Nature is the ultimate nano-manufacturer. The latest proof of that is an unusual shard of ancient Roman glass (dubbed the “wow glass”) that boasts a thin, golden patina. Roman glass shards are notable for their iridescent shades of blue, green, and orange—the result of the corrosion process that slowly restructures the glass to form photonic crystals– and this very sherd’s shimmering mirror-like gold sherd is a rare example with unusual optical properties, according to a new paper published in Proceedings of the National Academy of Sciences.
It is another example of naturally occurring structural color. As previously reported, the bright iridescent colors in butterfly wings, soap bubbles, opals or beetle shells come not from any pigment molecules, but from how they are structured – naturally occurring photonic crystals. In nature, for example, scales of chitin (a polysaccharide common to insects) are arranged like roof tiles. In essence, they form one diffraction gratingexcept photonic crystals only produce specific colors or wavelengths of light, while a diffraction grating will produce the entire spectrum, much like a prism.
Also known as photonic bandgap materials, photonic crystals are “tunable,” meaning they are precisely ordered to block certain wavelengths of light while letting others through. Change the structure by changing the size of the tiles and the crystals become sensitive to a different wavelength. They are used in optical communications as waveguides and switches, as well as in filters, lasers, mirrors and various anti-reflection stealth devices.
Researchers can make their own structurally colored materials in the lab, but scaling the process up to commercial applications without sacrificing optical precision can be challenging. So creating structural colors like those found in nature is an active area of materials research. For example, earlier this year, researchers from the University of Cambridge developed an innovative new plant-based film that becomes cooler when exposed to sunlight, making it ideal for cooling buildings or cars in the future without the need for any external power source. The films formed are colored, but they are structural colors in the form of nanocrystals, not due to the addition of pigments or dyes.
And last year, Massachusetts Institute of Technology scientists adapted a 19th-century holographic photography technique invented by physicist Gabriel Lippmann to develop chameleon-like films that change color when stretched. The films would be ideal for making bandages that change color in response to pressure and let doctors know if they’re packing a wound too tightly — an important factor in treating conditions such as venous ulcers, pressure ulcers, lymphedema and scarring. Children would love to wear bandages that change color, providing a boon for pediatricians. And being able to make large sheets of the material opens up applications in clothing and sportswear.
Fiorenzo Omenetto, a materials scientist at Tufts University who co-authored the new paper, discovered the unique shard while visiting the Italian Institute of Technology’s Center for Heritage Technology and decided it warranted further scientific investigation. “This beautiful sparkling piece of glass on the shelf caught our attention,” said Omenetto. “It was a fragment of Roman glass found near the ancient city of Aquileia, Italy.” The center’s director nicknamed it the “wow glass”.
Aquileia was founded by the Romans in 181 BCE, initially as a military outpost, but it quickly flourished as a center for trade, including wrought metal, Baltic amber, wine and ancient glass. “The discovery of a wooden barrel containing 11,000 glass fragments in the wreck of a Roman ship in the seawater off Aquileia demonstrates the city’s leading position in the exchange and processing of recycled glass along commercial routes,” the authors wrote. In the second century AD, at its peak, the city had a population of 100,000. Its fortunes declined after being sacked by Attila and his Huns in 452 and again by the Lombards in 590. Today the city has only about 3,500 inhabitants, but it remains a significant archaeological site.
Archaeologists found the “wow glass” in the topsoil of an agricultural field during a field walk survey in 2012 – most likely brought to the surface thanks to recent plowing – and were immediately struck by its distinctive polychromatic appearance. About 780 glass fragments were collected at the same time, but they had the usual iridescent patinas common to ancient Roman glass. This shard, while mostly dark green in color, was covered in a millimeter-thick golden patina that was almost mirror-like in its reflective properties. To learn more, Omenetto and his co-authors subjected the shards to both optical microscopy and a new form of scanning electron microscopy (SEM) that reveals not only a material’s structure with nanoscale resolution, but also its elemental composition.
Chemical analysis dated the glass to between the first century BCE and the first century BCE. There was a high level of titanium, suggesting that the sand used to make the glass was of Egyptian origin, which typically had more impurities. As for the dark green color still present in the bulk of the fragment, the authors suggest that it is due to the presence of iron. Until about the middle of the second century AD Roman glass was made with either Syro-Levantine raw glass made with relatively pure sand – resulting in a black/purple color – or a high-magnesium glass made with impure iron-rich sand and the addition of vegetable ash to give a dark green color. That’s consistent with this new analysis of the “wow glass.”
The SEM analysis showed the presence of precise hierarchical ordering to form so-called “Bragg stacks” – essentially one-dimensional photonic crystals with alternating layers of high-refractive and low-refractive-index materials that result in structural color. In an ideal Bragg stack, the layers have the same thickness. But one layer was thicker and denser than the other in the “wow glass”, giving it that brilliant metallic look. Specifically, each Bragg stack reflected a different narrow wavelength of light, and ten of them stacked together resulted in the highly reflective golden patina of the shard.
This is evidence that the glass artifact formed through “a pH-driven chemical alteration of silica that does not impose the same strict material constraints found in natural animal-based systems,” the authors wrote. According to to Omenetto, if they could figure out a way to speed up this process so that it didn’t take centuries to form such a patina, “we can find a way to grow optical materials instead of manufacturing them.”
“This is probably a process of corrosion and rebuilding,” said co-author Giulia Guidetti, also at Tufts. “The surrounding clay and the rain determined the diffusion of minerals and a cyclic corrosion of the silica in the glass. At the same time, assembly of 100-nanometer-thick layers combining silica and minerals also occurred in cycles. The result is an incredibly ordered arrangement of hundreds of layers of crystalline material. The crystals that grow on the surface of the glass are also a reflection of the changes in conditions that occurred in the soil as the city developed – a record of its environmental history.”
PNAS, 2023. DOI: 10.1073/pnas.2311583120 (About DOIs).