Rose-colored glass

My wife, Carol Bloomgarden, is an amazing artist, and participates in art shows all over the northeastern United States.  (Her work is called micrography -- it's drawings made from patterns of tiny handwritten text.  You can, and should, check it out at her website.)  Because showing framed art work requires moving lots of stuff around -- not only the work itself, but the canopies, frames, and stands on which to display it -- I frequently accompany her to her shows.

My usefulness is best summed up in a line from a t-shirt a student of mine used to wear: "I May Not Be Very Smart, But I Can Lift Heavy Objects."

In any case, in between setup and breakdown, I usually have lots of time to wander around the show and see what the other artists are selling.  Last year, one of the booths belonged to a very talented jeweler who made jewelry out of (amongst other things) fragments of Roman glass.

Carol hinted at me that she loved this jeweler's work, so for her birthday I got her a necklace and matching set of earrings made from chunks of turquoise-colored glass dating to about 300 C.E.

The Romans were outstanding glassmakers, and a lot of their work survives (unfortunately, much of it in fragmentary form).  And one curious thing about a lot of Roman glass is that it has a patina -- an iridescent sheen on the surface, sometimes refracting light and creating a metallic or rainbow appearance.  There is nothing in the existing writing from that era indicating that those effects were created deliberately; it seemed to be some sort of byproduct of the aging of the piece.

Fourth century C.E. Roman glass from a glassworks in Syria, showing the gold patina over pale green glass [Image is in the Public Domain courtesy of its creator, Marie-Lan Nguyen]

Researchers in materials science at Tufts University became curious about how these coatings were produced, and did microscopic analysis of the surfaces of pieces of Roman glass.  They came to a surprising conclusion; the gold, silver, or rainbow-colored coatings were (1) naturally produced after the pieces were buried, and (2) were photonic crystals -- regular, periodic microlayers of precisely-arranged molecules, of the same sort used in semiconductors and solar cells, which have the effect of generating light interference and an opalescent or iridescent appearance.

It turns out that the interaction between the glass surface, rainwater, and the minerals in the soil results in a very slow, orderly deposition of thin films on the artifact's surface, and in two thousand or so years, you have something truly spectacular.  "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," said Fiorenzo Omenetto, who co-authored the study.  "While the age of the glass may be part of its charm, in this case if we could significantly accelerate the process in the laboratory we might find a way to grow optic materials rather than manufacture them."

"This is likely a process of corrosion and reconstruction," said Giulia Guidetti, also a co-author.  "The surrounding clay and rain determined the diffusion of minerals and a cyclical corrosion of the silica in the glass.  At the same time, assembly of 100 nanometer-thick layers combining the silica and minerals also occurred in cycles.  The result is an incredibly ordered arrangement of hundreds of layers of crystalline material... [so] the crystals grown on the surface of the glass are also a reflection of the changes in conditions that occurred in the ground as the city evolved -- a record of its environmental history."

So here we have another example of the kind of fascinating crossover you see in the very best science -- in this case, between materials science and archaeology.  With possible applications to engineering.  

I know I'll think about this study every time Carol wears her Roman glass jewelry.  

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