Indigo dye is an important dyestuff with a distinctive blue color. The natural
dye comes from several species of plants, but nearly all indigo producedtoday is synthetic. Among other uses, it is used in the production of denimcloth for blue jeans. Indigo is a frequency range of visible light, from 440 to420 nanometers in wavelength, placing it between blue and violet. Thehuman eye is relatively insensitive to indigo frequencies, and someotherwise well-sighted people cannot distinguish indigo from blue andviolet. A variety of plants have provided indigo throughout history, but mostnatural indigo is obtained from plants in the genus Indigofera, which arenative to the tropics. In temperate climates indigo can be obtained fromwoad (Isatis tinctoria) and dyer\'s knotweed (Polygonum tinctorum),although the Indigofera species yield more dye. The primary commercialindigo species in Asia was true indigo (Indigofera tinctoria, also known asIndigofera sumatrana). In Central and South America the two speciesIndigofera suffructicosa and Indigofera arrecta (Natal indigo) were the mostimportant.Indigo is a dark blue crystalline powder that melts at 390°±392°C. It isinsoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene,or concentrated sulfuric acid. The chemical structure of indigo correspondsto the formula C16H10N2O2. The naturally occurring substance is indican,which is colorless and soluble in water. Indican can easily be hydrolyzed toglucose and indoxyl. Mild oxidation, such as exposure to air, convertsindoxyl to indigo.Several simpler compounds can be produced by decomposing indigo;these compounds include aniline and picric acid. The only chemicalreaction of practical importance is its reduction by urea to indigo white. Theindigo white is reoxidized to indigo after it has been applied to the fabric.Indigo treated with sulfuric acid produces a blue-green color. It becameavailable in the mid-1700s. Sulfonated indigo is also referred to as Saxonblue or indigo carmine.Indigo is a challenging dye to use because it is not soluble in water; to bedissolved, it must undergo a chemical change. When a submerged fabric isremoved from the dyebath, the indigo quickly combines with oxygen in theair and reverts to its insoluble form. When it first became widely available in
Europe in the sixteenth century,
European dyers and printers struggled withindigo because of this distinctive property. A preindustrial process for dyeing with indigo, used in
Europe, was todissolve the indigo in stale urine. Urine reduces the water-insoluble indigoto a soluble substance known as indigo white or leucoindigo, whichproduces a yellow-green solution. Fabric dyed in the solution turns blueafter the indigo white oxidizes and returns to indigo. Synthetic urea toreplace urine became available in the 1800s. Another preindustrial method,used in Japan, was to dissolve the indigo in a heated vat in which a cultureof thermophilic, anaerobic bacteria was maintained. Some species of suchbacteria generate hydrogen as a metabolic product, which can convertinsoluble indigo into soluble indigo white. Cloth dyed in such a vat wasdecorated with the techniques of shibori (tie-dye), kasuri, katazome, andtsutsugaki.Two different methods for the direct application of indigo were developed in
England in the eighteenth century and remained in use well into thenineteenth century. The first method, known as pencil blue because it wasmost 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 arseniccompound delayed the oxidation of the indigo long enough to paint the dyeonto fabrics.The second method was known as china blue due to its resemblance toChinese blue-and-white porcelain. Instead of using an indigo solutiondirectly, the process involved printing the insoluble form of indigo onto thefabric. The indigo was then oxidized in a sequence of baths of ferroussulfate. The china blue process could make sharp designs, but it could notproduce the dark hues possible with the pencil blue method