Leather and Skin

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THIS ENTRY IS A DRAFT

Leather and skin objects include artifacts made using the hides, tissues and inner membranes of animals. These may include objects made from parchment, guts, or rawhide as well as tawed, oil tanned, brain-tanned, vegetable tanned, or mineral tanned leathers.

Materials and Technology[edit | edit source]

The raw materials for skin and leather products are obtained from a wide range of animals. All of these animal products are composed of the protein collagen. Various procedures are carried out to prevent putrification and impart specific working properties to the finished product. Untanned and cured products can be manipulated by processes such as cutting or sewing. Decoration and protective coatings are often applied. The materials and technologies for the final products are found in the following sections.

===History ===
If appropriate: brief summary or introduction to historical context, art historic background, function, use, etc.

Materials[edit | edit source]

skin v. hide; types of animals

Identifying Leather[edit | edit source]

Many aspects can alter the appearance of animal skin leather such as tanning, leather treatments, and deterioration from use or burial. Despite this manipulation, identification of the type of skin the conservator is preserving may still be possible. The grain, hair follicle pattern, collagen fiber configuration, use of the leather, and infrared spectroscopy can offer clues to what animal the leather came from.

Grain and hair follicles[edit | edit source]

Perhaps the easiest method of identifying leather is by looking at it under magnification. With a microscope or even a loupe, the conservator can sometimes discern how the grain and hair follicle pattern is arranged. These two characteristics are sometimes good indicators of the animal that the leather came from. [1]

Leather from cattle skin has hair follicles that are mostly “equal in size and arranged in regular rows” (Haines 2006, 17). Calfskin follicles are smaller and closer together, and adult cattle skin has larger follicles. No matter the age of the animal from which the leather comes, the follicle pattern will be the same. Leather from goat skin has alternating rows of large and fine hair follicles that form clusters.The follicles in sheep skin leather are all the same size and are relatively small. Like goat skin, the follicles of sheep skin are arranged in clusters (Haines 2006, 19).

Collagen configuration[edit | edit source]

Under greater magnification, cross sections of the leather in questioncan reveal the configuration of the collagen layers, which can be diagnostic of the species from which the leather came.

In leather made from cattle, the nature of the skin changes depending on the age of the cattle. Mature cow skins are thicker, usually between 4 and 6 mm (Haines 2006, 12). The grain layer, which constitutes the outer portion of the skin, contains hairs that are equally spaced (Haines 2006, 13). The corium layer consists of thick fibers, which resemble a tangled mass beneath the grain layer.Figure 5. Cross section of mature cattle skin leather. (Haines 2006, 13).Skin and leather from calves is thinner than adult skin, but becomes thicker with age. While calf skin and leather has the same structure as adult skin, the corium layer is much thinner and has finer bundles of fibers (Haines 2006, 14).

Goat skins range in thickness between 1 and 3mm, and the grain layer takes up a greater portion of the skin (about a third of its thickness) than the grain layer in cow skin. Corium fiber bundles are fairly fine and the difference between the grain and corium layers is rather indistinct (Haines 2006, 14).

Sheep skin leather sometimes includes the wool, and this may be a useful clue to what species of mammal the skin is from. The type of wool the sheep produces determines what the skin and leather cross sections will look like. Sheep that grow light wool tend to have thinner skins (.8mm average) with a finer corium layer. Sheep that produce heavy, copious wool have thicker skins (2 to 3mm) with a coarser corium layer. The corium layer of sheep skin tends to be relatively loosely woven, and there is a layer of fat between the grain layer and corium that makes the two layers loose, and makes the leather drape well (Haines 2006, 14-15).

Deer skin is relatively thick (2 to 3mm), and has a thin grain layer with a loosely woven corium.

Pig skin has a coarse grain with indistinguishable grain and corium layers. The hair shafts pass all the way through the skin, rather than stopping in the grain layer. The fibers of the skin are compact and form a distinct basket weave pattern (Haines 2006, 15).

Use[edit | edit source]

When trying to identify leather, the conservator may sometimes be able to take into consideration how the leather was used. Thick leathers tend to be unacceptable for clothing or shoe uppers. Because of this, adult cow skin is often used for sole leather, harnesses and horse tack, and machinery belts. Adult cattle skin can be split to make it thinner and more malleable. When split, the leather can be used for shoe uppers, upholstery, or case leather (Haines 2006, 13). Calfskin, on the other hand, does not have to be split as it is already fairly thin leather. The compactness of the fiber weave means that the leather, although thin, is still unacceptable for clothing. Calfskin leather is strong and makes for good footwear, handbags, and bookbinding.

