Color Trails: Natural dyes in historic textiles get a closer look

Chemists have developed a way to extract natural dyes from ancient textiles while preserving the unique chemical characteristics of each dye. The technique enables the researchers to then identify the plant species from which the colorants came.

Determining the source of dyes could open a new window on how ancient people used natural resources, says chemist Richard Laursen of Boston University. What’s more, since plants grow within set geographic ranges, characterizing natural dyes could help archaeologists trace the movements of tribes or determine trade relations between distant communities.

Laursen and his colleague Xian Zhang, also of Boston University, used their new chemical method to analyze yellow plant dyes called flavonoids. Many dye flavonoids have attached sugar molecules that are specific to each plant.

Traditionally, textile manufacturers have used a substance known as a mordant to bind dye compounds to fibers. “It’s a practice that’s been used for thousands of years,” says Laursen. First, the textile is soaked in a solution containing the mordant, which, in most cases, is an aluminum salt. Aluminum ions penetrate the fabric’s fibers, and then, in a second bath, dye molecules bind to the ions. The result is a colored textile that holds on to its dye.

To extract the natural dyes from historical textiles for analysis, researchers have typically relied on harsh chemicals, such as hydrochloric acid, to separate the dye from the mordant. However, this process also strips away the flavonoids’ plant-identifying sugar molecules. “You lose a lot of the information this way,” says Laursen.

As they report in the April 1 Analytical Chemistry, he and Zhang extracted yellow dyes from test fabrics using different, milder reagents: ethylenediaminetetraacetic acid (EDTA) and formic acid. The Boston researchers tested their method on silk fibers that they had dyed with flavonoids from different natural sources, such as pagoda-tree buds from a local arboretum and onions from a supermarket.

Laursen and Zhang soaked their dyed silk samples in hydrochloric acid, EDTA, or formic acid, extracted the flavonoids, and chemically characterized the compounds. They found that the treatment in strong acid stripped away the distinguishing sugars of the flavonoid dyes, making all of the dyes appear chemically the same. The milder reagents, however, preserved the sugar signatures of the flavonoids’ sources.

The researchers also tested their method on textile fibers from a 1,000-year-old mummy in Peru. They found a new type of yellow dye, a flavonoid sulfate, that was previously unknown to archaeologists. “You wouldn’t see it using the traditional methods,” says Laursen. It turns out, he adds, that a certain group of plants in Peru and Argentina are rich in flavonoid sulfates and that there’s a long tradition of using these plants for dyeing textiles.

Irene Good, a specialist in ancient textiles at Harvard University, says the new dye-identifying technique “is extremely important and very promising.” In addition to pegging the specific plants used to make dyes, the method could reveal how natural dyes were processed by ancient people—for example, whether they dried plants or used them fresh, she says.

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