Dyeing of Silk Yarn with Lac and Indigo - A Venture for Textile Industries with Quality Improvement

Mukherjee, Atish, Director, Eco-N-Viron Enterprise, a Natural Dye producing unit based in West Bengal. He has developed an innovative system to extract natural dyes and subsequently preserve them in a dense liquid form; to obtain shade variations by the use of dyeing and mordanting baths without creating pollution; and developed a method to achieve a light and wash fastness.

Mukherjee, Arup, Research and Development Officer, Visva Bharati University, Sriniketan, Bholpur.

Natural dye has become very popular particularly in the foreign countries owing to their eco-friendliness and environment-friendliness. A few Indian traders are doing their export business exclusively on exportable handloom products dyed with natural dyes considering the fact that the price of the products may not be affordable for common people of our country. Before the synthetic dyes were introduced about 175 years back, natural dye was the only colorant in the world. It is very painful and unfortunate that popularity of natural dye has not yet reached the common people mainly due to the standard of washing and light fastness required for day to day laundering and drying. Though the dullness of natural dye is its inherent elegance, but is not liked by a class of people. Not only this reason is deemed as its drawback but also supply of raw materials for dyes as well as lengthy process of dyeing of natural colour are other reasons. Considering all these factors for the common interest of the silk exporters and customers of the country, the project was taken to develop the drawbacks by using only Lac & Indigo dye because of their beautiful colour and fastness quality and plenty of availability of the sources in nature. Natural dye product is very good for skin and it is used largely in baby care, health care, medicare and now it has become an aristocracy in fashion in the entire world. Natural dye application is best suitable for silk & wool industry. Country like Japan, Korea, England, America, Italy and Germany are much more interested to import the silk yarn particularly dyed with Indigo and Lac dyes especially from India.


It is known to all that natural dyes have very few colours mainly Yellow, Brown, Ash, Black, Orange, Pink and Blue. The best quality pink comes from Lac the resource of which is insects. Blue colour comes from Indigo which is a vat class dye and the resources are leaves of Indigofera tinctoria which is also known as Neel. Both the dyes are cultivated in huge quantity not only in India but also in other countries all over the world. These two dyes are also fulfilling the export criteria regarding the fastness properties and others. Lac is a natural resin of insect origin, which is non-toxic, biodegradable, and its sources are renewable. The basic raw material for lac industry is stick lac, which is obtained by scraping lac incrustation, deposited on certain host trees. Stick lac, apart from lac resin, also contains water-soluble lac dye (laccaic acid upto 1%). Lac dye is used for dyeing of wool and silk fabrics and yarns. Since it is non-toxic, it is also being used as food colouring material.  It is reported that Japan, China and Thailand are using pure lac dye for colouring beverages and products like ham, sausages, beers, jams etc. The present consumption of pure grade lac dye in Japan is reported to the tune of 5-6 ton per year. India at present produces about 20,000 ton of lac annually. Considering this annual production, nearly 200 ton lac dye is lost in effluents during washing. Thus, an enormous potential exists for recovery of the dye as by-product of lac industry. Even if half of the potential is exploited, it will be possible to turn trade of lac dye into highly profitable business as an exportable item with an assured foreign market. A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). Lac dye is mainly used in Textile, Food industry and Pharmaceuticals.  India was also the earliest major center for production and processing of Indigo. The Indigofera tinctoria variety of Indigo was domesticated in India. Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product. Indigo among all the dyes is the oldest one used for textile dyeing and printing. Many Asian countries, such as India, China, Japan and South East Asian nations have used indigo as a dye (particularly silk dye) for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Greece, Rome, Britain, Mesoamerica, Peru, Iran, and Africa. India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek world for the dye. The Romans Latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo. Indian silk industries should step forward and take the challenge for global demand of natural dye. It will also help the other small and big manufacturers of natural dye in India. 


Properties, History and Structure of Indigo dye

Lac is one of the most valuable gifts for mankind and is unique material of animal origin, being secreted by the tiny insect Laccifer lacca. The crude lac (Stick lac) contains 1per cent dye pigment. The colouring substance is the lac dye of the scarlet pigment present in the live, pre-emergent lac insects, which contain water soluble compound, laccaic acid. An article “the story of shellac” revealed that, during certain season of the year the host trees are prone to bear lac insects. The insects suck the sap from the host plants and the sap undergoes a chemical transformation in the body of the lac insects and this sap is eventually extruded through the pores. On contact with air, it forms a hard shell like coating which is known as ‘stick lac.’ The whole life span of these tiny insect is approximately six months and is completely devoted for eating, propagating and creating lac as a protective covering from their larvae.