Likewise, the compact weave of pig skin leather makes the substance ideal for bookbinding and case leather (Haines 2006, 15).

Sheep skin leather and leather from young goats are loosely woven and are ideal for making gloves and other clothing articles that require the leather to have a soft handle and good drape (Haines 2006, 14-15).

Spectroscopy[edit | edit source]

Infrared spectroscopy is yet another way of determining the species from which leather came. While this may be a useful technique, it does require that the surface layer of the leather be sloughed off to remove any scum that has collected over the years and any finishing agents that may skew the results. Although only a small section would have to be prepared in this way, the conservator would definitely have to weigh the costs and benefits of this method.

Although all mammalian skins are all primarily composed of collagen, the chemical signature of the types of collagen in different animal skins varies between species. The infrared spectrometer is able to pick up these differences, and report them in the form of a spectrum that can tell the conservator or scientist what the constituent parts of the leather are. In order to be able to discern what species the leather comes from, the scientist or conservator might have to make a database of known leather samples to compare to the unknown samples (Shao 2005, 49-50).

Tanning Techniques[edit | edit source]

When conservators must treat leather artifacts, it is important to be able to determine how the leather was tanned. The leatherís tanning and finishing may give the conservator some idea of how the object will continue to deteriorate, or how it may react to certain treatments.

Non-tanned leathers[edit | edit source]

Parchment and rawhide are two non-tanned leathers that may be encountered on a regular basis. Please see the books and paper chapter on parchment in this wiki for more information about parchment. Rawhide is comprised of de-fleshed and depilated animal skin that has been dried out. This type of leather has not been treated and is very tough and rigid. This makes it ideal for use in drum heads, parfleches, and shields (CCI Notes 8/4, 1).

Semi-tanned leathers[edit | edit source]

There are a variety of ways to produce semi-tanned leathers, but brain tanning, smoke tanning, and alum tawing are the most common methods. Well preserved or new brain tanned leathers tend to have ìa soft suede-like nap,î and are plum and flexible. When deteriorated, the leather becomes stiff, deformed, and turns from a light buff color to a grayish-brown color (CCI Notes 8/4, 1). Smoke tanned leathers exhibit many of the same tactile qualities as brain tanned leathers, but also frequently have a characteristic smoky smell and amber brown color (CCI Notes 8/4, 1). Alum tawed leather is flexible and light colored. It also tends to have a fine nap that makes it soft (CCI Notes 8/2, 1). Because of its flexibility and soft finish, semi-tanned leather is typically used in clothing, including jackets, moccasins, leggings, and gloves. This kind of leather is also used for accessories such as pouches (CCI Notes 8/4, 1).

Full-tanned leathers[edit | edit source]

The most popular methods for producing full-tanned leathers include mineral, aldehyde, and vegetable tanning methods. These methods can also be combined. It is sometimes difficult to differentiate between the different tanning methods, especially if the leather has been dyed or is very degraded. Vegetable tanned leather is the most common type of leather found in western cultures. There are a variety of tests that can indicate the presence of tannins that are characteristic of vegetable tanned leather, or differentiate between the types of tannins produced by different plants used in the tanning process.

The ferric test involves applying a ferric reagent to the leather which can indicate the presence of tannins in leather. If tannins are present, the area where the ferric reagent was applied will turn grey or black (Falcao and Araujo 2010, 152). Although the ferric test is a useful indicator of tannins, it is limited because it cannot detect what kinds of tannins are present in the leather and it can permanently mark the surface. There are two different kinds of tannins, condensed and hydrolysable, that are found in plants. The presence of one or the other kind of tannin may indicate what specie or group of plants was used to tan the leather. Determining the type of plants used might be of diagnostic use for artifacts whose origins are unknown.

Vanillin and acid butanol can be used to detect the presence of condensed tannins in leather. Plants that contain condensed tannins include quebracho, oak, and mimosa (Falcao and Araujo 2010, 151). The vanillin test consists of adding drops of vanillin reagent to the leather sample, and then adding two drops of HCl after the vanillin is absorbed. A red color in the test zone indicates that condensed tannins are present in the leather.

The acid butanol test must be carried out on individual leather fibers. Leather fibers are added to acid butanol reagent. Then, iron reagent is added, and the mixture is heated. If a red-orange to red-crimson product appears in the solution, condensed tannins are present (Falcao and Araujo 2010, 152-153). Nitrous acid can be used to detect the presence of ellagitannins, which are a type of hydrolysable tannin. Myrabolans, chestnut, Valona, oak, and divi-divi are plants that contain ellagitannins (Falcao and Araujo 2010, 151).

