It is the favorite color of most of my family and most of our customers. ( I have to exclude myself from this august group, because I only like it in small doses. Too much, and for some reason, it depresses me.) Nevertheless, I do like some blues. One of my favorite blues is indigo ( the other is a very pale blue/purple, sort of a periwinkle). But I am digressing. Today I want to write about indigo.
Indigo is a very old color. The oldest known indigo fabric was found in Peru and is approximately 6,000 years old. It was also known and used in many other ancient civilizations, although there were different sources for the color. India and other Asian countries used dye derived from the Indigo tinctoria plant (the pink flowered plant in the first picture), which produced a true indigo. Another source used in Europe and Asi was the murex sea shell. Dyes from this source contained a mixture of indigo and dibromoindigo (a red color). The combination resulted in beautiful purples, although exposure to light converts the dibromoindigo blue resulting in a purple-blue and colors like royal purple. The source of indigo in Europe was, and still is in some cases, woad. Woad (the yellow flowered plant in the third picture) contains the chemical containing indigo in very low doses and fabrics dyed with it sometimes have a greenish hue. Again, exposing it to sunlight brings out the true blue indigo everyone loves.
The process for deriving indigo from plant sources requires fermenting leaves, then using a strong basic solution to extract the dye. It is usually yellowish green at this point and can be fixed to the fiber without a mordant (usually a metal salt). Exposure to sunlight after dyeing brings out the blue. (This seems to be the opposite effect of leaving most dyed fabrics or fibers in the sun.)
Today, most indigo dyes are synthetic and the favorite use is for denim. The environmental consequences of manufacturing denim can be extreme--some streams and communities in China run blue (picture of the Pearl River, 4th picture below). It is almost enough to make one give up their jeans. (The second picture shows indigo dyed fabric.)