Leather
For other uses, see Leather (disambiguation).
Modern leather-working tools
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.
The leather and the fur industries are differentiated by the manufacturing importance of the raw materials used to make the wares. In the leather industry, the skin and rawhide are by-products of the meat industry, because the meat has greater commercial value than the rawhide and skin. In the fur industry, the meat is a by-product, because the skins and hides have greater commercial value. Moreover, in taxidermy, the raw materials usually are only the animal’s head and back; hide and skin also are the raw materials for manufacturing animal glue and gelatin.
Forms of leather
Several tanning processes transform hides and skins into leather:
• Vegetable-tanned leather is tanned using tannin and other ingredients found in vegetable matter, tree bark, and other such sources. It is supple and brown in color, with the exact shade depending on the mix of chemicals and the color of the skin. It is the only form of leather suitable for use in leather carving or stamping. Vegetable-tanned leather is not stable in water; it tends to discolor, and if left to soak and then dry it will shrink and become less supple and harder. In hot water, it will shrink drastically and partly gelatinize, becoming rigid and eventually brittle. Boiled leather is an example of this where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armor after hardening, and it has also been used for book binding.
• Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other salts of chromium. It is more supple and pliable than vegetable-tanned leather, and does not discolor or lose shape as drastically in water as vegetable-tanned. It is also known as wet-blue for its color derived from the chromium. More esoteric colors are possible using chrome tanning.
• Aldehyde-tanned leather is tanned using glutaraldehyde or oxazolidine compounds. This is the leather that most tanners refer to as wet-white leather due to its pale cream or white color. It is the main type of "chrome-free" leather, often seen in shoes for infants, and automobiles.
o Formaldehyde tanning (being phased out due to its danger to workers and the sensitivity of many people to formaldehyde) is another method of aldehyde tanning. Brain-tanned leathers fall into this category and are exceptionally water absorbent.
? Brain tanned leathers are made by a labor-intensive process which uses emulsified oils, often those of animal brains. They are known for their exceptional softness and their ability to be washed.
o Chamois leather also falls into the category of aldehyde tanning and like brain tanning produces a highly water absorbent leather. Chamois leather is made by using oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to make the fabric the color it is.
• Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types. This leather is white in color and was invented when vegetable tannins were in short supply during the Second World War. Melamine and other amino-functional resins fall into this category as well and they provide the filling that modern leathers often require. Urea-formaldehyde resins were also used in this tanning method until dissatisfaction about the formation of free formaldehyde was realized.
• Alum-tawed leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Purists argue that alum-tawed leather is technically not tanned, as the resulting material will rot in water. Very light shades of leather are possible using this process, but the resulting material is not as supple as vegetable-tanned leather.[1]
• Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tawing, rawhide is not technically "leather", but is usually lumped in with the other forms. Rawhide is stiffer and more brittle than other forms of leather, and is primarily found in uses such as drum heads where it does not need to flex significantly; it is also cut up into cords for use in lacing or stitching, or for making many varieties of dog chews.
Leather—usually vegetable-tanned leather—can be oiled to improve its water resistance. This supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Frequent oiling of leather, with mink oil, neatsfoot oil or a similar material, keeps it supple and improves its lifespan dramatically.
Leather with the hair still attached is called hair-on.
Leather types
In general, leather is sold in four forms:
• Full-grain leather refers to the upper section of a hide that previously contained the epidermis and hair, but were removed from the hide/skin. Full-grain refers to hides that have not been sanded, buffed, or snuffed (as opposed to top-grain or corrected leather) in order to remove imperfections (or natural marks) on the surface of the hide. The grain remains in its natural state allowing the best fiber strength and durability. The natural grain also has natural breathability, resulting in less moisture from prolonged contact. Rather than wearing out, it will develop a natural patina over time, with some cracking and splitting. The finest leather furniture and footwear are made from full-grain leather. For these reasons, only the best raw hide is used to create full-grain leather. One way to test if leather is full-grain is to lightly scratch its surface with your nail. If it leaves a lighter-colored streak, it's full-grain. Full-grain leathers are typically available in two finish types: aniline and semi-aniline.
• Top-grain leather is a misnomer: it gives the false impression that it is "top" quality. In fact, full-grain is the highest quality. Top-grain leather is the second-highest quality. Its surface has been sanded and refinished. As a result, it has a colder, plastic feel, less breathability, and will not develop a natural patina. However, it does have 2 advantages over full-grain leather: it is typically less expensive, and has greater resistance to stains.
