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Leather

Leather is a material created through the tanning of hides and skins of animals, primarily cattlehide. The tanning process converts the putrescible skin into a durable, long-lasting and versatile natural material for various uses.

Leather is an important material with many uses. Together with wood, leather formed the basis of much ancient technology. The leather industry and the fur industry are distinct industries that are differentiated by the importance of their raw materials. In the leather industry the raw materials are by-products of the meat industry, with the meat having higher value than the skin. The fur industry uses raw materials that are higher in value than the meat and hence the meat is classified as a by-product. Taxidermy also makes use of the skin of animals, but generally the head and part of the back are used. Hides and skins are also used in the manufacture of glue and gelatin.

Forms of leather
There are a number of processes whereby the skin of an animal can be formed into a supple, strong material commonly called leather.

  • Vegetable-tanned leather is tanned using tannin (hence the name "tanning") and other ingredients found in vegetable matter, tree bark, and other such sources. It is supple and brown in color, with the exact shade depending on the mix of chemicals and the color of the skin. 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 armour after hardening, and it has also been used for book binding. This is the only form of leather suitable for use in leather carving or stamping.

  • Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other 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 leather used in chrome-free leather often seen in infant's shoes and in automobiles that prefer a chrome-free leather. Formaldehyde tanning (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 oxidise easily to produce the aldehydes that tan the leather.

  • Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types. This leather is white in color and was invented when vegetable tannins were in short supply, i.e. during the Second World War. 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 dissatisfaction about the formation of free formaldehyde was realised.

  • Alum-tanned leather is tanned using aluminium salts mixed with a variety of binders and protein sources, such as flour, egg yolk, etc. Purists argue that alum-tanned leather is technically "tawed" and 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.

  • Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tanning, 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, and is 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, or for making many varieties of dog chews.

Leather usually vegetable-tanned 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.

Leather with the hair still attached is called hair-on.

Leather types
In general, leather is sold in three forms:

  • Full-Grain leather or Top-Grain leather is referring to the upper section of a hide that contains the epidermis or skin layer. It refers to hides that have not been sanded, buffed or snuffed(otherwise known as Corrected) in order to remove imperfections on the surface of the hide. Only the hair has been removed from the epidermis. The grain remains in its natural state which will allow the best fiber strength, resulting in greater durability. The natural grain also has natural breathability, resulting in greater comfort for clothing. The natural Full-Grain surface will wear better than other leather. Rather than wearing out, it will develop a natural "Patina" and grow more beautiful over time. The finest leather furniture and footwear are made from Full-Grain leather. For these reasons only the best raw hide are used in order to create Full-Grain or Top-Grain leather. Full grain leathers can mainly be bought as two finish types: aniline and semi-aniline.

  • Corrected-Grain leather is any Top-Grain leather that has had its surfaces sanded, buffed or snuffed in order to remove any imperfection on the surface due to insect bites, healed scars or brands. Top-Grain leather is often wrongly referred to as Corrected-Grain. Although Corrected-Grain leather is made from Top-Grain as soon as the surface is corrected in any way the leather is no longer referred to asTop-Grain leather. The hides used to create corrected leather are hides of inferior quality that do not meet the high standards for use in creating aniline or semi-aniline leather. The imperfections are corrected and an artificial grain applied. Most Correct 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.

  • Split leather is leather that is created from the fiberous part of the hide left once the Top-Grain of the raw hide has been separated from the hide. During the splitting operation the 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. Splits can 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. Suede is less durable than top-grain. Suede is cheaper because many pieces of suede can be split from a single thickness of hide, whereas only one piece of top-grain can be made. However, manufacturers use a variety of techniques to make suede appear to be full-grain. For example, in one operation, glue is mixed with one side of the suede, which is then pressed through rollers; these flatten and even out one side of the material, giving it the smooth appearance of full-grain. Latigo is one of the trade names for this product. 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 a true form of suede.

The International Union of Leather Technologists and Chemist Societies has a full glossary of leather terms that can be found at IULTCS

Other less-common leathers include:

  • Buckskin or brained leather is a tanning process that uses animal brains or other fatty materials to alter the 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.

  • Shagreen is also known as Stingray skin/leather. Applications used in furniture production date as far back as the art deco period. The word "Shagreen" originates from France and is commonly confused with a shark skin and stingray skin combination.

  • Vachetta leather is used in the trimmings of luggage and handbags, popularized by Louis Vuitton. The leather is left untreated and is therefore susceptible to water and stains. Sunlight will cause the natural leather to darken in shade, called a patina.

  • Slink is leather made from the skin of unborn calves. It is particularly soft, and is valued for use in making gloves.

  • Deer Skin is one of the toughest leathers, partially due to adaptations to their thorny and thicket filled habitats. Deerskin has been prized in many societies including indigenous Americans. Most modern deer skin 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 NY. Deer skin is used in jackets and overcoats, professional sporting equipment such as kendo bogu, as well as high quality personal accessories like handbags and wallets. It commands a high price due to its relative rarity and proven durability.

  • Nubuck is top-grain cattle hide leather that has been sanded or buffed on the grain side, or outside, to give a slight nap of short protein fibers, producing a velvet-like surface.
    There are two other descriptions of leather commonly used in specialty products, such as briefcases, wallets, and luggage.

