Honors
Project
Chemistry
1045
Webster’s dictionary defines dyeing as “the process of coloring fibers, yarns, or fabrics by using a liquid containing coloring matter for imparting a particular hue to a substance.” Indigo dye has an extensive history dating back many centuries. It has slowly evolved over time into the leading pigment in a multi-billion dollar dye industry. The coloration of every pair of blue jeans in the world is a result of indigo. This is because indigo is the only blue dye of permanence. Indigo can either be produced naturally or synthetically. In modern times, however, synthetic dyes are the ones of choice.
Natural indigo dye is perhaps the oldest dye known to man. Some of the oldest historic text mention indigo as one of the colors chosen for the Tabernacle of the Arc of the Covenant. The Chinese, Persians, and Indians used natural dyes, including indigo, many centuries ago. Man has used dyes to stain hides, decorate shells and feathers, and paint their stories on the walls of ancient caves. Scientists have been able to date the black, white, yellow, and reddish pigments that were used by primitive man in cave paintings to over 15,000 BC. With the development of fixed settlements and agriculture around 2,000-7,000 BC, man began to produce and use textiles, and would therefore add color to them as well. Dye analysis of textile fragments that have been excavated from archaeological sites in Denmark have been dated to the first century AD. Indigo has been used since the Neolithic times in Europe and also by the ancient Egyptians. In the 13th and 14th centuries, dyeing gained importance in Italy. From there, the methods used were carried to other parts of Europe. As new dyes became known, the dyeing industry flourished and grew rapidly. In the 18th and 19th centuries, the practice of colonialism insured that there would always be a supply of foreign dyes. The Industrial Revolution also helped the growth of the dye industry by meeting the demands of large-scale productions while at the same time, finding new ways to make the colors brighter and longer lasting to wear and wash repeatedly. Finally, in the 19th century, the first synthetic dyes were produced.
With the tremendous rise in the interest of chemistry in the mid 19th century, several important innovations in dyeing came about. William Henry Perkin, a student of the renowned European scientist Wilhelm von Hoffman, accidentally discovered the first synthetic dye, mauviene, in 1856. While trying to synthesize quinine, he discovered an aniline “mauve” color. Perkin, who was 18 years old, had been researching a cure for malaria at the time. This new dye was quickly put into industrial application, which allowed the young Perkin to start his own factory in London to commercially manufacture his newly discovered dye. This was the beginning of the synthetic dye industry and the decline of the natural dye demand.
The need for natural dyes dropped considerably after Perkin’s discovery for a number of reasons. Synthetic dyes have many advantages over natural dyes. For example, synthetic dyes have been proven to be much safer, more reliable, of far greater variety, and more consistent than natural dyes. Natural indigo is indistinguishable from its synthetic counterpart. Eventually, the old natural dyes lost popularity in favor of the new synthetic ones. Some purists still use natural dyes, but except for food and histological uses, few of them are important. By the end of the 19th century, a few Scottish tweed producers were the only ones still using natural dyes. These days, the use of natural dyes on a commercial scale barely exists. It can only be found in remote areas where people have either little access to synthetic dyes or a strong belief in retaining their ancient dyeing customs.
Over the years, indigo has been produced in many ways. Natural indigo is dyed in a manner different than any other dye. Most other natural dyes are affixed with a metallic salt, in a two-step process. Natural indigo is fermented, and then dyed in a process that affixes the indigo as it oxidizes in the air. Until the end of the 19th century, the sole source for indigo came from the woad (Isatis tinctoria) plant and the Dyer’s Knotweed (Polygonum tinctorum) plant in temperate climates and Indigofera species in the tropics. The woad was widely grown in Europe. Indigotin, the main constituent of indigo, is prepared from the leaves of several species of Indigofera. Only about four ounces of indigotin are extracted from 100 pounds of plant material. All natural dyes must be dissolved in a liquid before textile materials can absorb them. Water cannot be used since it will not dissolve indigo. When indigo is combined with a reducing agent, a compound is formed that will dissolve in an alkaline liquid. This process turns the indigo dye liquid yellow. When the material is dipped into this liquid, known as a dyebath, it absorbs the dye in this yellow reduced form. The dye is permanently fixed on the fibers by oxidation when the dye is exposed to air. During this step the blue color returns and the dye is once again insoluble in the fibers. This process is known as vat dyeing. The vat is the solution used to dye the material.
The main form of indigo produced since the end of the 19th century has been produced synthetically. The first synthetic indigo dye was discovered in 1897. This method can be done several ways. In one technique, indoxyl, the water-soluble dye that is reduced to indigo on contact with oxygen, is produced by coupling together two molecules of sodium phenylglycinate in a mixture of sodium hydroxide and sodamide. The sodium phenylglycinate is industrially obtained from aniline and the indoxyl is mainly found in the form of the glucoside indican or the ester isatan B. This process produces the vast majority of all indigo dyes in the world today. Today, indigo is also produced by the bacterium E. coli through genetic engineering. To produce indigo on a microscale, the Baeyer-Drewson reaction can be used. In this process, nirtobenzaldehyde is dissolved in acetone in a test tube. Water is then added and the solution is stirred. Sodium hydroxide, a base, is added with a dropper. As the base is added, the blue color of indigo can be seen almost at once. This causes an exothermic reaction and the solution may boil. After the reaction has cooled down, the precipitated indigo is collected by suction filtration. The indigo is then washed with water and ethyl alcohol.
Dyes are classified into three different groups according to how they are applied to the fabric: vat dyes, mordant dyes, and direct dyes. Indigo is an example of a vat dye. Mordant dyes require some sort of substance, usually a metal salt, to prevent the color from washing or a light bleaching out. Most dyes require a mordant at some point during the dyeing process, but indigo does not since it is a vat dye. In order for a dye to be useful, it must be fast, which in the dye industry means that the dye must remain in the fabric during washing. A fast dye is one that bonds to the fabric in some way. Indigo is one of the fastest dyes known to man, which is why the denim industry is so huge. Indigo is electrostatically held within the fiber molecule. Indigo is the only fast natural blue dye, which is why it is used in some form in all traditional cultures.
In the early part of the 19th century, indigo was often sold in the forms of dark cubes or cakes called “junks”. Since indigo was an expensive dye, the quality of these dyes was of great concern to the professional dyer. The price of indigo ran about $2.25 per pound in 1831. Opinions varied greatly on which country exported the best grade of indigo. Some dyers considered Bengal indigo the best. They claimed that it would color at least 10% more cloth than the best Spanish indigo, which was imported from the Spanish dominions in Central America. The criteria by which the 19th century dyer judged the quality of indigo included: light weight in relation to bulk, smoothness in fracture, and a bright violet, purple, or bronze color. There was a general agreement that the price for the indigo should be in proportion to its yield.
In 1914, ninety percent of the dyes in America were imported from Germany. Major changes have taken place during the last decade, and today, Asia has become the largest dye producer, accounting for about 42% of the value of the world’s dye market. The value of the global dye market in 1999 was estimated at $6.6 billion, an 18% drop in value from 1996. North America accounted for $1.2 billion, Central and South America for $0.7 billion, Western Europe for $1.2 billion, and Asia for $2.7 billion. The Buffalo Color Corporation claims to be the only United States manufacturer of indigo dye.
Indigo dye was highly prized for the color it produced. In the early days of America, indigo could be found in the coats worn by the American soldiers in the American Revolution. Today indigo can be found in the production of blue denim for blue jeans that are worn all around the world. The use and production of indigo dye has come a long way since its utilization in cave paintings and early biblical times.