Mute Swans (Cygnus olor) have only one kind of plumage which is molted once a year. As expected for waterfowl, their feathers are superhydrophobic, thanks to a combination of a natural hydrophobic coating (preen oil) and the micro-structural topography of the feather. Throughout one year their feathers are exposed to harsh weather conditions and are subject to wear and tear. What would happen if swans were not able to replace their feathers yearly? Would they still be able to float in lakes, to withstand rain, and to preen their feathers without damaging them?
Testing Water-Sensitivity
Observing how feathers in different condition states respond to water helps to understand the risks of wet cleaning. To explore this question, we used feathers aged in our accelerated aging chamber according to parameters1 that have been benchmarked against one of the Museum’s dioramas. One day in the chamber is roughly equivalent to one year in the diorama.
We aged four upper covert feathers of the Mute Swan for different periods equivalent to between eight and 25 years of display. A single droplet of water was applied with a pipette to each feather. Then we observed the shape of the droplet, the contact angle between the water and the feather surface, and the condition of the feather after removal of the water. We predicted that there would be changes in the contact angle with age because we know that our fresh swan feathers are very hydrophobic—a water droplet sits like a ball on the surface until it evaporates—and that keratin becomes more polar as it oxidizes.
Feather aged the equivalent of eight years (200 hours)
The water droplet maintained a steep contact angle while on the feather. After its removal, there was no visible change to the barbs and barbules in the test area.
Water drop on swan feather (left), and test area after water drop has evaporated, leaving no traces behind (right). Lighting: Full ring. ©AMNH/R.Riedler
Feather aged the equivalent of 12.5 years (300 hours)
The contact angle between the droplet and the feathers was high. After three seconds, the droplet was removed and some distortion of the distal barbules was observed in the test area.
Water drop test area on swan feather at 50x (left). Boundary between the test area and untreated feather, shown at 200x (right). Lighting: Full ring. ©AMNH/R.Riedler
Feather aged the equivalent of 21 years (500 hours)
The contact angle between the droplet and the feather was lower. After three seconds the droplet was removed and irreversible shrinkage of the distal barbules was observed in the test area.
Water drop test area on swan feather at 50x (left). Boundary between the untreated feather and the test area, shown at 200x (right). Lighting: Full ring. ©AMNH/R.Riedler
Feather aged the equivalent of 25 years (600 hours)
The contact angle between the droplet and the feather was lower still. After three seconds the droplet was removed and disintegration of the distal barbules was observed in the test area.
Water drop test area on swan feather at 50x (left). Boundary between the test area and the untreated feather, shown at 200x (right). Lighting: Full ring (left), partial ring (right) ©AMNH/R.Riedler
Water Sensitivity in Biopigmented Feathers
We repeated this investigation with biopigmented feathers from a Scarlet Ibis and a Common Raven, both aged for the equivalent of 12.5 years (300 hours in the aging chamber). After three seconds of exposure to the water droplet, the Scarlet Ibis feather (pigmented with carotenoids) showed some distortion of the distal barbules. The Common Raven feather (pigmented with melanins) did not.
Water drops newly pipetted onto an ibis feather (left), and shown after sitting for three seconds (right). ©AMNH/R.Riedler
Water drop test area on ibis feather at 50x (left). Boundary between the test area and untreated feather, shown at 200x (right). Lighting: Full ring. ©AMNH/R.Riedler
Water drops pipetted onto a raven feather (left); test area after water was removed, at 50x (right). Lighting: Full ring. ©AMNH/R.Riedler
Wet Cleaning Test
These tests demonstrated that water can have a severe effect on the distal barbules of degraded feathers, and that this water sensitivity increases as the keratin photooxidizes. In combination with water, we would expect that the mechanical action of cleaning can cause further damage. We simulated a cleaning treatment with water and brush on a feather that was aged for the equivalent of 25 years. After cleaning, a significant loss of distal barbules was observed in the area of cleaning. Barbs and proximal barbules appeared undamaged, perhaps because they were partially screened from light exposure by the distal barbules.
Looking Closer
Research by Ellen Pearlstein et al (2014, 2015) has demonstrated that UV-induced fluorescence is an early marker of oxidation in keratin and can be used to detect such change before it can be seen in visible light. With this in mind, we decided to use UV to further investigate the impacts of water on aged feathers in an effort to see changes beyond the structural damage visible under the microscope.
Three feathers were selected for observation:
1: Fresh swan feather
2: Swan feather aged 500 hours (equivalent to 21 years), then tested with several water droplets
3: Swan feather aged 600 hours (equivalent to 25 years), then cleaned in some areas with water applied by brush
Under UV, the fresh feather had a low fluorescence. The aged feathers were both highly fluorescent, though the feather aged 500 hours was brighter and had blue-white fluorescence, while the feather aged 600 hours was slightly less fluorescent and more yellow in color.
Areas where water was applied were readily visible in UV on both the aged feathers. On the feather aged 500 hours, fluorescent tide lines were present around the site where each droplet was placed, indicating that the water mobilized oxidation products on the surface of the feather. On the feather aged 600 hours, cleaned areas fluoresced more brightly and more blue than the surrounding keratin, suggesting the localized removal of yellow-fluorescing oxidation products.
Swan feathers (# 1, 2, and 3) shown in visible light (left) and UV-induced visible fluorescence (right). Feathers 2 and 3 have been treated with water droplets and brush-applied water. ©AMNH/R.Riedler
Water-Sensitivity of Naturally Aged Specimens
With this investigation, we learned that water can selectively damage feather barbules on its own (i.e., without the additional action of a brush or other cleaning implement). When damage was observed, it was associated with lower contact angles between the water droplet and the feather surface. These rough tests suggest that water sensitivity is impacted by the feather’s age, as well as the type of biopigmentation present.
The investigation also provides a model for a simple test that could be used to assess the water-sensitivity of a naturally aged specimen prior to executing a wet cleaning treatment. A small droplet of water can be applied in a discrete but representative area, followed by close observation of the test area using a microscope and/or UV source. If a low contact angle or changes in structure or fluorescence are observed, further consideration should be given to whether aqueous cleaning will cause unacceptable physical or chemical changes.
References
Ellen Pearlstein, Melissa Hughs, Joy Mazurek, Kevin McGraw, Christel Pesme, and M. Garcia-Garibay (2014) Correlations between photochemical damage and UV fluorescence of feathers, ICOM-CC 17th Triennial Conference, Melbourne, Australia September 15-19
Ellen Pearlstein, Melissa Hughs, Joy Mazurek, Kevin McGraw, Christel Pesme, Renée Riedler, and Molly Gleeson (2015) Ultraviolet-induced visible fluorescence and chemical analysis as tools for examining featherwork, Journal of the American Institute for Conservation, 54:3, 149-167
Footnotes
1 Accelerated Aging Parameters – Qsun Xe-3:
Spectral irradiance: 0.35W/m2-nm at 340nm
Radiant Exposure: 510 kJ/m² at 340nm
35°C chamber air temp, 48° black panel temperature 37% relative humidity
In Their True Colors 