"The Reincarnation of Natural Colourants A Review" B.J. AGARWAL* & B.H. PATEL Department of Textile Chemistry, Faculty of Technology and Engineering, M.S. University of Baroda, Vadodara. INTRODUCTION The practice of applying colour through dyeing and printing techniques has played a significant role in every civilization. Earlier dyestuffs were derived from natural resources, viz., plants, animals and minerals without any chemical processing. Vegetable dyes have been used for thousand of years by mankind(l). Natural dyes have been an integral part of human life since time immemorial. Egyptian mummies, documents of Mughal periods, etc. bear a testimony to the utilisation of these dyes. In India, Rajasthan and Kutch still possess a rich tradition in the use of natural dyes for textile dyeing and printing. In 1856, William Henry Perkin accidentally synthesised -- a basic dye, Mauveine. With the advent of coal tar dyes (now synthetic dyes), the use of natural dyes declined tremendously because the existing natural dyes failed to fulfil the demand of the market. Moreover, synthetic dyes were cheaper, more readily synthesised, give better and more reproducible shades with better fastness properties to various agencies. However, many synthetic dyes, particularly azo dyes that are prepared from ex-aryl amines have been found to be potentially carcinogenic (cancer inducing). Certain chemicals used in the synthesis of dyestuffs are also regarded to be carcinogenic, mutagenic as well as sensitising or allergic(2,3). Thus, based on "EC Control of Substance Hazardous to Health Act, 1989", a number of chemicals have been red-listed and have been banned in U.S. and Europe. German Government, in an ordinance dated June 16, 1994, has also banned twenty amines from use in any garment or any other article that come in contact with the skin. Due to this, the reincarnation of much less hazardous, eco-friendly natural dyes has been particularly realised by various textile manufacturers (including hand loom manufacturers). Germany, Australia and Denmark prefer import of Indian hand loom garments dyed with natural dyes (mostly vegetable dyes) to those coloured with synthetic dyes. The natural colourants are unsophisticated and harmo'nised with nature. They are obtained from renewable sources and their preparation involve a possibility of very little chemical reaction. Hence, they do not cause health hazards, but sometimes act as a health cure. Furthermore, the use of natural dyes offer no disposal problems(4). However, the natural dyes have their own limitations.
Limited number of suitable dyes, so the shade range is limited. Colour yield, dyeing efficiency, and cultivation efficIency. Reproducibility of shades. Complexity of dyeing process. Availability - natural resources are not available in all countries. Apart from these limitations, there are various other technical drawbacks also associated with the use of natural dyes (5,6). These are: Allow only wool, natural silk, linen and cotton to be dyed. Unsuitability for synthetic fibres. Great difficulty of blending dyes to produce compound shades. Lack of standardisation. Difficulty in collection of the dyes. Inadequate degree of fixation. Inadequate fastness properties, and Water pollution by heavy metals and large amounts of organic substances. CLASSIFICATION OF NATURAL COLOURANTS Natural dyes can be classified in various ways. The earlier classification was according to alphabetical aITangement of dyes. Later on, numerous other methods of classification were adopted, which are: Classification based on chemical structure Classification based on their origin or the sources from which they are obtained Classification based on their methods of application Classification based on their colour In Colour Index, the natural dyes are arranged and classified according to their chemical constitution as well as major application(7). Natural dyes comprises a separate section in Colour Index, where they are arranged according to hue within application class. Classification according to chemical constitution The natural organic dyes and pigments, constitute a wide range of chemical classes, such as indigoid, anthraquinonoids, naphthoquinones, po1ymethines, ketones, imines, quinones, flavones, flavanols, flavanones, and chlorophyll. Some of the important chemical classes are represented in Table 1.
Classification according to their sources Depending on the origin or sources from where they are produced, the natural dyes can be grouped into three distinct classes based on the dyes derived from
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Vegetable resources, Insects as well as animal resources, and Mineral resources. Dyes derived from vegetable sources Vegetable dyes are obtained from various parts of plants and herbs such as stem, wood, roots, bark, leaves, flowers, fruits, and skin of plants; which produce distinct pale to dark shades on both natural as well as synthetic fibres. Some important examples of dyes derived from plants or vegetables are logwood, turmeric, pine wood, catechu, madder, etc. Dyes derived from insects or animal sources Some of the most important red dyes, based on Anthraquinone structure, are obtained from insects or animals.