• Corrected-grain leather is any leather that has had an artificial grain applied to its surface. The hides used to create corrected leather are of inferior quality that do not meet the high standards for use in creating vegetable-tanned or aniline leather. The imperfections are corrected and an artificial grain applied. Most corrected-grain leather is used to make pigmented leather as the solid pigment helps hide the corrections or imperfections. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.
• Split leather is leather created from the fibrous part of the hide left once the top-grain of the rawhide has been separated from the hide. During the splitting operation, the grain and drop split are separated. The drop split can be further split (thickness allowing) into a middle split and a flesh split. In very thick hides, the middle split can be separated into multiple layers until the thickness prevents further splitting. Split leather then has an artificial layer applied to the surface of the split and is embossed with a leather grain (Bycast leather). Splits are also used to create suede. The strongest suedes are usually made from grain splits (that have the grain completely removed) or from the flesh split that has been shaved to the correct thickness. Suede is "fuzzy" on both sides. Manufacturers use a variety of techniques to make suede from full-grain. For example, in one operation, leather finish is applied to one side of the suede, which is then pressed through rollers; these flatten and even out one side of the material, giving it the smooth appearance of full-grain. Latigo is one of the trade names for this product. A reversed suede is a grained leather that has been designed into the leather article with the grain facing away from the visible surface. It is not a true form of suede.
The International Union of Leather Technologists and Chemist Societies has a glossary of leather terms that can be found at IULTCS.
Other less-common leathers include:
• Buckskin or brained leather is a tanning process that uses animal brains or other fatty materials to alter the leather. The resulting supple, suede-like hide is usually smoked heavily to prevent it from rotting.
• Patent leather is leather that has been given a high-gloss finish. The original process was developed in Newark, New Jersey, by inventor Seth Boyden in 1818. Patent leather usually has a plastic coating.
• Shagreen is also known as stingray skin/leather. Applications used in furniture production date as far back as the art deco period. The word "shagreen" originates from France and is commonly confused with a shark skin and stingray skin combination.
• Vachetta leather is used in the trimmings of luggage and handbags, popularized by Louis Vuitton. The leather is left untreated and is therefore susceptible to water and stains. Sunlight will cause the natural leather to darken in shade, called a patina.
• Slink is leather made from the skin of unborn calves. It is particularly soft, and is valued for use in making gloves.
• Deerskin is one of the toughest leathers, partially due to adaptations to their thorny and thicket filled habitats. Deerskin has been prized in many societies including indigenous Americans. Most modern deer skin is no longer procured from the wild, with deer farms breeding the animals specifically for the purpose of their skins. Large quantities are still tanned from wild deer hides in historic tanning towns such as Gloversville and Johnstown in upstate New York. Deerskin is used in jackets and overcoats, professional sporting equipment for martial arts such as kendo and bogu, as well as high-quality personal accessories like handbags and wallets. It commands a high price due to its relative rarity and proven durability.
• Nubuck is top-grain cattle hide leather that has been sanded or buffed on the grain side, or outside, to give a slight nap of short protein fibers, producing a velvet-like surface.
There are two other descriptions of leather commonly used in specialty products, such as briefcases, wallets, and luggage.
• Belting leather is a full-grain leather that was originally used in driving pulley belts and other machinery. It is often found on the surface of briefcases, portfolios, and wallets, and can be identified by its thick, firm feel and smooth finish. Belting leather is the only kind of leather used in luxury products that can retain its shape without the need for a separate frame; it is generally a heavy-weight of full-grain, vegetable-tanned leather.
• Nappa leather, or Napa leather, is chrome-tanned and is extremely soft and supple and is commonly found in higher quality wallets, toiletry kits, and other personal leather goods.
The following are not 'true' leathers, but contain leather material. Depending on jurisdiction, they may still be labeled as "Genuine Leather."
• Bonded leather , or "reconstituted Leather", is not really a true leather but a man-made material. Some types of bonded leather are composed of 90% to 100% leather fibers (often scrap from leather tanneries or leather workshops) bonded together with latex binders to create a look and feel similar to that of 'true' leather at a fraction of the cost. This bonded leather is not as durable as other leathers, and is recommended for use only if the product will be used infrequently. An example for the use of this type of bonded leather is in Bible covers. The term "bonded leather" when used to describe upholstered furniture is a different product and construction. Bonded leather upholstery is a vinyl upholstery that contains about 17% leather fiber in its backing material. Hence, there is no leather in the surface of this product. The vinyl is stamped to give it a leather-like texture.Bonded leather upholstery is durable and its manufacturing process is more environmentally-friendly than leather production.
