Under Construction, August 2011
Leather refers to the skins of animals that have been tanned or semi-tanned for use. There are many types of leather tanning (http://en.wikipedia.org/wiki/Leather#Forms_of_leather) which give skins different looks and feels. Since leather is an organic material, it is susceptible to many different forms of deterioration. Some can cause white stuff to appear on the surface, which may be difficult to distinguish from one another.
In Alaskan collections, there are various kinds of tanned and untanned skins and hides. Tlingit armor, fishing nets, gutskin parkas, babiche snowshoe lashings (made from moose or caribou rawhide), boots, tool lashings, model and full-sized kayaks, drums, and military gear are among the most common leather items.
2. POSSIBLE CAUSES
The most common white stuff we have seen on Alaskan leather items is white bloom resulting from fats, oils and waxes and may be referred to in the literature as ‘fatty bloom,’ ‘fat bloom,’ or ‘fatty spue (or spew)’. These terms all refer to the migration of fats/oils through the leather material that crystallize on the surface in the presence of air. When seeing fuzzy white growth on an object, people’s initial assumption is often that it is a mold or mildew. But this is not always the case. Blooms can sometimes have a feathery or matted fibrous look similar to mold, but microscopic examination and solubility tests can confirm the presence (or absence) of bloom.
Bloom can be considered as being Primary or Secondary. Primary bloom results from fats used during the tanning process and can be considered as an inherent vice of the material. Manufacturing flaws contribute to Primary bloom and can cause mineral salts to exude or fat bloom to develop through insufficient degreasing methods during production. Secondary bloom is caused by the application of fats and oils to the surface of the leather. At one time, it was believed that applying leather dressing or other kinds of soaps and oils to a leather surface would extend the life of a leather object. Now it is known that this is not the case and often the application of such substances can do quite a bit of damage
There are a number of hypotheses regarding the exact mechanism of the formation of these blooms. Some attribute it to free fatty acids migrating through the leather (Ordonez and Twilley 1998, 3-4). Analysis by Scott R. Williams (1988, 65-84) found bloom on objects to be primarily composed of a variety of fatty acids including palmitic, stearic, myristic and dicarboxylic acids (such as azelaic). These were present individually or occasionally as mixtures; however palmitic and stearic were the most commonly found (Williams 1988, 68-69).
Others have cited lactic acid, produced from the presence of potassium lactate in leather dressings, as the principle component of white efflorescence on leather (Gottlieb 1982, 39). In general, however, it is believed that temperature and humidity levels play important factors in the migration and crystallization of whatever is moving through and out of the leather.
Bloom can look powdery or gummy in appearance. Powdery bloom can be caused by either the natural fat of the hide or fatty materials applied to the leather. A number of variables are implicated in the formation of powdery bloom. These include: temperature, humidity, acidity of the leather, or materials used during the tanning process. Sticky or gummy bloom is believed to be caused by oils that are highly oxidizable, such as fish oils. If these kinds of oils were used during processing (and incompletely removed) or applied later, then they may cause sticky white bloom. High temperatures and humid environments, as well as exposure to air and light can accelerate these formations.
Fat bloom is often primarily found on areas of an object exposed to air. For example, on a leather-bound book the spine of the book (if it faces outward) may have the heaviest bloom. In some instances, it has been found that items closer to an air conditioning vent had a higher occurrence of bloom (Gottlieb 1982, 37) indicating that air circulation, temperature, and humidity play an important role.
Salt efflorescence is less common as a culprit on leather objects, but can occasionally be found on leather as inorganic salt spues. Leather items that have been worn (or in contact with perspiration in any way) may develop salt efflorescence as the salts migrate through the leather and crystallize on the surface.
Mold is typically described as having a fuzzy, velvety, or sometimes slimy appearance. When viewed under a microscope, the vegetative part of mold (known as mycelium http://en.wikipedia.org/wiki/Mycelium) can be seen as thin, thread-like branching hyphae and is very distinctive from the crystalline structure of salts. Mold growth generally begins to occur on organic materials when the environment is at 70% relative humidity or higher. The Canadian Conservation Institute (CCI) gives the following useful chart for mold growth on their “10 Agents of Deterioration” website http://www.cci-icc.gc.ca/crc/articles/mcpm/chap10-eng.aspx:
Metal items in contact with leather can react with the fats and oils, creating organo-metallic corrosion products. Quite frequently, the metal in question is a copper-alloy and therefore the corrosion products building up will be a bright green instead of white.
Up until the late 20th century, the application of toxic pesticides to organic materials in museum collections was a widespread and accepted practice. Compounds made of arsenic or mercury were sometimes sprayed or dusted onto artifacts to prevent pest damage. DDT was also common as were moth balls comprised of dichlorobenzene or naphthalene. The carcinogenic and hazardous nature of these chemicals is now known and they are no longer used. However, the residues of past applications remain and they can sometimes show up as white residues that may be confused with other salt formations. When handling objects made of organic materials such as skin, it is always better to err on the side of caution and protect yourself from possible exposure to toxic chemicals. Wear protective gloves and a lab coat or apron. You may wish to wear a dust mask to prevent breathing in toxic dust.
