the plants used for dye extraction are classified as medicinal, and some of these have recently been shown to possess antimicrobial activity. Punica granatum L. and many other common natural dyes are reported as potent antimicrobial agents owing to the presence of a large amount of tannins. Several other sources of plant dyes rich in naphthoquinones such as lawsone from Lawsonia inermis L. (henna), juglone from walnut and lapachol from alkanet are reported to exhibit antibacterial and antifungal activity.
Singh et al. studied the antimicrobial activity of some natural dyes. Optimized natural dye powders of Acacia catechu (L.f.) Willd, Kerria lacca, Rubia cordifolia L. and Rumex maritimus were obtained from commercial industries, and they showed antimicrobial activities. This is clear evidence that some natural dyes by themselves have medicinal properties. Another example is lycopene—a carotenoid pigment responsible for red colour in tomato, watermelon, carrot and other fruits—also used as a colour ingredient in many food formulations. It has received considerable attention in recent years because of its possible role in the prevention of chronic diseases such as prostate cancer.
Epidemiological studies have also shown that increased consumption of lycopene-rich food such as tomatoes is associated with a low risk of cancer. Also it is interesting to note that lycopene is the precursor to bixin and norbixin, pigments from Bixa orellena, commonly used for colouring foodstuff. Apart from dye-yielding property, some plants are also used traditionally for medicinal purposes.
NATURAL DYES OBTAINED FROM MINERALS
Ocher is a dye obtained from an impure earthy ore of iron or ferruginous clay, usually red (hematite) or yellow (limonite). In addition to being the principal ore of iron, hematite is a constituent of a number of abrasives and pigments.
NATURAL DYES OBTAINED FROM ANIMALS
Cochineal is a brilliant red dye produced from insects living on cactus plants. The properties of the cochineal bug were discovered by pre-Columbian Indians, who dried the female insects under the sun, and then ground the dried bodies to produce a rich red powder. When mixed with water, the powder produced a deep, vibrant red colour. Cochineal is still harvested today on the Canary Islands. In fact, most cherries today have a bright red appearance through the artificial colour ‘carmine’, which is obtained from the cochineal insect.
CHARACTERIZATION OF DYES
A dye can be defined as a highly coloured substance used to impart colour to an infinite variety of materials like textiles, paper, wood, varnishes, leather, ink, fur, foodstuff, cosmetics, medicine, toothpaste, etc. As far as the chemistry of dyes is concerned, a dye molecule has two principal chemical groups, viz. chromophores and auxochromes. The chromophore, usually an aromatic ring, is associated with the colouring property. It has unsaturated bonds such as –C=C, =C=O, –C–S, =C–NH, –CH=N–, –N=N– and –N=O, whose number decides the intensity of the colour.
The auxochrome helps the dye molecule to combine with the substrate, thus imparting colour to the latter
CHEMISTRY OF NATURAL DYES
Dyes are classified based on their chemical structure (Table 33.1), method of application, colour, etc. As a model study here the auth or explains chemistry as described by Vankar. They are classified into the following groups based on chemical structure:
- Indigo dyes: This is considered to be the most important dye obtained from the plant I. tinctoria L.
- Anthroquinone dyes: Some of the most important red dyes are based on the anthroquinone structure. These are obtained from both plants and insects. These dyes have good fastness to light. They form complexes with metal salts and the resultant metal complex dyes have good fastness.
- Alpha-hydroxy naphthoquinones: The most prominent member of this class of dye is henna or lawsone (L. inermis L.).
- Flavones: Most of the natural yellow colours are hydroxy and methoxy derivatives of flavones and isoflavones.
- Dihydropyrans: Closely related to flavones in chemical structure are substituted dihydropyrans.
- Anthocyananidins: Carajurin obtained from Bignonia chica Bonpl.
- Carotenoids: In these the colour is due to the presence of long conjugated double bonds. Typical examples for this group are annato (B. orellena) and saffron.
PREPARATION OF DYES
The dye is generally prepared by boiling the crushed powder with water, but sometimes it is left to steep in cold water. The solution then obtained is used generally to dye coarse cotton fabrics. Alum is generally used as a mordant. Flowers of Butea monosperma (Lam) Taubert yield an orange-coloured dye, which is not fast and is easily washed away. For the purpose of colouring, the material is steeped in a hot or cold decoction of the flowers. A more permanent colour is produced either by first preparing the cloth with alum and wood ash, or by adding these substances to the dye bath. The dye indigo is produced by steeping the plant in water and allowing it to ferment. This is followed by oxidation of the solution with air in a separate vessel. Mallotus philippinensis Muell. yields an orange colour, used for dyeing silk and wool. To prepare the annatto dye from B. orellena L., the fruits are collected when nearly ripe. The seeds and pulp are removed from the mature fruit and macerated with water. Thereafter, they are either ground up into an ‘annatto paste’ or dried and marketed as annatto seeds. Sometimes when the seeds and pulp are macerated with water, the product is stained through a sieve and the colouring matter which settles out is collected and partially evaporated by heat and finally dried in the sun.