In the nitrous acid test, leather fibers are added to HCl and heated. Then, aqueous sodium nitrate is added and the mix continues to be heated. After about 20 minutes, the formation of a progressive blue color in the solution indicates the presence of ellagitannins (Falcao and Araujo 2010, 153).

Rhodanine can be used to test for gallotannins, which are another type of hydrolysable tannin. Plants containing gallotannins include tara, Aleppo, sumac, and Chinese (Falcao and Araujo 2010, 151). In this test, leather fibers are tested with rhodanine reagent, followed by aqueous potassium hydroxide. If a pink formation appears on the tested fibers, gallic acid is present (Falcao and Araujo 2010, 153).If the leather tests negative for tannins, the leather may have been tanned using other methods. Mineral (e.g. chrome) tanning replaced vegetable tanning by the end of the 19th century, and was used widely throughout Europe (Falcao and Araujo 2010, 149).

Chrome tanned leather can sometimes be identified by ìa blue-green line on a cut edgeî that indicates where the mineral salts used for the tanning penetrated into the leather (CCI Notes 8/2, 1).

Mineral tanned leather can also be identified by burning a small piece of the leather until the residual carbon is burned off. The presence of a green ash indicates that chrome (or occasionally aluminum or zirconium) is present in the leather (Identifying Chrome Tanned Leather).

Aldehyde tanned leathers are those that were tanned using formaldehyde or other aldehyde compounds. After tanning, these leathers are white in color and are supple (Aldehyde Tannage).

Technology[edit | edit source]

Processing[edit | edit source]

Parchment

• [Composition/Chemistry/Characteristics]

• [Manufacture]

Gut

Rawhide

Leather and tanning

Archaeological Leather

Archaeological leather must be stabilized; this is necessary because it may be missing a percentage of its original tannins. Tannins, as the name implies, is part of the tanning process; it is a polyphenolic compound that binds to and precipitates proteins. Conservators must fully understand their leather in order to treat it, without understanding the tanning process or environment in which it was found, a conservator has less of a chance in being able to aid an artifact. Their goals, put simplistically are to: dry waterlogged leather and treat desiccated leather so it does not continue to lose its original integrity; this may include “humidifying” it in order for it to acquire a stable water balance.

Leather can be made in a variety of ways. A conservator needs to understand the tanning process before treating the archaeological leather because depending on the leather, different stabilizing techniques should be used (Smith 2003). There are a variety of archaeological leathers but the most common tanning processes found are: smoking, oil process, and vegetable tanning. Vegetable tanning, in conjunction with smoke tanning were most likely the first methods of tanning used (Florian 2007; May 2006). After the pre-tanning process, which includes cleaning, de-hairing, de-liming, bating and pickeling, the skins are placed in a tannin bath for 3-4 months (Florian 2007; May 2006). Vegetable tannins are organic compounds that can be found in most trees and shrubs; the tannins are water-soluble and contain polyphenolic substances. The hides are placed in multiple baths to create the desired grain color and to make sure they are fully tanned (May 2006).

Polyphenolic substances are polymers of phenol monomers. Vegetable tanning is effective because phenols are astringent, which means they are able to bind with the hydrogen bonds of collagen (Florian 2007). Collagen makes up most of the skin structure in leather; collagen is the fibres and fibre bundles that retain and are stabilized during the pre-tanning and tanning process (May 2006). Phenols are “water-soluble, weakly acidic and easily oxidized under alkaline conditions; they form complexes with metals (iron and aluminum in particular); they act as antioxidants; they are inhibitors in auto-oxidation of lipid compounds; and they are toxic to micro-organisms” (Florian 2007). Many of these properties are initially beneficial in the tanning process; they help stabilize the animal hide. However, over time, these vegetable tannins may contribute to leather deterioration (Florian 2007).

The smoking process is less complex for the tanner to complete. In simplest forms, the clean skin is exposed to smoke from burning wood. The skins become preserved because of an aldehyde mixture (formalehyde) given off from the partial dry distillation of the burning wood along with the other resinous material in the wood (Florian 2007). This process was popular among American Indians, not only because of the ease of making the leathers but also because of the positive qualities that came with smoke-tanned leather. The hide could conceivable be waterproofed because the polyphenolic tannage in wood could combine with formaldehyde; this combination formed an insoluble phenol formaldehyde resin (Florian 2007). The formaldehyde may continue to help the hide by precipitating the naturally occurring globular proteins; this may create a biocide because of the cross-linkage with amines (Florian 2007).

