A kaleidoscope of colors in ancient Roman glass

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 G. Guidetti    29-11-2023     Leggi in PDF
Fragment of the colorful patina observed on ancient Roman glass. The vibrant colors stem from intricate chemical processes that gradually transformed the glass into an ultra-thin lamellar structure (inset), functioning as photonic crystal. (Credits: G. Guidetti)

When delving into the depths of history and ancient artifacts, one might not immediately associate vibrant colors with the past. Typically, archaeological finds bear the marks of time with weathered appearances, subdued hues, and often fragmented states. Yet, picture a small, metallic fragment of golden and blue glass laying among other Roman artifacts – undoubtedly catching the eye. This intriguing piece of glass seized the attention of Fiorenzo Omenetto, Director of the Silklab laboratory at the Tufts University, USA, during a visit to Arianna Traviglia, Coordinator of the Centre for Cultural Heritage Technology (CCHT) at the Italian Institute of Technology (IIT) in Italy.

But why did this glass fragment, recovered from the outskirts of the ancient city of Aquileia in Italy, display such brilliant and unprecedented colors, unlike anything seen by conservation scientists at CCHT before? Glass, a transparent material made from sand, can be shaped into optical components, windows, containers, and decorative elements, with color typically added through pigments. Ancient glass often carries an iridescent patina, akin to the colors of soap bubbles, caused by thin-film interference of light with micrometer-thick layers of glass. This particular sample though, was quite different, as its outer surface was metallic golden and blue, and vividly changed color when tilted, akin to butterfly wings and jewel beetles. This fragment, once believed to be part of an ink container made from Egyptian sand and dating back to the 1st century BCE to the 1st century CE, held its secrets close. The unveiling began when the colorful patina was studied using optical and electron microscopes, revealing details a thousand times smaller than a hair's diameter.

The metallic patina, it turned out, was formed by thousands of exquisitely ordered nano-lamellae, resembling pages of a book when viewed from the side. The vibrant colors within the patina emerged from intricate chemical processes, gradually transforming the glass into a nanolayered structure acting as a photonic crystal. Buried for over two millennia, the glass's journey was marked by cyclic pH fluctuations in the surrounding soil, leading to gradual corrosion and the emergence of a remarkable nanostructured patina – over a thousand silica nanolamellae – that gave vibrant colors and a mirror-like appearance, transforming this relic into a scientific wonder.

Here, in this tiny fragment, a glimpse of history is preserved, captured in the form of a photonic crystal patina on ancient Roman glass. So, the next time you stroll barefoot on the beach, consider that the very sand beneath your feet holds the potential to become a mosaic of dazzling colors.

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Giulia Guidetti – Deputy Director and Research Assistant Professor at Silklab (Tufts University), with primary research focus on studying light management in natural systems. Her research interests revolve around biophotonic structures, structural colors, as well as optical and electronic imaging, and spectroscopy.