Sunday, October 1, 2023

2,000 years later, Roman glass reveals its secrets

2,000 years later, Roman glass reveals its secrets About 2,000 years ago, in ancient Rome, glass containers containing wine, water, or perhaps exotic perfumes would fall from a table in a market and shatter in the street. Over the centuries, these glass shards have been covered in dust and dirt, exposed to continuous changes in temperature, humidity, and surrounding minerals.

Today, these small pieces of glass are discovered at construction sites and archaeological excavations, revealing their extraordinary nature.

Hidden treasures in antique glass

On the surface of these fragments is a mosaic of iridescent blue, green, and orange colors, some of which feature shimmering gold mirrors. These beautiful glass artifacts are often incorporated into jewelry in the form of pendants or earrings, while larger, more complete objects are displayed in museums.

For Fiorenzo Omenetto and Giulia Guidetti, engineering professors at Tufts University’s Silklab and experts in materials science, what is fascinating is how glass molecules have rearranged and recombined with minerals over thousands of years to form what are called photonic crystals: ordered arrangements of atoms. that filter and reflect light in very specific ways.

Applications of photonic crystals

Photonic crystals have many applications in modern technology. They can be used to create waveguides, optical switches, and other devices for very fast optical communications in computers and over the Internet.

Because they can be designed to block certain wavelengths of light and allow others to pass through, they are used in filters, lasers, mirrors, and anti-reflective (stealth) devices.

A chance discovery

In a recent study published in Proceedings of the National Sciences USA, Omenetto, Guidetti and their collaborators report on the unique atomic and mineral structures that formed from the original mineral and silicate constituents of glass, modulated by the pH of the environment. surrounding area and fluctuations in groundwater levels in the soil.

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The project began by chance during a visit to the Cultural Heritage Technology Center of the Italian Institute of Technology (IIT).

« This beautiful shiny glass piece on the shelf caught our eye. », indicated the teacher. Omenetto. “ It was a fragment of Roman glass recovered near the ancient city of Aquileia, Italy. » Arianna Traviglia, director of the Center, said her team affectionately calls it the “ glass wow “. They decided to take a closer look.

Nature’s nanofabrication

The researchers quickly realized they were examining the natural nanofabrication of photonic crystals. “ It’s really amazing that you have glass that sits in mud for two millennia and you end up getting something that is a textbook example of a nanophotonic component. », added Prof. Omenetto.

Corrosion and reconstruction

The IIT team’s chemical analysis dated the glass fragment to between the 1st century BC and the 1st century BC. C. and the 1st century AD. C., originating in the sands of Egypt, an indication of global trade at that time. Most of the fragment retained its original dark green color, but on its surface was a millimeter-thick patina that had an almost perfect, mirror-like golden reflection.

The two scientists used a new type of scanning electron microscope that not only reveals the structure of the material, but also provides elemental analysis. “ Basically, it is an instrument that can tell you in high resolution what the material is made of and how the elements are assembled. », explained Prof. Guidetti.

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They could see that the patina had a hierarchical structure composed of very regular silica layers, micrometer in size, alternating between high and low densities, resembling reflectors known as Bragg stacks. Each Bragg stack strongly reflected different and relatively narrow wavelengths of light. The vertical stacking of dozens of Bragg piles gave the patina its gold mirror appearance.

A patient process

How did this structure form over time? Researchers suggest a possible mechanism that has been patiently developed for centuries.

« This is probably a corrosion and rebuilding process. », commented Prof. Guidetti. “ The surrounding clay and rain caused diffusion of minerals and cyclic corrosion of silica in the glass. At the same time, the 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. »


« While the glass age may be part of its allure, in this case, if we could significantly speed up the process in the lab, we could find a way to grow optical materials instead of manufacturing them. », added Fiorenzo Omenetto.

In modern times, streets and buildings are often built directly on ancient foundations. “ The crystals that formed on the surface of the glass are also a reflection of the changing conditions that occurred in the ground as the city evolved: a record of its environmental history. », concluded Giulia Guidetti.

For a better understanding

1. What is a photonic crystal?

A photonic crystal is an ordered arrangement of atoms that filters and reflects light in very specific ways. They have many applications in modern technology, including optical communications, filters, lasers, mirrors, and anti-reflective devices.

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Photonic crystals formed in ancient glass because glass molecules rearranged and recombined with surrounding minerals over thousands of years. This process was influenced by environmental pH and fluctuating groundwater levels in the soil.

3. What is the origin of the glass studied in this research?

Le verre étudié provient de l’ancienne Rome et a été daté entre le 1er siècle avant J.-C. et le 1er siècle après J.-C. Il a des origines dans les sables d’Égypte, ce qui indica un commerce mondial at the time.

The structure of photonic crystals was formed through a process of corrosion and reconstruction that took place patiently over centuries. The surrounding clay and rain determined the diffusion of minerals and the cyclical corrosion of silica in the glass, while the assembly of 100-nanometer-thick layers combining silica and minerals also occurred in cycles.

5. What is the relationship between photonic crystals and environmental history?

Photonic crystals formed on the surface of the glass reflect the changing conditions that occurred in the ground as the city evolved, providing a record of its environmental history.

Main illustration caption: Photonic crystals on ancient Roman glass seen up close. “If we could find a way to speed up this process by 2,000 years, reduce it to two weeks or two days, then we could produce these structures in the laboratory,” says Fio Omenetto. Photo: Giulia Guidetti – Tufts University

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