Birds of a Feather: Natural Structures Create the Colors of Qing Dynasty Featherworks

X-ray fluorescence (XRF) spectroscopy scan detects elements used in the fabrication of screen panels.

Additional pigments were used to enhance the bright blues of the feather designs.

Post-doctoral fellows Madeline Meier and Hortense de La Codre examined the screens with digital microscopes.

Meier and senior scientist Maria Kokkori set up Fourier transform infrared (FTIR) analysis to identify adhesives.

Elemental analysis indicated how tin and copper-zinc alloys were used to simulate more expensive gilding materials.

Bamboo paper enhanced the textures of this floral element on the screen.

In bright daylight, a kingfisher’s brilliant blue feathers or a peacock’s shimmering false-eye patterns might grab our attention in a flash. Under a microscope, away from oncoming light, those sapphires and turquoises will appear brown instead.

These feathers are not blue because of any inherent color. Rather, the individual strands of keratin on a feather’s vane—the parallel rows of “hairs” that grow in a V shape along the central shaft of the feather—are covered in microscopic ridges thinner than even a wavelength of light. These tiny ridges—called nanostructures—have no color of their own, but the semi-ordered, sponge-like shapes reflect and scatter the light in ways that we perceive as shimmering colors. Because light is the central ingredient for creating these colors, rather than something like a chemical or pigment, these nanostructures thus produce “structural colors.”

Two postdoctoral scholars from the Center for Scientific Studies in the Arts, Madeline Meier and Hortense de La Codre, have applied a combination of analytical techniques to study the materials and techniques used to create a calendar screen that dates from the Qing Dynasty in China (1644–1914), provided by the Field Museum of Natural History. These featherworks are an art practice called tian tsui, or “dotting with kingfishers.” Artists created intricate designs by painstakingly patterning various bird feathers into designs used for decorative items ranging from jewelry to headdresses to screens. These designs were enhanced with glue, gilding, and pigments to create miniature fantastic scenes and arrangements.

One can see the appeal of these beautiful iridescent feathers which caught the eyes of Chinese artists thousands of years ago. Yet there’s also value in looking deeper beyond this surface phenomenon to elucidate how these nanostructures produce color.

New ways of seeing structural color

Meier’s interest in investigating featherworks started during her PhD study, when she experimented with nanostructures herself in her work on studying semiconductor fabrication. “There was some knowledge to be transferred there,” she says.

The translation of analytical methods across fields allows her the flexibility to see these nanostructures in multiple ways. “Structural color in nature—and not just art—is not always simply a dye or a pigment. There’s these really complex nanostructures that these organisms are making themselves that are responsible for these fascinating, unique colors. Nature can do wild things to manipulate light that result in these beautiful objects,” Meier says.

Meier and de La Codre sought to identify the bird species whose feathers line these complex panels, as well as to distinguish which feathers were painted or otherwise enhanced. “In cultural heritage, we usually study paint pigments or dyes. Because structural colors are so different, it’s very interesting to learn more about how we can actually see colors,” says de La Codre.

The tian tsui scenes and screens vary in size. “It’s difficult to estimate the number of feathers used because artists were so meticulous in manipulating them,” Meier says. The kingfisher has striations of black and blue, so the feathers overlap in the application, often with beading and other gilding to further enhance their flashiness.

To detect the minuscule nanostructures they were looking for, Meier and de la Codre scraped back the topmost layers of the feathers and scanned them with a scanning electron microscope (SEM). This technique is capable of resolving images down to a single nanometer—thousands of times smaller than the width of a strand of hair—and generates a topography of the structure.

They employed another non-destructive technique of hyperspectral imaging (HSI), using a camera to scan different areas of the entire object to see how it absorbs and reflects light. The specific wavelengths at which light is absorbed, known as the reflectance spectra—a kind of fingerprint—reveal what kind of material is present.

The Field Museum was a fortuitous place to undertake this study because of the wide range of its collections. The featherworks from the Qing dynasty screen could be compared with feathers from the Field’s world-class taxidermied bird collection, which contains shelves and shelves of specimens. By scanning and “fingerprinting” the feathers of these actual birds, Meier and de la Codre could attach species names to the feathers used in the calendar screens—namely, common kingfishers, black-capped kingfishers, and mallard ducks. Before they started the study, researchers wondered if the greens in the screen were from peacocks or another bird; identifying the ducks lets them develop new questions about the relative price of obtaining different feathers and how that might have affected artists’ choices.

Using X-ray fluorescence (XRF), they were then able to map the chemical elements that make up materials used in the gilding, pigments, and other decorative elements of tian tsui. Fourier transform infrared (FTIR) indicated the presence of isinglass used as a glue. The multi-analytical approach revealed the presence of copper-zinc and tin alloys used as gilding materials that could mimic gold and silver. “They allowed the appearance of expensive materials while using more available elements,” says de La Codre.

The scans revealed portions of the screen that artists had touched up with pigments. These paints range from yellows to indigos to reds, providing further insight into artistic intent. It is possible Qing Dynasty artists wanted to modify these panels to match some sort of mental image better, or that they were working around the limitations of the medium, as feathers could be quite expensive to obtain.

Structural color gives an illusory quality to the objects, as the blues seem to change hue depending on the way the light glints on them. It may be difficult to capture this changeable quality in photographing these three-dimensional objects, but it also makes the work far more interesting. Between the flecks of paint artists left behind and this constantly shifting color scheme, these objects are not static. Animated by our constantly shifting perspectives as we observe them, the featherworks seem to come alive.

“Of course, when people were making these during the Qing Dynasty, they didn’t have scanning electron microscopes. But they recognized that these were really unique and beautiful colors they were working with, and they would write about how they changed with the sun. They still were trying to understand these materials, even though they didn’t have the same instrumentation as today,” Meier says.

Bright future for structural color studies

The Center will partner with Argonne National Laboratory to continue uncovering the nature of nanostructures in feathers. They will also undertake some comparative studies with Qing Dynasty headdresses at the Art Institute of Chicago. Just as oil paints crack and flake over time, structural colors undergo their own changes in color and composition. Whether the nanostructures responsible for producing structural colors might degrade over time, or given certain environmental conditions, is still unclear.

The versatility of structural colors offer various applications outside the world of cultural heritage. American dollar bills, for example, have the same iridescent structural colorants that catch onto and change under light, to protect against counterfeiting. There’s a considerable interest in biomimicry in materials sciences, where we can learn new ways of making material from these structural colors. The same principles that make a bird’s feathers shift colors in the daylight could eventually serve as a basis for developing more complex forms of defensive camouflage, through manipulating surrounding light.

Meier explains that “the project showed the challenges and complexities of assessing structural color with commonly used cultural heritage analysis techniques” that are more often used on paintings and sculptures. It provides a roadmap for other researchers to refine these techniques for work on other biological materials that contain structural color.