IMPORTANT
These dyes are characterised by good fastness to light. They combine with metal salts to form metal-complex dyes, which possess good wash fastness. Various examples of such dyes are Lac, Kermes, Cochineal, Lichen, etc. J
Dyes derived from mineral sources Natural dyes produced from mineral resources are chrome yellow, chrome orange, chrome green, iron buff, prussian blue, manganese brown, mineral khakhi, etc. Certain important minerals widely used as natural dyes are Cinebor (Sangraj), Red Lead (Sindur), Laminated Red earth (Geru), Ultramarine (Lajerd), Zinc white (Sajeda), etc. Classification according to their methods of application According to this classification, natural dyes can be classified as direct dyes and mordant dyes(8). The direct dyes may be further sub-divided into: * Direct dyes for cotton, e.g., turmeric, pomegranate, annatto, safflower, etc. * Direct dyes for wool and silk.
TABLE 1 CHEMICAL CLASSES OF NATURAL DYES
Natural dyes source
Colour produced
Indigo (indigofera tinctoria) Wood (isatis tinctoria) Tyrian purple (purple hoemastroma/murex brandaris)
Cotton, wool, silk Cotton, wool, silk Cotton, wool, silk
Blue Blue Blue purple/reddish purple
.Kermes (kermer vermilio) Cochineal (coccus cacti)
Wool Wool, silk, cotton (printing)
Pink, red, crimson, orange, brown and maroon* Red, scarlet, crimson and brown* Red and purple* Crimson, scarlet and pink*
Heena or lawsone (leaves of lawsonia inermis) Juglone (shells of juglans regia)
Wool and silk
Yellow to brown
Wool and silk
Weld (reseda luteola)
Cotton, wool and silk
Madder (rubia tinctorium) Manjith (rubia cordifolia) Lac (luccifer lacca)
}
Logwood (compeachy wobctWool, silk, cotton and leather haematoxylin campechianum) Brazilwood (red wood species, } Wool, silk and cotton caesalpinia echinate) Sappanwood (red wood species, caesalpinia sappan) Carajurin (leaves of bignonia chica) Awobanin (flowers of tsuyukusa cammelia communis) Annatto (bixa orelluna) Saffron (crocus sativus)
Wool and silk Wool a.nd silk
Yellow and orange Yellow
The direct textile fibres only to wool sub-stantivity cotton.
dyes for cotton can be applied to all natural as well; the acid dyes are mostly applicable and silk; while the basic dyes have a direct for wool and silk as well as tannic acid treated
I
On the other hand, the mordant dyes can be equally well fixed on both animal as well as vegetable fibres. Important mordant dyes include madder, logwood, cochineal, etc. Apart from these two classes of dyes, there are some other natural dyes, which are classified as vat dyes and are insoluble in water. These dyes include indigo, woad, Tyrian purple, etc.
