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Identifying the chemical makeup of pigments used in ancient documents, paintings, and watercolors is critical to restoring and conserving the precious artworks. However, despite numerous efforts, scientists had been unable to determine the source of folium, a popular blue dye used to color manuscripts in Europe during the middle ages — from the 5th to the 15th century. Now, a team of researchers from Portugal has finally uncovered the mysterious ingredient responsible for the gorgeous blueish-purple tint that helped bring ancient illustrations and texts to life.
"This is the only medieval color based on organic dyes that we didn't have a structure for," says study co-author Maria João Melo, an associate professor at the Department of Conservation and Restoration at NOVA University of Lisbon. "We need to know what's in medieval manuscript illuminations because we want to preserve these beautiful colors for future generations."
The interdisciplinary research team of chemists, conservation scientists, and botanists began by poring over recipes penned by European dye makers from the 12th, 14th, and 15th centuries. They found what they were seeking in a 15th-century text entitled The Book on How to Make All the Color Paints for Illuminating Books. However, deciphering the recipe was no easy task. It was written in the now extinct Judaeo-Portuguese language, and though the source of the dye was attributed to a plant, no name was mentioned.
However, by piecing together suggestions from the text, the scientists were able to determine that the dye was made from the bluish-green berries of the Chrozophora tinctoria plant. The unassuming herbal shrub is endemic to the Mediterranean, North Africa, and central and southwestern Asian countries. After an extensive search, the team found a few specimens of the plant growing along the roadside near the town of Monsaraz in south Portugal.
The detailed recipe gave the researchers critical clues — including the best time to pick the berries — to successfully recreate the dye. "It describes when to collect the fruits—in July," study co-leader Paula Nabais, a researcher at the NOVA University of Lisbon, says. "You need to squeeze the fruits, being careful not to break the seeds, and then to put them on linen." The scientist says the detail was important since broken seeds polluted the pigment, producing an inferior quality ink. The dyed linen, which was left to dry, was an efficient way to store and transport the pigment during ancient times. When needed, the artist would simply cut off a piece of the cloth and moisten it with water to squeeze out the blue color.
Once the key ingredient had been identified, the researchers began the next step and the primary purpose of their study — determining the dye's molecular structure. This entailed creating a concentrated sample of the blue pigment by immersing crushed berries into a methanol-water solution and stirring the mixture gently for two hours. The methanol was then extracted and the dye further purified. The team then used modern methods, such as nuclear magnetic resonance and mass spectrometry, to determine the pigment's chemical composition.
To their surprise, they found that folium was not like any other known permanent blue dyes — it was an entirely new class of color, one they named chrozophoridin.
"Chrozophoridin was used in ancient times to make a beautiful blue dye for painting, and it is neither an anthocyanin - found in many blue flowers and fruits - nor indigo, the most stable natural blue dye. It turns out to be in a class of its own," the team wrote in the study. "Thus, we believe that this will be not our final word on this amazing plant and its story and that further discoveries will follow soon."
The researchers, who published their findings in the journal Science Advances on April 30, 2020, believe their results will allow conservators and scientists to recreate the dye and further investigate its structure and reaction to environmental stresses over time. It will also enable them to easily identify the stain in medieval manuscripts.
Resources: advancessciencemag.org, phys.org, sciencealert.com