Roman glass keeps turning into photonic crystals. Scientists finally know why

Roman glass buried for 2,000 years is slowly transforming into a strange type of crystal that refracts light in surprising ways, and scientists finally know why.

This type of glass, used in jugs to carry water or wine, is often found with a thin patina of iridescent gold, blue, green and orange shimmering across its surface.

Now, after analyzing the microstructure of a Roman glass shard, scientists have found that Roman glass is a naturally occurring photonic crystal in a human-made material. They have also figured out how it likely acquired its opalescent gleam. The researchers published their findings Sept. 18 in the journal Proceedings of the National Academy of Sciences.

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"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," lead-author Fiorenzo Omenetto, a professor of engineering at Tufts University in Boston, said in a statement. 

Photonic crystals are materials made up of periodically dense and less dense layers with different refractive indices, meaning each layer reflects light of different wavelengths at different angles. As photonic crystals' color depends on the angle they are viewed from, the light they reflect is iridescent. 

Photonic crystals exist in nature: in opals; insects such as blue morpho butterflies (Morpho peleides, whose wings are bright blue despite not containing any blue pigment; and plants like blue-leafed begonias. 

Humans also manufacture them for use in cameras, sensors and light circuits, but scientists behind the new study wanted to see how they form naturally. 

The researchers analyzed the gold-hued surface of a shard of Roman glass found near the ancient city of Aquileia, Italy and dated between the first century B.C.and the first century A.D. 

By peering at the glass through an optical microscope and mapping its surface with a scanning electron microscope, the scientists discovered that the glass's structure consisted of nanometer-thick silica layers arranged into a pattern known as Bragg stacks — which altered between high and low refractive indexes to give the glass its distinctive shimmer.

"This is likely a process of corrosion and reconstruction," lead-author Giulia Guidetti, a professor of biomedical engineering at Tufts University, said in the statement.

The iridescent patina formed from many cycles of erosion and reconstruction; with therain and clay breaking some of the glass's silica down only to build it back up alongside caused some of the minerals present in the clay.

"At the same time, assembly of 100 nanometer-thick layers combining the silica and minerals also occurred in cycles," Guidetti added. "The result is an incredibly ordered arrangement of hundreds of layers of crystalline material."

The researchers say their next steps will be to investigate if the process that formed the Roman glass could be sped up, enabling people to grow photonic crystals instead of manufacturing them.