Caring for leather, skin and fur

Rawhide

The most basic treatment carried out on a flayed skin is to unhair and deflesh the skin mechanically and to dry it on a frame after having washed it or soaked it in water. This produces rawhide (Figure 3), a translucent material that is hard, stiff and tough.

Rawhide is stable and durable if kept dry, but it is susceptible to moisture, as it contains no added stabilizers (tanning agents). If it becomes wet enough, it will first swell and then will start to putrefy. As rawhide dries, it shrinks, a characteristic that has been used advantageously in the making of objects. For example, wooden boxes may be covered with rawhide in its wet state so that when the hide dries it exerts powerful contracting forces, pulling the wood joints tightly together. Similarly, drumheads are often made of wet rawhide that tightens on drying to produce a taut membrane (Figure 4).

Other typical rawhide objects include rattles, parfleches, shields and pouches.

Kayak coverings and sealskin floats are made of rawhide that is oiled to make it more resistant to moisture and water (Figure 5).

Sinew

Sinew, or tendon, an animal connective tissue that joins a muscle to a bone or other parts, was used by many cultures, including Native American, as a thread or string material in making clothing, drums, kayaks and other skin or fur objects. Its processing basically follows that of rawhide. “Babiche” is a type of lacing made of sinew or of long narrow strips of rawhide used, for example, to make the webbing in snowshoes.

Gut-skin

Membranes from animal internal organs (for example, the cecum, bladder or stomach) processed in a similar way to rawhide produce very thin, translucent “gut-skins” that have been used to make bags or clothing such as Inuit gut-skin parkas (Figure 6). Gut-skins were usually oiled to make them more flexible and water repellent.

Parchment and vellum

Parchment and vellum are used extensively for the preparation of manuscripts and as a bookbinding material. They are also used as musical membranes or as a covering material for wooden boxes. The skins of small animals are generally used, particularly calf (for vellum) or goat and sheep (for parchment). After cleaning, the skin is soaked in a lime bath and, while still in a swollen state, is stretched on a frame to dry. Simultaneous stretching and drying cause the skin fibre structure to dry in a “frozen,” stretched state and to be packed more tightly together. The result is a thin, flat and stiff skin. The surface is normally treated with an abrasive, such as pumice, to even out the texture. Various fillers, like chalk, are rubbed in to whiten the skin and make it more receptive to paint and ink.

Semi-tanned skins

Semi-tanning has a very long history and is still used by Indigenous peoples in North America and other peoples, including Australian aborigines and Mongol tribes. The material produced is often called buckskin and is strong and flexible with a soft, suede-like nap and a full plumpness within. It has been used extensively for making pouches or clothing items such as vests, jackets, leggings, mittens, moccasins, etc., often with added decorative elements such as paint, dyed porcupine quills or beadwork (Figure 7).

Pre-treatment involves cleaning, soaking in aqueous solutions, and various mechanical actions (flexing, scraping, etc., and sometimes chewing) to soften the skin and work in fatty substances. Indigenous peoples in North America typically work in fat from the brain or spinal column of the flayed animal; such skins are said to be brain-tanned. Bone marrow, egg yolk, milk or butter can also be used. There is little chemical interaction between the oil and the skin protein, and so this is not a tannage in the strict sense.

Brain-tanned skins are often smoked as well, producing a yellow- to amber-brown-coloured skin (Figure 8) with a characteristic smoky smell. Usually, green wood is burned in a low fire with the skin spread around it to form a tent. The smoke reacts chemically with the collagen and partially tans the skin, making it more stable and water-resistant. Oils need to be present in the smoked skin to prevent the fibres from sticking and hardening.

Black-tanned or iron-tanned skin

Black-tanned or iron-tanned skins are obtained by applying iron-rich mud or water, or a solution made from boiling nails, to semi-tanned skins. This produces a skin of a dark-brown colour. Objects made of black-tanned skins include pouches (Figure 9) and moccasins. Black rot is a form of severe chemical deterioration due to the long-term action of these iron salts present within the skin; the skin then becomes weak and brittle.