Use-Related White Stuff
Previous treatment of the artifact during use could be a cause of white accumulation on a leather surface. It is important to determine who performed the treatment and when, especially if an object is being considered for cleaning. If the treatment was done by the person who created the item – and it was part of the object’s use history, then it will likely be inappropriate to remove it. The loss of information involved in removing material will have to weighed against any benefit for cleaning an object. An example of an original treatment would the application of clay, flour, baking soda, or other similar substance by Native American groups to brighten and whiten a darkened or stained hide.
Catalog number II-A-77 is a Siberian Yupik pipe and tobacco bag in the Alaska State Museum collection. Tobacco and snuff production sometimes includes the addition of alkaline salts such as potash (often potassium carbonate). Could this be the white material we see on the leather bag? The pipe bowls, mouthpieces and decorative elements of the pipe are usually made from lead, which forms a white corrosion product. Museum leather care protocols decades ago called for applications of fats and oils to help leather remain supple. Proper identification of this white material would help us know if we ought to remove it or not.
Mold hyphae, image by Bob Blaylock
Fogle, Sonja. “Neat’s-Foot Oil in Commercial Products.” Leather Conservation News. Vol. 2, No 1, Fall 1985.
Crista Pack’s notes: Article provides overview and definitions for classes of neat’s foot-oil. More information can be found in his article “The Saddle Soap Myth,” which was reviewed in Leather Conservation News, No. 3.
Gottlieb, Jean S. (1982) “A Note on Identifying Bloom on Leather Bindings.” Journal of the American Institute for Conservation, Vol. 22, No. 1 (Autumn, 1982), pp. 37-40. Stable URL: http://www.jstor.org/stable/3179717. Accessed: 27/06/2011.
Crista Pack’s notes: This article discusses the finding of white bloom on a number of pre-1850 leather bound books. The author notes “a greater incidence in the general vicinity of one or two of the air conditioning ducts. “ Also that, “Sheepskin bindings and other older porous leathers showed the heaviest concentration of what appeared to be crystals, mostly on volumes that had been treated with potassium lactate and neatsfoot oil/lanolin within the past twenty-five years.” And “The surfaces exposed to the air (such as backbones) were those most densely coated with the bloom.” (p37)
The authors developed two hypotheses: “1) salts of lactic acid are always present in tanned leather, and may precipitate either in response to atmospheric changes, or from other causes; 2) the lactates identified in these samples appear to be residues of some substance introduced into or onto the leather (e.g., potassium lactate). “
Variations of tanning procedures and types of skin have to be considered with Hypothesis 1. For Hypothesis 2 “we must allow for variations in leather composition and condition, as well as the amount of substance applied, and stability of atmospheric conditions in which the books are kept.” (38)
Based on the NMR spectra obtained on samples analyzed, the author “identified a principal component of the efflorescence on the books as lactic acid, and have also pinpointed the source of this lactic acid as potassium lactate.” (39) And “the potassium lactate-neatsfoot oil/lanolin treatment was begun at the University of Chicago in the late 1950’s.” (39)
“There appears to be a correlation between the amount of efflorescence on leather volumes and their proximity to circulating air from ducts or vents. Since potassium lactate is deliquescent, air passing over surfaces holding a solution of potassium lactate and water would, by carrying the water off as vapor, cause the potassium lactate salts to be drawn to the surface: (KL · H₂O)hq à(air)à KLS + H2O (circulating air)” (40)
Doesn’t really clarify if any of the finding support Hypothesis 1, but seems to imply that Hypothesis 2 definitely has something to do with the production of bloom and, specifically, the presence of potassium lactate to create lactic acid is an important factor.
Ordonez, Eugenia and John Twilley, John. (1998) “Clarifying the Haze: Efflorescence on Works of Art” WAAC Newsletter 20 (1) 1998 pp 12-17.
Plenderleith, H.J. (1956) The Conservation of Antiquities and Works of Art. Oxford university Press London.
Ellen Carrlee’s notes: Mentions the museum use of potassium lactate solution for protection of vegetable tanned leather and British Museum Leather Dressing for enhancing flexibility of leather. Recipe for BM leather dressing includes lanolin, cedarwood oil, beeswax and hexane. Described as a yellow cream when applied.
Stambolov, T.(1969) “Manufacture, Deterioration and Preservation of Leather: A Literature Survey of Theoretical Aspects and Ancient Techniques.” ICOM, The International Council of Museums Committee for Conservation. Plenary Meeting. Amsterdam: Central Research Laboratory for Objects of Art and Science, September 15-19, 1969.
Williams, R. Scott. (1989) “Blooms, Blushes, Transferred Images and Mouldy Surfaces: What Are These Distracting Accretions on Art Works?” In Proceedings of the 14th Annual IIC-CG Conference 1988. Edited by Johanna G. Wellheiser. Ottawa. Pp 65-84
4. EXAMPLES IN ALASKA