Leather

Leather

Leather is a durable and flexible material created by tanning animal rawhide and skin, often cattle hide. Traditionally, tanning used tannin, an acidic chemical compound from which the process draws its name (tannin is in turn named for an old German word for oak or fir trees, which supplied it).

Rawhide is made by removing the flesh and fat and then the hair (leather with the hair still attached is called ‘hair-on’) by use of an aqueous solution (this process is called ‘liming’ when using lime or ‘bucking’ when using lye), then scraping over a beam with a somewhat dull knife, and then drying (often while stretched). Liming or bucking also cleans the fiber network of the skin to encourage penetration of the tanning agent.

Vegetable-tanned leather is tanned using tannin and other ingredients found in vegetable matter, such as tree bark prepared in bark mills, and other similar sources. 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, and if left to soak and then dry it will shrink and become less supple, and harder. In hot water, it will shrink drastically and partly gelatinize, 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 armor after hardening, and it has also been used for book binding. Leather 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.

Chrome-tanned leather, invented in 1858, is tanned using salts of chromium. 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 esoteric colors are possible using chrome tanning.

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 automobiles and shoes for infants. Formaldehyde tanning, which is being phased out due to its danger to workers and the sensitivity of many people to formaldehyde, is another method of aldehyde tanning. ‘Brain-tanned’ leathers fall into this category and are exceptionally water absorbent. Brain tanned leathers are made by a labor-intensive process which uses emulsified oils, often those of animal brains. They are known for their exceptional softness and their ability to be washed. Chamois leather also falls into the category of aldehyde tanning and like brain tanning produces a highly water absorbent leather. Chamois leather is made by using oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to make the fabric the color it is.

Rose tanned leather is a variation of vegetable oil tanning and brain tanning, where pure rose oil replaces the vegetable oil and emulsified oils. It has been called the most valuable leather on earth but this is mostly due to the high cost of rose oil and its labor-intensive tanning process.

Synthetic-tanned leather is tanned using aromatic polymers such as Novolac or Neradol (syntans, contraction for ‘synthetic tannins’). This leather is white in color and was invented when vegetable tannins were in short supply during the WWII. 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 until formaldehyde was regulated.

Alum-tawed leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Purists argue that alum-tawed leather is technically not tanned, as the resulting material will rot in water. Very light shades of leather are possible using this process, but the resulting material is not as supple as vegetable-tanned leather. Like alum-tawing, 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’s primarily found in uses such as drum heads 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.

In general, leather is sold in four forms: Full-grain, Top-grain, Split, and Corrected-grain. ‘Full-grain’ leather refers to hides that have not been sanded or buffed (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 will develop a patina over time. High quality leather furniture and footwear are often made from full-grain leather. Full-grain leathers are typically available in two finish types: aniline and semi-aniline.

Aniline leather is a type of leather dyed exclusively with soluble dyes without covering the surface with a topcoat paint or insoluble pigments. The resulting product retains the hide’s natural surface with the ‘grain’, i.e. visible pores, scars etc. of the complete original animal’s skin structure. Originally, the dyes used for this process were synthesized from aniline through chemical reactions. These dyes used to be called ‘aniline dyes’ or ‘tar dyes.’ In modern times, the dyes used are subject to laws and regulations in many countries, and the use of certain azo compounds is prohibited due to the health risk. Aniline leather may be referred to as ‘full aniline’ or ‘full sauvage’ leather to differentiate between this dye treatment and variants. ‘Semi-aniline’ leather is produced through a very similar process to full-aniline, but has a thin protective top coat added. ‘Pull up’ aniline leather has additional oil or wax applied to the leather to give it a distressed look.

‘Top-grain’ leather (the most common type used in high-end leather products) is the second-highest quality. It has had a layer removed called a ‘split,’ making it thinner and more pliable than full-grain. Its surface has been sanded and a finish coat added to the surface which results in a colder, plastic feel with less breathability, and it will not develop a natural patina. It is typically less expensive and has greater resistance to stains than full-grain leather, so long as the finish remains unbroken. ‘Nubuck’ is top-grain cattle hide 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 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, a partially leather product). 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 to be a true form of suede.

‘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 impressed 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.

‘Buckskin’ is another named for brained leather The resulting supple, suede-like hide is usually smoked heavily to prevent it from rotting. ‘Patent leather’ is leather that has been given a high-gloss finish. The original process was developed in Newark, New Jersey, by inventor Seth Boyden in 1818. Patent leather usually has a plastic coating. ‘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 will cause the natural leather to darken in shade. ‘Slink’ is leather made from the skin of unborn calves. It is particularly soft and is valued for use in making gloves. ‘Belting leather’ is a full-grain leather that was 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.

