Indigo, or indigotin, is a dyestuff originally extracted from the varieties of the indigo and woad plants. Indigo was known throughout the ancient world for its ability to color fabrics a deep blue. Egyptian artifacts suggest that indigo was employed as early as 1600 b.c. and it has been found in Africa, India, Indonesia, and China.
The dye imparts a brilliant blue hue to fabric. In the dying process, cotton and linen threads are usually soaked and dried 15-20 times. By comparison, silk threads must be died over 40 times. After dying, the yarn may be sun dried to deepen the color. Indigo is unique in its ability to impart surface color while only partially penetrating fibers. When yarn died with indigo is untwisted, it can be seen that the inner layers remain uncolored. The dye also fades to give a characteristic wom look and for this reason it is commonly used to color denim. Originally extracted from plants, today indigo is synthetically produced on an industrial scale. It is most commonly sold as either a 100% powder or as a 20% solution. Through the early 1990s, indigo prices ranged near $44/lb ($20/kg).
The name indigo comes from the Roman term indicum, which means a product of India. This is somewhat of a misnomer since the plant is grown in many areas of the world, including Asia, Java, Japan, and Central America. Another ancient term for the dye is nil from which the Arabic term for blue, al-nil, is derived. The English word aniline comes from the same source.
The dye can be extracted from several plants, but historically the indigo plant was the most commonly used because it is was more widely available. It belongs to the legume family and over three hundred species have been identified. Indigo tinctoria and I. suifruticosa are the most common. In ancient times, indigo was a precious commodity because plant leaves contain only about small amount of the dye (about 2-4%). Therefore, a large number of plants are required to produce a significant quantity of dye. Indigo plantations were founded in many parts of the world to ensure a controlled supply.
Demand for indigo dramatically increased during the industrial revolution, in part due to the popularity of Levi Strauss's blue denim jeans. The natural extraction process was expensive and could not produce the mass quantities required for the burgeoning garment industry. So chemists began searching for synthetic methods of producing the dye. In 1883 Adolf von Baeyer (of Baeyer aspirin fame) researched indigo's chemical structure. He found that he could treat omega-bromoacetanilide with an alkali (a substance that is high in pH) to produce oxindole. Later, based on this observation, K. Heumann identified a synthesis pathway to produce indigo. Within 14 years their work resulted in the first commercial production of the synthetic dye. In 1905 Baeyer was awarded the Nobel Prize for his discovery.
At the end of the 1990s, the German based company BASF AG was the world's leading producer, accounting for nearly 50% of all indigo dyestuffs sold. In recent years, the synthetic process used to produce indigo has come under scrutiny because of the harsh chemicals involved. New, more environmentally responsible methods are being sought by manufacturers.
The raw materials used in the natural production of indigo are leaves from a variety of plant species including indigo, woad, and polygonum. Only the leaves are used since they contain the greatest concentration of dye molecules. In the synthetic process, a number of chemicals are employed as described below.
The Manufacturing Process
- 1 Plant extraction of indigo requires several steps because the dye itself does not actually exist in nature. The chemical found in plant leaves is really indican, a precursor to indigo. The ancient process to extract indican from plant leaves and convert it to indigo has remained unchanged for thousands of years. In this process, a series of tanks are arranged in a step wise fashion. The upper-most tank is a fermentation vessel into which the freshly cut plants are placed. An enzyme known as indimulsin is added to hydrolyze, or break down, the indican into indoxyl and glucose. During this process carbon dioxide is given off and the broth in the tank turns a murky yellow.
2 After about 14 hours, the resulting liquid is drained into a second tank. Here, the indoxyl-rich mixture is stirred with paddles to mix it with air. This allows the air to oxidize the indoxyl to indigotin, which settles to the bottom of the tank. The upper layer of liquid is siphoned away and the settled pigment is transferred to a third tank where it is heated to stop the fermentation process. The resultant mixture is filtered to remove impurities and dried to form a thick paste.