Classification of natural dyes on the basis of colour (9) I
It is often quite customary to divide different sources of natural dyes on the basis of the colour which they impart on the fibre substrate. Yellow natural dyes Yellow colour symbolises growth and chear and is perhaps the most abundant hue in nature. The number of plants which yield yellow dyes are much higher than those yielding other colours. In India, yellow colour has special importance since it is still considered to be an auspicious colour having deep religious significance. Some of the yellow dyes which have been most commonly used are: Flavanoid dyes * French marigold * Flame of the forest
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Tagetes patula, Tagetes erecta Butea monosperma, Butea frondosa Artocarpus heterophyllus, integrifolia Mallotus phillipinensis Myrica esculenta Datisca cannabina Allium cepa Delphinium zalil
* Annatto * Saffron * Harsinghar * Indian Mahagany
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Bixa orellana Crocus sativus Nyctanthes arbortristis Cedrela toona
Diaroyl methane dyes * Turmeric
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Curcuma tinctoria, Curcuma longa, Rotunda, and Viridiflora
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*
Kamala Kaiphal * Hemp * Onion * Yellow larkspur Carotenoid dyes
Alkaloid dyes * Barberry
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Berberis aristata, berberis vulgaris
Quinanoid dyes * Dolu (Himalayan rhubard) * Heena
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Rheum emodi Lawsonia inermis
Red natural dyes 32 Red natural dyes have been listed in the Colour Index. Red dyes are extracted from the roots or bark of plants. They are also camouflaged in the bodies of dull grey insects. However, the sources of these dyes are limited. Since ancient times, the most beautiful red colour of legendary importance has been provided to these dyes by nature. Almost all natural red dyes have a basic quinone structure. They mainly constitute anthraquinones (alizarin, purpurin, munjistin, laccaic, etc.), or naphthoquinones (alkannin and shikonin), and there is only one benzoquinone dye (carthamin). Some of the prominent red natural dyes, as obtained from various sources, are listed below: Plant sources * Saffron * Madder
Animal sources * Lac * Cochineal * Kermes
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Carthamus tinctorius Indian madder (chay; oldenlandia umbellata) English madder (Rubia tinctorium) Caesalpinia sappan
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Luccifer lacca Coccus cacti Kermes vermilo
Mineral sources In some parts of India, shades of red were obtained with mineral products like laminated red earth (Geru) and sulphide of mercury (Cinnabart)(lO). Natural blue dyes The Colour Index lists only three natural blue dyes, viz., natural indigo, sulphated natural indigo and the flowers of the Japanese "Tsuykusa" used mainly for paper making. The most brilliant and the fastest blue shades are obtained from indigo on all fibres. The principal colouring matter is indigotin, whose main sources are:
Natural black dyes One important black natural dye is Logwood (Haemotoxylin Campechianum) which is also known as Campeachy wood because it was discovered by the Spaniards on the bay of Campeachy in Mexico. It is used even today for dyeing
silk in deep shades on an iron tannate mordant. It also gives excellent depth and fastness on most natural and synthesis fibres(ll ).
(i) Metal salts or metallic mordants, (ii) Tannins and tannic acid, and (iii) Oils or oil-mordants.
Tannins are important sources of black dyes. Pomegranate rind contains the hydrolysable tannic flavogallol, which combines with iron salts to give deep blacks.
Cotton can be treated with these mordants and thus acquire an affinity for basic dyes. Tannic acid and oil-mordants also act as a primary mordants for metallic salts. For instance, cotton mordanted with tannic acid becomes capable of absorbing all types of metallic mordant and can be readily dyed with mordant dyes. In such cases, the metallic mordant forms complexes with the carboxylic groups of tannic acid(ll).
Natural brown dyes Majority of browns are obtained from quinone based dyes, naphthoquinones and anthraquinones. Generally, copper and iron salts are used as mordants and they tend to turn the colour to dull and deep shades, particularly browns. Napthoquinones Sanderswood Anthroquinones Cochineal Chayroot Manjith Madder Lac Dolu
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Coccus cacti OZdenZandia umbellata Rubio cordifoZia Rubia tinctorium Luccifer Zacca Rheum emodi
Apart from these dyes, another natural dye (Flavanoid ~annin based) - Cutch: Acacia catechu also produce rich brown shades with copper and chromium salts. Mordants Natural dyes are either substantive, needing no mordant or adjective requiring a mordant. Adjective dyes only dye the material mordanted with a metallic salt or with the addition of a metallic salt to the dyebath itself. Examples of such dyes are logwood, madder, cochineal, fustic, etc. In pure state, the adjective dyes are generally only slightly coloured and give poor dyeings when used alone. Mordants (Latin, mordere - to bite; the mordant ate away the surface of the fibre so that the dye can skin in) are the chemicals in the form of metallic salts which are generally used to create an affinity between the fibre and the pigment. The main objective of the mordant is to open up the pores and render the fibres more suitable for the entrance and penetration of the colouring matter, thereby helping in the fixation of the dyestuffs on the substrate in case of adjective dyes. However, the mordants can also be used in case of dyes which are capable of being applied directly, where they form an insoluble compound with the dyestuff within the fibre itself, thereby improving the fastness properties of the dyed material. Generally: mordants can be classified into three categories, namely,
