Examination and Analysis: Feathers and Fur

A feather next to color analysist charts. © AMNH

Examining a feather object or bird specimen involves investigation not only of its form and construction, but also its history of use and condition. Conducting background research and talking to source communities, curators, and collection managers can help us better understand the object or identify species. Those conversations, coupled with examination for evidence in and on the object, can also help us determine how the object was used. Looking for signs of deterioration on the object leads to an assessment of condition. Together this understanding informs decisions about treatment, rehousing and storage/display conditions. 

Conservators do not rely solely on eyesight and normal illumination under the fluorescent lights of a conservation lab. Additional techniques can be used to understand feathers and fur in a way that is not possible using the human eye alone, to answer research questions, and to inform conservation judgements and decisions.

Learn more about how these techniques are used in the examination of fur and feather objects:

Multi-Band Imaging (MBI)

Digital Microscopy

Fourier-Transform Infrared Spectroscopy (FTIR)


Accelerated Aging

Multiband Imaging for Fur and Feathers

Digital imaging is an essential technique for graphic representation in conservation, and is readily used to create detailed, accurate, and durable records describing objects and specimens composed of fur and feathers.

Multi-band imaging (MBI) extends the capabilities of digital imaging, enabling the visualization of surface information beyond the visible region of the electromagnetic spectrum, including reflected ultraviolet (UVR), ultraviolet-induced visible fluorescence (UVF), visible light (VIS), and often infrared (IR). This offers the ability to observe material differences and monitor changes over time that may otherwise be difficult to detect under normal lighting conditions.

Multi-band imaging can be used to visualize and capture degradation in keratin, as well as changes in certain associated materials like preening oils and biopigments. Unpigmented keratin in good condition has a low blue-white fluorescence. Previous study has shown that fluorescence in feather keratin increases in brightness with exposure to light and oxygen (photo-oxidation).  Change can be observed using ultraviolet before it can be detected in the visible range. Conservators can use MBI images to document and track this chemical change.

Some natural materials such as the preening oils and biopigments found in or on bird feathers fluoresce under ultraviolet (UV) radiation. Examination using UV aids in their detection and identification, which may in turn facilitate better decision-making about treatment. For example, UVF can be used to confirm or exclude the presence of certain fluorescent pigments, like porphyrins, a group of fugitive biopigments present in some owls, grouse, and pigeon species.

Ultraviolet reflectance (UVR) images, which capture wavelengths below the visible range (<400nm), are rendered in black and white to produce a visible image with high contrast that helps to show small structural features and fine texture, such as the topography on a feather’s surface. Physical details which are otherwise difficult to distinguish are sometimes more readily apparent in the UVR image.

Digital Microscopy for Fur and Feathers

Microscopy is a powerful tool that conservators often rely on to identify the animal origins of feathers and fur, and to inform a nuanced assessment of a specimen’s construction and condition. 

Digital photomicroscopy couples a microscope with a digital camera, providing the ability to capture what you see through the optics of the microscope in still and/or video formats. Many higher end digital microscopes integrate Z-stacking capabilities that are beneficial in examining and imaging the textured and topographical surfaces of fur and feathers under magnification. Microscopy is best used in combination with other techniques that help to contextualize information gathered at different scales.

With sufficient magnification (100x – 400x), characteristic anatomical features in fur and feathers may be observed and used to make a general or exact species identification. In fur, such features include the shape and size of scales on the fiber cuticle; the size and character of the medulla (a central open network present in some animal fibers); and the average diameter of the fiber overall. Diagnostic features can be also observed in feathers. Villi, fine protrusions at the barbule base, may be present in some species and absent in others; and nodes, minute features at the junction between adjacent cells on the plumulaceous barbules of certain feathers, are characteristic in their shape, pigment location, and pigment distribution. These clues can be used in conjunction with reference libraries to narrow down the animal from which the fiber or feather originated.

Microscopy also aids in identifying coloration mechanisms. Both non-iridescent and iridescent structural colors found in feathers are resilient to color fading because they derive from optical phenomena rather than light-sensitive biopigments. To determine whether structural color is present, a feather can be viewed in transmitted light, neutralizing the structural colors and leaving only the biopigmentation visible. In reflected light, one can also clearly see whether coloration is located in the feather barb (non-iridescent structural color), the barbule (iridescent structural color), or is dispersed in both (biopigmentation).

