In Qing dynasty China, artisans augmented decorative pieces by incorporating iridescent kingfisher feathers—a technique known as tian-tsui. Scientists at Northwestern University’s Center for Scientific Studies in the Arts have used high-energy x-ray imaging to achieve unprecedented nanoscale resolution of the unique structure of those feathers, presenting their findings at the annual meeting of the American Association for the Advancement of Science.
As previously reported, nature is the ultimate nanofabricator. The bright iridescent colors in butterfly wings, soap bubbles, opals, or beetle shells don’t come from any pigment molecules but from how they are structured—naturally occurring photonic crystals. In nature, scales of chitin (a polysaccharide common to insects), for example, are arranged like roof tiles. Essentially, they form a diffraction grating, except photonic crystals only produce specific colors, or wavelengths, of light, while a diffraction grating will produce the entire spectrum, much like a prism. In the case of kingfisher feathers, the color is due to the microscopic ridges that cover the parallel rows of keratin strands that grow along the central shaft.
Also known as photonic band-gap materials, photonic crystals are “tunable,” which means they are precisely ordered to block certain wavelengths of light while letting others through. Alter the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength. They are used in optical communications as waveguides and switches, as well as in filters, lasers, mirrors, and various anti-reflection stealth devices.
The 19th century poet Gerard Manley Hopkins paid homage to the kingfisher’s brilliant plumage in his poem “As Kingfishers Catch Fire,” but Chinese poets and artists were extolling their praises long before that. Tian-tsui (“dotting with kingfishers”) is a prime example of how much the feathers were valued. The feathers were cut and glued onto gilt silver and used as inlays for things like fans, hairpins, screens and panels, or headdresses—carefully oriented in intricate patterns to enhance the dazzling hues. The feathers were so popular, in fact, that kingfisher populations were declared endangered following the Chinese Communist Revolution. The last tian-tsui studio closed in 1933.
A spongy nanostructure
The Northwestern team started looking at kingfisher feathers in tian-tsui objects via postdoc Madeline Meier, who has a background in chemistry and nanostructures, and was interested in combining that expertise with studies of cultural heritage. The first step was to identify the bird species that provided the feathers used in Qing dynasty screens and panels, as well as other materials used. Researchers carefully scraped away the topmost layers and imaged the feathers with scanning electron microscopy to get a better look at the underlying nanostructure. Hyperspectral imaging revealed how different areas of the screens absorbed and reflected light.
The team also made use of the center’s partnership with Chicago’s Field Museum, comparing the screen feathers with the museum’s vast collection of taxidermied bird species. The screens and panels contained feathers from common kingfishers and black-capped kingfishers, as well as mallard ducks (used to add green hues). Finally, x-ray fluorescence and Fourier-transform infrared spectroscopy enabled them to create a map of the various chemicals used in the gilding, pigments, glues, and other materials.
Most recently, the lab has partnered with Argonne National Laboratory and used synchrotron radiation to get an ever-better look at the nanostructure of kingfisher feathers. Synchrotron radiation differs from conventional X-rays in that it’s a thin beam of very high-intensity X-rays generated within a particle accelerator. Electrons are fired into a linear accelerator (linac), get a speed boost in a small synchrotron, and are injected into a storage ring, where they zoom along at near-light speed. A series of magnets bend and focus the electrons, and in the process, they give off X-rays, which can then be focused down beam lines.
That makes it ideal for noninvasive imaging, since in general, the shorter the wavelength used (and the higher the energy of the light), the finer the details one can image and/or analyze. It has become a popular technique for imaging fragile archaeological artifacts without damaging them—like Qing dynasty headdresses with inlays of kingfisher feathers. In this case, the imaging revealed that the feathers’ microscopic ridges have an underlying semi-ordered, porous, sponge-like shape that reflect and scatter light, thereby giving the feathers their gloriously brilliant hues.
“Long admired in Chinese poetry and art, kingfisher feathers have amazing optical properties,” co-author Maria Kokkori said. “Our discoveries not only enhance our understanding of historical materials but also reshape how we think about artistic and scientific innovation, and the future of sustainable materials.”
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