In short, mordants can be precisely considered as an integral part of the natural dyeing process by most dyers of natural dyes. Metallic mordants Naturally occurring metal salts were being used as mordants since long. Today, metal salts of aluminium, chromium, iron, copper and tin are also used. Some of the importan metallic mordants are alum, potassium dichromate, ferTous sulphate, copper sulphate, stannous chloride and stannic chloride. Most of the natural dyes are capable of forming metalcomplexes and thereby produce different shades (hues). Therefore, in actual practice, virtually all types of metal salts can be used for this purpose. However, some restrictions have been put by the famous 'German Ban' to the use of metal salts. The maximum permissible amounts of different metals in the final product are as follows(3): As (1 ppm), Cd (2 ppm), Co (4 ppm), Cr (2 ppm), Pb (l ppm), Cu (50 ppm), Ni (4 ppm), Zn (20 ppm). However, the upper limits of the presence of metals is variable for different products and it also depends invariably on the eco-standard chosen. Fortunately, there is no upper limit on aluminium, iron and tin and that on copper is also fairly high. So, these salts can be safely used for complexing an~. mordanting but their quantities should be optimised so as to minimise the pollution load. Tannins and tannic acid Tannins are the important ingredient in the dyeing of natural dyes producing yellow, brown, gray and black colours. They also improve the affinity of fibres towards different dyes. Tannins are mainly used in the preservation of leather, glues, stains and mordants. Vegetable tannins are bitter and astringent substances occurring as excretions in the bark, leaves, fruits, galls, etc. of plants. These excretions may be used directly or in the concentrated form.
Tannins are naturally occurring phenolic compounds of high molecular weight (ranging from 500 to 3000). The phenolic hydroxyl group enables them to form effective crosslinks between proteins and other macromolecules. Tan~in which contain o-dihydroxy (Catechol) group are capable of forming chelates giving different colours with different metals. Numerous ta'nnin-containing substances are employed as mordants in dyeing of textile fibres. The vegetable tannins are divided structurally into two distinct classes depending upon the type of phenolic nuclei involved and the manner in which they are joined together(3): (i) Hydrolysable tannins (ii) Condensed tannins Hydrolysable tannins These tannins are distinguished by having as a core a polyhydric alcohol, such as glucose, the hydroxyl groups of which are esterified either partially or wholly by gallic acid or its cogener. Such tannins are readily hydrolysed by acids, bases or enzymes to give carbohydrate and a number of isolable crystalline phenolic acids: thus they are called as hydrolysable tannins. The other acid isolated from these tannins is ellagic acid. Some of the important raw materials for these tannins are: Myrabolan fruit (Terminalia chebula) Oak bark and Wood (Quercus alba and other species) Sumach leaves (Rhus typhina) Gall nuts (Quercus infactoria) Pomegranate rind (Punica granatum) Myrabolan and sumach contain ellagitannic acid and gallotannic acid respectively, whereas Gall nuts contain 6077% tannic acid. The aqueous solution of tannic acid decomposes by fermentation on long standing, which can be inhibited by the addition of boric acid. The alkaline solution of tannic acid rapidly absorb oxygen from atmosphere which turns brown due to decomposition. Condensed tannins Tannins of this class contain only phenolic nuclei. When treated with hydrolytic reagent, particularly in acid solution, they tend to polymerise into insoluble, amorphous red coloured compounds known as phlobaphenes. These tannins are mostly formed by the condensation of two or more molecules of flavon-3-0Is, viz., catechin. Catechin was first iso-
lated by Runge, 140 years ago, from the tannins of Acacia catechu. It produces copper red colours on cotton, wool and silk with superior washing and light fastness.
Oil-mordants Oil mordants are mainly used in the dyeing of Turkey Red colour from madder. The main object of the oil-mordant is to form a complex with alum used as the main mordant. Alum is soluble in water and does not have affinity for cotton, so it is easily washed out from the treated fabrics. The naturally occurring oils contain fatty acids such as palmitic, stearic, oleic, ricinoleic, etc. and their glycerides. The -COOH group of fatty acids react with metal salts and get converted into -COOM (M denotes the metal; e.g. in case of alum it is Aluminium). It has also been found that when concentrated sui phonic acid is treated with oils, sulphonated oils are produced which have better metal binding capacity than the natural oils due to the introduction of sulphuric acid group -S03H. This can react with metal salts to produce -S03M. The bound metal can form a complex with the mordant dye such as madder to give Turkey Red colour having superior fastness and hue.
