The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Leather
Leather is a durable and flexible material created by the tanning of animal rawhide and skin, often cattle hide. It can be produced through manufacturing processes ranging from cottage industry to heavy industry.
Leather is used for various purposes 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 and is decorated by a wide range of techniques.
Several tanning processes transform hides and skins into leather:
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 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" organic leathers, but are materials that contain leather fibre. Depending on jurisdiction, they may still be labelled 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:
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, 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 favoured 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, 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. 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 rawhide is also used as grips on Chinese swords, Scottish basket hilted 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 matches 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 colour (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 colouring 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 some environmental impact, most notably due to:
One ton of hide or skin generally leads to the production of 20 to 80 m3 of wastewater including chromium levels of 100–400mg/L, sulphide 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 8kg 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" 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. 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 neighbourhood 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 neighbourhood 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 USD$70–100 per ton of raw hides processed against USD$43/t for irresponsible behaviour.
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.
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 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
The natural fibres 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. Although it is chemically irreversible, there are treatments that 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.
Various treatments are available such as conditioners. Saddle soap is used for cleaning, conditioning and softening leather. Leather shoes are widely conditioned with shoe polish.
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 discolour and stain the leather, oxidize (penetration and expansion of oils including displacement and weakening of fibres) and stiffen, leave a sticky surface, collect dust, wick into adjacent materials, form unstable surface spews, encourage biological deterioration and mould 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.
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.
The related term Cuir de Cordoue refers to painted and gilded (and sometimes embossed) leather hangings manufactured in panels and assembled for covering walls as an alternative to tapestry.
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.
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 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.
Jainism prohibits the use of leather since it is obtained by killing animals.
Some vegetarians, vegans and animal rights activists such as PETA, boycott 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.
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