Experts Solve 1000-Year-Old Mystery of Rare Medieval Blue Ink

Close-up of Chrozophora tinctoria fruits collected in Portugal and used to recreate the medieval blue ink. Source: Paula Nabais et al. / Science Advances

Portuguese researchers believe that they have identified the long-lost process and plant that provided a unique purple-blue pigment in the Middle Ages . This was used to color many things including cloths, but it was particularly used in illuminated manuscripts, which were masterpieces of the medieval world. The experts were able to recreate the medieval blue ink based on a book written in an extinct language.

The pigment was known as folium and it was famous for its hue and its long-lasting properties. Science News reports that “long-lasting blues are relatively rare among dyes,” and this made folium so prized in the Middle Ages. This blue hue is not like the indigo still widely in use, or those pigments produced from some types of flowers. Atlas Obscura reports that this distinctive blue dye was “responsible for coloring everything from Bible scenes to, later, the rind of a popular Dutch cheese.”

Lost Blue Pigment

Folium was one of the main sources of blue pigment, along with indigo. After the production of the ink, it was soaked onto a piece of linen. This allowed it to be transported all over the European continent.  However, after Gutenberg developed the world’s first printing press, the popularity of books, meant that illuminated manuscripts slowly fell out of use. Over time, folium was used less and less and the process for producing it was lost after the invention of synthetic dyes.
The recipe for the hue was lost since at least the 19 th century.  Conservation scientists from the New University of Lisbon decided to solve the mystery of how to make the purplish-blue dye. They decided to decipher a 15 th century medieval text on how to produce colors for illuminated books, which were typically produced by Christian monks.

Translating the Extinct Language

However, this was not an easy process. The book was written in an extinct language Lusitanic, that was spoken by the Judeao-Portuguese community. Portugal had a vibrant Jewish population in the Middle Ages until its members were expelled or driven underground by the Inquisition.
There were problems with translating some of the instructions and Atlas Obscura reports that “there’s more than one way to skin a cat, and other sources provided different instructions.” However, the team of researchers retrieved enough information to attempt to resurrect folium after at least a century.
They knew that the small silvery-green herb Chrozophora tinctoria was the ink’s sole ingredient.  While the work did not state the name of the plant, it left behind a vivid description and this allowed the researchers to understand what plant to use. Paula Nabais,  the study’s lead author, stated that “it says how the plant looks, how the fruits look … it’s very specific, also telling you when and where the plant grows, and when you can collect it,” reports Atlas Obscura 
An illuminated manuscript from the Middle Ages. One of these manuscripts included the recipe for recreating the medieval blue ink. (Paula Nabais et al. / NOLA)

Deciphering the Mystery

The manuscript gave detailed instructions and even gave a location for the best plants to use. Nabais and her colleagues travelled to Monsaraz, in southern Portugal. They collected the fruit of the plant, which is the size of a small nut along the roadside in the fall. After a preliminary examination, it became clear that it contained a blue fluid. Based on their research, they knew that the seeds could not be crushed because this could impact on the extraction of the pigment.
They brought the specimens back to their laboratory. Science News reports that “the team used a suite of analytical techniques to zero in on the dye molecule’s structure.”  They were able to show that the chemical compound of the purplish-blue hue was identical to the plant’s fruit on the molecular level. Then they examined molecules from the fruit of the plant and “simulated the light’s interaction with the candidate molecule, to check whether it would give them their desired blue,” according to Science News . This was the case and then the scientists decided to extract the pigment that was so valued in the Middle Ages.
The Chrozophora tinctorial herb used to recreate the medieval blue pigment. (Paula Nabais et al. / Science Advances )

Trial and Error Helps Resurrect the Recipe

Then the team decided to copy the procedures that were outlined in the medieval texts. Melo told Atlas Obscura that “part of our expertise is to make this conversion from what is actually written and sometimes not so clear enough for us, and what they were making.” The vague nature of the instructions meant that the process was time-consuming. After a period of trial and error, they were able to extract the pigment from the tiny fruit.
Melo told Science News that “it was great fun to recover these recipes.” There is more work that needs to be done on the formula. However, the team now hope that folium can be once again used by experts who are preserving rare illuminated books, as the medieval dye could last centuries.
Melo is quoted by Atlas Obscura as saying that “we don’t have such paints now. So, this is part of our research—to know as much as possible about this material that was completely lost.” The discovery could help in the development of pigments that are more long-lasting and durable than synthetic dyes.


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