For the oil process, oils were simply trampled into the skins; traditionally the tawyer worked the mixture in by using his bare feet (May 2006). This process is effective because oily unsaturated fats oxidize once exposed to air; “the tanning action is due to hydrolytic and oxidation products reacting with the collagen” (Florian 2007). Once fully oiled, the skins were hung in warm, spacious stoves for the oxidation process to occur; the oiling and heating process occurs about four time before being completely washed off in alkaline liquors (May 2006). The alkaline is added to remove excess oil; the hide is then worked till it is softened. Oil-tanned leather does not tend to be affected by red rot; it is tough, washable and durable (Florian 2007).

Once the tanning process is complete, leathers may be subject to “fat-liquoring, staining, dying, graining or embossing, plating, boarding, enamelling or abrading” (Florian 2007). The conservator must be aware of the leather’s life history after the tanning process in order to account for other factors that may affect the treatment of leather. Without stabilization, leather can continue to worsen its condition post excavation; frequent changes in relative humidity can negatively affect the archaeological material. The tanning process is such that it does not allow conservators to simply add more tannins to the leather in order to properly conserve the artifact (Volken 2001). This is why is it so important to understand the tanning process of archaeological leather found; it adds insight in how to treat it.

Alum Tawing

Oil Tanning

Brain tanning

Vegetable Tanning

• What is Vegetable Tanning?

• Process used for Historic Vegetable Tanning

Mineral Tanning

Finishing Treatments[edit | edit source]

Historically, coatings have been applied to seal, waterproof, or decorate leather. On horse-drawn vehicles, coated leathers have been used to create hoods, dashboards, and fenders as well as decorative trimming. Coated leathers are also found on apparel.

Japanned or Patent Leather
The process of rendering leather waterproof through application of multiple layers of a baked-on colored glossy coating was first patented in England by Charles Frederick Mollersten in 1805 (Anon 1806, 229). In the United States, Newark, NJ inventor Seth Boyden began producing a similarly made waterproofed leather c. 1819, referred to as patent leather (Bishop, Freedley, and Young 1864). Distribution of patent leather became more wide spread after 1822. This same material is also referred to as japanned leather in literature (Proctor 1922, 473). Though what results is a glossy, often black colored smooth surface similar to what one sees on lacquered or japanned decorative arts, the manner in which this coating was produced was different.

Splits of vegetable tanned cow hide were used in the 19th century as a substrate for coated leathers to be used on vehicles (FitzGerald 2007, 30). Various historic accounts supply recipes for the coating material thus variations on artifacts are likely to exist, but typically contained linseed oil as a major component with whale oil, horse grease, Prussian blue, lamp black, gum benzoin, gamboge, and litharge as possible additives (Seeley and Sutherland 1991). According to period literature, prior to coating, the hides were degreased to enhance the bond with the coating. Some makers applied a coarse grade of japan directly against the leather as a sort of primer before applying subsequent coats. Each layer was baked in a stove at temperatures between 60-93°C (140-200°F) and polished before the next layer was applied (Proctor 1922, 477.) After a number of layers of this coating was built up, the surface was finished with a top coat, referred to as a varnish. Depending on the time period and the location in which the japanned leather was produced, the varnish layer may have consisted of resins such as copal, amber or shellac; combined with gums; linseed oil; and/or asphaltum with red lead or litharge used as driers (Seeley and Sutherland 1991) or nitrocellulose (Proctor 1922, 481).

During their period of use, japanned leather on horse drawn vehicles may have been polished using a slurry of egg white, sugar and lamp black mixed with spirit of wine as a maintenance procedure (FitzGerald 2007, 314). Another period recommendation was to use a mixture of wax, olive oil, lard, oil of turpentine, and oil of lavender to polish coated leathers (Seeley and Sutherland 1991).

While 19th and early 20th century trade manuals describe the manner in which japanned and patent leather was produced, little technical analysis has been done to determine whether the descriptions are accurate (Ravenel 2011).

Enameled Leather
Coated leathers that are referred to as enameled have the coating applied to the grain side of the leather, often used for hoods on vehicles. FitzGerald points out that the grain split may be used for coated leathers other than enameled, but in these cases the coating is not applied to the grain but the split side (FitzGerald 2007, 31). While japanned leather became available commercially after 1822, enameled leather was not produced until after 1836 (Proctor 1922, 476).

Other Coatings on Leather

Paint
Nitrocellulose

As early as 1897, nitrocellulose was being used as a coating material on leather (Proctor 1922, 483).