ADVANTAGES AND LIMITATIONS OF NATURAL DYES
Natural dyes are less toxic, less polluting, less health hazardous, non-carcinogenic and non-poisonous. Added to this, they are harmonizing colours, gentle, soft and subtle, and create a restful effect. Above all, they are environment friendly and can be recycled after use. Although natural dyes have several advantages, there are some limitations as well. Tedious extraction of colouring component from the raw material, low colour value and longer time makes the cost of dyeing with natural dyes considerably higher than with synthetic dyes. Some of the natural dyes are fugitive and need a mordant for enhancement of their fastness properties. Some of the metallic mordants are hazardous. Also there are problems like difficulty in the collection of plants, lack of standardization, lack of availability of precise technical knowledge of extracting and dyeing technique and species availability. Tyrian purple is obtained from the rare Mediterranean molluse Murex brandavis. In order to obtain 14 g of the dye, about 1,200 molluses are needed.
TECHNOLOGY FOR PRODUCTION OF NATURAL DYES
Technology for production of natural dyes could vary from simple aqueous to complicated solvent systems tosophisticated supercritical fluid extraction techniques depending on the product and purity required. Purification may entail filtration or reverse osmosis or preparatory HPLC, and drying of the product may be by spray or under vacuum or using a freeze-drying technique. Use of biotechnological methods to increase the yield of colourants in plants is also being attempted in several laboratories in India.
GENETIC VARIATION AND DYE CONTENT
Siva and Krishnamurthy studied an important dye-yielding plant, B. orellena, for understanding the relationship between degree of genetic diversity (using isozymes) of various populations and their pigment content. Bixin (C25H30O4 ) and norbixin (C24H28O) are carotenoid pigments that form the main components of B. orellena. The total amount of these two pigments in seed materials collected from 10 different geographical localities was estimated using HPLC. It was interesting to learn that the lowest band frequency shows the least total pigment and bixin content. Similarly, greater band frequency (i.e., genetic diversity) shows greatest dye content. In other words, it is likely that individuals with greater genetic diversity may have high dye content. Further critical study is needed to establish the relationship between the geographical localities with the dye content.
CONCLUSIONS
Nowadays, fortunately, there is increasing awareness among people towards natural products. Due to their non-toxic properties, low pollution and less side effects, natural dyes are used in day-to-day food products. Although the Indian subcontinent possesses large plant resources, only little has been exploited so far. More detailed studies and scientific investigations are needed to assess the real potential and availability of natural dye-yielding resources and for propagation of species in great demand on commercial scale. Biotechnological and other modern techniques are required to improve the quality and quantity of dye production. Due to lack of availability of precise technical knowledge on the extraction and dyeing technique, it has not commercially succeeded like synthetic dyes. Also, low colour value and longer time make the cost of dyeing with natural dyes considerably higher than with synthetic dyes.
Mahanta and Tiwari identified a few rare, endangered and endemic dye-yielding plant species during their study in Arunachal Pradesh. They reported that species of Ilex embelioides, Phaius tankervilliae and Entada purseatha are rare treasures amidst the rich floral diversity of Arunachal Pradesh. Numerous plant species are found to have an important role in the day-to-day life of the ethnic and local people. However, it is a matter of concern that the indigenous knowledge of extraction, processing and practice of using of natural dyes has diminished to a great extent among the new generation of ethnic people due to easy availability of cheap synthetic dyes. It has been observed that the traditional knowledge of dye-making is now confined only among the surviving older people and few practitioners in the tribal communities of Arunachal Pradesh. Unfortunately, no serious attempts have been made to document and preserve this immense treasure of traditional knowledge of natural dye-making associated with the indigenous people. Lack of a focused conservation strategy could also cause a depletion of this valuable resource. It is time that steps are taken towards documenting these treasures of indigenous knowledge systems. Otherwise, we are bound to lose vital information on the utilization of natural resources around us. To conclude, there is an urgent need for proper collection, documentation, assessment and characterization of dye-yielding plants and their dyes, as well as research to overcome the limitation of natural dyes.
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