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Researchers Rediscover Elusive Natural Blue Pigment Used in Medieval Illuminations

Arkansas Natural Heritage Commission - Friday, May 29, 2020

Scientists, historians, and conservationists spend a lot of time espousing the value of primary source records, natural history collections, herbarium specimens, and other historical documents in restoration work. These records often end up putting together the missing puzzle piece needed to understand what came before us and the best means of preserving or restoring it. One such example that was only recently solved involves a blue-purple pigment used in medieval manuscripts.

Illuminated medieval manuscripts have long been treasured for their colorful artwork, each painstakingly done by hand. Many of the materials used to create these vivid colors have since been identified and their usage can be repeated to this day. However, one bluish-purple color, called folium or tornasol (turnsole) by medieval authors, had alluded art conservationists for decades.

A team of chemists, conservators, and a biologist have been able to follow primary source records and old publications to rediscover the method and the plant used by medieval artists to create a blue watercolor called folium or tornasol (turnsole). The tale of how it was rediscovered further illustrates the value of herbarium specimens, natural history collections, and other primary source records.

Chrozophora tinctoria, a grayish-green plant that grows in southern Portugal, the Mediterranean, north Africa, and central and southwestern Asia, was used to produce a blue pigment that fell out of fashion sometime in the 19th century when book printing became widespread. The recipe for creating the blue was thought to be lost. It was known that C. tinctoria could produce a blue pigment, but modern methods had been unable to recreate the vivid, fade-resistant color used by medieval artists.

Part of the reason that the recipe was lost is probably due to confusion with one of the commonly used names for the blue dye, tornasol (turnsole). In medieval times, blue and purple solutions were extracted from plants, applied to cloth, dried, and stored. To use the color, a piece of the cloth was cut, and the color extracted with a binding medium. The term tornasol came to refer to any water-based color stored in cloth. Over time, C. tinctoria became confused with a lichen of a similar name, Roccella tinctoria, which was also associated with blue/purple pigments.

Another reason that the method for creating the blue pigment was lost to history is likely due to the limited number of surviving written accounts that describe how to create it. Although it was well-known that the plant could produce a blue color, modern methods had been unable to recreate the same fade-resistant blue used by medieval artists. Three sources were able to help modern researchers discover the method of extracting the blue pigment: The Book on How to Make All the Colour Paints for Illuminating Books from the 15th century, a manual written by an artisan named Theophilus in the 12th century, and a painting handbook from the 14th century. Unlike most sources from the time, these explained that the pea-sized fruits of the plant were needed to make the pigment, but also gave instructions not to release the seeds inside the fruit. According to researchers, the 15th century book was the most useful because it describes in detail what C. tinctoria looks like, what the fruits look like, where it grows, and when to collect the fruits.

The fruits of the plant mature in summer and early fall, so through July to September 2016, 2017, and 2018, researchers collected samples and transported them back to the lab. Once back at the lab, the team followed the medieval recipe, soaking the fresh fruit in methanol and water. They stirred the fruit for two hours and had to be careful not to release the seeds inside the fruit, which would make the mixture gummy and impossible to purify. The methanol was evaporated, and the sample was purified, creating a dried powder. The scientists then used chromatography, mass spectrometry, and nuclear magnetic resonance to determine the molecular structure of the color, which they named chrozophoridin. The blue pigment from C. tinctoria is quite different from the pigments created by other known sources of long-lasting blue dyes (indigo and anthocyanins), which can be explained by the different molecular structure.

Knowing how to extract the blue pigment and identifying its molecular structure is invaluable to art conservationists because it gives them an understanding of how the color is derived, allowing them to better preserve these vivid colors for future generations. Yet, without historical documentation, the secret of this color would forever be lost to the past. Much the same can be said for the many natural history and herbarium specimens that have been housed all over the world, but not documented in one, easily accessible place. Today’s digital transcription projects are bringing the data from these collections out into the light, allowing for the possibility of even more scientific discoveries.

Sources

“75367 Dispersion K 9 - Kremer Pigmente GmbH & Co. KG.” n.d. Accessed May 27, 2020. https://www.kremer-pigmente.com/media/pdf/75367.pdf.

Machemer, Theresa. 2020. “Researchers Follow a 15th-Century Recipe to Recreate Medieval Blue Ink.” Smithsonian.com. Smithsonian Institution. April 21, 2020. https://www.smithsonianmag.com/smart-news/researchers-followed-15th-century-recipe-recreate-medieval-blue-ink-180974709/.

Nabais, P., J. Oliveira, F. Pina, N. Teixeira, V. De Freitas, N. F. Brás, A. Clemente, M. Rangel, A. M. S. Silva, and M. J. Melo. 2020. “A 1000-Year-Old Mystery Solved: Unlocking the Molecular Structure for the Medieval Blue from Chrozophora Tinctoria, Also Known as Folium.” Science Advances 6 (16). https://doi.org/10.1126/sciadv.aaz7772.

Starr, Michelle. n.d. “Scientists Discover a New Compound in Medieval Ink That Was Once Lost to Time.” ScienceAlert. Accessed May 27, 2020. https://www.sciencealert.com/we-finally-know-how-medieval-artists-created-this-gorgeous-delicate-blue-pigment.

Yirka, Bob. 2020. “Medieval Blue Dye's Molecular Structure Identified.” Phys.org. Phys.org. April 20, 2020. https://phys.org/news/2020-04-medieval-blue-dye-molecular.html.

Photos:

Top left: An illuminated page from The Book of Hours, a Medieval Christian devotional book. Copyright The Master and Fellows of Trinity College, Cambridge and licensed under CC BY-NC 4.0 image resized to fit space.

Right: Chrozophora tinctoria, photo by Galmiche JMG MG, Copyright CC BY-SA 4.0, image resized to fit space.

Middle left: Fruits of Chrozophora tinctoria, photo by Aviplot, CC BY 2.0, image resized to fit space.

Bottom left: Herbarium specimen of Chrozophora tinctoria, © copyright of the Board of Trustees of the Royal Botanic Gardens, Kew. POWO (2019). "Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; http://www.plantsoftheworldonline.org/ Retrieved 05/29/2020." 

 



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