Application of mordants on various substrates On cotton The metallic mordants are soluble in water and are only loosely held by cotton fibres. So, these mordants have to be precipitated onto the fabric by converting them into insoluble form or by first treating the fibres with oil or tannic acid and then impregnating the treated fabric with the solution of a mordant, whereby the metallic mordants are held onto cotton via oil or tannic acid. Alum is a widely used mordant for cotton. In actual practice, addition of alkali to alum solution produces aluminium sulphate which is used as a mordant. This mordant is extensively used for dyeing of Turkey Red colour on cotton. For the successful application of chrome mordant, it is essential that dichromate is reduced to chromic oxide before the mordanted material is dyed. In case the goods are treated with dichromate after dyeing, the chromic acid produced acts as an oxidising agent and chromic oxide which is thus generated acts as the mordant. Iron salts are very widely applied as mordants in dyeing and printing. These mordants are generally applied on tannin treated cotton fabric. Copper salts are used in cotton dyeing as oxidising agents for the production of cutch browns and logwood blacks. Copper sulphate is frequently employed in the dyeing of black shades on cotton and for this purpose, it is generally fixed
with the help of tannic acid. Tin mordants produce exceptionally brilliant colours. Stannous salts are very powerful reducing agents and are usually used in discharge printing. Due to their powerful reducing action, they cannot be used alongwith oxidising mordant such as copper sulphate. The natural dyes which are susceptible to reduction should not be dyed on tin mordants otherwise they may get decolourised. Stannous salts are not frequently employed as mordants on cotton. Persian berries yield a good yellow to orange shade on the materials previously mordanted with tannin and stannous chloride. Stannic salts are extensively used as mordants for cotton. Many natural dyes, for example, logwood, fustic, quercitron and weld are fixed on cotton with stannic oxide produced on mordanting. On wool Wool is comparatively more receptive than cotton towards natural dyes and mordants. It can absorb both acids and bases effectively due to its amphoteric nature. When treated with a metallic salt, wool hydrolyses the salt into an acidic and a basic component - the basic component being absorbed at the -COOH groups while the acidic component is removed in the course of washing. Aluminium sulphate is quite effective as a mordant for wool and sometimes it is used without any addition. However, when used along with tartar (tartaric acid), full depth of shade and brilliant colours are obtained which have good rubbing fastness. For full depth of shade, about 6 to 8% aluminium sulphate and 5 to 7% tartar are necessary. On the other hand, alum is less effective as a mordant for wool than aluminium sulphate. Due to low cost and ease of application, mordanting of wool with sodium or potassium dichromate find wide applications. It gives full bright shades of fairly good fastness properties. The brightness of the shade can be further improved by addition of rin organic acid such as formic, oxalic or tartaric acid. Ferrous sulphate does not find much application as a mordant for wool. Copper sulphate is used in conjunction with aluminium sulphate and ferrous sulphate in the dyeing oflogwood blue and logwood black. Among the tin salts, stannous chloride is used as a mordant for wool. On silk Silk possess amphoteric behaviour like wool and can absorb both acids and bases. Potassium dichromate cannot be effectively used as a mordant for silk because silk does not contain thiol (-SH) group like wool (present in the cystine
amino acid), which behaves like a reducing agent and help in the reduction of hexavalent chromium of potassium dichromate to trivalent form which can thereby form a complex with the fibre and the dye. Alum is generally not preferred as a mordant for silk since it diminishes the lustre and pliability of silk. Iron salts are also used .both for mordanting and weighing of silk. Excellent black colours are produced on iron mordanted silk. Stannic chloride is regarded as one of the most important mordant for silk and can also be used as a weighing agent. . FASTNESS PROPERTIES OF NATURAL DYES Generally, most natural dyed materials do not possess excellent fastness properties as exhibited by present day natural dyes. Although mordanting and after treatments improved fastness, the intrinsic susceptibility of the chromophore of the natural colouring matters to photochemical degradation resulted in low fastness to