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    Here’s why some Renaissance artists egged their oil paintings

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    Art historians often wish that Renaissance painters could shell out secrets of the craft. Now, scientists may have cracked one using chemistry and physics.

    Around the turn of the 15th century in Italy, oil-based paints replaced egg-based tempera paints as the dominant medium. During this transition, artists including Leonardo da Vinci and Sandro Botticelli also experimented with paints made from oil and egg (SN: 4/30/14). But it has been unclear how adding egg to oil paints may have affected the artwork.  

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    “Usually, when we think about art, not everybody thinks about the science which is behind it,” says chemical engineer Ophélie Ranquet of the Karlsruhe Institute of Technology in Germany.

    In the lab, Ranquet and colleagues whipped up two oil-egg recipes to compare with plain oil paint. One mixture contained fresh egg yolk mixed into oil paint, and had a similar consistency to mayonnaise. For the other blend, the scientists ground pigment into the yolk, dried it and mixed it with oil — a process the old masters might have used, according to the scant historical records that exist today. Each medium was subjected to a battery of tests that analyzed its mass, moisture, oxidation, heat capacity, drying time and more.

    In both concoctions, the yolk’s proteins, phospholipids and antioxidants helped slow paint oxidation, which can cause paint to turn yellow over time, the team reports March 28 in Nature Communications. 

    In the mayolike blend, the yolk created sturdy links between pigment particles, resulting in stiffer paint. Such consistency would have been ideal for techniques like impasto, a raised, thick style that adds texture to art. Egg additions also could have reduced wrinkling by creating a firmer paint consistency. Wrinkling sometimes happens with oil paints when the top layer dries faster than the paint underneath, and the dried film buckles over looser, still-wet paint.

    The hybrid mediums have some less than eggs-ellent qualities, though. For instance, the eggy oil paint can take longer to dry. If paints were too yolky, Renaissance artists would have had to wait a long time to add the next layer, Ranquet says.

    “The more we understand how artists select and manipulate their materials, the more we can appreciate what they’re doing, the creative process and the final product,” says Ken Sutherland, director of scientific research at the Art Institute of Chicago, who was not involved with the work.

    Research on historical art mediums can not only aid art preservation efforts, Sutherland says, but also help people gain a deeper understanding of the artworks themselves. More

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      in Physics

      These chemists cracked the code to long-lasting Roman concrete

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      MIT chemist Admir Masic really hoped his experiment wouldn’t explode.

      Masic and his colleagues were trying to re-create an ancient Roman technique for making concrete, a mix of cement, gravel, sand and water. The researchers suspected that the key was a process called “hot mixing,” in which dry granules of calcium oxide, also called quicklime, are mixed with volcanic ash to make the cement. Then water is added.

      Hot mixing, they thought, would ultimately produce a cement that wasn’t completely smooth and mixed, but instead contained small calcium-rich rocks. Those little rocks, ubiquitous in the walls of the Romans’ concrete buildings, might be the key to why those structures have withstood the ravages of time.

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      That’s not how modern cement is made. The reaction of quicklime with water is highly exothermic, meaning that it can produce a lot of heat — and possibly an explosion.

      “Everyone would say, ‘You are crazy,’” Masic says.

      But no big bang happened. Instead, the reaction produced only heat, a damp sigh of water vapor — and a Romans-like cement mixture bearing small white calcium-rich rocks.

      Researchers have been trying for decades to re-create the Roman recipe for concrete longevity — but with little success. The idea that hot mixing was the key was an educated guess.

      Masic and colleagues had pored over texts by Roman architect Vitruvius and historian Pliny, which offered some clues as to how to proceed. These texts cited, for example, strict specifications for the raw materials, such as that the limestone that is the source of the quicklime must be very pure, and that mixing quicklime with hot ash and then adding water could produce a lot of heat.

      The rocks were not mentioned, but the team had a feeling they were important.

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      “In every sample we have seen of ancient Roman concrete, you can find these white inclusions,” bits of rock embedded in the walls. For many years, Masic says, the origin of those inclusions was unclear — researchers suspected incomplete mixing of the cement, perhaps. But these are the highly organized Romans we’re talking about. How likely is it that “every operator [was] not mixing properly and every single [building] has a flaw?”

      What if, the team suggested, these inclusions in the cement were actually a feature, not a bug? The researchers’ chemical analyses of such rocks embedded in the walls at the archaeological site of Privernum in Italy indicated that the inclusions were very calcium-rich.

      That suggested the tantalizing possibility that these rocks might be helping the buildings heal themselves from cracks due to weathering or even an earthquake. A ready supply of calcium was already on hand: It would dissolve, seep into the cracks and re-crystallize. Voila! Scar healed.

      But could the team observe this in action? Step one was to re-create the rocks via hot mixing and hope nothing exploded. Step two: Test the Roman-inspired cement. The team created concrete with and without the hot mixing process and tested them side by side. Each block of concrete was broken in half, the pieces placed a small distance apart. Then water was trickled through the crack to see how long it took before the seepage stopped.