Oil-tanned skin (chamois)

Oil tannage is similar to brain tannage, except that the oils typically used react further and create chemical links throughout the skin structure. Fish oil is most often used, particularly cod-liver oil. The modern process involves heating the oiled skin in the range of 30–50°C to effect oxidation and crosslinking of the oil. This reaction prevents the fibres from sticking upon drying and so imparts suppleness. “Chamois” is a well-known product of this process and is used to make gloves, purses and other such objects (Figure 10). Chamois is strong, resistant to water and very absorbent. Oil tanning is also a common process for furs.

Alum-tawed skin

Alum tawing is an ancient skin-processing method used mostly for making gloves and other personal items (Figure 11). A mixture of potash alum, sodium chloride, egg yolk, flour and water is worked into softened skins. It is not a true tannage, as the salts and other materials applied are not chemically bonded to the skin and can be washed out to some extent. It can therefore be damaged by contact with water. Alum tawing produces a very white material, which is soft and capable of taking a polish but difficult to dye. Alum-tawed skin can also be smoked to increase its durability. The process is also used for dressing fur-skins.

Vegetable-tanned leather

Vegetable tanning is commonly used to make a wide variety of objects, including bags, belts, purses (Figure 12), boots, straps, harnesses, saddles, furniture and vehicle upholstery, painted or gilt wall panels and bookbindings. It is an old technique, used to this day, although mineral tanning, which was introduced at the end of the 19th century, is also now common. Vegetable-tanned leathers retain the impressions formed by tools and so are used to make decoratively tooled items and for fine bookbindings.

The basic process involves suspending a prepared skin in tanning liquors consisting of infusions of bark, leaves or twigs in water. The minimum soaking time is two days, but it can be done for a year or more with regular beating and manipulation of the skin. At the end of the process, the leather is hard and inflexible, so fats and oils, such as tallow or cod-liver oil, are worked in to provide flexibility. The tanning process stabilizes the skin to provide a higher degree of resistance to water and wear. The tannins in the bark infusions that bind to the collagen substrate are of two main types: some wood species predominantly have the condensed type of tannins (e.g. mimosa), some have the hydrolysable-type (e.g. sumac) and some have a combination of both (e.g. oak). These different tannin types affect the extent of a vegetable-tanned leather’s resistance, or vulnerability, to inherent chemical degradation (oxidation and acid hydrolysis). In particular, vegetable-tanned leathers can suffer from “red rot,” which is a severe degradation characterized by the leather becoming quite weak and powdery (consult the section Pollutants, red rot for more information).

Mineral-tanned leather and combination tannages

Nowadays, a large part of the world’s supply of leathers is mineral-tanned. Chromium salts are predominantly used, but other salts, such as zirconium, have come into use as well. The salts in the tanning liquor bind strongly to the skin’s collagen and provide crosslinks, creating chemically stable and water-resistant leathers. A fully chrome-tanned leather will not shrink or alter even when exposed to hot water. It is necessary to add a lubricant after tanning before the leather is dried, otherwise it will be stiff. The first modern mineral tanning process, chrome tanning, dates back to about 1884, so only relatively recent museum objects may be made in this way. (Note: the older tanning processes of iron-tanning and alum-tawing [described above] are, technically, also “mineral-tanning.”)

Combinations of the above processing techniques are common. For example, a skin may be vegetable tanned to impart a firm yet flexible structure, followed by mineral tanning to provide higher resistance to water.

Furs

Furs provide an ideal protection against the cold, and their use as clothing and blankets is probably as ancient as the art of hunting. Fur-skins processed as rawhide are generally stiff. Within Indigenous North American cultures, furs are often prepared using semi-tanning processes (brain-tanning with or without smoking) and with extensive scraping and manipulation, resulting in a soft, supple material suitable for clothing. Inuit amauti are prime examples (Figure 13).

Other common processing methods for fur-skins include oil-tanning, alum-tawing and pickling. Pickling involves immersing the skin in baths of strong acids and salts; the term “pickling” is used by furriers when the chemical agent used does not confer complete resistance to water, while “tanning” implies a more water-resistant processing treatment. Certain factors or methods when processing a fur-skin are critical for preserving the hairs. These include the time of year the animal was killed, as well as avoiding physical damage to the hairs during the scraping and manipulation, quick cooling, and prompt and thorough cleansing of the skin to prevent bacterial growth that would risk loosening the hairs.