Historically, the Japanese have used another method which involves extracting indigo from the polygonum plant. In this process the plant is mixed with wheat husk powder, limestone powder, lye ash, and sake. The mixture is allowed to ferment for about one week to form the dye pigment which is called sukumo.
- 3 A variety of synthetic chemical processes have been used to produce indigo. All these processes involve combining a series of chemical reactants under controlled conditions. The reactants undergo a series of reactions which result in the formation of the indigo molecule. A number of other chemical byproducts are also produced in this reaction.
- 4 These synthesis reactions are conducted in large stainless steel or glass reaction vessels. These vessels are equipped with jackets to allow steam or cold water to flow around the batch as the reactions progress. Because of the complexity of these chemical processes, the dye is usually made in batch quantities. There are, however, a few methods invented by the Germans for continuous process manufacturing.
Types of reactions
- 5 The first commercial method of producing indigo was based on Heumann's work. In this method, N-phenylglycine is treated with alkali to produce indoxyl, which can be converted to indigotin by contact with air. However, the amount of dye yielded by this process is very low. Another, more efficient, synthesis route utilizes anthranilic acid. This process was popular with major manufacturers, such as BASF and Hoechst, for over 30 years. A variation of this method (which has become widely used) involves the reaction of aniline, formaldehyde, and hydrogen cyanide to form phenylglycinonitrile. This material is then hydrolyzed to yield phenylglycine which is then converted to indigotin. Currently, a method which uses sodamide with alkali to convert phenylglycine to indoxyl. Sodamide reacts with excess water, thus lowering the overall reaction temperature from almost 570°F (300°C) to 392°F (200°C). This results in a much more efficient reaction process.
6 After the chemical reaction process is complete, the finished dye must be washed to remove impurities and then dried. The dried powder can be packed in drums or reconstituted with water to form a 20% solution and filled in pails.
During indigo manufacture, the reaction process is continuously monitored to ensure the chemicals are combined in the proper ratios. Key elements that must be controlled include the pH (or acid/base quality of the batch), the temperature (which controls the speed of the reaction), and the reaction time (which determines the degree of completion). If any of these variables deviate from specifications, the resulting reaction product can be affected. Typically, poor quality control results in lower yield of the dye, which increases costs for the manufacturer.
To ensure that manufacturers can consistently purchase the same shade of dye, indigo is assigned a Color Index number that defines its shade. It is designated as "CI Natural Blue CI 75780."
Indigo production produces a variety of waste products which must be handled carefully. In addition to the reactants described above, there are other reaction side products that are produced along with the indigo. Some of these materials are considered to be hazardous and must be disposed of in accordance with local and federal chemical waste disposal guidelines. These waste chemicals can enter the environment in at least three different ways. The first is during the actual manufacture of the molecule. The second is when the dye is applied to the yarn, and the third is when the dye is eluted into the wash water during the initial stonewashing or wet processing of the fabric. This last route typically occurs during the production of denim fabric.
Much of the need for indigo is being met with other types of blue dyes and today most of the indigo used by the world is made out-side the United States. Researchers are concentrating on new methods of indigo manufacture that are more environmentally friendly. One promising future method involves using biocatalysts in the dye reaction process. Indigo dye may be one of the first high-volume chemicals made through a biological route. Genencor International, of Rochester New York, is evaluating a process to produce indigo using biotechnology. According to Charles T. Goodhue, Genencor's Program Director/Biocatalysis Research and Development, indigo produced by this method is chemically the same as the regular synthetic dye and behaves identically in dyeing tests. However, at this time the technology is expensive and production costs could be prohibitive. Genencor is seeking a major market partner to work with them in the development of this new technology.
