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

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.

researchers turned to medieval texts that described the source plant. With the
help of a botanist, they discovered Chrozophora
, 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

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

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.