Minute anatomical features like feather barbules and hooklets are too small to see clearly without magnification, but they impact how the feather behaves on the macroscopic level. Is the vane uniform in its opacity, or does it have thin, translucent areas? Does the vane “zip” together into a cohesive sheet, or do the barbs splay apart? Translucency and unzipping occur when barbules in an area are broken off or clumped in a way that prevents the barbs from properly engaging with one another. Understanding structure on the microscopic level supports a more informed assessment of condition issues visible to the naked eye, and makes for better informed judgements. 

FTIR Spectroscopy for Fur and Feathers

Fourier-Transform Infrared Spectroscopy (FTIR) measures the intensity of absorbed or transmitted infrared (IR) radiation as it passes through a sample. Peaks in the resulting FTIR spectrum represent chemical groups present within the sample, and collectively are characteristic for any given material. In many materials, aging and degradation are accompanied by characteristic changes in the relative intensity or position of peaks. By measuring these changes, FTIR may be used to monitor deterioration.

As keratin degrades, the protein undergoes chemical changes in its primary and secondary structure. One of those changes is the oxidation of the amino acid cystine to form cysteic acid, which correlates with loss of strength and breakage of the disulphide bonds that stabilize its helical form. Because both cystine and cysteic acid are represented by peaks in keratin’s IR spectrum, infrared spectroscopy can be used to track degradation over time by looking at relative change in these markers. Their ratio to one another can be compared before and after an event (such as a chemical treatment, light exposure, etc.) to reveal how that event has impacted the chemistry of the keratin.

Spectrophotometry for Fur and Feathers

Spectrophotometry measures the amount of light reflected or transmitted by a material at individual wavelengths of the spectrum. Using algorithmic conversions, the measured reflectance spectrum can be translated into a scientific description of color, or colorimetry.  This non-invasive analytical technique allows the conservator to describe color in a sample quantitatively, which in turn enables us to monitor color change, such as fading from light exposure, over time.

The complex structure and topography of fur and feathers makes them difficult candidates for measurement with typical fiber optic reflectance spectrophotometry probes, which were designed to measure the color of flat surfaces. Structural color in feathers, which changes with orientation and viewing angle, is particularly difficult to measure. While some ornithologists have successfully adapted this equipment to measure feather color in the field and lab, it requires one to be meticulously consistent in the orientation of the sample relative to the probe, and even then, error can be high enough to overshadow change. 

In place of a fiber optic probe, an integrating sphere streamlines capture and improves the repeatability of measurements from bulk fur and feather samples. The integrating sphere provides a perfectly diffuse interior chamber where light reflected from the sample is equally distributed by multiple scattering reflections. Through diffusion, the light’s intensity becomes uniform before it enters the detector, minimizing the impact of directionality in the sample.

Accelerated Aging for Fur and Feathers

Analytical techniques like FTIR and spectrophotometry may be coupled with accelerated aging to develop a general understanding of how keratin materials will change chemically or aesthetically as they age over time. Accelerated light aging rapidly reproduces the damage in keratin that is caused by light in real environments over longer periods. Aging is conducted inside of a chamber containing high-powered xenon arc lamps. Light output, as well as temperature and humidity, can be set, and filters can be used to adjust the incident spectrum to include or exclude UV.

Accelerated light aging is a very effective means of inducing photo-oxidation in keratin. As aging takes place, a number of physical changes may be observed, including yellowing and/or bleaching of the keratin; increase in ultraviolet-induced fluorescence; fading of organic biopigments, particularly carotinoids, which are not as light stable as melanins; increased sensitivity to water; brittleness and loss of strength. In feathers, loss of strength may initially be associated with breakage and loss of distal barbules. However eventually in both feathers and fur, continued light aging weakens keratin to the point that fibers and feather barbs will simply shatter when handled.

The rate at which accelerated aging takes place is determined by the set points (light output, temperature, humidity) and lighting condition (UV-included vs. UV-excluded) selected. Associated damage can be tracked with complimentary techniques that record or measure those changes directly.