People use leather to make various goods—including clothing (e.g., shoes, hats, jackets, skirts, trousers, and belts), bookbinding, leather wallpaper, and as a furniture covering. It is produced in a wide variety of types and styles, decorated by a wide range of techniques.
Several tanning processes transform hides and skins into leather:
- Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other chromium salts. 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 exotic colors are possible when using chrome tanning. The chrome tanning method usually only takes a day to finish, and the ease and agility of this method make it a popular choice. It is reported that chrome-tanned leather adds up to 80% of the global leather supply.
- Vegetable-tanned leather is tanned using tannins and other ingredients found in different vegetable matter, such as tree bark prepared in bark mills, wood, leaves, fruits, and roots. 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, so if left to soak and then dried it shrinks and becomes harder. In hot water, it shrinks drastically and partly congeals—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 armour after hardening, and it has also been used for book binding.
- 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.
- Formaldehyde tanning (being phased out due to danger to workers and sensitivity of many people to formaldehyde) is another aldehyde tanning method. Brain-tanned leathers fall into this category, and are exceptionally water absorbent.
- Brain tanned leathers are made by a labor-intensive process that uses emulsified oils, often those of animal brains such as deer, cattle, and buffaloes. They are known for their exceptional softness and washability.
- Chamois leather also falls into the category of aldehyde tanning, and like brain tanning, produces a porous and highly water-absorbent leather. Chamois leather is made using marine oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to color it.
- Rose-tanned leather is a variation of vegetable oil tanning and brain tanning, where pure rose otto replaces the vegetable oil and emulsified oils. Rose-tanned leather tanned leaves a powerful rose fragrance even years from when it is manufactured. It has been called the most valuable leather on earth, but this is mostly due to the high cost of rose otto and its labor-intensive tanning process.
- 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 before people realized the hazards that formaldehyde presents to tanners and consumers.
- Alum-tanned leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Alum-tanned leather is technically not tanned, as tannic acid is not used, and the resulting material reverts to rawhide if soaked in water long enough to remove the alum salts.
- Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tanning, 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 is primarily found in uses such as drum heads and parchment 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 currying process after tanning supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Russia leather was an important international trade good for centuries. 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 these four forms:
- Full-grain leather 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 develops a patina during its expected useful lifetime. High quality leather furniture and footwear are often made from full-grain leather. Full-grain leathers are typically available in two finish types: aniline, semi-aniline.
- Top-grain leather (the most common type in high-end leather products) is the second-highest quality. It has 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, which produces a colder, plastic feel with less breathability, and it does not develop a natural patina. It is typically less expensive and has greater stain resistance than full-grain leather if 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 embossed 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 a true 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 returning to a rawhide state, if wetted. It is easier to soften, and helps repel leather-eating bugs.
- Patent leather is leather that has been given a high-gloss finish. Inventor Seth Boyden developed the original process in Newark, New Jersey in 1818. Patent leather usually has a plastic coating.
- Fish leather is popular for its motifs and its pigmentation. Mainly used for making shoes and bags, the fish skin is tanned like other animal skins. The species used include salmon, perch, sturgeon, etc.
- Salmon : farmed in Iceland and Norway, salmon skin has fine scales. Its strength and elegant look make it the most popular fish leather.
- Perch : from the Nile, its skin is recognizable with large, round and soft scales
- Wolffish : smooth, without scales, with dark spots, and stripes due to the friction of marine rocks
- Cod : finer scales than salmon, but more varied texture, sometimes smooth and sometimes rough
- Sturgeon : known for its eggs (caviar), its leather is quite expensive
- Eel : without scales, its skin is shiny
- Tilapia : originally from Africa and farmed in many places, tilapia leather is beautiful, with resistant qualities similar to salmon and perch
- 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. It is known as the most difficult leather to work due to dished scales of the animal, and it is one of the most expensive leathers.
- Shark is covered with small, close-set tubercles, making it very tough. Shark skin handbags were once in vogue, but interest has fallen as the material and production costs is very high. Moreover, this skin is more difficult to work. (Do not confuse with sharkskin, a woven textile product.
- 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 makes the natural leather darken in shade (develop a patina).
- Slink is leather made from the skin of unborn calves. It is particularly soft and is valued for making gloves.
- Deerskin is a tough, water-resistant 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 bogu, as well as personal accessories such as handbags and wallets.
- Goatskin is soft but tough, and is used for items such as thorn-resistant gardener's gloves.
- 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.
