Leather

Drying of leather in East Timor

Leather tanning in Fes, Morocco

Tanned leather in Marrakech

The leather manufacturing process is divided into three fundamental subprocesses: preparatory stages, tanning, and crusting. A further subprocess, finishing, can be added into the leather process sequence, but not all leathers receive finishing.

The preparatory stages are when the hide is prepared for tanning. Preparatory stages may include soaking, hair removal, liming, deliming, bating, bleaching, and pickling.

Tanning is a process that stabilizes the proteins, particularly collagen, of the raw hide to increase the thermal, chemical and microbiological stability of the hides and skins, 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, will putrefy, while tanned material dries to a flexible form that does not become putrid when rewetted.

Many tanning methods and materials exist. The typical process sees tanners load the hides into a drum and immerse them in a tank 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. Once the process achieves even penetration, workers slowly raise the liquor's pH in a process called basification, which fixes the tanning material to the leather. The more tanning material fixed, the higher the leather's hydrothermal stability and shrinkage temperature resistance.

Crusting is a process that thins 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 splitting, shaving, dyeing, whitening or other methods.

For some leathers, tanners apply a surface coating, called "finishing". Finishing operations can include oiling, brushing, buffing, coating, polishing, embossing, glazing, or tumbling, among others.

Leather 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. Frequent oiling of leather, with mink oil, neatsfoot oil, or a similar material keeps it supple and improves its lifespan dramatically.[2]

In general, leather is produced in the following grades:

• Top-grain leather includes the outer layer of the hide, known as the grain, which features finer, more densely packed fibers, resulting in strength and durability. Depending on thickness, it may also contain some of the more fibrous under layer, known as the corium. Types of top-grain leather include:
  • Full-grain leather contains the entire grain layer, without any removal of the surface. Rather than wearing out, it develops a patina during its useful lifetime. It is usually considered the highest quality leather. Furniture and footwear are often made from full-grain leather. Full-grain leather is typically finished with a soluble aniline dye. Russia leather is a form of full-grain leather.
  • Corrected grain leather has the surface subjected to finishing treatments to create a more uniform appearance. This usually involves buffing or sanding away flaws in the grain, then dyeing and embossing the surface.
  • Nubuck is top-grain leather that has been sanded or buffed on the grain side to give a slight nap of short protein fibers, producing a velvet-like surface.
• Split leather is created from the corium left once the top-grain has been separated from the hide, known as the drop split. In thicker hides, the drop split can be further split into a middle split and a flesh split.
  • Bicast leather is split leather that has a polyurethane or vinyl layer applied to the surface and embossed to give it the appearance of a grain. It is slightly stiffer than top-grain leather but has a more consistent texture.
  • Patent leather is leather that has been given a high-gloss finish by the addition of a coating. Dating to the late 1700s, it became widely popular after inventor Seth Boyden developed the first mass-production process, using a linseed-oil-based lacquer, in 1818. Modern versions are usually a form of bicast leather.
  • Suede is made from the underside of a split to create a soft, napped finish. It is often made from younger or smaller animals, as the skins of adults often result in a coarse, shaggy nap.
• Bonded leather, also called reconstituted leather, is a material that uses leather scraps that are shredded and bonded together with polyurethane or latex onto a fiber mesh. The amount of leather fibers in the mix varies from 10% to 90%, affecting the properties of the product.[3]

Phone cases in ostrich leather

Today, most leather is made of cattle hides, which constitute about 65% of all leather produced. Other animals that are used include sheep (about 13%), goats (about 11%), and pigs (about 10%). Obtaining accurate figures from around the world is difficult, especially for areas where the skin may be eaten.[4][5] Other animals mentioned below only constitute a fraction of a percent of total leather production.

Horse hides are used to make particularly durable leathers. Shell cordovan is a horse leather made not from the outer skin but from an under layer, found only in equine species, called the shell. It is prized for its mirror-like finish and anti-creasing properties.

Lamb and deerskin are used for soft leather in more expensive apparel. Deerskin is widely used in work gloves and indoor shoes.

Reptilian skins, such as alligator, crocodile, and snake, are noted for their distinct patterns that reflect the scales of their species. This has led to hunting and farming of these species in part for their skins.

Kangaroo leather is used to make items that must be strong and 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.[6] Kangaroo leather is also used for falconry jesses, soccer footwear,[7] and boxing speed bags.[8]

Although originally raised for their feathers in the 19th century, ostriches are now more popular for both meat and leather.[9] Ostrich leather has a characteristic "goose bump" look because of the large follicles where the feathers grew. Different processes produce different finishes for many applications, including upholstery, footwear, automotive products, accessories, and clothing.

In Thailand, stingray leather is used in wallets and belts. Stingray 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).

For a given thickness, fish leather is typically much stronger due to its criss-crossed fibers.[10]

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, phthalate esters, nonyl phenol ethoxylate soaps, pentachlorophenol and solvents)
• Air pollution due to the transformation process (hydrogen sulfide is formed during mixing with acids and ammonia liberated during deliming, solvent vapors)

Chemical waste disposal

Rajasthani-style leather jooti, Jaipur, India

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[15] 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.[16] In 2003 for instance, the main tanneries' effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open.[17]

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.[18] After approximately 15 years of ignoring high court rulings, the government shut down more than 100 tanneries the weekend of 8 April 2017 in the neighborhood.[19]

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.[20] In November 2009, one of Uganda's main leather making companies was caught directly dumping waste water into a wetland adjacent to Lake Victoria.[21]

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.[22]

Lipases are used in the degreasing operation to hydrolyze fat particles embedded in the skin.[23]

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 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.

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.[24]

Elize Ryd wearing a leather jacket

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.

Leather's flexibility allows it to be formed and shaped into balls and protective gear. Subsequently, many sports use equipment made from leather, such as baseball gloves and the ball used in American football.

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 1970s and 80s) are well known for wearing leather clothing. Extreme metal bands (especially black metal bands) and Goth rock groups have extensive black leather clothing. Leather has become less common in the punk community over the last three decades, as there is opposition to the use of leather from punks who support animal rights.

Many cars and trucks come with optional or standard leather or "leather faced" seating.

In countries with significant populations of individuals observing religions which place restrictions on material choices, 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 labeling helps a Muslim to not accidentally purchase pigskin or a Hindu to avoid cattleskin; it facilitates religious observance and respect. Many vegetarian Hindus do not use any kind of leather. Such taboos increase the demand for religiously neutral leathers such as ostrich and deer.

Judaism forbids the comfort of wearing leather shoes on Yom Kippur, Tisha B'Av, and during mourning.[25] Also, see Teffilin and Torah Scroll.

Jainism prohibits the use of leather, since it is obtained by killing animals.

Many forms of artificial leather have been developed, usually involving polyurethane or vinyl coatings applied to a cloth backing. Many names and brands for such artificial leathers exist, including "pleather", a portmanteau of "plastic leather", and the brand name Naugahyde.[26]

Another alternative is cultured leather which is lab-grown using cell culture methods,[27][28] mushroom-based materials and gelatin-based textile made by upcycling meat industry waste.[29][30]

• Beeby, K.J. The Wonderful Story of Leather (PDF). UK: Harmatan.
• Lefroy, George Alfred (1884). The leather-workers of Daryaganj . Delhi: Cambridge Mission to Delhi.
• Leathers for Bookbinding and Upholstery (PDF). UK: Harmatan. 2002.
• Leather for Libraries (PDF). UK: Harmatan.
• Parsons, F. G. (1911). "Leather" . Encyclopædia Britannica. 16 (11th ed.). pp. 330–345. (includes several diagrams)