Lac dye is based on anthraquinoid type of structure and composed of laccaic acid and erythrolaccin. Laccaic acid is water soluble compound whereas erythrolaccin is water insoluble. The insect lac, Laccifer lacca or Kerria lacca belongs to family Lacciferidae. Lac insect is soft-bodied, round tiny creature, which completes its life cycle on host plants viz., Kusum, Palas, Ber, Moduga and Cajanus. The insect survives on the succulent green branches of these plants and completes its life cycle within six months. During the life cycle it secretes reddish brown gelatinous substance around the branches of host plants, which gives stick lac. The stick lac having gravid female is ready to produce larvae. Stick lac harvesting is performed by cutting off the twigs before the emergence of larva. These pregnant lac insets contain high amount of laccaic acid which is the main colouring substance in lac dye. These gravid insects are killed by exposing the stick lac to direct solar rays, (http://www.fao.org). Stick lac is used for various purposes such as production of shellac; seed lac as well as lac dye. Commercially, shellac and seed lac are the better economic earning products than lac dye. Lac dye is usually extracted from the by-products of shellac industry. Stick lac is made into shellac by thoroughly washing in plain water. From the water soluble dye pigment, laccaic acid is taken for further extraction of lac dye.  Lac dye is acidic in nature, soluble in water and other solvents such as methyl alcohol, amyl alcohol, acetone, acetic acid, formic acid, but insoluble in ether, chloroform and benzene. Since lac dye is acidic in nature can safely be applied to protein substrates such as wool, silk and other animal hair (Indian Lac Research Institute, 2006).



Picture 1: Lac Stick in Tree


Picture 2: Lac dye in cake form


Properties, History and Structure of Indigo dye

Indigo dye is an important dyestuff with a distinctive blue color. The natural dye comes from several species of plants, but nearly all indigo produced today is synthetic. Among other uses, it is used in the production of denim cloth for blue jeans. Indigo has a frequency range of visible light from 440 to 420 nanometers in wavelength placed between blue and violet. The human eye is relatively insensitive to indigo frequencies and some otherwise well-sighted people cannot distinguish indigo from blue and violet. A variety of plants have provided indigo throughout history, but most natural indigo is obtained from plants in the genus Indigofera, which are native to the tropics. In temperate climates indigo can be obtained from woad (Isatis tinctoria) and dyer's knotweed (Polygonum tinctorum), although the Indigofera species yield more dye. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as Indigofera sumatrana). Indigo is a dark blue crystalline powder that melts at 390°–392°C. It is insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid. The naturally occurring substance is indican, which is colorless and soluble in water. Indican can easily be hydrolyzed to glucose and indoxyl. Mild oxidation, such as exposure to air, converts indoxyl to indigo.


Several simpler compounds can be produced by decomposing indigo; these compounds include aniline and picric acid. The only chemical reaction of practical importance is its reduction by urea to indigo white. The indigo white is reoxidized to indigo after it has been applied to the fabric. Indigo treated with sulfuric acid produces a blue-green color. It became available in the mid-1700s. Sulfonated indigo is also referred to as Saxon blue or indigo carmine.

Indigo is a challenging dye to use because it is not soluble in water. If it is to be dissolved and it must undergo a chemical change. When a submerged fabric is removed from the dyebath, the indigo quickly combines with oxygen in the air and reverts to its insoluble form. When it first became widely available in Europe in the sixteenth century, European dyers and printers struggled with indigo because of this distinctive property.


A preindustrial process for dyeing with indigo, used in Europe, was to dissolve the indigo in stale urine. Urine reduces the water-insoluble indigo to a soluble substance known as indigo white or leucoindigo, which produces a yellow-green solution. Fabric dyed in the solution turns blue after the indigo white oxidizes and returns to indigo. Synthetic urea to replace urine became available in the 1800s. Another preindustrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture of thermophilic, anaerobic bacteria was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which can convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki. There are different methods for the direct application of indigo were developed in England in the eighteenth century and remained in use well into the nineteenth century.


Picture 3: Indigo Cake (Extract from indigo leafs)



Fig-3: The compound found in the leaves of the indigo plant that is used to make indigo dye.


