Can we solve(nt) the problem of cleaning feathers?
A solvent is strong or weak depending on the dirt which needs to be dissolved!
As we saw in our aqueous testing, feathers (especially those that have been aged) can experience significant and irreversible structural changes when exposed to water. With this in mind, as well as the fact that over half of our survey respondents report using non-aqueous solvents and solvent mixtures at least some of the time, we had to explore whether the types of damage associated with non-aqueous treatments were as extreme.
According to our survey, nonpolar solvents like mineral spirits are reportedly used for the removal of oily or tenacious soil and grime from specimens. Acetone, by comparison, is used cautiously and primarily in a localized manner for the removal of stains and accretions. Alcohols like ethanol, isopropanol, and IDS (industrial denatured alcohol) are widely used in various mixtures for cleaning in general, but also to lower the surface tension of a water bath and to address mold. Alcohol is also used in high concentrations in water, both to rinse and to assist with drying after a wet cleaning treatment involving surfactants.
Changes in the physical structure, like the loss or deformation of barbules and the unzipping of barbs, are frequently observed types of damage associated with solvent cleaning. Some survey respondents noted a change in feather color and texture, like the loss of gloss or lightening of color. It was indicated that biopigments can be extracted from degraded keratin, a phenomenon more generally described as the “bleeding of biopigments”. Strong polar solvents were observed to dehydrate feather keratin, resulting in a rough matte surface texture. Other risks of solvent cleaning reported in the survey include over-cleaning, the removal of preen oil (including pigmented preen oil), and the redeposition of dirt.
Advantages of Organic Solvents
Organic solvents are capable of removing fatty/unpolymerized oily/greasy/resinous dirt, bituminous material, and waxes. Pure solvents contain only trace amounts of water, if any, so the keratin hydrolysis that has been documented due to aqueous cleaning (see previous blog post) can be avoided. Risk of damage to any underlying water-sensitive skin is also reduced. Most organic solvents quickly wet the feather and soak in due to their low surface tension, requiring little manipulation to push the liquid through the zipped feather structure. The drying time is short because many of the solvents used in cleaning are highly volatile.
The smaller the molecule of the solvent, the lower the boiling point, the higher the vapor pressure, and the faster the evaporation rate (Rivers 2003).
Disadvantages of Organic Solvents
Compared to water, organic solvents have low to moderate polarity, and thus may not effectively remove the most polar degradation products. Sugars, starches, vegetable gums, clayey materials, and salts do not dissolve and polymerized oils are not easily dissolved in organic solvents.
Strong solvents like alcohols and ketones not only remove free water, but can also extract physically bound water, which can permanently damage the keratin structure.
Dehydration: Drop of acetone released through a pipette to a 500 hours aged upper covert swan feather. Image taken after air drying. @AMNH/Leslie Vilicich
When solvents penetrate the structure and evaporate quickly, they carry the dirt only to the location where evaporation takes place. This can produce a clear line of dirt and degradation products, called a tideline.
Some organic solvents containing aromatic hydrocarbons may remove the natural preen oils found on a bird’s feathers. Preen oils are complex species-specific mixtures of predominantly aliphatic mono- and diester waxes. While preen oils can entrap dust and dirt on the feather surface, it may still be undesirable to remove it, especially from specimens used in scientific research. Preen oils are believed by some to impart a protective benefit to the feather, and have aesthetic importance as biopigmented colorants. Highly processed feathers (e.g., commercial poultry feathers) generally have been stripped of any preen oil (and, indeed, not all birds have it to begin with), so in these cases, the concern is unnecessary.
Environmental Impact | Health & Safety
Organic solvents contribute to green-house gas emissions and impact our environment. Their flammability and toxicity also require consideration for safe use and disposal. For a summary of conservation solvents with information on manufacture, environmental impact, disposal implications, and recycling possibilities, we recommend consulting the AIC-wiki site ‘Sustainability and Chemicals’.
Solubility
The array of non-aqueous solvents and solvent mixtures reported by our survey respondents fell into three families: ketones, alcohols, and hydrocarbon solvents. Ketones and alcohols can further be considered using three solubility parameters fd, fp and fh (representing dispersive/van der Waals, polar, and hydrogen-bonding characteristics respectively) which are sometimes charted in two dimensions on a so-called Teas plot. Hydrocarbons, which have variable composition, are more usefully differentiated by their volatility and aromatic content.
Solubility parameters of solvents and solvent mixtures used to clean feathers
In an initial open-ended investigation of non-aqueous solvents for feather cleaning, we aged several swan feathers (upper coverts) in the Q-Sun Xe-3, our accelerated aging chamber (see earlier post). Drops of acetone, ethanol, isopropanol, and VM&P Naphtha were released onto the aged feathers through a pipette. The solvent penetrated quickly, and after it evaporated, the condition of the feather surface was evaluated visually and documented with the 3D Keyence microscope.
In these preliminary tests, only acetone caused significant deformation of the barbules, which remained clumped together after the solvent evaporated. This was accompanied by an increase in the macroscopic translucency of the feather in the area tested. A more limited increase in translucency was observed in tests with isopropanol, indicating the presence of subtle changes in the barbule shape or arrangement. Ethanol and VM&P naphtha did not cause these changes in the test area.
Left: Acetone (left side of green line). Swan feather aged 600 hours. @AMNH/R.Riedler
Right: Ethanol (left side of green line). Swan feather aged 600 hours. @AMNH/R.Riedler
Left: Isopropanol (left side of green line). Swan feather aged 600 hours. @AMNH/R.Riedler
Right: V.M&P Naphtha (left side of green line). Swan feather aged 600 hours. @AMNH/R.Riedler
While the higher volatility solvents were associated with irreversible physical changes in the barbule structure, the slower-evaporating solvents correlated with a much lesser degree of damage, if any. But more work is needed to fully understand how whole feathers respond to different solvents and solvent mixtures in cleaning, and which methods are least likely to cause unacceptable damage.
Stay tuned as we stay on the case and explore surfactants next!
Recommended Reading
Taylor A.M., Bonser R.H.C, and Farrent J.W. 2004. The influence of hydration on the tensile
and compressive properties of avian keratinous tissues JOURNAL OF MATERIALS SCIENCE 39. pp. 939– 942.
Banik G. and Krist G. (eds.) 1984. Lösungsmittel in der Restaurierung. Wien: Der Apfel
Phenix A. 2007. Generic Hydrocarbon Solvents: a guide to nomenclature. WAAC Newsletter 29(2), pp. 13-22
Rivers S and Umney N. 2003. Conservation of Furniture. Oxford: Butterworth-Heinemann. p. 513
Torraca G. 2005. Solubility and solvents for conservation problems. Rome: ICROMhttps://www.iccrom.org/publication/solubility-and-solvents-conservation-problems
In Their True Colors 