I came across a fascinating article in the online magazine Ars Technica, written by Jennifer Ouellette about a nerdy glass topic: nanofabrication, or the precise construction of crystals or molecular structures on an extremely small scale, and I hope you’ll find it as interesting as I did. What makes this so amazing is that nature itself can build these structures and needs no help from us.
According to Ouellette, in 2012, a fragment of ancient Roman glass caught the eye of archeologists as they walked through a freshly-plowed field near the city of Aquileia, Italy. The glass was green, but it had an especially brilliant golden patina. Ancient glass fragments often have varied patinas on their surfaces. But not like this piece. It was stunning.
Oullette recounts how it ended up on a shelf at the Italian Institute of Technology’s Center for Cultural Heritage Technology, where it also captivated a visiting materials scientist from Tufts University, Fiorenzo Omenetto, who decided he’d study it further; indeed, the CCHT director had named the fragment “the wow glass,” and Omenetto would soon discover the scientific reason why using two types of Scanning Electron Microscopy, or SEM.
Photonic crystals on the surface of ancient Roman glass. (Guidetti, cf Ouellette)
Turns out this glass, dated within 100 years before or after the birth of Jesus, displays a very ordered construction of photonic crystals, which occur in nature in many places and produce a metallic sheen on insects’ bodies or wings, for instance. (If you want to read more on photonic crystals, check out Ouellette’s “Shiny Things: An Ode to Photonic Crystals” in Scientific American.)
The realization that glass could assimilate various crystalline formations to block, scatter, transmit, or reflect light was pioneered by S. Donald Stookey and others in the 1950s and 1960s at Corning Glassworks. The technology at the time that allowed these maverick scientists to see the resulting crystalline structures built by adding elements to glass was in its infancy, but it worked well enough for these men to make great discoveries including photonic crystals and new ways to harness glass. (More on some promising breakthroughs with exciting applications in the blog posts to come.)
Photonic crystals don’t possess a pigment, but instead transmit or even scatter light in various hues depending on their makeup and thickness. Ouellette recounts how these structures, according to Omenetto’s SEM observations, very slowly altered the typical (and random) silica molecular structure of the “wow” glass with many minute ordered layers only a nanometer thick, and this over centuries.
Highly regular nanometer-thick s-ilica layers forming metalic patina on a Roman glass fragment. (Silklab-Tufts University, cf Ouellette)
Omenetto and co-author Giulia Guidetti published the results of their SEM exploration of the “wow glass” in a paper appearing in Proceedings of the National Academy of Sciences last year. Guidetti states:
“This is likely a process of corrosion and reconstruction [. . .] The surrounding clay and rain determined the diffusion of minerals and a cyclical corrosion of the silica in the glass. At the same time, assembly of 100 nanometer-thick layers combining the silica and minerals also occurred in cycles. The result is an incredibly ordered arrangement of hundreds of layers of crystalline material. The crystals grown on the surface of the glass are also a reflection of the changes in conditions that occurred in the ground as the city evolved—a record of its environmental history (cf Ouellette).
Nature’s precision is often realized in its slow, methodical, and hidden work. Through these processes, looking deeply into the gold mirrored surface of this special artifact gives us a glimpse back in time to a city that, though it still exists, is now an echo of its former self--a bustling place once filled with the glory of glass.
Ouellette, Jennifer. "Ancient Roman 'wow glass' has Photonic Crystal Patina Forged Over Centuries." Ars Technica. 9-18-23.