Indigo, manufacture and history of a thousand-year-old pigment
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The indigo tree, the green plant that gives blue
Indigo dye can be produced from a dozen different plants. The one giving the richest dye (we speak of "true indigo") is the indigo tree (Indigofera tinctoria). It is a bushy shrub of the fabaceae family and of the indigofera genus (which counts no less than 700 species). It can reach 1,20 meter and gives small pink flowers. Its name comes from the Latin indicum meaning "from India". This name was given by the Greco-Romans who brought the plant back from India, where traces of cultivation dating back more than 4,000 years can be found. Since the Renaissance, the indigo tree was also exploited in the Near East and Egypt, where indigo blue bands were found around some mummies. Note that in 2016 in Peru, archaeologists have uncovered a fabric dyed with indigo 6,000 years old. Researchers believe that the dye used comes from the Indigofera, one of the indigo plants found in South America. As for the epithet dyeing, it attests that the indigo tree was originally associated with its dyeing use, although its roots and leaves are also used in several traditional medicines. The shrub was then exported to other tropical regions of Asia, America and Asia.
Indigofera also has a medicinal use to treat various pathologies. The leaves and roots are used in traditional Ayurvedic, Chinese and South African medicine. In Europe, in the first century, the Greek physician Dioscorides recommends the use of indigo for therapeutic purposes.
Indigotine : the blue gold of the dyers
You may wonder how a green plant can produce a purplish-blue dye. The coloring principle of indigo is called indican. This organic compound, extracted from the indigo tree, is colorless. Through chemical reactions, indican is transformed into indigotine, which is the blue dye present in indigo dye. There are several processes to obtain the indigo dye, but the principle is always the same. The ancestral method, still used in Africa, is the following: Harvested before flowering, the shrubs are plunged into a tank filled with water where it occurs a fermentation. This step allows the extraction of the indican. Then, the liquid is transferred to another tank, where it is stirred for a long time. This oxygen supply allows the oxidation and the formation of indigotine. Then the liquid is left to rest and the indigotine settles to the bottom. The pit is then emptied and the pigment can be recovered. The problem is that this pigment is insoluble, which does not allow it to be used as a dye at this stage. In order to dissolve it in water, a chemical transformation called reduction must first be carried out. To do this, the traditional method will use products such as lime, iron sulfate, glucose, or fermented urine. This ancient process being long and complicated, the modern industry uses a powerful chemical reducer called sodium hydrosulfite, a quick solution but, you guessed it, polluting.
As we have seen, several different plants are used to obtain indigo dye, as indican is present in all species of the genus indigofera. If the pigment made from the indigo tree is called true indigo, it is because it is the one with the deepest blue, the intensity of the pigment being determined by the concentration of indican in the plant. In the Middle Ages in Europe, we cultivated a species called the woad of the dyers or woad (Isatis tinctoria). In France, the Lauragais region was specialized in its cultivation and many woad makers made their fortune. But the woad trade was damaged in the 16th century by the arrival in Europe of dye from India, whose pigments are 20 times more active. Native to North America, we have the species of the genus Amorpha, also called false indigo. Finally, let's mention the Mayan blue, which is a pigment that was used by the pre-Columbian Mayan and Aztec civilizations as a dye for sculptures, frescoes, paintings and pottery. A color associated with sacrifice for the ancient Maya, the pigment would have been used to paint the bodies of the sacrificed. Archaeologists have been surprised by the pigment's great stability, its first use dating back to 800 BC.
From human exploitation to the exploitation of nature
In the 17th and 18th centuries, the western demand for indigo was very important. Indigo production was an economic issue for the great powers of the time. To be able to produce quality indigo in quantity, it is necessary to develop indigo crops. To do this, a suitable climate and a consequent workforce are needed. These problems were quickly solved by the creation of indigoteries in North America, the French West Indies and India (then a British colony) and the massive use of slavery.
En 1878, le chimiste allemand Adolph Von Bayer réalise le premier indigo de synthèse. En 1897, premier pigment indigo de synthèse est commercialisé par l'entreprise BASF. Au cours du XXe siècle, la teinture synthétique remplaça peu à peu l'indigo naturel.
In 1878, the German chemist Adolph Von Bayer produced the first synthetic indigo. In 1897, the first synthetic indigo pigment was marketed by the company BASF. During the 20th century, the synthetic dye gradually replaced natural indigo.