Specialty of Our Dye
Generally natural dye application is very time taking process. Because raw resources need to be crushed, then boiled and finally filtered. Our natural dye is special kind of dye. It is water soluble and ready to use for dyeing as well as printing in most innovative method and most important it is that the dye can be stored more than three years without any growth of fungus which is the main enemy for spoiling natural dyes kept in storage. 

Dyeing Process
This experiment has been done by using some eco-friendly and nontoxic organic compounds specially manufactured. Mulberry silk yarn has been used as the main raw material for dyeing.

Chemicals and Reagents used

Soda ash, Glauber’s salt, Sodium Nitrite, Sulphuric Acid, Alum, Ferrus Sulphate, Acetic Acid, Sodium Hydrosulphite and Hydrogen Peroxide (all of commercial grade) have been used.


Degumming Process of Silk Yarn

The raw silk yarn was degummed with our special degumming agent  Scourex 10 gm/lit in addition of 5-10 gm/lit of soda ash at 75-80˚c for 30 minutes.  Sodium Hydrosulphite 10g/l was added in the same bath for bleaching at 60˚c for another 15 minutes. If necessary for better whiteness, an addition of 2gm/lit Hydrogen Peroxide may be alternatively used in the same bath for another 15 minutes in the same temp for more delicate or light shades of the silk. Silk yarn is now washed in cold water and 2gm/lit of Acetic Acid is used for softening the silk yarn and to remove alkalinity. Now the yarn is ready for dyeing.


Dyeing Process of Lac dye

30gm/ lit lac dye is taken in a dye bath and the temperature of the bath is raised at 50˚c. Then silk yarn is put into the bath and dyed for 20 - 30 minutes at the temperature of 80˚c with addition of 2gm/lit Acetic acid for better dyeing and absorption of dye.


Dyeing Process of Indigo dye

50gm/ lit Indigo dye is taken in a dye bath and the temperature of the bath is raised at 50˚c and silk yarn is put into the bath and dyed for  20 -30 minutes at  60˚c with addition of 10gm/ lit Glauber’s salt and 25gm/lit Sodium Nitrite in the dyeing bath.


Mordanting and Development

Mordant class natural dyes need a metal salt to create insoluble coloured lakes inside the fiber according to necessity of shade. These salts are known as “Mordants”. Only Alum AlK(SO4)2, and Ferrus Sulphate (FeSO4) have been used here to get different  shades. The quantity of mordant salts is taken 1/6th of lac dye used at 60˚c for 15 mints. The mordating process should be post mordanting to get the best result. It should be remembered that the premordanting and simultaneous mordanting processes are totally unsuitable for silk industry. There is no question for mordanting in Indigo dye as it is a vat class natural dye. Only 10gm/lit Sulphuric Acid and 10g/l Glauber’s Salt at 50˚c is required for developing the blue shade of indigo. Hydrochloric acid and common salt should not be used for developing the blue shade of Indigo dye in case silk material.

After treatment process of the Silk
The silk yarn should be after treated with non-ionic detergent. It is a necessary procedure to remove remaining surface dyes and chemicals after dyeing. First cold wash then soaping with 1gm/lit Non-ionic detergent for 10 mints at 50˚c is normally done. The dyed yarn should be dried in shade particularly for natural class dyed materials. Softener may be used for better finishing, if required.

Result and Discussion
Fastness properties are tested both for the Lac and Indigo dyed materials and the result were observed very good. Washing fastness of the Lac dyed materials is found 4-5 in ISO-2 method and Light fastness of the same is observed 5-6 both of which are very satisfactory for exportable standard. The washing fastness of Indigo dyed materials is 5 in ISO-2 method and Light fastness of the same is 6 both of which are in exportable standard. The breaking strength of the dyed yarn is also very good due to use of special degumming agent and procedure. Different shades of color and tone can be obtained on the silk yarn by changing the ratios of mordants when taken in combination. Indigo dye is not suitable for mordanting as it belongs to Vat class dyes.

The dyeing process of natural dye should be very simple as well as scientific for implementation in Textile Industry. Natural dye has now been survived not only in cottage sector but also tends to enter medium size Textile Silk Industries. Hence training workshop and awareness programme on natural dye are necessary to be undertaken. Governments of all countries are encouraging to take some initiatives to save the heritage dye because the dyes are not only eco-friendly but also environ-friendly and will be helpful to balance the green-house gases due to more cultivation of plants and trees.

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