Objects decorated with beads, tinklers and other ornaments

Leather, skin and fur objects with heavy decorative panels (glass beads are especially heavy) are vulnerable to tears and distortions because they may not be able to bear the weight. Tears can result if the item is handled unsupported (Figure 18).

Beads, tinklers, sequins, feathers, quills and other decorative elements often attached to skin and leather objects may be at risk of loss during handling and moving because the thread or other means of attachment may now be weak or broken (Figure 19). If glass beads or metal tinklers are corroding, the thread or sinew that is carrying them may be attacked by the degradation products and thus become weak. Also, objects with fringes and thongs may have become worn, stretched or damaged and unable to carry their weight. Objects with any such decorations should generally be stored in boxes or trays to capture any loose elements. To prevent detachments, decorative components should be examined to assess if they need to be locally secured; for example, if they need to be loosely bound with cloth tape. For more information on the care of objects decorated with glass beads, consult CCI Note 6/4 Care of Objects Decorated with Glass Beads.

Fur hair slippage

Furs of the deer family can suffer from hair slippage, which is caused by the loosening of the hair root anchoring system. The exact cause is not well understood, and it is not possible to detect which skin is likely to suffer from this over time or to prevent it from happening. Any handling or movement should be minimized to prevent losses or further dislodging of the hairs.

Loss of flexibility and leather dressings

A common question is whether leather dressings or other oil-based formulas should be applied to restore flexibility to a stiff vegetable-tanned leather. The answer is no. Museum objects are usually not flexed or used to the extent that non-museum pieces are and so do not require the additional flexibility that such oiling imparts. These dressings can be harmful because, in time, oils oxidize and may produce deteriorative effects within the skin structure, such as further stiffening and darkening. Oils and dressings can also lead to sticky surfaces that attract dust and soiling, and to dehydration within the material. An excellent discussion on leather dressings can be found in Leather Dressing: To Dress Or Not To Dress (PDF format), published by McCrady and Raphael (1993). Saddle soap, which contains oils plus a soap element that is also supposed to clean leather, should not be used for the additional reason that these products are alkaline and harmful to leathers, which are naturally acidic in nature.

For more preservation strategies to prevent damage by physical forces, consult Agent of deterioration: physical forces.

Recommendations

• Provide sufficient horizontal and vertical storage space to avoid cramming, compression or creases.
• Ensure no fringe, tinkler and other such decorative component hangs from storage shelves (Figure 20).

• Inspect decorative components (e.g. tinklers, fringes, beadwork) to identify weak or damaged threads, strings or attachments and secure (e.g. with ties), if necessary. Support and contain objects with decorative components to prevent losses (Figure 21).
• Provide boxes, trays, padding and securing nests or ties for fragile or vulnerable objects on storage shelves or in drawers (Figure 22).
• Provide inner padded supports for rounded objects such as bags, purses, hats, boots, moccasins (Figure 22), gloves, mittens, coats, vests, jackets and leggings. Pad out objects to preserve their three-dimensional shape, but avoid overstuffing, which produces tension and can stretch the skin, loosen stitching or cause weakening and tears. For an example, consult Storage protection for a collection of moccasins.
• Provide padded hangers for items in hanging storage, such as described in CCI Note 13/5 Hanging Storage for Costumes.
• Identify fragile items such as skins or leather suffering from red rot or black rot, or otherwise severely degraded, and provide them with protection (handling tray, box) and supports, for example as in Display mount for fire apparatus suffering from red rot.
• Avoid handling fragile, brittle or stiff objects directly; provide them with supports, padded trays or boxes (Figures 21, 22 and 23).
• Provide temporary trays with padding for use during handling and relocations.
• Identify furs with hair slippage, contain them within boxes or trays and avoid any handling.
• Train staff and volunteers to recognize the most vulnerable objects and how to provide extra care.

Mould

Leather, skins and fur objects can be attacked by mould; this is discussed further under Incorrect relative humidity.

Insects

Leathers, skins and furs are nutrients for many species of insects, including dermestids (from the carpet beetle family). Furs are even more prone to infestations because the hair is quite attractive food to some insects, particularly dermestids and moths, and because furs provide shelter where the insects can hide and develop. Soiled or undisturbed objects are most prone to attack. Signs of insect infestations include grazed surfaces, holes (Figures 28a, 28b and 28c) and the presence of frass and casings (Figure 29). In furs, the signs of infestation also include hair loss.

