Nature is the ultimate nanofabricator. The latest evidence of this is a rare fragment of ancient Roman glass (dubbed “wow glass”) that boasts a pale golden patina. The pieces of Roman glass are notable for their iridescent hues of blue, green, and orange. This is the result of the glass slowly being rebuilt by the corrosion process. photonic crystal— and, according to some, this particular fragment’s shimmering, mirror-like gold luster is a rare example with unusual optical properties. new paper Published in the Proceedings of the National Academy of Sciences.
This is another example of naturally occurring structural color. As previously reported, the bright iridescent colors of butterfly wings, soap bubbles, opals, and beetle shells do not originate from pigment molecules, but from their structure, which is naturally occurring. photonic crystal. For example, in nature, scales of chitin (a polysaccharide common to insects) are arranged like tiles.Essentially, they are Diffraction gratingHowever, photonic crystals only produce a specific color or wavelength of light, whereas diffraction gratings, like prisms, produce the entire spectrum.
Photonic crystals, also known as photonic bandgap materials, are “tunable.” This means that they are precisely aligned to block certain wavelengths of light and allow others to pass through. Changing the structure by changing the size of the tile makes the crystal sensitive to different wavelengths. They are used in optical communications as waveguides and switches, as well as in filters, lasers, mirrors, and various anti-reflection stealth devices.
Scientists can create their own structurally colored materials in the lab, but scaling up the process for commercial applications without sacrificing optical precision can be difficult. Therefore, creating structural colors similar to those found in nature is an active area of materials research. For example, earlier this year scientists at the University of Cambridge developed An innovative new plant-based film cools down when exposed to sunlight, making it perfect for cooling buildings and cars in the future without the need for external power. Although the films created are colored, it is not due to the addition of pigments or dyes, but is a structural color in the form of nanocrystals.
And last year, scientists at the Massachusetts Institute of Technology applied a 19th-century holographic photography technique invented by physicist Gabriel Lippmann to create a chameleon-like film that changes color when stretched. The film is ideal for creating bandages that change color in response to pressure, letting medical professionals know if they’re wrapping a wound too tightly. This is an important factor in treating conditions such as venous ulcers, bedsores, lymphedema, and scarring. Children love wearing color-changing bandages, which is a boon for pediatricians. It also allows for the creation of large sheets of material, increasing its use in apparel and sportswear.
Tufts University materials scientist Fiorenzo Omenetto, a co-author of the new paper, discovered the unique fragment during a visit to the Italian Institute of Technology’s Heritage Technology Center and decided that further scientific research was needed. did. “This beautiful shiny glass on the shelf caught our attention.” Omenet said.. “It was a fragment of Roman glass recovered near the ancient city of Aquileia in Italy.” The center’s director called it “amazing glass.”
Founded by the Romans in 181 BC, Aquileia was initially a military outpost, but quickly became a thriving center for trade in wrought metals, Baltic amber, wine, ancient glass, and more. “The discovery of a wooden barrel containing 11,000 pieces of glass from the wreck of a Roman ship in the waters before Aquileia is a sign that Aquileia is a leader in the exchange and processing of recycled glass along commercial routes. ”, the authors write. By the 2nd century AD at its peak, the city’s population reached her 100,000 people. Its fortunes diminished after it was sacked by Attila and his Huns in 452 and again by the Lombards in 590. Today, the town has only about 3,500 residents, but it remains an important archaeological site.
Archaeologists discovered the “ring crow” in the topsoil of a farmland (probably brought to the surface thanks to recent plowing) during a field trip in 2012, and soon discovered its distinctive multicolored appearance. I was shocked. About 780 glass shards were collected at the same time, and they had an iridescent patina common in ancient Roman glass. The fragment was dark green overall, but covered in a several-millimetre-thick golden patina with almost mirror-like reflective properties. To learn more, Omenet and his co-authors examined the fragments with both an optical microscope and a new type of scanning electron microscope (SEM), revealing not only the structure of the material with nanoscale resolution, but also its The elemental composition was also revealed.
Chemical analysis determined that the glass dates from the 1st century BC to the 1st century AD. It contains high levels of titanium, suggesting that the sand used to make the glass came from Egypt and generally contains more impurities. The authors suggest that the dark green color still present in the bulk of the debris is due to the presence of iron. Until about the mid-2nd century AD, Roman glass was divided into two types: Syro-Levantine raw glass made from relatively pure sand (resulting in a black/purplish color), or high-grade glass made from impure iron-rich sand. It was manufactured using magnesium glass and additives. of plant ash giving a dark green color. This is consistent with this new analysis of “Wow Glass”.
SEM analysis showed the existence of a precise hierarchical order to form the so-called “Bragg stack”. It is an essentially one-dimensional photonic crystal characterized by alternating layers of high and low refractive index materials that provide structural color. In an ideal Bragg stack, the layers are of equal thickness. But in “Wow Glass,” one layer was thicker and denser than the other, giving it that shiny metallic look. Specifically, each Bragg stack reflected a different narrow wavelength of light, and stacking dozens of them created a highly reflective golden patina on the debris.
This is evidence that the glass artifacts were “formed by pH-induced chemical changes in silica, which do not impose the severe material constraints seen in natural animal-based systems,” the authors wrote. ing.according to To Mr. Omenet If we can find a way to speed up this process so that it doesn’t take centuries to form such a patina, “we might find a way to grow optical materials rather than make them.”
“This is probably a process of corrosion and rebuilding.” Co-author Julia Gudetti said:, also available at Tufts. “The surrounding clay and rain determined the diffusion of minerals and the periodic erosion of the silica within the glass. At the same time, the assembly of 100-nanometer-thick layers of combined silica and minerals also occurred periodically. The result is hundreds of layers of crystalline material arranged in an incredibly regular manner. The crystals that grow on the glass surface also reflect the changing conditions that occurred on the ground as the city evolved, a record of its environmental history. doing.”
PNAS, 2023. DOI: 10.1073/pnas.2311583120 (About DOI).