Indians have been considered forerunners in the art of natural dyeing. Natural dyes find use in the colouring of textiles, drugs, cosmetics, etc. Owing to their non- toxic effects, they are also used for colouring various food products. In India, there are more than 450 plants that can yield dyes. In addition to their dye-yielding characteristics, some of these plants also possess medicinal value. Though there is a large plant resource base, little has been exploited so far. Due to lack of availability of precise technical knowledge on the extracting and dyeing technique, it has not commercially succeeded like the synthetic dyes.
Although indigenous knowledge system has been practised over the years, the use of natural dyes has diminished over generations due to lack of documentation. Also there is not much information available on databases of either dye-yielding plants or their products.
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 BC. Dyeing was known as early as in the Indus Valley period (2500 BC); 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 BC). 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
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.
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 ab- sorbed. 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.
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. 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.
Natural dyes obtained from plants:
Many natural dyestuff and stains were obtained mainly from plants and dominated as sources of natural dyes, producing different colours like red, yellow, blue, black, brown and a combination of these (Table 1). Almost all parts of the plants like root, bark, leaf, fruit, wood, seed, flower, etc. produce dyes. It is interesting to note that over 2000 pigments are synthesized by various parts of plants, of which only about 150 have been commercially exploited. Nearly 450 taxa are known to yield dyes in India alone, of which 50 are considered to be the most important; ten of these are from roots, four from barks, five from leaves, seven from flowers, seven from fruits, three from seeds, eight from wood and three from gums and resins.
Natural dyes are less toxic, less polluting, less health hazardous, non-carcinogenic and nonpoisonous. 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 make 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 mordents are hazardous(copper). 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.
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 alkannet 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.
Ocher is a dye obtained from an impure earthy ore of iron or ferruginous clay, usually red (hematite) or yellow (limo- nite). In addition to being the principal ore of iron, hematite is a constituent of a number of abrasives and pigments.
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 & Orissa. Unfortunately, no serious attempts have been made to document and preserve this immense treasure of traditional knowledge of natural dyemaking 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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