washing and light. From ancient available literature, it becomes evident that man was aware of the fleeting nature of natural dyes available to him and made keen effort to improve or restore their fastness properties(12). Gebhard attempted to correlate the chemical constitution of dyes with their light fastness. He stated that the fastness to light depended on the number, nature and position of the substituent groups on the dye chromophore(l3) However, latter on, it was realised that light fastness is a complicated phenomenon and cannot be related to the invariable effect of substituent groups in a dye molecule. Several other factor which affect the fastness to light of natural dyed materials are(8): Chemical structure of the colourant Concentration of the dyestuff Nature of fibre Nature of incident light Composition of surrounding atmosphere Effect of mordants Presence of foreign materials. The poor washing fastness of many natural dyes is mainly due to Weak dye-fibre bond between natural dye and fibre Change in hue due to the breaking of the dye-metal complex during washing, and Ionisation of natural dyes during alkaline washing. The washing fastness of some of the natural dyes can be improved by a post-treatment with alum or a dye-fixing agent resulting in the formation of dye-fibre complex or a crosslink between the dye and fibre, respectively. The fastness properties of a few natural dyes are discussed below.
Yellow natural dyes Vegetables yellow dyes generally have low tinctorial value and the shades are pale. Hence, their fading is quicker which is also greatly influenced by'the mordant used during their application. Aluminium and tin mordants cause more light fading than chrome, iron or copper mordants as in the case of onion, tesu, etc.(l4,15). Flavonoids constitute a major class of yellow natural dyes. The basic flavonoid chromophore is susceptible to photochemical attack and probably leads to the formation of quinones. Due to this reason, the yellow colour becomes a dull brown as commonly seen in the old museum textiles(16).
Red dyes based on pigments other than anthraquinones, viz., sappanwood safflower, sandalwood and can wood ex~ hibit poor fastness to light of the grade 1-2.
Blue natural dyes The most brilliant and fastest blue shades are produced by . famous Indigo on all fibres. The washing fastness of indigo is excellent since it is applied in soluble leu co form which oxidise to insoluble form inside the fibre which then hold it firmly. An unusual fact of the photochemical behaviour of indigo is that its light fastness on wool(7 -8) is slightly higher than that on silk and much higher than that on cotton(3-4).
Brown natural dyes The nature and position of the substituent group on the chromophore as well as their number also affect the intensity and light fastness. For example, flavonic pigments present in weld, sandalwood and plumes exhibit a light fastness rating much higher than flavonol pigments, such as those present in quercitron, persian berries and onion skin. This is due to highly photosensitive hydroxyl group in position 3 present in flavonic pigments. Turmeric is the most brilliant natural yellow dye available to date. Despite being fugitive to light, it continues to be used for its brilliance. Berberis is a basic dye and being fugitive to light exhibit a light fastness grade of I (17). In this case, the ammonium group, which impart the basic dyeing property is probably responsible for the photosensitivity, which is further increased by fluorescence. Anthraquinonoid based yellow and orange mordant dyes, such as rhubarb, manjith, morinda, madder, chayroot, etc. exhibit very good fastness to light. These colours are quite resistant to photofading due to the presence of anthraquinone chromophore which is intrinsically fast to light fading. The wash fastness properties of yellow dyes range from fair to excellent.
Red natural dyes Red natural dyes are mostly stable to light and washing, but sometimes the choice of mordant may affect the washing fastness. For instance, the washing fastness of cochineal dyed wool is much higher with chromium acetate mordant than with alum mordant. Hydroxyanthraquinone pigments, the most important sources of red dyes (both plant as well as animal based), possess good fastness to light. These compounds form stable metal complexes with several mordants, yielding a wide range of deep and fast shades. Mordants usually affects the fastness of anthraquinone based dyes, e.g., cochineal and kermes when used with alum have a lightfastness grade equal to 3, whereas with tin mordant, the light fastness is improved to 5-7. The nature of the substrate also affects the light fastness of these dyes(l8).