Manufacturers who use indigo in dying operations are also seeking to improve their use of the dye. For example, Burlington's Denim Division introduced a technology in 1994 they call "Stone Free," which allows indigo dye in the fabric to break down 50% faster in the stonewash cycle. Compared to traditional methods of stonewashing fabric dyed with indigo, their new process uses few, if any, pumice stones which help give the fabric its faded look. Therefore, pumice stone handling and storage costs are reduced, along with time required to separate pumice from garments after stonewashing. It also uses much less bleach. Therefore, this new process not only reduces garment damage, but also reduces waste produced by the stones and bleach.
Where to Learn More
Kirk, R. E., and D. F. Othmer (ed.) Encyclopedia of Chemical Technology: Alkoxides, Metal to Antibiotics (Peptides). Wiley-Interscience, John Wiley and Sons, 1978.
Guilbaut, G. B., and D. W. Kramer. "Resorufin Butyrate and Indoxyl Acetate as Fluorogenic Substrates for Cholinesterases." Analytical Chemistry 37 (1965):120-23.
McCurry, John. "Burlington Debuts Stone Free Denim." Textile World 144, no.3 (March 1994): 120-123.
Rotman, David, and Emma Chynoweth. "The Quest for Reduced Emissions, Greener Processes." Chemical Week 153, no.1 (July 7,1993): 117.
Indigo is a dyestuff known for its blue hue. The color comes from indigotin, a dye derived from the glucoside indican found in some fifty related plants, mostly in the leaves. Of these, Indigofera tinctoria, native to India, has the highest concentration of indican that makes deep, dark blues practicable. Less well-endowed varieties grow in Mexico and South America, Europe, Egypt, West Africa, Sumatra, Central Asia, China, and Japan where from ancient times they were used, it is thought, with varying degrees of influence from India.
Woad and Indigo
Woad (Isatis tinctoria) was used as a blue dye from as early as the fifth century b.c.e. in Europe where a textile fragment from a Celtic grave has been verified as having been dyed with this source of indigotin. The East India companies introduced indigo into Europe in the 1500s; although Europeans recognized its virtues, they outlawed it because it threatened economic disaster to their woadbased economies.
Separating indican, the basis of the dye, from the rest of the plant and changing it into indigotin, the dye, was a laborious undertaking begun by fermenting plant material in vats of water. One hundred pounds of plants yielded about four ounces of the insoluble, granular blue precipitate called "indigo," containing from 10 percent to 80 percent indigotin. The best came from Bengal.
Spices and calico were the focus of the India trade, but traders established hundreds of indigo plantations in the East Indies to supply the growing demand. The Spanish introduced indigo production to Central and South America and the West Indies, where it flourished. In America, the English colonists found indigo to be a lucrative crop, and during the difficult period of the Revolution, for lack of stable currency, lumps of indigo served the purpose.
Supplies of processed indigo dyestuff are nearly nonexistent in the early 2000s because its manufacture is labor intensive and expensive; there is little profit in it now that synthetic indigo prevails.
Unlike madder and most other natural dyes, indigo permanently attaches to fibers, particularly cotton, without mordant (a chemical that fixes a dye). Silk, wool, linen, and cotton may be successfully dyed with indigo. The dyestuff, however, cannot simply be stirred into the vat because indigo is insoluble. This property confers permanence, but carries the challenge of getting it onto the cloth in the first place.
Undissolved indigo does not adhere to fibers. The purchased chunk must be laboriously ground to a very fine powder and placed in a vat of water containing an agent to reduce, or chemically remove, oxygen molecules from the indican suspended in the liquid. The color of the indigo then changes from blue to a near white known as "indigo white." In this state the indican may be dissolved, but only if the liquid is alkaline. The art of indigo dyeing lies in maintaining the vat in a state of balance between reduction and solution in such a way that the cloth is successfully dyed without suffering alkali damage.