• Bycast leather is a split leather with a layer of polyurethane applied to the surface and then embossed. Bycast was originally made for the shoe industry and recently was adopted by the furniture industry. The original formula created by Bayer was strong, but expensive. Most of the Bycast used today is very strong and durable product. The result is a slightly stiffer product that is cheaper than top grain leather but has a much more consistent texture and is easier to clean and maintain.
The vast majority of leather is sold according to its area. The leather is placed through pin-wheel or electronic measuring machines and its surface area is determined. The unit of measurement is square meter, square decimeter or square foot. The thickness is also important, and this is measured using a thickness gauge (the unit of measurement is millimeters, e.g., 1.8 mm is a standard thickness for a school shoe).
In some parts of the world, top-grain thicknesses are described using weight units of ounces. Although the statement is in ounces only, it is an abbreviation of ounces per square foot. The thickness value can be obtained by the conversion: 1 oz/ft² = 1/64 inch (0.4 mm).
Hence, leather described as 7 to 8 oz is 7/64 to 8/64 inches (2.8 to 3.2 mm) thick. The weight is usually given as a range because the inherent variability of the material makes ensuring a precise thickness very difficult. Other leather manufacturers state the thickness directly in millimeters.
Leather from other animals
Tanned leather in Marrakech
Today, most leather is made of cattle skin, but many exceptions exist. Lamb and deer skin are used for soft leather in more expensive apparels. Deer and elk skin are widely used in work gloves and indoor shoes. Pigskin is used in apparel and on seats of saddles. Buffalo, goats, alligators, snakes, ostriches, kangaroos, oxen, and yaks may also be used for leather.
Kangaroo skin is used to make items which need to be strong but flexible — it is the material most commonly used in high quality bullwhips. Kangaroo leather is favored by some motorcyclists for use in motorcycle leathers specifically because of its lighter weight and higher abrasion resistance compared with cowhide, thus providing greater protection in case of a fall on the roadway.[citation needed] Kangaroo leather is also used for high performance soccer footwear.[citation needed]
Leather made from more exotic skins has at different times in history been considered very beautiful. For this reason certain snakes and crocodiles have been hunted to near extinction.[citation needed]
In the 1970s, ostrich farming for their feathers became popular, and ostrich leather became available as a side product. There are different processes to produce different finishes for many applications, i.e., upholstery, footwear, automotive products, accessories and clothing. Ostrich leather is considered one of the finest and most durable in the world and is currently used by many major fashion houses such as Hermès, Prada, Gucci, and Louis Vuitton. Ostrich leather has a characteristic "goose bump" look because of the large follicles from which the feathers grew.
In Thailand, sting ray leather is used in wallets and belts in the same way as regular bovine leather. Sting ray leather is as tough and durable as hard plastic. The leather is often dyed black and covered with tiny round bumps in the natural pattern of the back ridge of an animal. These bumps are then usually dyed white to highlight the decoration. Leather clothing is also popular in Thailand. Sting ray leather is also used as grips on Japanese katana.
Leather production processes
Barrel for leather tanning, Igualada Leather Museum, Spain
Leather tanning in Fes, Morocco
Main article: Leather production processes
The leather manufacturing process is divided into three fundamental sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating can be added into the leather process sequence but not all leathers receive surface treatment. Since many types of leather exist, it is difficult to create a list of operations that all leathers must undergo.
The preparatory stages are when the hide/skin is prepared for tanning. Preparatory stages may include: preservation, soaking, liming, unhairing, fleshing, splitting, reliming, deliming, bating, degreasing, frizing, bleaching, pickling and depickling.
Tanning is the process which converts the protein of the raw hide or skin into a stable material which will not putrefy and is suitable for a wide variety of end applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that when re-wetted (or wetted back) putrefy, while tanned material dries out to a flexible form that does not become putrid when wetted back. There is a large number of different tanning methods and materials that can be used, the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is the highly-polluting[citation needed] chromium, which leaves the leather once tanned a pale blue color (due to the chromium), this product is commonly called “wet blue”. The hides once they have finished pickling will typically be between pH of 2.8-3.2. At this point the hides would be loaded in a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be made to see the penetration by cutting the cross-section of a hide and observing the degree of penetration. Once a good even degree of penetration exists, the pH of the float is slowly raised in a process called basification. This basification process fixes the tanning material to the leather and the more tanning material fixed the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned would typically finish somewhere between 3.8-4.2.