Recommendations

• Implement a full integrated pest management system, including monitoring, housekeeping, building and equipment features.
• For the most pest-prone part of your collection, investigate further preventive treatment options, such as cold storage or anoxic storage, as discussed under Furs: cold storage versus anoxic storage.
• Keep storage areas clean and tidy.
• Keep the collection dust free by using dust covers or enclosures (boxes, storage cabinets, display cases, etc.).
• Quarantine incoming objects before integrating them in the collection.
• Inspect storage areas at least twice a year for signs of insect activity (Figure 29). Inspect objects most vulnerable to infestation twice a year (in spring and fall).

If an infestation occurs, implement basic pest management procedures as outlined in Agent of deterioration: pests under Respond.

For more preservation strategies to prevent damage by insects and other pests, consult Agent of deterioration: pests.

Dust

Dust is obtrusive and abrasive and can also increase the risk of infestation. Furs in particular need protection since the hairs trap dust and are difficult to clean. Soiling and staining from dust or hand contact is a general concern, but especially a problem for light-coloured skins, such as brain-tanned skin, and for parchment and vellum.

Pollutants, red rot

Pollutants can cause or significantly affect the oxidation or hydrolysis of skins and leathers. The risk category is ranked in general as moderate to low for most skins and leathers, but will increase the higher the pollution levels. Vegetable-tanned leathers are most vulnerable, more so than alum-tawed skins or rawhide for example, while chrome-tanned leathers are the least vulnerable (Haines 1991). Sulfur dioxide (SO₂) is considered the main source of the acidic environment that causes hydrolytic breakdown in leather, while ozone, sulfur dioxide, nitrogen oxides and peroxides are key causes of oxidative breakdown processes. Visible light and ultraviolet (UV) radiation, which can create high-energy free radicals, are also significant factors in initiating oxidation processes. Heat, moisture and the presence of acids or acidic gases also play roles in increasing the rates of both oxidative or hydrolytic breakdown, as well as the presence of metal ions, in particular iron or copper ions (Florian 2006). Sources of metal ion contaminants include iron-based black dyes or mordants, iron-gall ink, some pigments (such as verdigris), blood or rust stains, dust, soiling, metal attachments, or machining and processing impurities.

“Red rot” is a term that refers to the condition whereby vegetable-tanned leather has undergone severe chemical degradation (and not rot, per se, which implies a biological origin) due to inherent tanning compounds as well as atmospheric pollutants [usually sulfur dioxide (SO₂)]. Red-rotted leather (Figure 30) is more acidic (typical pH in the range of 3.0-3.5). It is often damaged and worn and has a powdery surface, and physically, it is quite weak and vulnerable to abrasion damage or tearing, especially if improperly supported during handling, storage or display. In the more advanced states, it is noticeably red.

In the 1930s, when coal was more widely used than today and was the main cause of SO₂ pollution in the atmosphere, it was observed in the United Kingdom that leather bookbindings in urban locations were suffering more from red rot than those in cleaner rural settings (Brimblecombe 1996). In Canada, SO₂ levels were high in industrialized areas of the country from the start of industrialization up to the 1970s when acid-rain legislations greatly reduced SO₂ levels in the air. Many examples of historic vegetable-tanned leather have suffered from SO₂ deterioration over time and now are weak and degraded.

Degradation due to metal parts

Stains and chemical weakening of leather, skins and fur-skins can be caused by their oils or by leather dressing applications in intimate contact with metal parts, usually iron or copper or their alloys (Figures 31a and b). The oils attack the metals, and the resulting rust or corrosion products in turn can stain and attack or weaken the skin and leather around it. In the case of copper and its alloys, these corrosion products are typically waxy and green (copper stearate).