Browns are generally obtained from quinone based dyes. Naphthoquinones have only moderate fastness, while anthraquinone based browns have excellent fastness due to the stability of dye chromophore. Copper and iron salts are sometimes referred to as "saddening" agent since they tend to turn the colours to dull and deep shades, particularly browns. Both these mordants enhances the light fastness of dyes. Some yellow dyes, viz., dolu gives deep rich browns when mordanted with copper salts. Tannins combine with ferrous salts to form complexes which give a range of grey-brown shades.
Black natural dyes Anthraquinone based cochineal dye produce black shade on protein fibres with moderate light fastness, but the dye is no longer used now. Silk can be dyed in deep black by Logwood dye on an iron tannate mordant. This method is popular even today and Logwood is considered to be a mordant dye giving excellent depth and fastness on most natural and synthetic fibres(7). Tannins are an important source of black dyes. Pomegranate rind contains the hydrolysable tannin flavogallol, which combines with iron salt to produce deep black shades with excellent light fastness. FUTURE SCOPE OF NATURAL DYES Inspite of numerous limitations associated with the use of natural dyes, viz., its availability, limited shade range and . average fastness properties; natural dyes still possess the potential to change the textile wet processing scenario owing to their synthesis without using hazardous chemicals, eco-friendliness and number of other advantage~. In this new millennium, maintenance of a safe environmental balance will become a necessary requirement. Most of the synthetic dyes are not only based on toxic raw materials and intermediates but their use in textile wet processing also produce effluent which causes environmental pollution. Natural dyes are free from such problems. Moreover, if steps are taken to com-
mercialise the cultivation of plants from which most natural dyes are generated, it will definitely assist in preserving the eco-balance. These vegetable resources are not only replaceable but also bio-degradable compared to limited and irreplaceable petrochemical resources of synthetic dyes. There has also been much interest recently in the pharmacological effects and possible health benefits of the use of natural dyes. The natural dyes were being neglected for about 150 years, but currently they have again came into limelight. However, the main requirement for their revival to be accepted, by and large, on a commercial scale is to develop some new techniques which are effective enough to provide maximum utilisation of available natural dyes. A greater emphasis is needed to improve the fastness properties and widen the shade range of these dyes. More research is definitely required to develop the market to its full potential.
REFERENCES 1. Mohanty, B.C.; "National Seminar on Natural Dyes", Jaipur, NHDC, (1989). 2. Ramakrishna K., Colourage, Vol. XLVI (July 1999) 29. 3. Gulrajani M.L., Colourage, Vol. XLVI (July 1999) 19. 4. Nahr Uwe and Schmitt Michael., Ludwigshafen, many (Private Circulation).
Ger-
5. Glower Brian., J. Soc. Dyers. Col., 114 (1998) 4. 6. Schweppe H., "Natural dyes", Ecomed Publications, Landsberg (1992). 7. Colour Index, Vol. III 3rd ed., Society of Dyers and Colourists, Bradford: (1971). 8. Sekar N., Colourage, Vol. XLVI, (July 1999) 33. 9. Paul R., Malanker lV, Naik S.R., Textile Dyer & Printer, (Oct. 23, 1996) 16. 10. Mohanty B.c., Naik H.D., Chandramouli K.V, "Natural Dyeing Process of India ", Calicut Museum of Textiles, Ahmedabad, (1987). II. Knecht E., Rawson C., and Loewenthal R., "Mannual of Dyeing ", Vol. I; Charles Griffin and Co. Ltd. Lon/ don, (1933). '-" 12. Forrester S.D., 1. Soc. Dyers Colourist, 7 (1975) 217. 13. Gebhar, J. Soc. Dyers Colourist, 25 (1909) 305.
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14. Padfield P, and Landi S., Stud. In Conserv., 11 (1966) 161. 15. Crew pc., J. Am. Inst. Conserv., 21 (1982) 43. 16. Grierson S., et. aI., 1. Soc., Dyers Colourist, 101 (1985) 220. 17. Venkatraman K., The Chern,. ofSyn. Dyes, Vol. III (Academic Press: New York: 1952) 1210. 18. Saito M., et. aI., Text. Res. Jr., 58 (8) (1988) 450.
