Explained: Indigo Dye

We all remember learning about indigo in grade school - the bright blue color used for dyeing cloth, and one of the seven colors of Sir Isaac Newton’s rainbow. Indigo has been used as a coloring agent since ancient times, and is best known for the distinctive color it gives to blue jeans.

A Block of Indigo Dye

The Origins of Indigo:

Indigo is found naturally in tropical plants of the indigofera family, thought most of the indigo used today is a synthetic variety. Indigo is produced today mainly for dyeing cotton and for food coloring. You might see it on food packaging as “Blue No. 2″.

The natural form of indigo is among the oldest dyes used to color textiles, and has a rich tradition in clothing, design, and the arts in India, China, the United States and Latin America, as well as ancient civilizations in Mesopotamia, Egypt, Greece, Rome, Peru, and Africa.

Indigo plants were first domesticated in India, which became the major hub for indigo production and trade. India provided the main supply of dye to Europe as early as the Ancient Greeks, who referred to the color as indikon or “indian dye.” 

Because the dye had to travel on precarious overland routes from India into Europe and incur tariffs from shipping merchants, indigo was expensive and difficult to acquire, which helped it earn the nickname “blue gold.” Wearing indigo blue clothing became a sign of wealth (as opposed to now, as “blue collar” refers to the working class), which is why one particular shade is now known as “royal blue”.

Indigo continued to command a premium even through the Middle Ages, and many Western European countries began to cultivate a similar plant called woad as an alternative. It wasn’t until Vasco de Gama discovered a trading route to India by sea that the price was greatly reduced.

In the 1700s, many European powers began to cultivate true indigo in their territories in the Carribean and North America. American plantation owners grew and sold indigo to England, until British companies began buying indigo from India in accordance with their imperial interests, and indigo fields were replaced with cotton crops.

The industry began to see radical change in 1878, when German chemist Adolf von Baeyer (of Bayer Aspirin fame) perfected a process for synthetically manufacturing indigo dye, which largely replaced plant dye by the turn of the 20th century.

Indigo Ropes

Why is Indigo Best for Denim?

Indigo is perfect for dyeing cotton because it only partially penetrates into the fibers, lending a rich surface color that does not affect the innermost parts of the fabric. Cloth dyed with indigo turns a lighter color when worked or rubbed over time, and remains the preferred dye to achieve the signature worn-in look of denim.

Indigo is not soluble in water, so it must be chemically changed to “white indigo” for the dyeing process. During manufacturing, cotton fibers are woven into ropes and dipped in the white indigo saturated water. Once they are taken out of the water, the dye then reacts with oxygen in the air and takes on its characteristic blue hue again.

Normally, the cotton yarns go through at least 4 to 8 “dips”, or passages through the dye bath. The more dips, the darker the fabric. After the initial dyeing process, the ropes are washed or additional chemicals are added to change the color of the yarns or improve the fastness of the dye to the fibers.  Once the dyeing process is complete, the yarns are woven into the cotton cloth known as denim. While there are many ways to treat denim after it is woven, cut, and sewn into the pair of jeans you see at your local boutique, the initial dyeing process accounts for much of the variability in denim color.

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