      “The results were stunning,” Masic says. The blocks incorporating hot mixed cement healed within two to three weeks. The concrete produced without hot mixed cement never healed at all, the team reports January 6 in Science Advances.

      Cracking the recipe could be a boon to the planet. The Pantheon and its soaring, detailed concrete dome have stood nearly 2,000 years, for instance, while modern concrete structures have a lifespan of perhaps 150 years, and that’s a best case scenario (SN: 2/10/12). And the Romans didn’t have steel reinforcement bars shoring up their structures.

      More frequent replacements of concrete structures means more greenhouse gas emissions. Concrete manufacturing is a huge source of carbon dioxide to the atmosphere, so longer-lasting versions could reduce that carbon footprint. “We make 4 gigatons per year of this material,” Masic says. That manufacture produces as much as 1 metric ton of CO2 per metric ton of produced concrete, currently amounting to about 8 percent of annual global CO2 emissions.

      Still, Masic says, the concrete industry is resistant to change. For one thing, there are concerns about introducing new chemistry into a tried-and-true mixture with well-known mechanical properties. But “the key bottleneck in the industry is the cost,” he says. Concrete is cheap, and companies don’t want to price themselves out of competition.

      The researchers hope that reintroducing this technique that has stood the test of time, and that could involve little added cost to manufacture, could answer both these concerns. In fact, they’re banking on it: Masic and several of his colleagues have created a startup they call DMAT that is currently seeking seed money to begin to commercially produce the Roman-inspired hot-mixed concrete. “It’s very appealing simply because it’s a thousands-of-years-old material.” More

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      in Heart

      This eco-friendly glitter gets its color from plants, not plastic

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      All that glitters is not green. Glitter and shimmery pigments are often made using toxic compounds or pollutive microplastics (SN: 4/15/19). That makes the sparkly stuff, notoriously difficult to clean up in the house, a scourge on the environment too.

      A new, nontoxic, biodegradable alternative could change that. In the material, cellulose — the main building block of plant cell walls — creates nanoscale patterns that give rise to vibrant structural colors (SN: 9/28/21). Such a material could be used to make eco-friendly glitter and shiny pigments for paints, cosmetics or packaging, researchers report November 11 in Nature Materials.

      The inspiration to harness cellulose came from the African plant Pollia condensata, which produces bright, iridescent blue fruits called marble berries. Tiny patterns of cellulose fibers in the berries’ cell walls reflect specific wavelengths of light to create the signature hue. “I thought, if the plants can make it, we should be able to make it,” says chemist Silvia Vignolini of the University of Cambridge. 

      Vignolini and colleagues whipped up a watery mixture containing cellulose fibers and poured it onto plastic. As the liquid dried into a film, the rodlike fibers settled into helical structures resembling spiral staircases. Tweaking factors such as the steepness of those staircases changed which wavelengths of light the cellulose arrangements reflected, and therefore the color of the film.

      That allowed the researchers, like fairy-tale characters spinning straw into gold, to transform their clear, plant-based slurry into meter-long shimmery ribbons in a rainbow of colors. These swaths could then be peeled off their plastic platform and ground up to make glitter.

      This gleaming ribbon contains tiny arrangements of eco-friendly cellulose that reflect light in specific ways to give the material its color.Benjamin Drouguet

      “You can use any type of cellulose,” Vignolini says. Her team used cellulose from wood pulp, but could have used fruit peels or cotton fibers left over from textile production.

      The researchers need to test the environmental impacts of their newfangled glitter. But Vignolini is optimistic that materials using such natural ingredients have a bright future. More

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      in Heart

      Many cosmetics contain hidden, potentially dangerous ‘forever chemicals’

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      A new chemical analysis has revealed an ugly truth about beauty products: Many may contain highly persistent, potentially harmful “forever chemicals” called PFAS.

      PFAS, short for per- and polyfluoroalkyl substances, include thousands of chemicals that are so sturdy they can linger in the body for years and the environment for centuries. The health effects of only a few PFAS are well known, but those compounds have been linked to high cholesterol, thyroid diseases and other problems.

      “There is no known good PFAS,” says chemist and physicist Graham Peaslee of the University of Notre Dame in Indiana.

      In the first large screening of cosmetics for PFAS in the United States and Canada, Peaslee and colleagues found that 52 percent of over 200 tested products had high fluorine concentrations, suggesting the presence of PFAS, the researchers report online June 15 in Environmental Science & Technology Letters.

      The potential health risks of PFAS in makeup are not yet clear, Peaslee says. But besides people ingesting or absorbing PFAS when wearing makeup, cosmetics washed down the drain could get into drinking water (SN: 11/25/18).

      Peaslee’s team measured the amount of fluorine, a key component of PFAS, in 231 cosmetics. Sixty-three percent of foundations, 55 percent of lip products and 82 percent of waterproof mascara contained high levels of fluorine — at least 0.384 micrograms of fluorine per square centimeter of product spread on a piece of paper. Long-lasting or waterproof products were especially likely to contain lots of fluorine. That makes sense, since PFAS are water-resistant.