India Rubber

Seeley and Sutherland describe black pigmented finishes containing india rubber used as early as 1858 (Seeley and Sutherland 1991, 24).

===Identification===
If appropriate: visual examination, microscopy, spot tests, instrumental techniques, etc. In order to reduce overlap, generic identification issues could refer to the appropriate RATS wiki section such as Materials Testing, Analytical Techniques, or Technical Studies.

Deterioration[edit | edit source]

Deterioration of leather

Deterioration of Patent Leather Coatings[edit | edit source]

Red-brown patent leather fender on a c. 1900 Jourbert & White (Glens Falls, NY) Buckboard Surrey showing one kind of surface coating deterioration seen on patent leather panels in transportation collections. Dr. and Mrs. W. Seward Webb collection, Shelburne Museum, 1947-18.4. Photo by Laura Brill.
Patent leather panel on a c. 1886 Brewster & Co. Victoria sleigh(Brewster #746). The bottom portion of the panel is covered with carpet, removed for the purpose of this photograph, which has protected the coating from degradation. Dr. and Mrs. W. Seward Webb collection, Shelburne Museum,1947-18.27. Photo by Laura Brill.
The same patent leather panel as that shown to the left, on a c. 1886 Brewster & Co. Victoria sleigh (Brewster #746), as normally displayed. Photo by Laura Brill.

Nineteenth-century patent leather coatings soften in the presence of water or heat, despite claims of their makers of the material's resistance to water and hot materials. In his carriage-trimmer's manual originally published in 1881, FitzGerald included instructions for separating patent leather panels that have been stored with their coated sides together in warm conditions (FitzGerald 2007, 313). As the coating ages, it may drip, sag, crack and/or form raised scabby islands on the surface of the leather support. Various panels may exhibit different kinds of deterioration on the same vehicle, depending on the nature of the panel's exposure to light and the environment. Note that the coating remains smooth and glossy where the patent leather panel had been protected by a rug on the patent leather panel on Shelburne Museum's Brewster & Co. Victoria sleigh. On the same panel the area just above where it is more exposed, but still somewhat protected by the wool fall under the seat, the coating has formed a fine traction crackle with small raised islands and a pebbly texture. Where the coating was fully exposed to the environment, a wide traction crackle has formed with large scabby islands.

Sample site F from a c. 1886 Brewster & Co. Victoria sleigh's fender. Dr. and Mrs. W. Seward Webb collection Shelburne Museum, 1947-18.27. Photo by Laura Brill. The site was sampled with the hope of revealing the stratigraphy of the coating in the raised islands as well as the the glossy valleys.
Sample F as viewed under visible light, 100x. The sample included the leather substrate, which can be seen at the bottom of the section. The coating layers consist of a grey colored ground layer against the leather, a lighter brown layer that has flowed around the darker layer above it to create the outer wall of the raised coating island. Sample preparation and photo by Laura Brill.

Sample site N from a patent leather panel on a c. 1886 Brewster & Co. Victoria sleigh (Brewster #746) Dr. and Mrs. W. Seward Webb collection, Shelburne Museum,1947-18.27. Photo by Laura Brill. This protected area was sampled with the hope of revealing the stratigraphy of a minimally deteriorated coating.
Sample N as viewed under visible light,100x. The sample included the leather substrate which can be seen at the bottom of the section. The coating layers consist of a black colored ground layer against the leather, a coarse grey colored layer above the ground and a thick black translucent layer on top of the grey. There may be a thin varnish layer above the thick black layer. Sample preparation and photo by Laura Brill.
Sample N as viewed under ultraviolet light,100x. The thick black layer appears to be heterogeneous. Sample preparation and photo by Laura Brill.

Deterioration of Waterlogged Archaeological Leather[edit | edit source]

