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.
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Several tanning processes transform hides and skins into leather:
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:
Less-common leathers include:
There are two other types of leather commonly used in specialty products, such as briefcases, wallets, and luggage:
The following are not "true" leathers, but contain leather material. Depending on jurisdiction, they may still be labeled as "Genuine Leather":
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.[3] Kangaroo leather is also used for falconry jesses and soccer footwear.[4]
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.[5] 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.
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:
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.[6]
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[7] 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" with 10,000 tanneries as of 2011 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.[8] In 2003 for instance, the main tanneries' effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open.[9] 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.[10]
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.[11]
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.[12]
Lipases are used in the degreasing operation to hydrolyze fat particles embedded in the skin.[13]
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.
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 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.[14][15]
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.[16]
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.[17] Meanwhile, scientific experiments have shown no substantial benefits.[18] The main authorities on the subject therefore discourage it, with a caveat for special cases done under the direction of a conservator.[14][16][17]
Leather can be decorated by a variety of methods, including pyrography and beading.
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.[19] 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,[20] its place taken in part by chintz hangings and printed wallpapers. Cordwainer is still used to describe someone in the profession of shoemaking.
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.
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 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.
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.
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.[21]
Jainism prohibits the use of leather since it is obtained by killing animals.
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.
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