When it comes to the blue pigment used in medieval manuscripts we have known for years that lots of manuscripts were colored with indigo and lots with something called folium. Until recently however we didn’t know what the word folium meant exactly; we didn’t have a recipe for making it. Turns out folium is derived from a quite common plant which can be found growing alongside Portuguese roadways.

Researchers worked from recipes for preparing the blue pigment found in medieval manuscripts. Using the sledgehammer approach they collected many possible plants along with their fruits and subjected all of them to the procedures described in the manuscripts. Then they waited to see which one turned out to provide the correct blue.

Chrozophora tinctoria’s fruit is the missing ingredient. It’s apparently about the size of a walnut. Atlas Obscura reports that the recipe for creating folium was mined from a manuscript about making colors written in Judeao-Portugeuse, the extinct language used by the Jews of medieval Portugal. The discovery has been published in a paper in Science Advances, which describes the plant thus: C. tinctoria (L.) A.Juss. (Euphorbiaceae) is an annual herb native to the Mediterranean region, north Africa, and central and southwestern Asia. This species can be found on dry and disturbed lands, ruderal habitats, fallows, and along the edges of cultivated fields, mostly in limestone. The plants are 10 to 40 cm tall, gray-green, and tomentous (densely covered with stellate hairs). Stems are erect and branched and leaves are alternate, rhombic to ovate, cuneate at the base, and with sinuate leaf margin.”

Here, from the Hungarian Medieval Manuscript Manual is a listing of the sources of some of the pigments used by the medieval scribe/illuminator. Even better, because well illustrated, is this page from the Fitzwilliam Museum’s Illuminated display.