Physics Magazine joins Tufts University researcher Giulia Guidetti who has studied a glass shard that was broken and buried shortly after 100 B.C.E. that over 2,000 years gained the iridescent sheen of butterflies’ wings by becoming a naturally created nanomaterial:
Its high magnesium and titanium content suggested that the principal ingredient, sodium-rich sand, came from Egypt and that the glass was made between 100 BCE and 100 CE. The substrate’s dark green color is original and arose from the glassmaker’s use of vegetable ash as a reducing agent. The blue and gold colors, however, arose later during the degradation process.
To understand the origin of these colors, Guidetti and her collaborators examined the shard with optical and electron microscopes, discovering structures on several length scales. At the largest scale are micrometer-wide concave domains that are randomly distributed over the surface, like craters on the Moon. Parallel to the surface are thousands of thin layers, mostly made of silica, that alternate in density—and therefore refractive index—between high and low. The thickness of the layers decreases from 320 to 90 nm as the distance from the outer surface increases.
The shard’s layered structure resembles that of an artificial photonic crystal. In a photonic crystal the refractive index is engineered to vary periodically on a length scale comparable to the wavelength of light—that is, a few hundred nanometers. The periodicity creates interference effects that cancel some wavelengths from being reflected back out of the crystal. With one or more of its components blocked, white light that enters a photonic crystal acquires color on its way out.
Glass resists acids, but when it’s subjected to highly alkaline solutions, the hydroxyl ions react with the glass’s silicon atoms, dissolving the surface to form pits. The dissolution also occurs when the solution is mildly alkaline, but it proceeds slowly enough that silicon and oxygen atoms have the chance to reprecipitate to form nanoparticles.
Just how those nanoparticles assemble into layers was elucidated in 2021 by Olivier Schalm of the University of Antwerp, Belgium, and colleagues. Their crucial insight was to identify the role of a chemical feedback loop that alters the local pH of the soil at the reaction front. The local pH cycles between high and low values, leaving in its wake alternating layers of loosely packed and densely packed silica nanoparticles.