RGB (75, 0, 130) CMYK (42, 100, 0, 49)Indigo
RGB (75, 0, 130) CMYK (42, 100, 0, 49)
Indigo is the color of light between 440 to 420 nanometers in wavelength, placing it between blue and violet. Like many other colors (orange and violet are the most well-known), it gets its name from an object in the natural world - the plant named indigo used for dyeing cloth.
Indigo is neither an additive primary color nor a subtractive primary color. It was named and defined by Isaac Newton when he divided up the optical spectrum (which is a continuum of frequencies).
The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people cannot distinguish indigo from blue and violet. For this reason some commentators including Isaac Asimov have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet.
Sources and uses
Indigo dye is an important dyestuff with a distinctive blue color. The natural dye comes from several species of plant, but nearly all indigo produced today is synthetic.
A variety of plants, including woad, have provided indigo throughout history, but most natural indigo is obtained from those in the genus Indigofera, which are native to the tropics.
In temperate climates indigo can also 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). In Central and South America the two species Indigofera suffruticosa (Anil) and Indigofera arrecta (Natal indigo) were the most important.
Natural indigo was the only source of the dye until about 1900. Within a short time, however, synthetic indigo had almost completely superseded natural indigo, and today nearly all indigo produced is synthetic.
Indigo is among the oldest dyes to be used for textile dyeing and printing. Many Asian countries, such as India, China, and Japan, have used indigo as a dye for centuries.
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 word for the dye, which was indikon. The Romans used the term indicum, which passed into Italian dialect and eventually into English as the word indigo.
The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.
Indigo remained a rare commodity in Europe throughout the Middle Ages; woad, a dye derived from a related plant species, was used instead.
Woad
In the late fifteenth century, the Portuguese explorer Vasco da Gama discovered a sea route to India. This led to the establishment of direct trade with India, China, and Japan. Importers could now avoid the heavy duties imposed by Persian and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly.
Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England.
Spain imported the dye from its colonies in South America. Many indigo plantations were established by European powers in tropical climates; it was a major crop in Jamaica and South Carolina. Indigo plantations also thrived in the Virgin Islands. However, France and Germany outlawed imported indigo in the 1500s to protect the local woad dye industry.
Indigo was the foundation of centuries-old textile traditions throughout West Africa. The use of indigo here pre-dated synthetics. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba of Nigeria and the Manding of Mali particularly well known for their expertise. Among the Hausa male dyers working at communal dye pits were the basis of the wealth of the ancient city of Kano, and can still be seen plying their trade today at the same pits.
In 1865 the German chemist Johann Friedrich Wilhelm Adolf von Baeyer began working with indigo.
His work culminated in the first synthesis of indigo in 1880 from o-nitrobenzaldehyde and acetone upon addition of dilute sodium hydroxide, barium hydroxide, or ammonia and the announcement of its chemical structure three years later.
BASF developed a commercially feasible manufacturing process that was in use by 1897, and by 1913 natural indigo had been almost entirely replaced by synthetic indigo. In 2002, 17,000 tons of synthetic indigo were produced worldwide.
Indigo is a challenging dye to use because it is not soluble in water; to be dissolved, 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. It was also a toxic substance that, by requiring many chemical processes, had many opportunities to injure many workers.
A pre-industrial 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, 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.
Two 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. The first method, known as pencil blue because it was most often applied by pencil or brush, could be used to achieve dark hues. Arsenic trisulfide and a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.
The second method was known as china blue due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then oxidized in a sequence of baths of Iron(II) sulfate. The china blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.
Around 1880 the glucose process was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the china blue method.
In the United States, the primary use for indigo is as a dye for cotton work clothes and blue jeans. Over one billion pairs of jeans around the world are dyed blue with indigo. For many years indigo was used to produce deep navy blue colors on wool.
Indigo does not bond strongly to the fiber, and wear and repeated washing may slowly remove the dye.
Also known as Royal purple, Tyrian purple, purple of the ancients, this ancient dyestuff, mentioned in texts dating about 1600 BC, was produced from the mucus of the hypobranchial gland of various species of marine molluscs, notably Murex. Although originating in Tyre (hence the name), man's first large scale chemical industry spread throughout the world. With the decline of the Roman Empire, the use of the dye also declined and large scale production ceased with the fall of Constantinople in 1453 (29 May, actually). It was replaced by other cheaper dyes like lichen purple and madder.
Tyrain purple dye was derived from the shellfish Murex brandaris - 12,000 Murex shells were needed to make only 1.4 grams of dye, making it immensley laborious and expensive to produce.
The fast, non-fading dye was an item of luxury trade, prized by Romans, who used it to color ceremonial robes. Pliny the Elder described the dyeing process of two purples in his Natural History. The Roman mythographer Julius Pollux, writing in the second century CE, asserted that the purple dye was first discovered by Heracles, or rather, by his dog, whose mouth was stained purple from chewing on snails along the coast of the Levant. The myth has been discredited as mere cultural boasting. Recently, the archaeological discovery of substantial numbers of Murex shells on Crete suggests that the Minoans may have pioneered the extraction of Royal purple centuries before the Tyrians. Dating from colocated pottery, suggests the dye may have been produced during the Middle Minoan period in the 20th–18th century BCE.
