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.
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, such as tree bark prepared in bark mills, 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 automobiles and shoes for infants.
- 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.
- 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.
- 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.
- Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types (syntans, contraction for synthetic tannins). 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.
- 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; it's 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 and for making many varieties of dog chews.
Leather—usually vegetable-tanned—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.
In general, leather is sold in four forms:
- Full-grain leather refers to the leather which has not had the upper "top grain" and "split" layers separated. 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) to remove imperfections (or natural marks) on the surface of the hide. The grain remains allowing the fiber strength and durability. The grain also has breathability, resulting in less moisture from prolonged contact. Rather than wearing out, it will develop a patina over time. High quality leather furniture and footwear are often made from full-grain leather. Full-grain leathers are typically available in two finish types: aniline and semi-aniline.
- Top-grain leather is the second-highest quality. It's had the "split" layer separated away, making it thinner and more pliable than full grain. Its surface has been sanded and a finish coat added to the surface which results in a colder, plastic feel with less breathability, and will not develop a natural patina. It is typically less expensive and has greater resistance to stains than full-grain leather, so long as the finish remains unbroken.
- Corrected-grain leather is any leather that has had an artificial grain applied to its surface. The hides used to create corrected leather do not meet the standards for use in creating vegetable-tanned or aniline leather. The imperfections are corrected or sanded off and an artificial grain impressed into the surface and dressed with stain or dyes. 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 top 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. 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 considered to be a true form of suede.
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.
- Vachetta leather is used in the trimmings of luggage and handbags. 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 a tough leather, possibly due to the animal's adaptations to its thorny and thicket-filled habitats. Deerskin has been used by many societies, including indigenous Americans. Most modern deerskin 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, martial arts equipment such as kendo and bogu, as well as personal accessories like handbags and wallets.
- 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 types 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 found on the surface of briefcases, portfolios, and wallets, and can be identified by its thick, firm feel and smooth finish. Belting leather is generally a heavy-weight of full-grain, vegetable-tanned leather.
- Napa leather is chrome-tanned and is soft and supple. It is commonly found in 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 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 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. Bonded leather upholstery is a vinyl upholstery that contains about 17% leather fiber in its backing material. 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.
Leather from other animals
Today most leather is made of cattle skin but many exceptions exist. Lamb and deerskin are used for soft leather in more expensive apparel. Deer and elkskin are widely used in work gloves and indoor shoes. Pigskin is used in apparel and on seats of saddles. Buffalo, goats, alligators, dogs, 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 bullwhips. Kangaroo leather is favored by some motorcyclists for use in motorcycle leathers specifically because of its light weight and abrasion resistance. Kangaroo leather is also used for falconry jesses and soccer footwear.
Although originally raised for their feathers in the 19th century, ostriches are now more popular for both meat and leather. There are different processes to produce different finishes for many applications, i.e., upholstery, footwear, automotive products, accessories, and clothing. Ostrich leather 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. Sting ray leather is tough and durable. 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. Sting ray leather is also used as grips on Chinese swords and Japanese katanas.
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. Many different tanning methods and materials can be used; the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is 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 2.8 and 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 an 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 and 4.2.
Crusting is the process by which the hide/skin is thinned, retanned, and lubricated. Often a coloring operation is included in the crusting subprocess. The chemicals added during crusting must be fixed in place. The culmination of the crusting subprocess 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.
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).
- Leather biodegrades slowly; it takes 25–40 years to decompose.
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of 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.
Tanning is especially polluting in countries where environmental regulations are lax, such as in India, the world's third-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. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization 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. In 2003 for instance, the main tanneries' effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open. Scientists at the Central Leather Research Institute in India have developed biological methods for pretanning as well as better chromium management.
The higher cost associated to the treatment of effluents that to untreated effluent discharging leads to illegal dumping to save on costs. For instance, in Croatia in 2001, proper pollution abatement cost 70-100 USD/t of raw hides processed against 43 USD/t for irresponsible behavior.
No general study seems to exist but the current news is rife with documented examples.[clarification needed] 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.
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.
Lipases are used in the degreasing operation to hydrolyze fat particles embedded in the skin.
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.
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.
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 and will 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. Meanwhile, scientific experiments have shown no substantial benefits. The main authorities on the subject therefore discourage it, with a caveat for special cases done under the direction of a conservator.
Working with leather
Leather can be decorated by a variety of methods, including pyrography and beading.
Cordwain, "Cordovan" or "Spanish leather"
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. Leather was even more protection against draughts and dampness than tapestry and 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, and Ghent), the major handicraft center for this cordwain was Mechelen, where it was mentioned as early as 1504.
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, 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
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. Top-quality motorcycle leather is superior to any practical man-made fabric for abrasion protection and is still used in racing. Many sports still use leather to help in playing the game or protecting players; its flexibility allows it to be formed and flexed.
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 Napa leather, found on luxury car brands like Mercedes-Benz, BMW, and Audi.
Leather is used exclusively by publishers like The Easton Press to bind books, for both practical and aesthetic purposes.
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 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.
Jainism prohibits the use of leather since it is obtained by killing animals.
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 to be 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.
Types of leather
- Aniline leather, a leather treated with aniline as a dye
- Artificial leather, a fabric of finish intended to substitute for leather
- Bicast leather, a synthetic upholstery product
- Boiled leather, a historical construction material
- Bonded Leather, man-made material composed of leather fibers
- Chamois leather, leather made from the skin of the mountain antelope or Chamois
- Composition leather, man-made leather made from recycled leather offcuts, trimmings, or shavings
- Corinthian leather, a marketing term used by Chrysler in the 1970s
- Crocodile leather, leather from a crocodile
- Morocco leather, a type of goatskin dyed red
- Napa leather, a full-grain leather
- Ostrich leather, leather from an ostrich
- Patent leather, leather with a high gloss and shiny finish
- Pleather, a term for artificial leather
- Poromeric imitation leather, a group of synthetic leather substitutes
- Vegan leather, an artificial alternative to traditional leather
- British Museum leather dressing, a conservator's treatment for display items
- Leather carving, a process of cutting and stamping to give a three-dimensional appearance
- Leather crafting, the practice of making leather into crafts or pieces of art
- Liming (leather processing), a process of treating leather
- Adarga, a hard leather shield
- Henry Burk – inventor of the alum and sumac tanning process
- Horse tack, various equipment and accessories worn by horses, much of which is made of leather
- Leather skirt
- Leather subculture
- Mink oil, leather treatment
- Neatsfoot oil, leather treatment
- Saddle soap, leather cleaning and conditioning
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