Fish leather is mainly used for making shoes and bags; the fish skin is tanned like other animal skins. Salmon skin has fine scales. Its strength and elegant look make it the most popular fish leather. Sturgeon is a fish well known for its eggs (caviar), which make it rare. Its leather is thus quite expensive. The eel is a fish without scales, its skin has a shiny appearance. ‘Shagreen’ is also known as stingray skin/leather. Applications used in furniture production date as far back as the art deco period (1930s). The word ‘shagreen’ originates from France. Known as the most difficult leather to work due to dished scales of the animal, it is one of the most expensive leathers. Shark skin is covered by small, close-set tubercles, making it very tough. The handbags made of shark skin used to be in vogue but interest has since fallen as the costs of production and of the leather itself are very high. Moreover, this skin is more difficult to work.

Deerskin is a tough 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 (e.g. Japanese Kendo armor), as well as personal accessories such as handbags and wallets.

There are several non-leather products that contain some leather material. ‘Bycast leather’ is a split leather with a layer of polyurethane applied to the surface and then embossed. Bycast was originally made for the shoe industry and recently was adopted by the furniture industry. The original formula created by Bayer was strong but expensive, though  costs have since come down. The product is slightly stiffer than top grain leather but has a much more consistent texture and is easier to clean and maintain. ‘Bonded leather,’ or ‘reconstituted leather,’ is composed of 90% to 100% leather fibers (often scrap from leather tanneries or leather workshops) bonded together with latex binders to create a look and feel similar to that of leather at a fraction of the cost. This bonded leather is not as durable as other leathers and is recommended for use only if the product will be used infrequently. Bonded leather upholstery is a vinyl upholstery that contains about 17% leather fiber in its backing material. The vinyl is stamped to give it a leather-like texture. Bonded leather upholstery is durable and its manufacturing process is more environmentally-friendly than leather production.

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 also favored by some motorcyclists for use in motorcycle leathers specifically because of its light weight and abrasion resistance. Additionally, kangaroo leather is used for falconry and soccer footwear.

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 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, is optional. 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. Enzymes 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 (skin proteins), but should be able to remove other molecules like 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.’

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.’

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 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 requires maintenance to retain its condition. The natural fibers of leather will break down with the passage of time. Acidic leathers are particularly vulnerable to ‘red rot’ (leather degradation), 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. ‘Saddle soap’ (a mild soap and softening ingredients such as neatsfoot oil, glycerin, and lanolin) 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 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. Scientific experiments have shown no substantial benefits to the use of dressings.

Leathercraft is the practice of making leather into craft objects or works of art, using shaping techniques, coloring techniques or both. Leather can be decorated by a variety of methods, including pyrography (burning) and beading. ‘Cordwain’ (‘from Córdoba’) is 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,’ from the Libyan town of Ghadames, while ‘cordobanes’ signified soft goat leather. Leather was even more protection against droughts and dampness than tapestry and unaffected by insects. From the fourteenth century, the technique in which panels of wet leather were shaped over wooden molds  painted, then oil-gilded and lacquered, reached Flanders and even farther north. Though there were craftsmen in several cities, the major handicraft center for this cordwain was Mechelen (north of Brussels), where it was mentioned as early as 1504. Patterns for these panels followed fashions in silk damask (wall coverings), at some lag in time, since the high-relief wooden molds were laborious to make. After the second half of the 18th century, this luxurious artisan product was no longer made, its place taken in part by chintz (glazed calico cloth) 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. Extreme metal bands (especially black metal bands) and Goth rock groups have extensive leather clothing, i.e. leather pants, accessories, etc.

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 wearing leather on Yom Kippur (Day of Atonement), Tisha B’Av (commemoration of the destruction of the First Temple), and during mourning. Jainism prohibits the use of leather since it is obtained by killing animals. Some 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 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. However, environmental and safety issues exist with the choice of vegans. Microfiber textiles tend to be flammable and emit toxic gases when burning. They are made with synthetic fibers such as polyester and nylon which are made from petrochemicals. Microfibers are not made from a renewable resource and are not biodegradable.

Leather is also a product with some environmental impact, most notably due to: the impact of livestock, the use of chemicals in the tanning process, and air pollution due to the transformation process (vapors of hydrogen sulfide during dehairing and ammonia during deliming). Leather biodegrades slowly; it takes 25–40 years to decompose. However, vinyl and petro-chemical derived materials will take 500 or more years to break down and return to the earth. 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. In Kanpur, the self-proclaimed ‘Leather City of World’ with 10,000 tanneries 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 2009. In 2003 for instance, the main tanneries’ effluent disposal unit was dumping 22 tons 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.

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