Leather is a product created from the skin of animals, which is processed in order to make it “non-putrescible even under warm moist conditions” (Thomson 2006). Made from the corium, the thickest layer of the skin, leather derives its strength from the collagen fibers that compose it. When viewing leather that is in good condition, the smooth “grain” side shows a pattern of hair follicles, while the rough “flesh” side shows the ends of collagen fibers. Water makes the skin pliable during an animal’s life, This flexibility is maintained in leather by replacing the water with another substance, such as oil, during the tanning process (Cronyn 1990). This process also helps prevent leather from rotting, even if it becomes wet (Thomson 2006). Some leather products only undergo a partial tanning process, resulting in semi-tanned leather. Other products, like rawhide, and parchment, are processed without tanning (Cronyn 1990). But even in the best of situations, leather does not survive burial well. Waterlogged leather is even less likely to survive a long burial. Waterlogged environments create a unique set of issues for leather, and its condition when excavated is influenced by both pre- and post-depositional factors (Cameron et al 2006). The tannins used to tan some types of leather can react with iron in the environment, darkening the leather as iron tannates are formed. As the pH rises, the color of the leather will darken (Cronyn 1990). Leather can also darken due to saturation by other components of the burial environment (Cameron et al 2006). As a result, the original color of the leather can be obscured, especially if it was light colored. In spite of these color changes, leather may appear to be in good shape, even after a long period of burial. This being said, the oils and tannins used to tan the leather may be leached out, making it less flexible as it dries. Penetration by water can also cause hydrolysis of collagen fibers. This process makes the leather weaker, increasing the likelihood that it will crumble if mishandled. Fragmentation is a greater problem as hydrolysis and detanning progresses. Splitting is another type of damage often encountered with waterlogged leather. The leather splits into layers due to discontinuity between the collagen fibers of the grain and flesh layers of leather. When tanning agents fail to penetrate the leather completely, splitting is more likely to occur. As a result, many waterlogged leather artifacts are found in many pieces, if they are found at all (Cronyn 1990). Some types of leather products, such as rawhide or semi-tanned leather, rarely survive damp conditions. This is due to the hydrolysis and breakdown of collagen fibers in the leather. However, there are some conditions that will help preserve leather. In wet environments, the presence of copper and heart oak slows decay (Cronyn 1990). This is due to the toxic nature of copper; where copper is present, bacteria cannot survive, preventing them from breaking down the leather. Certain woods, such as heart oak, contain tannins, so the penetration of these tannins from the environment can help maintain the leather. In marine environments, the lack of bacteria may help preserve leather (Cronyn 1990). Generally, only vegetable-tanned leathers survive wet anaerobic deposits, though the reason for this preservation is not completely understood (Cameron et al 2006). The treatment of waterlogged leather is necessary for several reasons. When leather dries, it shrinks. This shrinkage increases the brittleness of the leather, also increasing the likelihood that the leather will fragment. Additionally, salt and silt can penetrate the leather, which will cause abrasion whenever the leather is flexed, wet or dry. Wet leather is also prone to fungal growth, as the components of leather provide a good food source for other organisms. Finally, wet leather, when untreated, will continue to react with the water it contains and continue undergoing hydrolysis (Cronyn 1990). Although the preservation of leather in waterlogged environments is limited, even fragmented leather can provide valuable information to archaeologists. In order to be able to decide on the proper treatment for waterlogged leather, it is important for the conservator to understand the general properties of leather and how its components are affected by the introduction of water. Although waterlogged environments can cause irreversible damage to leather artifacts, certain conditions exist that can also help preserve leather.

Chemical Deterioration[edit | edit source]

Leather is usually a chemically stable material; it tends to physically deteriorate before being severely chemically altered. Chemical deterioration in leather occurs when the environment in which it is stored becomes unstable or polluted. The chemical deterioration of leather is due to a combination of oxidation and hydrolysis (May 2006). "Oxidation of the amino acid involves altering the side chains which can result in breakdown products and finally ammonia" (Florian 2007). This process can begin with: oxygen, pollutant atmospheric gases, heat, light or free radical action (Florian 2007) (May 2006). "Hydrolysis involves breaking the amino acid and carboxyl bonds and can cause either separation of the chain or the release of the amino acids from the chain" (Florian 2007). Once hydrolysis begins, the rate may exponentially increase. Hydrolysis and oxidation can interact and drastically deteriorate leather. Leathers are not all equal; leathers processed with condensed tannins do not age as well as leathers tanned with hydrolysable tannins (May 2006). Rapid decay can occur when the environment alters significantly, especially when there are influxes of pollution. Sulfur dioxide in the air, for example, may oxidize and hydrate to form sulfuric acid within a leather structure (May 2006). Sulphuric acid is formed from sulphur dioxide air pollution. Sulfuric acid in leather is known as red rot. Sulfuric acid may either originate from pollution or had been added during the tanning processes (May 2006). Acid hydrolyses may be caused by the absorption of sulphur dioxide by the tannin in leather; alkaline soaps have the potential to concentrate and cause alkaline hydrolysis (Florian 2007). Free radicals may arise from the autoxidation of lipids; free radicals may begin to start the oxidation process and arise if the tannin polyphenols break down (May 2006). Deterioration in cleaner environments is caused by oxidative change (May 2006). Leathers in polluted areas absorb sulfur dioxide twice as rapidly if they were produced with condensed tanning materials as opposed to processed with hydrolysable tannins (May 2006). Acidic environments in highly polluted atmosphere can suppress oxidative reactions (May 2006). Oxidation and hydrolytic reactions occur in chemical degraded leather, this can occur in both polluted and unpolluted environments. The exact degradation is dependent to the conditions in which the leather is subject to as well as the process in which the leather was tanned. Leather exposure to adverse conditions may cause or increase the rate of oxidation and hydrolysis as well as initiate new reactions, which would in turn, further the deterioration of the leather; some of these factors are: extremes in water vapor, heat, chemicals from the tanning process, light and air pollutants (Florian 2007).