The earliest method of reducing indigo was to induce fermentation by the introduction of vegetable matter such as seeds into the vat. Later, woad, bran, or madder were used, but the fermentation vat was slow to "come on." Because it was effective and worked faster than fermentation, orpiment (arsenic trisulphide) came into favor although it was a deadly poison. When ferrous sulphate, or copperas, was found to work better and safely, it became the reducing agent of choice. Toward the end of the nineteenth century, a form of glucose was preferred. For all of these methods, lime was added to the vat to raise the alkalinity sufficiently to allow the white indigo to completely dissolve. The well-known urine vat was best suited to dyeing on a domestic scale; fermenting nutrients in the urine simultaneously generated a reducing and an alkaline solution.
As dyed cloth is removed from the vat, atmospheric oxygen immediately combines with the white indigo to restore the insoluble blue form again, thereby fixing the color mechanically within and upon the surface of the fibers and forming a weak chemical bond. If the dye is not completely reduced, it is not completely dissolved, and in that state cannot adhere to the fibers. An excess of undissolved—hence, unattached—indigo would crock, or rub off. The deepest shades of blue are achieved by successive dips so that the color will be even and rich.
The Indians are recognized as the originators of resist dyeing with indigo, accomplished by applying wax to protect the cloth wherever it was not to be colored blue, even if that meant most of the cloth. The Japanese and Indonesians employed tie-dye and clamping techniques to resist indigo.
Europeans developed a resist paste made of starch mixed with copper or lead salts that were best applied by block. Discharge pastes that bleached or removed the blue color to make white designs came into use on indigo in the 1820s. These were more suitable than resists for cylinder printing. Resist and discharge pastes bearing mordants or lead salts for chrome colors were later developed, permitting the incorporation of colored shapes neatly fitted into the blue ground.
Obtaining a white ground with fine or small blue designs posed a problem. To do so, "pencil blue" —indigo mixed with lime and orpiment—was applied like water-color with a brush or "pencil." Variations called China or Fayence blue were repeatedly revised for block and cylinder printing, but never produced a very dark or even color and usually gave trouble.
Adolph von Bayer first synthesized indigo in 1880, but it was not until the end of the century that a commercially viable process became available. The synthetic indigo was chemically identical to the natural form, minus the organic impurities. It was very much cheaper and little reason remained to resist; by 1920, natural indigo was little more than a memory in commercial print works.
The efficiently aggressive East India traders launched the final three hundred years of intensive commercial exploitation of all aspects of indigo production. By 1900, the venerable role of the "drug" that had shaped cultures and ages was usurped by a distinctly unromantic, synthetic replacement. The ubiquitous blue jeans of the early 2000s testify, undaunted, to the endurance of a global heritage called "indigo blue." Natural indigo is used only by artisans producing special custom yarns and fabrics. Most distinctively, the Japanese continue to employ ageold tie-dyeing, resist, and stencil techniques to make masterful indigo-patterned fabrics for kimonos.
Irwin, John, and P. R. Schwartz. Studies in Indo-European Textile History. Ahmedabad, India: Calico Museum of Textiles, 1966.
Liles, J. N. The Art and Craft of Natural Dyeing. Knoxville: University of Tennessee Press, 1990.
Pettit, Florence. America's Indigo Blues. New York: Hastings House, 1974.
Sandberg, Gosta. Indigo Textiles: Techniques and History. Asheville, N.C.: Lark Books, 1989.
The cultivation of indigo (tinta añil) in the New World dates to pre-Hispanic times. The Indians of northern Central America harvested wild indigo plants, called xiquilite, which they traded extensively and used as a rich blue dye for textiles.
During the late sixteenth century, the Spaniards domesticated the cultivation of indigo in Central America and established indigo plantations in Yucatán and along the fertile Pacific coasts of present-day El Salvador, Guatemala, and Nicaragua. Spaniards exported their first indigo in 1576, when a shipload was sent from Nicaragua to New Spain, where it was used in Mexico's nascent textile industry. By 1600, indigo had replaced cacao—until that time Central America's most profitable commodity—as the leading export. In the early seventeenth century, indigo also became the leading source of hard currency for the isthmus.
