Oldest indigo-dyed fabric found

Ancient South Americans made blue fabrics to dye for. A piece of approximately 6,000-year-old woven cotton material from Peru gets its blue hue from indigo dye, making it the oldest known example of the colorfast dye’s use anywhere, researchers find.

Until now, the earliest indigo-dyed fabrics dated to around 4,400 years ago in Egypt and about 3,000 years ago in what’s now western China, say archaeologist Jeffrey Splitstoser of George Washington University in Washington, D.C., and his colleagues. Chemical analyses of fabric unearthed at Huaca Prieta, an ancient site on Peru’s northern coast, unveiled the presence of indigo dye roughly 1,600 years before this fabric coloring showed up in Egypt, Splitstoser’s team reports online September 14 in Science Advances.

Huaca Prieta was first excavated in the 1940s. Radiocarbon dating of charcoal from hearths and other material indicates that people occupied Huaca Prieta from around 14,500 to 4,000 years ago. From roughly 7,600 to 4,000 years ago, residents built carefully designed layers of stone dwellings, plazas, burial chambers and other structures that now form a huge stone and earthen mound.

Splitstoser’s group identified indigo blue in 3-centimeter- to 5-centimeter-long yarn strands from five of eight woven, blue-striped cloth fragments excavated at Huaca Prieta in 2009. One patch of material was found in sediment with an estimated age of 6,200 to 6,000 years. Other pieces dated to no more than around 4,100 years ago. It’s not known what types of items these swatches came from.

Blue dye in ancient cotton patches at Huaca Prieta most likely came from an indigo-producing plant native to South America, the scientists say. Other such plants grow in many parts of the world, including Asia and Europe. Indigo blue has also been produced from sea snails in the genus Murex.

“What surprises me is that the indigo dye process was discovered at all and developed independently so early in multiple parts of the world,” Splitstoser says.

Indigo blue is not easy to make, he explains. It takes several steps to extract the dye from plants. This process includes soaking and fermenting leaves to produce a colorless substance before stirring the mixture in the open air, which eventually yields the main blue-dye component, indigotin.

It’s hard to know precisely when and where people first dyed fabrics with indigo blue, Splitstoser says. It wouldn’t be surprising if the earliest textiles tinged with indigo blue came from the Middle East, in his view, because some the earliest civilizations emerged there. “Based on physical evidence, however, the earliest known use of indigo blue was on the northwestern coast of South America.”  

The new discovery joins previous evidence of other “firsts” at Huaca Prieta, says archaeologist Daniel Sandweiss of the University of Maine in Orono. For instance, the Peruvian site hosted the Americas’ earliest known farmers and corn cultivation. “This shows the value of multidisciplinary, long-term research at a single site by a large, well-funded team,” Sandweiss says.