The first known use of indigo dye

This square of striped cotton, and a few others like it, represents the first known instance of people using indigo to dye a textile blue.
The ancient Peruvian fabric is more than 1,500 years older than the earliest known Egyptian fabrics with indigo-dyed borders and 3,000 years older than the first blue-dyed textiles in China, according to a study published this week in the journal Science Advances.
“It is possible it is the earliest known example of cloth dyeing in the world,” said Jeffrey Splitstoser, a textile expert in the department of anthropology at George Washington University. “I don’t know of anything older.”..
The blue-tinged pieces of cloth were unearthed at Huaca Prieta, an ancient ceremonial mound on the north coast of Peru that was occupied between 14,500 and 4,000 years ago. Thousands of squares of the prehistoric textiles have been found at the site. Splitstoser said he has personally examined 800 of them...
The cloth pieces were not used for clothing because they had no arm, leg or head holes, and the edges were not treated or hemmed the way you would expect for even a simple item of clothing like a poncho, he said. Instead, he suspects that they may have been used to carry items to the site.
“If you got to the Andes today people will take a square of fabric about the same size as what we saw, put whatever they want to carry in the center and then wrap it up,” he said. “I think they were carrying things in the bag to the temple and then ritually depositing or using them there and leaving the textiles there as well.” ..
He added that the find is a little surprising because indigo is not the most intuitive dye. Indigotin, the blue component in indigo, is not soluble in water, so it’s not like you can just throw some Indigofera flowers in a vat of boiling water and extract the dye. Instead, you have to ferment the leaves, which turns the indigotin into another chemical that is soluble in water, but is not blue.
“It’s actually kind of a yellowish color,” he said. “In order to get the blue, you dip the clothes in the water with the dissolved indigo molecule, then when you pull it out it oxidizes, and that’s when it turns blue.” Further details at the Los Angeles Times.