- Russia leather is a particular form of bark-tanned cow leather. It is distinguished by an oiling step, after tanning, where birch oil is worked into the leather to make it particularly hard-wearing, flexible and resistant to water.
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 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" organic leathers, but are materials that contain leather fiber. Depending on jurisdiction, they may still be labeled as "Genuine Leather", even though the consumer generally can only see the outer layer of the material and can't actually see any of the leather content:
- Bonded leather or reconstituted leather is an economical material that uses leftover organic leather (from tanneries or workshops) that are shredded and bonded together with polyurethane or latex on to a fiber sheet. The varying degree of organic leather in the mix (10% to 90%) affects the smell and texture. Its reduced cost makes it popular for furniture upholstery, especially for commercial furniture that requires durability—though durability can vary widely depending on the formulation.
- Bycast leather is a split leather with a layer of polyurethane laminated to the surface and then embossed. Bycast was originally made for the shoe industry, and later adopted by the furniture industry. The original formula created by Bayer was strong but expensive. The result is a material that is slightly stiffer but cheaper than top-grain leather but has a much more consistent texture. Because its surface is completely covered in plastic, is easier to clean and maintain, but is not easily repaired.
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, goat, alligator, snake, ostrich, kangaroo, ox, and yak skins may also be used for leather.
Kangaroo leather is used to make items that must be strong but flexible. It is the material most commonly used in bullwhips. Some motorcyclists favor kangaroo leather for motorcycle leathers because of its light weight and abrasion resistance. Kangaroo leather is also used for falconry jesses, soccer footwear, and boxing speed bags. At different times in history, leather made from more exotic skins has been considered desirable. For this reason, certain species of snakes and crocodiles have been hunted.
Although originally raised for their feathers in the 19th century, ostriches are now more popular for both meat and leather. Different processes 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 where the feathers grew.
In Thailand, stingray 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. Stingray rawhide is also used as grips on Chinese swords, Scottish basket hilted swords, and Japanese katanas. Stingray leather is also used for high abrasion areas in motorcycle racing leathers (especially in gloves, where its high abrasion resistance helps prevent wear through in the event of an accident.)
The leather manufacturing process is divided into three fundamental subprocesses: preparatory stages, tanning, and crusting. All true leathers undergo these subprocesses. A further subprocess, 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 a process that stabilizes the protein of the raw hide or skin so it does not putrefy, making it suitable for a wide variety of end applications. The principal difference between raw and tanned hides is that raw hides dry out to form a hard, inflexible material that, when rewetted (or wetted-back) putrefy, while tanned material dries to a flexible form that does not become putrid when wetted-back.
Many tanning methods and materials exist. The choice ultimately depends on the end application for the leather. The most common tanning material is chromium, which leaves the tanned leather a pale blue color (due to the chromium). This product is commonly called wet blue. The hides, when finished pickling, are typically between pH 2.8 and 3.2. At this point, tannery workers load the hides into a drum and immerse them in a float that contains the tanning liquor. The hides soak while the drum slowly rotates about its axis, and the tanning liquor slowly penetrates through the full thickness of the hide. Workers periodically cut a cross-section of a hide and observe the degree of penetration. Once the process achieves even penetration, workers slowly raise the float's pH in a process called basification, which fixes the tanning material to the leather—and the more tanning material fixed, the higher the leather's hydrothermal stability and shrinkage temperature resistance. Chrome-tanned leather pH is typically between pH 3.8 and 4.2.
Crusting is a process that thins, retans, and lubricates leather. It often includes a coloring operation. Chemicals added during crusting must be fixed in place. Crusting culminates with a drying and softening operation, and may include:
For some leathers, workers apply a surface coating. Tanners call this finishing. Finishing operations can include:
- Roller coating
- Curtain coating
- Ironing, ironing-combing (for hair-on)
Leather produces some environmental impact, most notably due to:
- The carbon footprint of cattle rearing
- Use of chemicals in the tanning process (e.g., chromium, formic acid, mercury and solvents)
- Air pollution due to the transformation process (hydrogen sulfide during dehairing and ammonia during deliming, solvent vapors)
One estimate of the carbon footprint of leather goods is 0.51 kg of CO2 equivalent per £1 of output at 2010 retail prices, or 0.71 kg CO2eq per £1 of output at 2010 industry prices.
One ton of hide or skin generally produces 20 to 80 m3 of waste water, including chromium levels of 100–400 mg/l, sulfide levels of 200–800 mg/l, high levels of fat and other solid wastes, and notable pathogen contamination. Producers often add pesticides to protect hides during transport. With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process represents a considerable strain on water treatment installations.