Crusting is when the hide/skin is thinned, retanned and lubricated. Often, a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations: wetting back, sammying, splitting, shaving, rechroming, neutralization, retanning, dyeing, fatliquoring, filling, stuffing, stripping, whitening, fixating, setting, drying, conditioning, milling, staking, and buffing.
For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include: oiling, brushing, padding, impregnation, buffing, spraying, roller coating, curtain coating, polishing, plating, embossing, ironing, ironing/combing (for hair-on), glazing and tumbling.
Environmental impact
Leather is a product with high environmental impact, most notably due to:
• the impact of livestock
• the heavy use of polluting chemicals in the tanning process
• air pollution due to the transformation process (hydrogen sulfide during dehairing and ammonia during deliming, solvent vapors).
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of turbid and foul-smelling wastewater including chromium levels of 100-400 mg/L, sulfide levels of 200-800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport. With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.[2]
Tanning is especially polluting in countries where environmental norms are lax, such as in India - the world's 3rd largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg. VOC emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. Very clearly, the process remains highly polluting all the same. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization [3] posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that « Even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries. »
In Kanpur, the self-proclaimed "Leather City of World" and a city of 3 million people on the banks of the river Ganges, pollution levels were so high that despite an industry crisis, the pollution control board has decided to seal 49 high-polluting tanneries out of 404 in July 2009 [4]. In 2003 for instance, the main tanneries effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open [5].
The higher cost associated to the treatment of effluents that to untreated effluent discharging leads to environmental dumping to save on costs. For instance, in Croatia in 2001, proper pollution abatment cost 70-100 USD/t of raw hides processed against 43 USD/t for irresponsible behaviour.[6]
No general study seems to exist but the current news is rife with documented examples. In November 2009 for instance, it was discovered that one of Uganda's main leather producing companies directly dumped its waste water in a wetland adjacent to Lake Victoria. [7]
Role of enzymes in leather production
Enzymes like proteases, lipases and amylases have an important role in the soaking, dehairing, degreasing, and bating operations of leather manufacturing.
Proteases are the most commonly used enzymes in leather production. The enzyme used should not damage or dissolve collagen or keratin, but should be able to hydrolyze casein, elastin, albumin and globulin-like proteins, as well as non-structured proteins which are not essential for leather making. This process is called bating.[8]
Lipases are used in the degreasing operation to hydrolyze fat particles embedded in the skin[9]..
Amylases are used to soften skin, to bring out the grain, and to impart strength and flexibility to the skin. These enzymes are rarely used.
Preservation and conditioning of leather
The natural fibers of leather will break down with the passage of time. Acidic leathers are particularly vulnerable to red rot, which causes powdering of the surface and a change in consistency. Damage from red rot is aggravated by high temperatures and relative humidities, and is irreversible.
Exposure to long periods of low relative humidities (below 40%) can cause leather to become desiccated, irreversibly changing the fibrous structure of the leather.
Various treatments are available such as conditioners, but these are not recommended by conservators since they impregnate the structure of the leather artifact with active chemicals, are sticky, and attract stains.
Leather in book binding
Leather used in book binding has many of the same preservation needs: protection from high temperatures, high relative humidity, low relative humidity, fluctuations in relative humidity, light exposure, dust buildup, pollution, mold, and bug infestation.[10][11]
For books with red rot, acid-free phase boxes and/or polyester dust jackets (Dupont Mylar Type D or ICI Mellinex 516) are recommended to protect the leather from further handling damage and as well as to prevent the residues from getting on hands, clothes, the text block, and nearby books.[12]
The debate on the use of dressings for preservation of book bindings has spanned several decades as research and experimental evidence have slowly accumulated. The main argument is that, done incorrectly, there are multiple disadvantages and that, done correctly, there is little to no preservation advantage. Pamphlets and guidelines give numerous downsides to dressings use, including: the dressing becoming increasingly acidic, discolor and stain the leather, oxidize (penetration and expansion of oils including displacement and weakening of fibers) and stiffen, leave a sticky surface, collect dust, wick into adjacent materials, form unstable surface spews, encourage biological deterioration and mold growth, block surface porosity, impede further treatment, wet and swell the leather, affect surface finishes, and desiccate or dry out the leather.[13] Meanwhile, scientific experiments have shown no great benefits.[14] The main authorities on the subject therefore discourage it, with a caveat for special cases done under the direction of a conservator.[10][12][13]
Working with leather
Main article: Leather crafting
Leather can be decorated by a variety of methods, including pyrography and beading.
