When birds bathe to maintain their feathers, they sit or stand in shallow water and flap their wings and tails to scatter water onto the feathers and skin. Water droplets scatter from their feathers due to the water’s high surface tension and the feathers’ tendency to repel water. That water repellency, or hydrophobicity, is owed to a combination of material and structural properties, as well as the presence of a thin film of preen oil used by some species. On the fresh feathers of a living bird, water cleans by picking up and trapping loosened dirt particulates as well as insects and mites and then easily rolls off the hydrophobic keratin instead of soaking into it and weighing down the bird.
Water is readily available, cheap, and non-hazardous to humans and the environment. So, if birds use water for cleaning their own feathers, should conservators consider it a safe and effective cleaning agent?
Survey Results
If birds use water for cleaning their feathers, one might assume that water is a safe and effective cleaning agent. In our survey we asked people about whether and how they use water to clean feathers. About two-thirds of people reported using aqueous solutions often or sometimes. The rest said that they rarely or never use water. The latter shared concerns about its potential to damage degraded feathers or the underlying skin in bird taxidermy or study skins. Some noted that they would consider using water to clean recently prepared feathers, but not historic feather objects. Others use water only in specific circumstances, such as the removal of localized accretions.
In some cases, respondents included very specific details about the degree of purification or pH of the water they use. Some also described manipulating water temperature to increase the efficacy of the treatment; warm water applied by brush is sometimes used to simultaneously clean and align feather barbs and barbules while some use warm vapor to soften hardened grease and oil residues.
Our survey respondents reported that when using water, they like to apply it with brushes, cotton swabs, and sponges. Some brushes can hold large quantities of water due to their bristle material and structure, making them well-suited to washing dirt off a feather quickly. However, there is also a case to be made for brushes that hold onto less water within their bristles because they can be used in more targeted cleaning and leave less water sitting on the surface of the feather. Larger quantities of water can also be released through a pipette. Both methods are often used in combination with an absorbent blotter under the feather or a vacuum unit to collect excess liquid. Swabs and sponges can be used to release more controlled amounts of water, reabsorbing the dirty water through capillary forces (see this post). The use of microfiber cloths is also mentioned. In some cases, people have undertaken full immersion of single or detachable feathers, and even entire feather objects, in a water bath. The impacts of different drying methods used after wet cleaning were noted too, such as the use of forced versus passive air of different temperatures.
Water
In wet cleaning, water can act as a solvent alone and/or as a medium to carry additional cleaning agents like surfactants; here we will focus on water as a solvent only.
The water molecule is a dipole with oppositely charged poles on the hydrogen and oxygen atoms. This polar nature gives water the ability to form hydrogen bonds, which in turn allow it to dissolve or swell organic soils that contain polar groups themselves (e.g. sugar, certain polysaccharides, proteins). The high polarity of water also provides the capacity to dissolve ionic compounds.
As a consequence, water obtained directly from the tap generally contains unknown impurities. These impurities can originate from water soluble compounds in the ground, rocks, and pipes, or from pollutants in the atmosphere. Past research in textile conservation (Ágnes Tímár-Balázsy 1999, 2000) has shown that these impurities may impact cleaning: cations like Ca2+, Mg2+, Na+, K+, Mn2+, Fe2+, or Fe3+ can catalyze reactions that contribute to deterioration and photo-oxidize or react with soils to form colored compounds.
– Cations will reduce the cleaning power of the washing solution for soils containing the given ion.
– Compounds of heavy and transition metals are catalysts for chemical reactions, contributing to further deterioration of textiles.
– Compounds of these metals may turn into colored compounds by photo-oxidation or form colored compounds with other soiling on the textile.
(Tímár-Balázsy, 2000, pp. 47)
Anions, like sulphates (SO42-), carbonates (CO32-), hydrogen carbonates (HCO3-), nitrates (NO3-) or chlorides (Cl–), can change the pH of the water, making it acidic or alkaline (Tímár-Balázsy 1999, 2000). Water stored for weeks or months in leaky bottles or containers can easily absorb acidic pollutants from the atmosphere. It is advisable to measure the pH of any type of such stored water before use.
Purity
One measure of water’s purity is its conductivity or resistivity, i.e., its ability to conduct or resist an electric current, which is determined by its ion content. Conductivity can be measured using a handheld conductivity meter, and can be directly linked to the total dissolved solids (TDS) present in water. Water purified of all contaminants has no ionic content, and thus, very low conductivity. Purified water used in conservation is very powerful and can be generated through distillation, reverse osmosis, or ion exchange. These processes remove inorganic ions, organic compounds, bacteria, endotoxins, nucleases, particulates, and gases.
