Solving the mystery of a medieval blue hue

To add blue illuminations to their texts, medieval artists used folium, a watercolor pigment derived from the pea-sized fruits of the plant Chrozophora tinctoria. Over time, artists abandoned the pigment. By the 19th century its recipe was considered lost, and researchers have been unable to determine the structure of the compound responsible for folium’s blue hue—until now.

“This is the only medieval color based on organic dyes that we didn’t have a structure for,” says Maria João Melo, who studies conservation and restoration at NOVA University of Lisbon and was one of the lead sleuths, along with the University of Porto’s Joana Oliveira and Victor de Freitas. “We need to know what’s in medieval manuscript illuminations because we want to preserve these beautiful colors for future generations.”

The color compound, which the researchers named chrozophoridin, is structurally distinct from other known natural blue colorants like anthocyanins and indigo (Sci. Adv. 2020, DOI: 10.1126/sciadv.aaz7772). It consists of a dimer of hermidine and features a sugar group. Chrozophoridin has two stereochemical forms arising from hindered rotation around one of its bonds (shown in red).

Paula Nabais, who works in Melo’s lab and is the report’s first author, says a 15th-century text, The Book on How to Make All the Color Paints for Illuminating Books, was indispensable in re-creating the pigment. The original book is written phonetically in Portuguese with Hebrew characters. Melo’s group began using it to reconstruct pigments a few years ago.

“It describes when to collect the fruits—in July,” Nabais says. “You need to squeeze the fruits, being careful not to break the seeds, and then to put them on linen.” Those details turned out to be critical, Melo and Nabais say. Broken seeds release polysaccharides, which form a gummy material that was impossible to purify.

Once the researchers had the purified pigment in hand, they used a combination of liquid and gas chromatography, mass spectrometry, and nuclear magnetic resonance to figure out its structure.

The study shows “how the combination of historical literature and current scientific methods and instrumentation can sort out with pinpoint precision the chemical nature of the artist’s or scribe’s palette,” says Patrick Ravines, who studies art conservation at Buffalo State College.

Arie Wallert, a curator and scientist at the Rijksmuseum, says he tried to solve the mystery of this pigment in the 1990s but was stumped. “I decided to shelve it up, for after retirement,” he says. “But now, through the combined brain power of this group of Portuguese researchers, this problem has been fully, and beautifully, resolved. I can spend my retirement on other things.”