Conservation and care[edit | edit source]

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

To find a conservator, please visit AIC's Find a Conservator page.

Documentation[edit | edit source]

There are several resources for identifying leather based on follicle pattern and grain cross sections that include diagnostic images (Florian 2007, Kite and Thomson 2006). When examining an object that includes a leather component, it can be useful to include the answers to the following questions:

Was the leather tanned? Can you determine the tanning or treatment method?
Is a follicle pattern visible? Can you identify the animal with that follicle pattern?
What sections of the skin are present (grain layer, corium layer, etc.). Can you see the collagen configuration?
How thick is the leather or skin?
Does the leather have a finishing treatment?
Are there any signs of previous repair, either during the original use of the object, or once it entered museum collections?

For general recommendations, please refer to the Objects wiki article on Conservation Practices or for general conservation work practices, please refer to the main AIC wiki section on Work Practices.

Preventive conservation[edit | edit source]

For material/object type specific issues regarding recommendations for storage and display, handling, inhibitive conservation measures, supports, mounts, labelling, transport, condition surveys, monitoring, etc. In order to reduce overlap, general preventive care issues should refer to or be discussed in the appropriate Preventive Care section of the main AIC wiki.

Interventive treatments for Leather and Semi-tanned Hide/Fur[edit | edit source]

As with any other type of object, each leather or hide object must be considered unique in its conservation needs. Care must be taken to identify any culturally significant deposits so that they not be inadvertently removed along with post-collection alterations. What follows are brief descriptions of common solutions to the most frequently encountered problems, and should not be taken as prescriptions for treatment (more detailed discussion of specific treatments can be found in the references at the bottom of the page). It is recommended that any treatment be undertaken by a trained conservation professional.

Cleaning
Cleaning is done mechanically if at all possible. The preferred method is vacuuming under low suction while gently brushing dirt and debris loose with a soft-bristled brush. Often, the end of the vacuum hose is reduced in size and covered with fine netting to prevent loss of material and it never actually makes contact with the basket. Foamed latex rubber sponges are also helpful in removing some soiling when gently applied to soiled surfaces.

Degraded leather dressings applied in the past can pose tough cleaning challenges; solvent cleaning can be appropriate for some leather, if it is fully tanned and in good condition. Any liquid used should be applied sparingly. For degraded leather and semi-tanned hide (either brain or smoke tanned) aqueous solutions should be avoided if possible as they can reverse many of the chemical bonds created during the tanning process.

Stabilization
Consolidation, desalination, deacidification, corrosion inhibitors; etc.

Structural treatments
Degraded leather can sometimes require consolidation—especially where it has lost the cohesive strength between fibers . In the case where the leather is friable and powdery, it can be consolidated with various adhesives that can be applied either in a spray (using a nebulizer) or with a brush.

Breaks/tears or losses to a leather or hide piece can affect its stability as well as distracting from the legibility of the piece. In some cases, the breaks can be knit back together by applying adhesive just along the break edges—especially if the break does not need to support its own weight; however, most breaks require a bridge applied to the back (or front depending on the type of object and its construction) of the break. Bridge materials vary greatly and are based on both the weight and condition of the leather or hide adjacent to the break. Bridge materials include Japanese tissue, nylon netting, nylon gossamer, spun bonded synthetics, and goldbeater’s skin. Adhesives used also can vary widely and include heat-set films, solvent- and water-based adhesives (for example BEVA 371, Lascaux 360HV/498HV, Aquazol and Butvar B-98, Elvace 40705, Jade 403, methyl cellulose, and arrowroot paste). Care must be taken if a water-based adhesive is selected that it does not denature the tannins present.

Where losses are present, it is sometimes appropriate to fill these. In other cases, it is more appropriate to leave the losses visible due to cultural sensitivity and/or the history of the piece. Many materials have been used as fills including Japanese tissue, polyester felt, and other spun bonded synthetic materials. Silicon molds of the surface of leather with similar grain and wear patterns (not accessioned or historically important leathers) can be used to cast fills using acrylic paint and Japanese tissue.

