Ancient recipes led scientists to a long-lost natural blue

Medieval texts and modern techniques helped unveil the blue watercolor’s identity

fruit from the plant Chrozophora tinctoria

Medieval recipes instructed scientists to prepare the blue watercolor from the fruits of Chrozophora tinctoria. That provided enough dye to reveal its structure using a suite of chemical analysis techniques.

Paula Nabais/NOVA Univ.

Scientists have resurrected a purple-blue hue whose botanical origin had been lost to time.

The pigment, called folium, graced the pages of medieval manuscripts. But it fell out of use, and the watercolor’s identity has eluded scientists for decades. Now, after tracking down folium’s source, researchers have mapped out the chemical structure for its blue-producing molecule.

Such chemical information can be key to art conservation. “We want to mimic these ancient colors to know how to … preserve them,” says Maria Melo, a conservation scientist at Universidade Nova de Lisboa in Caparica, Portugal. But to unmask folium’s identity, Melo and her team first had to find where it came from.

The researchers turned to medieval texts that described the source plant. With the help of a botanist, they discovered Chrozophora tinctoria, a tiny herb with silvery-green foliage. In a village in the south of Portugal, the team found the wild plant growing along the roadside and in fields after harvest. Back in the lab, researchers extracted the pigment from its pebble-sized fruits by following directions detailed in the medieval manuscripts. “It was really great fun to recover these recipes,” Melo says. 

an ancient book
During the Middle Ages, a blue watercolor called folium was popular for illustrating texts such as this book of hours from the 15th century. Now that researchers have tracked down the color’s source, prepared the dye and studied its chemistry, they may be able to find examples of it on ancient pages.Palácio Nacional de Mafra collection

The team used a suite of analytical techniques to zero in on the dye molecule’s structure, it reports April 17 in Science Advances. The scientists also simulated light’s interaction with the candidate molecule, to check whether it would give them their desired blue.

Long-lasting blues are relatively rare among dyes, and this one is neither like the indigo (SN: 9/14/16) used in denim jeans nor an anthocyanin, such as those that show up in many flowers (SN: 7/26/17). This newfound hue is in its own class of blues.

Carolyn Wilke is a freelance science journalist based in Chicago and former staff writer at Science News for Students. She has a Ph.D. in environmental engineering from Northwestern University.

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