Cordwain, "Cordovan" or "Spanish leather"
Fragment of Cordwain
Cordwain, once a synonym of cordovan (through Old French cordewan) meaning "from Córdoba" describes painted or gilded embossed leather hangings manufactured in panels and assembled for covering walls as an alternative to tapestry. Such "Cordovan leathers" were a north African style that was introduced to Spain in the ninth century (hence it is sometimes referred to as 'Spanish leather'); in Spain such embossed leather hangings were known as guadamecí or guadamecil, from the Libyan town of Ghadames, while cordobanes signified soft goat leather.[15] Leather was even more proof against draughts and dampness than tapestry, and it was unaffected by insects. From the fourteenth century, the technique in which panels of wet leather were shaped over wooden moulds, painted, then oil-gilded and lacquered, reached Flanders and Brabant in the Low Countries. Though there were craftsmen in several cities (such as Antwerp, Brussels, Ghent), the major handicraft center for this cordwain was Mechelen, where it was mentioned as early as 1504.
Embossed gilded leather hangings in a Dutch interior, ca 1730, painted by Philip van Dijk (Mauritshuis)
Patterns for these panels followed fashions in silk damask, at some lag in time, since the high-relief wooden moulds were laborious to make. After the second half of the 18th century, this luxurious artisan product was no longer made,[16] its place taken in part by chintz hangings and printed wallpapers. Cordwainer is still used to describe someone in the profession of shoemaking.
Leather in modern culture
Handmade leather art created by J.C. Velasquez
Due to its excellent resistance to abrasion and wind, leather found a use in rugged occupations. The enduring image of a cowboy in leather chaps gave way to the leather-jacketed and leather-helmeted aviator. When motorcycles were invented, some riders took to wearing heavy leather jackets to protect from road rash and wind blast; some also wear chaps or full leather pants to protect the lower body. Many sports still use leather to help in playing the game or protecting players; its flexibility allows it to be formed and flexed.
The term leathering is sometimes used in the sense of a physical punishment (such as a severe spanking) applied with a leather whip, martinet, etc.
Leather fetishism is the name popularly used to describe a fetishistic attraction to people wearing leather, or in certain cases, to the garments themselves.
Many rock groups (particularly heavy metal and punk groups in the 1980s) are well-known for wearing leather clothing. Leather clothing, particularly jackets, are common in the heavy metal and Punk subculture. Extreme metal bands (especially black metal bands) and Goth rock groups have extensive leather clothing, i.e. leather pants, accessories, etc.
Many cars and trucks come with optional or standard 'leather' seating. This can range from cheap vinyl imitation leather, found on some low cost vehicles, to real Nappa leather, found on luxury car brands like Mercedes-Benz, BMW, and Audi.
Leather biodegrades slowly, and takes 25–40 years to decompose.
Religious sensitivities to leather
In religiously diverse countries, leather vendors are typically careful to clarify the kinds of leather used in their products. For example, leather shoes will bear a label identifying the animal from which the leather was taken. In this way, a Jew or a Muslim would not accidentally purchase pigskin leather, and a Hindu could avoid cow leather. Many Hindus who are vegetarians will not use any kind of leather.
Such taboos increase the demand for religiously neutral leathers like ostrich and deer.
Judaism forbids the comfort of wearing shoes made with leather on Yom Kippur, Tisha B'Av, and during mourning.[17]
Concern for animals and the environment and alternatives
Vegans and animal rights activists boycott the use of all items made from leather, believing the practice of wearing animal hides is unnecessary and cruel in today's society. Animal rights groups such as PETA have called for boycotts and encourage the use of alternative materials such as synthetic leathers.
Many pseudo-leather materials have been developed, allowing those who wish to wear leather-like garments to do so without actually wearing leather. One example of this is vegan microfiber, which claims to be stronger than leather when manufactured with strength in mind. Vinyl materials, pleather, Naugahyde, Durabuck, NuSuede, Hydrolite, and other alternatives exist, providing some features similar to leather.[citation needed]