Natural photonic crystals on Roman glass

The party was in full swing in the Roman city of Aquileia in northern Italy. Musicians played the flute and tambourine, and the guests danced and drank wine. In the joyful commotion, a misplaced elbow caused a cup to fall, shattering. The servant picked up the pieces and threw them away… Two thousand years later, a fragment was found in a layer of clay during archaeological excavations. We can clearly see its original green color. But what’s most surprising is its unconventional appearance, with a metallic and gold reflection. By analyzing this stunning Roman piece of glass, Giulia Guidetti from Tufts University in the US and her colleagues have shown that it is a rare example of a natural ‘Prague Mirror’.

Bragg mirrors belong to the family of photonic crystals, that is, materials whose nanoscopic periodic structure gives them certain optical properties. Normally, they only reflect certain wavelengths, often giving them metallic colours, like the skin of some insects or the wings of a morpho butterfly. In all these cases, the color comes not from pigments but from structure. “Photonic crystals are very useful, for example, in reflecting very intense laser beams,” explains Antoine Moreau, from the University of Clermont-Auvergne. They are produced by depositing layer by layer on small surfaces. However, they are not easy to produce at low cost on large surfaces while maintaining their optical properties. » To solve these challenges, researchers draw a lot of inspiration from natural photonic crystals. “Opals, known for their exquisite iridescence, are an example of how tiny identical glass beads form periodic arrangements,” the researcher points out. The glass fracture from Aquileia is a new example.

Glass pieces are considered informative archaeological artifacts. They provide information on the level of technological mastery of the population or on the trade routes through which glass objects were traded. This is the case in Aquileia, which was founded in 181 BC. Initially a military camp established to cut off the barbarians who were threatening Rome’s eastern borders, Aquileia soon became a thriving trading center thanks to its ideal location near the Adriatic Sea and at the crossroads of several major roads. Iron and amber from the Baltic Sea, wine and glass pass through it. Attila sacked the city in 452, an event that marked the beginning of the city’s decline. Today, Aquileia, a small city with a population of 3,000, is above all a rich archaeological site.

Chemical analysis of the gold piece confirms that it dates back to the Roman era, between the first century BC and the first century AD. Its high titanium content indicates that the sand used in its manufacture comes from Egypt, and the presence of iron is the origin of its green color. Patina reveals other information. Glass buried in the ground is exposed to chemical aging processes. The metal ions it contains are replaced by hydrogen atoms brought by the flowing water. The silicone melts and forms nanoparticles. These transformations depend on various conditions, such as pH. The patina deposited on the surface, through the aggregation of nanoparticles, consists of overlapping layers of different thicknesses (nanometer and micrometer) and densities. They often have an opal structure that produces a milky (due to light diffusion) and iridescent (through interference) effect. This is the look we generally see on Roman objects, hence the interest shown by Giulia Guidetti in the piece at Aquileia nicknamed the “dazzling glass.”

Researchers studied its structure under a microscope. They found the phenomenon of natural formation of dentin, but with a high level of regularity and a large number of layers, which led to the formation of a photonic crystal. An ideal Prague mirror consists of an array of layers of the same thickness alternating between low and high refractive index. The reflection is very strong in green. But in the Roman Fissure, the thickness of the layers varies, and the layers are thicker on the surface and thinner in depth. Different wavelengths are not reflected at the same depth (red near the surface, orange and green below). The blue color continues to pass through the structure. The result: we get a highly reflective structure, similar to the surface of gold, that absorbs the blue color but reflects the rest of what is visible.

For the researchers, this graduated Bragg mirror was formed through pH-controlled chemical glass-altering processes. Their goal now is to understand whether this process can be accelerated and better controlled for industrial applications.

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