Indigo was highly valued in the Old World, where it supplanted more expensive blue dyes from the Far East and inferior dyes produced from woad in northern Europe. From 1580 to 1620 the Central American indigo industry thrived. Añileros (indigo planters) employed Indians for harvesting and processing the plant. Because the work was difficult and dangerous (extended contact with the toxins present in the indigo plant often proved fatal, as did constant exposure to the swamp waters used to extract dye from the plants), the industry was limited by the size of the indigenous labor supply. Although the work was dangerous, labor scarcity gave the indigenous workers some power in the colonial order, according to recent research. The repartimiento (distribution or assessment) system, traditionally explained as a way of forcing indigenous communities to buy unwanted goods, may have provided an incentive to keep the laborers producing indigo. This interpretation suggests that the repartimiento was a form of forced credit that intended to keep the labor force motivated. By the mid-seventeenth century, the indigo industry had stagnated. In the late seventeenth century, however, a resurgence in the size of the Indian population allowed the industry to expand again, at which time African slaves were imported to augment the labor force.
By 1635, San Vicente, El Salvador, had become the hub of the indigo trade, although other important centers were located in Guatemala and Nicaragua. The locus of the trade later shifted to Sonsonate, El Salvador, once the center of the colonial cacao trade, and to Guatemala. In addition to the European trade, Central American indigo also made its way to Mexico and Peru.
Indigo remained the most important export of the Kingdom of Guatemala throughout the colonial period. The industry reached its peak during the second half of the eighteenth century, when the expanding textile industry in northern Europe created a significant demand for the rich blue hue. The "free trade" economic policies of the Spanish Bourbons in the eighteenth century also enhanced the indigo trade, and in 1782 añileros formed an Indigo Growers Society based in Guatemala City. By the last years of the colonial period, indigo generated more than 2 million pesos worth of profits per year.
By the last decades of the eighteenth century, however, indigo exports from Central America began to decline, although the dye continued to bring heavy profits for planters in South Carolina, Venezuela, and the East Indies. In the mid-eighteenth century, Brazil's reformer, the Marquis de Pombal, introduced indigo production to Rio de Janeiro and Maranhão, where the dye's high price helped compensate for the decline of sugar production in that country.
Indigo's importance as a commodity diminished worldwide during the nineteenth century, as cheaper synthetic dyes became available. El Salvador continued to export some indigo into the late nineteenth century.
Ralph Lee Woodward, Jr., Class Privilege and Economic Development: The Consulado de Comercio of Guatemala, 1793–1871 (1966).
Murdo J. Mac Leod, Spanish Central America: A Socioeconomic History, 1520–1720 (1973).
William L. Sherman, Forced Native Labor in Sixteenth Century Central America (1979).
Woodward, Central America: A Nation Divided (1985).
Fernández M., José Antonio. Pintando el mundo de azul: El auge añilero y el mercado centroamericano, 1750–1810. San Salvador: Dirección de Publicaciones e Impresos, Consejo Nacional para la Cultura y el Arte, 2003.
Indigo 1984- (Alyssa Ashley, Alyssa Nichols, Alyssa A. Nichols)
Indigo 1984- (Alyssa Ashley, Alyssa Nichols, Alyssa A. Nichols)
Original name, Alyssa Ashley Nichols; born June 25, 1984, in Los Angeles, CA; daughter of Lance E. Nichols. Avocational Interests: Music and art.
Agent—Innovative Artists, 1505 Tenth St., Santa Monica, CA 90401. Manager—Stevenson Talent Management, 22838 Epsilon St., Woodland Hills, CA 91634.
Actress. Appeared in television commercials, including Armour hot dogs, c. 1989, Domino's Pizza, 2004, Oscar Meyer Lunchables, McDonald's, AT&T Yellow Pages, and Emporium department stores; appeared in radio commercials, including McDonald's, U.S. Army, Nestle's Crunch candy, and Geico insurance; also appeared in student and industrial films.
