Samples of 6,200-year-old cloths that were unearthed in an archeological site in Peru provide the earliest discovered evidence of using indigo dye.
The fabrics were first unearthed from the ancient ceremonial mound Huaca Prieta in Northern Peru in 2007. Experts think that this site was a temple where people placed a variety of offerings such as textiles as part of a ritual.
The fabrics were so dirty they appeared colorless. After gently washing the cloths and using a technique known as high-performance liquid chromatography, which isolates parts of a mixture, researchers discovered that the textiles have the oldest indigo-dyed fabric in the world.
Five of the eight samples that the researchers put through tests were found to have traces of indigo. The rest of the samples may have degraded over time.
The indigo-dyed fabrics were also 6,2000 years old, which makes them almost 2,000 years older than previously known oldest sample of the indigo dye that was made in Egypt.
"6,000-year-old cotton fabrics from the Preceramic site of Huaca Prieta on the north coast of Peru retained traces of a blue pigment that was analyzed and positively identified as an indigoid dye (indigotin), making it the earliest known use of indigo in the world," researchers reported in their study. "This predates by ~1,500 years the earliest reported use of indigo in the Old World, from Fifth Dynasty Egypt."
Traditional indigo dye comes from the organic compound indigoid, which can be found in plants such as the Indigofera. Researchers said that ancient Peruvians likely sourced indigo dye from this particular plant. The ancient Egyptians, on the other hand, extracted indigo coloring from sea snails.
Study researcher Jeffrey Splitstoser from the George Washington University, said that making indigo can be quite complicated.
While many dyes can be made by simply boiling flowers in the water to extract the color, indigo dye uses leaves that need to be fermented. After being fermented, the mixture needs to be aerated so that a solid compound from the mixture would fall to the bottom of a tub.
Reconstituting the mixture, which can be dried and stored, needs an alkaline substance such as urine, which makes white indigo. Yarns that are dipped in white indigo will turn yellow, green and finally blue.
Splitstoser said that the findings revealed sophisticated textile technologies that the ancient Andean people developed 6,200 years ago.
The study was published in the journal Science Advances on Sept. 14.