Electron microscope reveals Roman-era glass shard structure

The surface (patina) of this remarkable shard features intricate patterns of iridescent colors such as blues, greens, and oranges, while some areas depict reflected gold-toned mirrors that sparkle. 

Electron microscope scanning technique

The researchers utilized an electron microscope scanning technique to determine the nano-level structure of the patina, which is a thin layer that accumulates on the surface of objects over time due to continuous exposure to various environmental variables. 

“Basically it's an instrument that can tell you with high resolution what the material is made of and how the elements are put together,” said Giulia Guidetti, professor at Tufts. 

The piece exhibited unique atomic and mineral hierarchical structures that emerged from the glass's initial silicate and mineral components. These structures were likely influenced by the soil's pH and changing groundwater levels in the vicinity. 

This revealed that the glass fragment is a result of "nanofabrication of photonic crystals by nature.” Photonic crystals are macroporous materials and are particularly used in the realm of optics.

“It's really remarkable that you have glass that is sitting in the mud for two millennia and you end up with something that is a textbook example of a nanophotonic component,” said Fiorenzo Omenetto. 

The chemical analysis of the Roman glass piece places it between the 1st century BC and the 1st century AD. It was most likely created from the "sands of Egypt," implying that it was traded to this other region of the world. 

Corrosion contributed to this process

In this case, the photonic crystals formed naturally as a result of "corrosion and crystallization over centuries."  

“This is likely a process of corrosion and reconstruction,” said Guidetti. “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.”

Photonic crystals find numerous applications in contemporary technology, including the development of optical switches and other high-speed optical communication equipment used in computers and the internet.

The results were published in Proceedings of the National Academy of Sciences.

Study abstract:

Ancient glass objects typically show distinctive effects of deterioration as a result of environmentally induced physicochemical transformations of their surface over time. Iridescence is one of the distinctive signatures of aging that is most commonly found on excavated glass. In this work, we present an ancient glass fragment that exhibits structural color through surface weathering resulting in iridescent patinas caused by silica reprecipitation in nanoscale lamellae. This archaeological artifact reveals an unusual hierarchically assembled photonic crystal with extremely ordered nanoscale domains, high spectral selectivity, and reflectivity (~90%), that collectively behaves like a gold mirror. Optical characterization paired with nanoscale elemental analysis further underscores the high quality of this structure providing a window into this sophisticated natural photonic crystal assembled by time.