      Twenty-nine products further tested for specific PFAS all contained at least four of these chemicals, but only one product listed PFAS among its ingredients. In addition to posing their own potential health risks, these compounds can break down in the body into other PFAS, such as perfluorooctanoic acid, which has been linked to cancers and low birth weights (SN: 6/4/19).

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      in Heart

      A new technique could make some plastic trash compostable at home

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      A pinch of polymer-munching enzymes could make biodegradable plastic packaging and forks truly compostable.

      With moderate heat, enzyme-laced films of the plastic disintegrated in standard compost or plain tap water within days to weeks, Ting Xu and her colleagues report April 21 in Nature.

      “Biodegradability does not equal compostability,” says Xu, a polymer scientist at the University of California, Berkeley and Lawrence Berkeley National Laboratory. She often finds bits of biodegradable plastic in the compost she picks up for her parents’ garden. Most biodegradable plastics go to landfills, where the conditions aren’t right for them to break down, so they degrade no faster than normal plastics.

      Embedding polymer-chomping enzymes in biodegradable plastic should accelerate decomposition. But that process often inadvertently forms potentially harmful microplastics, which are showing up in ecosystems across the globe (SN: 11/20/20). The enzymes clump together and randomly snip plastics’ molecular chains, leading to an incomplete breakdown. “It’s worse than if you don’t degrade them in the first place,” Xu says.

      Her team added individual enzymes into two biodegradable plastics, including polylactic acid, commonly used in food packaging. They inserted the enzymes along with another ingredient, a degradable additive Xu previously developed, which ensured the enzymes didn’t clump together and didn’t fall apart. The solitary enzymes grabbed the ends of the plastics’ molecular chains and ate as though they were slurping spaghetti, severing every chain link and preventing microplastic formation.

      Filaments of a new plastic material degrade completely (right) when submerged in tap water for several days.Adam Lau/Berkeley Engineering

      Adding enzymes usually makes plastic expensive and compromises its properties. However, Xu’s enzymes make up as little as 0.02 percent of the plastic’s weight, and her plastics are as strong and flexible as one typically used in grocery bags.

      The technology doesn’t work on all plastics because their molecular structures vary, a limitation Xu’s team is working to overcome. She’s filed a patent application for the technology, and a coauthor founded a startup to commercialize it. “We want this to be in every grocery store,” she says. More

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      in Heart

      A new iron-based catalyst converts carbon dioxide into jet fuel

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      Today, airplanes pump a lot of climate-warming carbon dioxide into the atmosphere. But someday, carbon dioxide sucked from the atmosphere could be used to power airplanes.
      A new iron-based catalyst converts carbon dioxide into jet fuel, researchers report online December 22 in Nature Communications. Unlike cars, planes can’t carry batteries big enough to run on electricity from wind or solar power. But if CO2, rather than oil, were used to make jet fuel, that could reduce the air travel industry’s carbon footprint — which currently makes up 12 percent of all transportation-related CO2 emissions.
      Past attempts to convert carbon dioxide into fuel have relied on catalysts made of relatively expensive materials, like cobalt, and required multiple chemical processing steps. The new catalyst powder is made of inexpensive ingredients, including iron, and transforms CO2 in a single step.
      When placed in a reaction chamber with carbon dioxide and hydrogen gas, the catalyst helps carbon from the CO2 molecules separate from oxygen and link up with hydrogen — forming the hydrocarbon molecules that make up jet fuel. The leftover oxygen atoms from the CO2 join up with other hydrogen atoms to form water.
      Tiancun Xiao, a chemist at the University of Oxford, and colleagues tested their new catalyst on carbon dioxide in a small reaction chamber set to 300° Celsius and pressurized to about 10 times the air pressure at sea level. Over 20 hours, the catalyst converted 38 percent of the carbon dioxide in the chamber into new chemical products. About 48 percent of those products were jet fuel hydrocarbons. Other by-products included similar petrochemicals, such as ethylene and propylene, which can be used to make plastics. More

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      in Physics

      Molecular jiggling may explain why some solids shrink when heated

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      When things heat up, most solids expand as higher temperatures cause atoms to vibrate more dramatically, necessitating more space. But some solid crystals, like scandium fluoride, shrink when heated — a phenomenon called negative thermal expansion. Now, by measuring distances between atoms in scandium fluoride crystals, scientists think that they have figured out how that […] More

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      in Physics

      Plant-based fire retardants may offer a less toxic way to tame flames

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      Flame retardants are going green. Using compounds from plants, researchers are concocting a new generation of flame retardants, which one day could replace the fire-quenching chemicals added by manufacturers to furniture, electronics and other consumer products. Many traditional synthetic flame retardants have come under fire for being linked to health problems like thyroid disruption and […] More