Leather is a durable and flexible material created by tanning animal rawhide and skin, often cattle hide. It can be produced at manufacturing scales ranging from cottage industry to heavy industry.
- From other animals
- Production processes
- Environmental impact
- Carbon footprint
- Water footprint
- Chemicals used in tanning
- Role of enzymes
- Preservation and conditioning
- Leather book binding
- Cordwain and Cuir de Cordoue
- In modern culture
- Religious sensitivities
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:
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:
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 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:
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:
Leather produces some environmental impact, most notably due to:
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.
Leather biodegrades slowly—taking 25 to 40 years to decompose. However, vinyl and petrochemical-derived materials take 500 or more years to decompose.
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.
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 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.
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.
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. Many sports still use leather equipment. Its flexibility facilitates forming and shaping it into balls and protective gear.
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.
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.
Judaism forbids the comfort of wearing leather shoes 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 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 synthetic 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.