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Science reveals secrets of a mummy’s portrait

How much information can you get from a speck of purple pigment, no bigger than the diameter of a hair, plucked from an Egyptian portrait that’s nearly 2,000 years old? Plenty, according to a new study. Analysis of that speck can teach us about how the pigment was made, what it’s made of — and maybe even a little about the people who made it. The study is published in the International Journal of Ceramic Engineering and Science.

“We’re very interested in understanding the meaning and origin of the portraits, and finding ways to connect them and come up with a cultural understanding of why they were even painted in the first place,” says materials scientist Darryl Butt, co-author of the study and dean of the College of Mines and Earth Sciences.

Faiyum mummies

The portrait that contained the purple pigment came from an Egyptian mummy, but it doesn’t look the same as what you might initially think of as a mummy — not like the golden sarcophagus of Tutankhamen, nor like the sideways-facing paintings on murals and papyri. Not like Boris Karloff, either.

The portrait, called “Portrait of a Bearded Man,” comes from the second century when Egypt was a Roman province, hence the portraits are more lifelike and less hieroglyphic-like than Egyptian art of previous eras. Most of these portraits come from a region called Faiyum, and around 1,100 are known to exist. They’re painted on wood and were wrapped into the linens that held the mummified body. The portraits were meant to express the likeness of the person, but also their status — either actual or aspirational.

That idea of status is actually very important in this case because the man in the portrait we’re focusing on is wearing purple marks called clavi on his toga. “Since the purple pigment occurred in the clavi — the purple mark on the toga that in Ancient Rome indicated senatorial or equestrian rank- it was thought that perhaps we were seeing an augmentation of the sitter’s importance in the afterlife,” says Glenn Gates of the Walters Art Museum in Baltimore, where the portrait resides.

The color purple, Butt says, is viewed as a symbol of death in some cultures and a symbol of life in others. It was associated with royalty in ancient times, and still is today. Paraphrasing the author Victoria Finlay, Butt says that purple, located at the end of the visible color spectrum, can suggest the end of the known and the beginning of the unknown.

“So the presence of purple on this particular portrait made us wonder what it was made of and what it meant,” Butt says. “The color purple stimulates many questions.”

Lake pigments

Through a microscope, Gates saw that the pigment looked like crushed gems, containing particles ten to a hundred times larger than typical paint particles. To answer the question of how it was made, Gates sent a particle of the pigment to Butt and his team for analysis. The particle was only 50 microns in diameter, about the same as a human hair, which made keeping track of it challenging.

“The particle was shipped to me from Baltimore, sandwiched between two glass slides,” Butt says, “and because it had moved approximately a millimeter during transit, it took us two days to find it.” In order to move the particle to a specimen holder, the team used an eyelash with a tiny quantity of adhesive at its tip to make the transfer. “The process of analyzing something like this is a bit like doing surgery on a flea.”

With that particle, as small as it was, the researchers could machine even smaller samples using a focused ion beam and analyze those samples for their elemental composition.

What did they find? To put the results in context, you’ll need to know how dyes and pigments are made.

Pigments and dyes are not the same things. Dyes are the pure coloring agents, and pigments are the combination of dyes, minerals, binders and other components that make up what we might recognize as paint.

Initially, purple dyes came from a gland of a genus of sea snails called Murex. Butt and his colleagues hypothesize that the purple used in this mummy painting is something else — a synthetic purple.

The researchers also hypothesize that the synthetic purple could have originally been discovered by accident when red dye and blue indigo dye mixed together. The final color may also be due to the introduction of chromium into the mix.

From there, the mineralogy of the pigment sample suggests that the dye was mixed with clay or a silica material to form a pigment. According to Butt, an accomplished painter himself, pigments made in this way are called lake pigments (derived from the same root word as lacquer). Further, the pigment was mixed with a beeswax binder before finally being painted on linden wood.

The pigment showed evidence suggesting a crystal structure in the pigment. “Lake pigments were thought to be without crystallinity prior to this work,” Gates says. “We now know crystalline domains exist in lake pigments, and these can function to ‘trap’ evidence of the environment during pigment creation.”

Bottom of the barrel, er, vat

One other detail added a bit more depth to the story of how this portrait was made. The researchers found significant amounts of lead in the pigment as well and connected that finding with observations from a late 1800s British explorer who reported that the vats of dye in Egyptian dyers’ workshops were made of lead.

“Over time, a story or hypothesis emerged,” Butt says, “suggesting that the Egyptian dyers produced red dye in these lead vats.” And when they were done dyeing at the end of the day, he says, there may have been a sludge that developed inside the vat that was a purplish color. “Or, they were very smart and they may have found a way to take their red dye, shift the color toward purple by adding a salt with transition metals and a mordant [a substance that fixes a dye] to intentionally synthesize a purple pigment. We don’t know.”