The most common standard used in describing water purity is ASTM D1193-06 (2018). ASTM Type I water has been purified through a series of treatment steps and is sometimes called ultrapure (though different industries use different standards in the production of ultrapure water). Type I water is a very powerful solvent that rapidly pulls ions into solution and degrades quickly in storage as it absorbs contaminants from the air. Its strength is derived from the highly reactive hydroxyl-radical, a species shown to mutate DNA, denature proteins, disrupt cell membranes, and chemically alter critical neurotransmitters (Griesser and Bayerova 2007).
As such, it may be too aggressive for certain cleaning treatments (Heald 1995). The feather research team has access to ultrapure water due to a filter system for cooling the Q-sun Xe-3 accelerated aging chamber (see this post). Another source for ultrapure water is Milli-Q water, which is accessible through the genomics lab at the Museum.
ASTM Types II, III, and IV water have higher levels of conductivity and ionic and particulate content.
Purification can be achieved to different extents through processes including ion exchange, distillation, and reverse osmosis that remove inorganic ions, particulates, organic compounds, and/or micro-organisms.
| Purification Process | Conductivity μS/cm | Resistivity M•cm |
| Tap water | 500 – 1000 | 0.001- 0.002 |
| Deionized (DI) water | 0.056 – 10 | 0.1 – 18 |
| Distilled water | 0.5 – 3 | 0.33 – 2 |
| Reverse osmosis (RO) water | 0.056 – 200 | 0.005 – 18 |
Deionized (DI) water
Source water is run through a bed of electrically charged resin, exchanging positive and negative contaminant ions for positive hydrogen and negative hydroxyl ions. In areas where water is particularly hard or contaminated, filters may need to be replaced frequently, which can be expensive. Deionization removes all ions, but does not remove uncharged organic molecules or particles (including most micro-organisms). The resulting water is reactive and begins to absorb CO2 as soon as it is exposed to air.
Distilled water
Source water is boiled. The steam is cooled, collected, and condensed, leaving behind most biological material, particulates, salt, and mineral impurities. This process can be energy intensive and may not remove all volatile components.
Reverse osmosis (RO) water
Reverse osmosis uses a semipermeable membrane to remove ions, unwanted molecules (including biological ones), and particulates from source water. Source water is pressurized to diffuse through the membrane, overcoming osmotic pressure from purified water retained on the other side. This technique is cost effective, but may discharge large quantities of wastewater.
So, what about cleaning feathers with water?
In our research, we observed that aqueous solutions almost always caused some damage to our artificially aged feathers. The water permanently distorted the barbules, causing them to flatten themselves against the central ramus of the barb. This deformation of the barbules in turn caused the vane to become irreversibly unzipped and splayed. In fresh (unaged) feathers, damage associated with aqueous cleaning was much more limited.
The influence of condition is clear: deteriorated feathers are more sensitive to water. Before considering an aqueous cleaning method, it is important to make a thorough assessment of condition to understand how the feather is likely to respond. We will describe one technique for investigating water sensitivity in our next post. Stay tuned!
References
Banik, G. and Brückle, I. 2018. Paper and Water: A Guide for Conservators. Siegl: München.
Griesser, M. and Bayerova, T. 2007. Dihydrogenmonoxide – About the Danger of Applying Highly Purified Water in Conservation. In: Wasser. 20. Tagung des Österreichischen Restauratorenverbandes. 10–11. November 2006 (Hofmann, Ch. and Schäning, A.), Vienna 2007, pp. 84––93
Heald, S. 1995. Deionized water and its reactivity – could it be damaging? In: Postprints of the Conference of the AIC Textile Speciality Group, AIC 1995 (Ewer, P. and McLaughlin, B. eds.), pp.12––14
Hofmann, Ch. and Schäning, A. (eds.) 2007. Wasser. 20. Tagung des Österreichischen Restauratorenverbandes. 10-11. November 2006, Wien
Tímár-Balázsy, Á. 2000. Wet cleaning of historical textiles: surfactants and other wash bath additives, Studies in Conservation, 45(1): 46–64
Tímár-Balázsy, Á. and Eastop, D. 1999. Chemical principles of textile conservation. Oxford: Butterworth-Heinemann.
In Their True Colors 