Chemicals used in tanning
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, "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"—with 10,000 tanneries as of 2011 and a city of three million on the banks of the Ganges—pollution levels were so high, that despite an industry crisis, the pollution control board decided to shut down 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.
In the Hazaribagh neighborhood of Dhaka in Bangladesh, chemicals from tanneries end up in Dhaka's main river. Besides the environmental damage, the health of both local factory workers and the end consumer is also negatively affected. Besides local sales of products made with leather from the Hazaribagh neighborhood of Dhaka, the leather is also bought by huge Western companies and sold in the developed world.
The higher cost associated with the treatment of effluents than to untreated effluent discharging leads to illegal dumping to save on costs. For instance, in Croatia in 2001, proper pollution abatement cost US$70–100 per ton of raw hides processed against $43/t for irresponsible behavior.
No general study seems to exist, but recent media reports are rife with examples. In November 2009, for example, one of Uganda's main leather companies was caught directly dumping waste water into a wetland adjacent to Lake Victoria.
Role of enzymes
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 must not damage or dissolve collagen or keratin, but should hydrolyze casein, elastin, albumin, globulin-like proteins, and nonstructural proteins that are not essential for leather making. This process is called bating.
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
The natural fibers of leather 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. Although it is chemically irreversible, treatments can add handling strength and prevent disintegration of red rotted leather.
Exposure to long periods of low relative humidities (below 40%) can cause leather to become desiccated, irreversibly changing the fibrous structure of the leather. Chemical damage can also occur from exposure to environmental factors, including ultraviolet light, ozone, acid from sulfurous and nitrous pollutants in the air, or through a chemical action following any treatment with tallow or oil compounds. Both oxidation and chemical damage occur faster at higher temperatures.
Leather 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 insect 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 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, when done incorrectly, multiple disadvantages exist, and when done correctly, little to no preservation advantage occurs. Pamphlets and guidelines give numerous downsides to dressings use, including: the dressing becoming increasingly acidic and discolors and stains the leather, oxidizes (penetration and expansion of oils including displacement and weakening of fibers) and stiffens, leave a sticky surface, collects dust, wicks into adjacent materials, forms unstable surface spews, encourages biological deterioration and mold growth, blocks surface porosity, impedes further treatment, wets and swells the leather, affects surface finishes, and desiccates 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. Leather can be decorated by a variety of methods, including pyrography and beading.
Cordwain and Cuir de Cordoue
Cordwain, once a synonym of cordovan (through Old French cordewan) meaning "from Córdoba" describes a certain kind of fine leather, originally from Córdoba. Cordwainer is still used to describe someone in the profession of shoemaking.
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. Many sports still use leather equipment. Its flexibility facilitates forming and shaping it into balls and protective gear.
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.
Many cars and trucks come with optional or standard "leather" seating. These days most car manufacturers due to consideration of durability and cost use synthetic PU leather, including luxury car brands like Mercedes-Benz, BMW, and Audi.
In religiously diverse countries, leather vendors typically clarify the kinds of leather in their products. For example, leather shoes bear a label that identifies the animal from which the leather came. This helps a Muslim not accidentally purchase pigskin, and a Hindu avoid cattle. Many vegetarian Hindus do not use any kind of leather. Such taboos increase the demand for religiously neutral leathers such as ostrich and deer.
Jainism prohibits the use of leather, since it is obtained by killing animals.
Some vegetarians, vegans and animal rights activists and groups such as PETA, boycott and promote the boycotting of all leather products, arguing that the use of leather is unjustifiable. They encourage the use of alternative materials such as synthetic leathers.
Many pseudo-leather materials have been developed. Some published claims assert that certain versions of artificial leather are stronger than real leather when manufactured with strength in mind. Ranges of synthetic polymeric materials provide features rivalling or exceeding those of various types of leather in particular applications; they include vegan microfiber, pleather and Naugahyde.
- Aniline leather, a leather treated with aniline as a dye
- Boiled leather, a historical construction material for armour
- 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, traditionally this is goatskin leather prepared for softness and decorative finish
- Ostrich leather, leather from an ostrich
- Patent leather, leather with a high gloss and shiny finish
- Artificial leather, a fabric of finish intended to substitute for leather
- Bicast leather, a synthetic upholstery product
- Kirza, developed in the Soviet Union
- 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 subculture
- Mink oil, leather treatment
- Neatsfoot oil, leather treatment
- Saddle soap, leather cleaning and conditioning
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