Over time, environmental and physical stresses placed on leather can cause distortions. Depending on how extensive the damage and how structurally sound the component materials, leather and hide objects can sometimes be restored to their original shape. This is almost always achieved with gentle, controlled humidification to increase flexibility, followed by the use of wadding or weights to hold the object in the correct shape until all introduced humidity has been dissipated. [AKS3] Depending on the type of object, permanent supports may be required. For example, with shoes or moccasins, custom cushions can be made using Tyvek and polyester batting. For garments, either storage pillows or custom mounts can be used to support the leather/hide and prevent further distortions. For full hides that sit flat, custom trays can be made with pillows supporting areas where the hide is does not lay flat.

Aesthetic reintegration
Where loss compensation is required strips of non woven polyester fabric (e.g. Reemay®) and Japanese Tissue paper, coated with conservation-grade acrylic adhesives can be used to fill losses. Water colours can be used to colour matching of missing areas. Losses can also be sorted filling with new leather, matched according to the grain, thickness and colour, using conservation-grade adhesives to insert new patches.

References[edit | edit source]

Aldehyde Tannage. ND. Bogazici University Chemistry Department. [www.chem.boun.edu.tr/webpages/courses/leathertechnology/deri26.htm] (accessed 03/21/13).

Anon. 1806. Retrospect of philosophical, mechanical, chemical and agricultural discoveries: Being an abridgment of the periodical and other publications, English and foreign relative to arts, chemistry, manufactures, agriculture and natural philosophy, vol. 1. London: J. Wyatt.

Bishop, J. L., E. T. Freedley, and E. Young. 1864. History of American manufactures, from 1608 to 1860: Exhibiting ... comprising annals of the industry of the United States in machinery, manufactures and useful arts, with a notice of the important inventions, tariffs, and the results of each decennial census. Philadelphia: Edward Young & Co.

Cameron, E., J. Spriggs, E. Wills, 2006. Chapter 22: The Conservation of Archaeological Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp. 244-261.

Cronyn, J. M. 1990. Elements of Archaeological Conservation. New York, New York: Routledge.

CCI Notes 8/2: Care of Alum, Vegetable, and Mineral Tanned Leather. 1992. Canadian Conservation Institute. [2] (accessed 10/2/19).

CCI Notes 8/4: Care of Rawhide and Semi-Tanned Leather. 1992. Canadian Conservation Institute. [3] (accessed 03/25/13).

Falcao, L and M. E. Araujo. 2010. Tannins Characterisation in New and Historic Vegetable Tanned Leathers Fibres by Spot Tests. Journal of Cultural Heritage 12: 149-156.

FitzGerald, W. N. 2007. The carriage trimmers' manual and guide book. Mendham, NJ: Astragal Press. ISBN 1931626235

Florian, M. E. 2007. Protein Facts: Fibrous Proteins in Cultural and Natural History Artifacts, pp. 87-94.

Haines, B. 2006. Chapter 3: The Fibre Structure of Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp.11-21.

Identifying Chrome Tanned Leather. 1998. Cool Conservation OnLine. [4] (accessed 03/21/13).

May, E., and M. Jones. 2006. Conservation Science: Heritage Materials, pp. 92-119.

Procter, H. R. 1922. The principles of leather manufacture. New York: D. Van Nostrand Company.

Ravenel, N. 2011. Notes on Laura Brill's unpublished analytical work on patent leather at the Shelburne Museum in 2009. Shelburne Museum, Shelburne, VT.

Seeley, N. and A. Sutherland. 1991. Enamelled, Japanned and patent carriage leathers. In Conservation of leather in transport collections. London: United Kingdom Institute for Conservation. 24-26. ISBN 9781871656138

Shao, Y. 2005. Chapter 3: Chemical Analysis of Leather. In: Fan (ed.), Chemical Testing of Textiles, pp.47-71.

Smith, C. W. 2003. Archaeological Conservation Using Polymers: Practical Applications for Organic Artifact Stabilization, pp. 60-69.

Thomson, R. 2006. Chapter 1: The Nature and Properties of Leather. In: Kite and Thomson (eds.), Conservation of Leather and Related Materials, pp. 1-3.

Volken, M. 2001. Practical Approaches in the Treatment of Archaeological Leather. In: Wills, B. (ed.) Leather Wet and Dry: Current Treatments in the Conservation of Waterlogged and Desiccated Archaeological Leather, pp.37-43.