Screen Actors Guild Award nomination (with others), outstanding performance by an ensemble in a comedy series, 2007, for Weeds; Governor's Medallion, California State Summer School for the Arts.
Girl, Killer per caso (also known as The Good Bad Guy), 1997.
Television Appearances; Series:
(As Alyssa A. Nichols) Julie, Minor Adjustments, UPN, 1995-96.
(As Alyssa Ashley Nichols) April Gregory, Any Day Now, Lifetime, 2000.
Cheyenne Webb, Boston Public, Fox, 2002-2003.
Rona, Buffy the Vampire Slayer (also known as BtVS, Buffy, and Buffy the Vampire Slayer: The Series), UPN, 2003.
Vaneeta, Weeds, Showtime, 2005—.
Television Appearances; Specials:
Fatima, Nelly's Bodega, PBS, 2001.
Television Appearances; Pilots:
Voice of Huey, Boondocks (animated), 2005.
Television Appearances; Movies:
(As Alyssa Ashley Nichols) Jackie, Zooman, Showtime, 1995.
(As Alyssa A. Nichols) Agnes, "Summer Bummer," Sister, Sister, The WB, 1996.
(As Alyssa Ashley Nichols) Cherise Martin, "McNeil and Pray," Chicago Hope, CBS, 1998.
(As Alyssa A. Nichols) Sylvia Dobkin, "Everyone in the Poole," NYPD Blue, ABC, 2001.
(Uncredited) Girl at Sanctuary House, "The Right Thing to Do," Judging Amy, CBS, 2001.
Keisha, "Someone to Count On," Crossing Jordan, NBC, 2002.
Red Beans, Girlfriends, 2002.
Tisha Graves, "Brothers in Arms," 10-8: Officers on Duty (also known as 10-8 and 10-8: Police Patrol), ABC, 2003.
Tyra, "The Badlands," Cold Case, CBS, 2004.
Strong Medicine, Lifetime, 2004.
Voice, "Don't Make Me Over," Family Guy (animated), Fox, 2005.
Voice, "Petarded," Family Guy (animated), Fox, 2005.
Voice, "Hell Comes to Quahog," Family Guy (animated), Fox, 2006.
Voice, "Barely Legal," Family Guy (animated), Fox, 2006.
Cha Cha Romero, "Fannysmakin'," CSI: Crime Scene Investigation (also known as CSI, CSI: Las Vegas, CSI: Weekends, and Les experts), CBS, 2006.
Indigo is a deep blue dye used to color cotton, wool, and other textiles. Today it is manufactured synthetically, but in earlier times it was derived from the indigo plant, a member of the legume family. The plant was chiefly grown in India (hence its name). In the Caribbean indigo was cultivated by European colonists. During the 1600s it was a principle item of export from the region. In the United States the indigo plant was cultivated in the low country of South Carolina and Georgia beginning in 1741. It was introduced by Elizabeth Lucas Pinckney (1722–93), the daughter of a plantation owner whose family later figured prominently in American politics. Pinckney had brought the plant with her from the Caribbean island of Antigua, where she had lived. Since indigo is a labor-intensive crop, plantations required numerous slaves to cultivate the plant. Growers sold indigo overseas to be used in the European dyestuffs industry. By the 1760s indigo had become an important crop for the southern plantation owners—its export value was on a par with rice and wheat. It continued to be grown in the region until the American Civil War (1861–65). Synthetic indigo was produced in 1880, and it was first used commercially in 1897. This launched a synthetic dye industry that completely eliminated the need for the dye to be derived from plants.
See also: Plantations, Triangular Trade
in·di·go / ˈindiˌgō/ • n. (pl. -os or -oes) 1. a tropical plant (genus Indigofera) of the pea family, which was formerly widely cultivated as a source of dark blue dye. 2. the dark blue dye obtained from this plant. ∎ a color between blue and violet in the spectrum.