  • Belting leather is a full grain leather that was originally used in driving pulley belts and other machinery. It is often found on the surface of briefcases, portfolios, and wallets, and can be identified by its thick, firm feel and smooth finish. Belting leather is the only kind of leather used in luxury products that can retain its shape without the need for a separate frame; it is generally a heavy-weight of full-grain, vegetable-tanned leather.

  • Nappa leather, or Napa leather, is extremely soft and supple and is commonly found in higher quality wallets, toiletry kits, and other personal leather goods.
    The following are not 'true' leathers, but contain leather material.

  • Bonded leather , or "Reconstituted leather", is not really a true leather but a man-made material 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 genuine leather at a fraction of the cost. Bonded leather is not as durable as other leathers, and is recommended for use only if the product will be used infrequently. One example of bonded leather use is in Bible covers.

  • Bicast leather is a man-made product that consists of a thick layer of polyurethane applied to a substrate of low-grade or reconstituted leather. Most of the strength of bicast leather comes from the polyurethane coating. Bicast was originally made for the shoe industry and recently was adopted by the furniture industry. The original formula created by Bayer was fairly strong, but creating Bicast from the original recipe is expense. Most of the Bicast used today is created using inferior generic chemicals resulting in an inferior material. The result is a much stiffer product that tends to delaminate resulting in bubbles and cracking.

The vast majority of leather is sold according to its area. The leather is placed through pin-wheel or electronic measuring machines and its surface area is determined. The unit of measurement is square metre, square decimetre or square foot. The thickness is also important, and this is measured using a thickness gauge (the unit of measurement is millimetres, e.g., 1.8 mm is a standard thickness for a school shoe).

In some parts of the world top-grain thicknesses are described using weight units of ounces. Although the statement is in ounces only, it is an abbreviation of ounces per square foot. The thickness value can be obtained by the conversion:
1 oz/ft² = 1/64 inch (0.4 mm)
Hence, leather described as 7 to 8 oz is 7/64 to 8/64 inches (2.8 to 3.2 mm) thick. The weight is usually given as a range because the inherent variability of the material makes ensuring a precise thickness very difficult. Other leather manufacturers state the thickness directly in millimetres.

Leather from other animals
Today, most leather is made of cattle skin, but many exceptions exist. Lamb and deer skin are used for soft leather in more expensive apparels.

Kangaroo skin is used to make items which need to be strong but flexible, it is the material most commonly used in high quality bullwhips. Kangaroo leather is favored by some motorcyclists for use in Motorcycle Leathers specifically because of its lighter weight and higher abrasion resistance compared with cowhide, thus providing greater protecting in case of a fall on the roadway. Kangaroo leather is also used for high performance soccer footwear.

Leather made from more exotic skins has at different times in history been considered very beautiful. For this reason certain snakes and crocodiles have been hunted to near extinction.

In the 1970s, ostrich farming for their feathers became popular, and ostrich leather became available as a side product. There are different processes to produce different finishes for many applications, i.e., upholstery, footwear, automotive products, accessories and clothing. Ostrich leather is considered one of the finest and most durable in the world and 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 in the same way as regular bovine leather. Sting ray leather is as tough and durable as hard plastic. 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. Leather clothing is also popular in Thailand.

In the United States, bison leather has become popular. It is used for gloves, jackets and some baseball gloves. It is rugged but supple and has a waxy feel.

Overall, leather comes from a variety of other sources, including the skins of cattle, hogs, goats, sheep, alligators, ostriches, kangaroos, Yak.

There is quite a wide range of different animal leather used both for Leather garments as well as Leather Goods such as Ladies Handbags, Gents wallets, Ladies Purses, Leather Belts, File bags and other customized leather articles.

The most commonly used leather types are Cow leather, Sheep leather, Buffalo Leather and Ox leather. The most expensive is Cow leather and then Buffalo leather, Ox leather and Sheep leather respectively. The Sheep leather is quite famous for its softness and mostly used in leather garments; however due to certain restrictions of its overall size; it cannot be used for long coats and there Cow leather and Buffalo leather is widely used.

Leather production processes
The leather manufacturing process is divided into 3 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. It's difficult to have a list of operations that all leathers must undergo, as there are so many types of leather.

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 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, whilst tanned material dries out to a flexible form that does not become putrid when wetted back. There is a large number of different tanning methods and materials that 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 of 2.8-3.2. 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 a good 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 3.8-4.2.

Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations: wetting back, sammying, splitting, shaving, rechroming neutralisation, retanning, dyeing, fatliquoring, filling, stuffing, stripping, whitening, fixation, 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)and glazing.

Role of enzymes in leather production
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 hydrolyse casein, elastin, albumin and globulin-like proteins, as well as non-structured proteins which are not essential for leather making. It is especially important to hydrolyse the elastin if the leather is to be limed, or treated with calcium hydroxide; if not treated properly before liming, the elastin will harden and the grain will be loose. Vasudeo Zambare and his team have done ample of work on dehairing protease. Vasudeo Zambare is one of the inventor of this work and the team has filled patent application in Indian, German and United States Patent Offices.

Lipases are used in the degreasing operation to hydrolyse 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. This process is called bating.

Preservation and conditioning of leather
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.

 

Information provided by Wikipedia