Recommendations

• Protect from dust, grime and pollutants using display cases, dust covers, etc. (Figure 32).
• Wear gloves and avoid direct handling to avoid transferring oils or grime from the hands. In particular, take care with parchment, alum-tawed skins and semi-tanned skins as they are easily soiled.
• Measure pollutants levels, assess risks and, if necessary, reduce airborne pollutants.
• Identify all sensitive leather and skin objects, including items suffering from red rot. Provide enhanced pollution protection for these items. For storage, this can include wrapping red rot–affected leather objects in acid-free tissue paper, padded out as required and enclosing them in acid-free, alkaline-buffered boxes.
• Avoid any contact between metals (e.g. shelving, tags, clips) and skins and leathers. Where metal is part of the object and the metal is in contact with the skin or leather, insert a Melinex interleaf or other barrier, if possible. If the metal is actively corroding, consult a conservator for further advice.

For more preservation strategies to prevent damage by dust and pollutants, consult Agent of deterioration: pollutants and Technical Bulletin 32 Products Used in Preventive Conservation.

Recommendations

• Eliminate UV radiation from light sources.
• Protect coloured materials from light, including the smoky colour of smoked-tanned skins.
• Identify light sensitivity levels for objects in the collection, including decorative details.
• For the most light-sensitive objects, adopt strategies to minimize light exposure (e.g. temporary displays, limiting length of time on display, rotation, timers and motion detectors for lights). In storage, keep them in the dark, for example, in opaque containers. Keep some representative objects out of the rotation so that they remain pristine indefinitely, for future research and to make facsimiles.

For more preservation strategies to prevent damage by visible light and ultraviolet radiation, consult Agent of deterioration: light, ultraviolet and infrared.

Recommendations

• Assess the different sensitivity levels of your skin, leather and fur collection to the various RH risks, including the sensitivity to various ranges of RH fluctuations, and choose appropriate levels of control (consult Table 2 in Agents of deterioration: incorrect relative humidity).
• Avoid large RH deviations with respect to normal stable conditions. Monitor to detect these occurrences, so as to eliminate if possible.
• Keep the most susceptible objects in environmentally stable areas of the building.
• In storage, wrap or cover objects with humidity-buffering materials such as cotton or acid-free (non-buffered) tissue. For furs and other objects susceptible to pest infestations, full wrapping need not be provided if the priority is to have the object sealed within a transparent plastic bag so as to allow easy visual inspection. Place wrapped objects in boxes, enclosures or cabinets to further help slow down and buffer changes in RH. The more layers and enclosures, the better the protection.
• Use well-sealed display cases (Figure 36b).
• Take extra precautions in handling in dry environments (below 30% RH) because loss of moisture may lead to loss of flexibility and risk of physical damage.
• Provide good air circulation.
• Avoid damp conditions due to the high risk of mould growth. Monitor with alarms for RH above 65%, and prepare an action plan to respond to such circumstances, such as trained personnel and equipment to increase ventilation or to dehumidify.

For more preservation strategies to prevent damage caused by RH, consult Agent of deterioration: incorrect relative humidity.

Cold storage

The PWNHC has practised the cold storage method for a collection of fur garments for many years following specific parameters (fur vault temperature of 4°C, RH 50–60%, good air circulation and back-up control system; Figure 39). The objects are stored wrapped in layers of buffering material and sealed in plastic bags, with minimal air entrapped that could cause condensation problems (Figure 40). Since the objects continue to be accessed, the environmental standard for the cold room’s temperature was recently changed from 4°C to 10°C to minimize the potential problems from condensation or acclimatization when the materials are removed from the cold to the regular (room temperature) storage unit.

Objects need to be bagged before they are moved from cold storage to room temperature so that moisture does not condense on the cool surface of the object. Objects can be bagged before going into cold storage, but there is a potential for condensation forming on the inside surface of the bag. The amount of moisture will depend on the RH of the environment the object was bagged in and the change in temperature. The objects are wrapped with cloth or tissue paper to absorb this moisture. Bagged objects need to warm to room temperature before being opened.

Anoxic storage

Anoxic storage (Figures 41 and 42) consists of depleting the oxygen locally around the object to such an extent that insects cannot survive (because they need to breathe). PWNHC’s assessment of the anoxic storage alternative was based on the guidelines provided by Burke (1996). Specialized laminate barrier film (e.g. Escal) is suggested as a transparent, long-term application barrier film. Bagging would be adjusted for 20% decrease in volume since the most feasible (e.g. cost-effective, non-labour-intensive) protocol for PWNHC is to use air without diluting it with nitrogen.

Conclusion

The above considerations led to the decision to store the Nunavut collection using anoxic storage.