Broader impacts

This isn’t Butt’s first time using scientific methods to learn about ancient artwork. He’s been involved with previous similar investigations and has drawn on both his research and artistic backgrounds to develop a class called “The Science of Art” that included studies and discussions on topics that involved dating, understanding and reverse engineering a variety of historical artifacts ranging from pioneer newspapers to ancient art.

“Mixing science and art together is just fun,” he says. “It’s a great way to make learning science more accessible.”

And the work has broader impacts as well. Relatively little is known about the mummy portraits, including whether the same artist painted multiple portraits. Analyzing pigments on an atomic level might provide the chemical fingerprint needed to link portraits to each other.

“Our results suggest one tool for documenting similarities regarding time and place of production of mummy portraits since most were grave-robbed and lack archaeological context,” Gates says.

“So we might be able to connect families,” Butt adds. “We might be able to connect artists to one another.”

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This fruit attracts birds with an unusual way of making itself metallic blue

There’s a reason why blue fruits are so rare: the pigment compounds that make fruits blue are relatively uncommon in nature. But the metallic blue fruits of Viburnum tinus, a popular landscaping plant in Europe, get their color a different way. Instead of relying solely on pigments, the fruits use structural color to reflect blue light, something that’s rarely seen in plants. Researchers reporting August 6 in the journal Current Biology show that the fruits use nanostructures made of lipids in their cell walls, a previously unknown mechanism of structural color, to get their striking blue — which may also double as a signal to birds that the fruits are full of nutritious fats.

“Structural color is very common in animals, especially birds, beetles, and butterflies, but only a handful of plant species have ever been found to have structural color in their fruits,” says co-first author Miranda Sinnott-Armstrong, a postdoctoral researcher at the University of Colorado-Boulder. “This means that V. tinus, in addition to showing a completely novel mechanism of structural color, is also one of the few known structurally colored fruits.”

Senior author Silvia Vignolini, a physical chemist at the University of Cambridge, has been interested in the plants for nearly 10 years. “I actually found this Viburnum in a garden in Italy and observed that they looked weird, so we measured them at the time but didn’t have conclusive results. It was kind of always on the back of my mind,” she says. As her team grew, they become more interested in V. tinus and eventually had the capability to examine the structure of the fruits using electron microscopy. “Before we got the images, we were just seeing all these blobs,” she says. “When we found out that those blobs were lipids, we got very excited.”

While most plants have cell walls made of cellulose, used to make cotton and paper, V. tinus fruit cells have much thicker walls with thousands of globular lipids arranged in layers that reflect blue light. The structure formed by this so-called lipid multilayer allows the fruits to create their vibrant blue color while containing no blue pigment. “This is very strange because globular lipids like these are not usually found in this arrangement in the cell wall, as they are normally stored inside the cell and used for transport,” says co-first author Rox Middleton, a physicist who studied the optical response of the fruits during her PhD and is now a postdoctoral researcher at the University of Bristol. “We also believe that this lipid may contribute to the fruit’s nutrition. That means that the fruit can demonstrate how nutritious it is by being a beautiful, shiny blue.”

This extra nutrition would be important for V. tinus’s main consumers: birds that disperse the plant’s seeds. Although the researchers can’t say for sure whether the lipids are used as fat by the birds that consume them, there is reason to believe they might be. If so, the researchers suggest that the metallic blue color made by the lipid multilayer could indicate to the birds that if they see this striking blue, the fruit in question will have enough nutrients to make it a worthwhile meal. “While birds have been shown to be attracted to blue fruits,” says Vignolini, “other blue fruits that we have studied essentially don’t have any nutritional value.”

Going forward, the researchers want to see how widespread blue structural color is in fruits to understand its ecological significance. They had never seen this type of lipid multilayer in a biomaterial before, but since their discovery, they’ve begun to take notice of other species. “We actually realize now that there are some older electron microscopy pictures from other plants where you can see the blobs. The researchers didn’t know that they were lipids at the time, or that lipids could even form this type of structure, but our research suggests that they very well could be, meaning this structure may not be limited to Viburnum,” Vignolini says.

Additionally, learning how V. tinus can use such a unique mechanism to make color may have implications for how we color our own foods. “There are lots of problems connected to food coloration,” says Vignolini. She adds that once this mechanism is better understood, it could potentially be used to create a healthier, more sustainable food colorant.

But right now, Vignolini is just excited her initial hunch paid off: “I’ve been working on this type of photonic structure for quite a while, and I was beginning to think there were no new ways to make it — at some point you’ve seen so many that you think, ‘This is more or less the end, it’s going to be difficult to find something new,'” she says. “Instead, we discovered much more than what we expected.”

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