Tanning, or hide tanning, is the process of treating skins and hides of animals to produce leather. A tannery is the place where the skins are processed.
Tanning hide into leather involves a process which permanently alters the protein structure of skin, making it more durable and less susceptible to decomposition and coloring.
Before tanning, the skins are often dehaired, then have fat, meat and connective tissue removed. They are then washed and soaked in water with various compounds, and prepared to receive a tanning agent. They are then soaked, stretched, dried, and sometimes smoked. Historically this process was considered a noxious or "odoriferous trade" and relegated to the outskirts of town.
Historically, vegetable based tanning used tannin, an acidicchemical compound derived from the bark of certain trees, in the production of leather. An alternative method, developed in the 1800s, is chrome tanning, where chromium salts are used instead of natural tannins.
Tanning, 1880Tanner, Nuremberg, 1609Peeling hemlock bark for the tannery in Prattsville, New York, during the 1840s, when it was the largest in the world
The English word for tanning is from medieval Latin tannāre, derivative of tannum (oak bark), from French tan (tanbark), from old-Cornish tann (oak). These terms are related to the hypothetical Proto-Indo-European *dʰonu meaning 'fir tree'. (The same word is source for Old High Germantanna meaning 'fir', related to modern German Tannenbaum).
The process of tanning was also used for boats and fishing vessels: ropes, nets, and sails were tanned using tree bark.[2]
Formerly, tanning was considered a noxious or "odoriferous trade" and relegated to the outskirts of town, among the poor. Indeed, tanning by ancient methods is so foul-smelling that tanneries are still isolated from those towns today where the old methods are used. Skins typically arrived at the tannery dried stiff and dirty with soil and gore. First, the ancient tanners would soak the skins in water to clean and soften them. Then they would pound and scour the skin to remove any remaining flesh and fat. Hair was removed by soaking the skin in urine,[3] painting it with an alkalinelime mixture, or simply allowing the skin to putrefy for several months then dipping it in a salt solution. After the hair was loosened, the tanners scraped it off with a knife. Once the hair was removed, the tanners would "bate" (soften) the material by pounding dung into the skin, or soaking the skin in a solution of animal brains. Bating was a fermentative process that relied on enzymes produced by bacteria found in the dung. Among the kinds of dung commonly used were those of dogs or pigeons.[4]
Historically the actual tanning process used vegetable tanning. In some variations of the process, cedar oil, alum, or tannin was applied to the skin as a tanning agent. As the skin was stretched, it would lose moisture and absorb the agent.
Following the adoption in medicine of soaking gut sutures in a chromium (III) solution after 1840, it was discovered that this method could also be used with leather and thus was adopted by tanners.[5]
Preparation
Tanned rabbit pelt. The fur has been left on, apart from small patches exposing leather.
The tanning process begins with obtaining an animal skin. When an animal skin is to be tanned, the animal is killed and skinned before the body heat leaves the tissues.[6] This can be done by the tanner, or by obtaining a skin at a slaughterhouse, farm, or local fur trader.
Curing
Preparing hides begins by curing them with salt to prevent putrefaction of the collagen from bacterial growth during the time lag from procuring the hide to when it is processed. Curing removes water from the hides and skins using a difference in osmotic pressure. The moisture content of hides and skins is greatly reduced, and osmotic pressure increased, to the point that bacteria are unable to grow. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing, the hides are agitated in a saltwater bath for about 16 hours. Curing can also be accomplished by preserving the hides and skins at very low temperatures.
Beamhouse operations
The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They include, in order, soaking, liming, removal of extraneous tissues (unhairing, scudding and fleshing), deliming, bating or puering, drenching, and pickling.[7][8]
Soaking
In soaking, the hides are soaked in clean water to remove the salt left over from curing and increase the moisture so that the hide or skin can be further treated.
To prevent damage of the skin by bacterial growth during the soaking period, biocides, typically dithiocarbamates, may be used. Fungicides such as TCMTB may also be added later in the process, to protect wet leathers from mold growth. After 1980, the use of pentachlorophenol and mercury-based biocides and their derivatives was forbidden.[9]
After soaking, the hides are treated with milk of lime (a basic agent) typically supplemented by "sharpening agents" (disulfide reducing agents) such as sodium sulfide, cyanides, amines, etc.
This:
Removes the hair and other keratinous matter
Removes some of the interfibrillary soluble proteins such as mucins
Causes the fibers to swell up and split up to the desired extent
Removes the natural grease and fats to some extent
Brings the collagen in the hide to a proper condition for satisfactory tannage
The weakening of hair is dependent on the breakdown of the disulfide link of the amino acid cystine, which is the characteristic of the keratin class of proteins that gives strength to hair and wools (keratin typically makes up 90% of the dry weight of hair). The hydrogen atoms supplied by the sharpening agent weaken the cystine molecular link whereby the covalentdisulfide bond links are ultimately ruptured, weakening the keratin. To some extent, sharpening also contributes to unhairing, as it tends to break down the hair proteins.
The isoelectric point of the collagen (a tissue-strengthening protein unrelated to keratin) in the hide is also shifted to around pH 4.7 due to liming.
Traditional hand scudding in Marrakech, Morocco
Any hairs remaining after liming are removed mechanically by scraping the skin with a dull knife, a process known as scudding.
The pH of the collagen is then reduced so the enzymes may act on it in a process known as deliming. Depending on the end use of the leather, hides may be treated with enzymes to soften them, a process called bating. In modern tanning, these enzymes are purified agents, and the process no longer requires bacterial fermentation (as from dung-water soaking) to produce them.[10]
Pickling
Pickling is another term for tanning, or what is the modern equivalent of turning rawhide into leather by the use of modern chemical agents, if mineral tanning is preferred. Once bating is complete, the hides and skins are treated by first soaking them in a bath containing common salt (sodium chloride), usually 1 quart of salt to 1 gallon of hot water.[11] When the water cools, one fluid ounce of sulfuric acid is added. Small skins are left in this liquor for 2 days, while larger skins between 1 week and as much as 2 months.[11]
In vegetable tanning, the hides are made to soak in a bath solution containing vegetable tannins, such as found in gallnuts, the leaves of sumac, the leaves of certain acacia trees, the outer green shells of walnuts, among other plants. The use of vegetable tanning is a process that takes longer than mineral tanning when converting rawhides into leather. Mineral tanned leather is used principally for shoes, car seats, and upholstery in homes (sofas, etc.). Vegetable tanned leather is used in leather crafting and in making small leather items, such as wallets, handbags and clothes.
Process
Chrome tanning
A modern electric tanning drum in Germany
Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) has long been regarded as the most efficient and effective tanning agent.[12][13] Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium, although the latter arises in inadequate waste treatment. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H2O)6]3+, which at higher pH undergoes processes called olation to give polychromium(III) compounds that are active in tanning,[14] being the cross-linking of the collagen subunits. The chemistry of [Cr(H2O)6]3+ is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the amino acids, and masking agents. Masking agents are carboxylic acids, such as acetic acid, used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes.
Collagen is characterized by a high content of glycine, proline, and hydroxyproline, usually in the repeat -gly-pro-hypro-gly-.[15] These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2−) are formed by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). Later during pickling, collagen carboxyl groups are temporarily protonated for ready transport of chromium ions. During basification step of tanning, the carboxyl groups are ionized and coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters.
Tanning increases the spacing between protein chains in collagen from 10 to 17 Å.[16] The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.
Possible chromium(III) tanning mechanisms
Before the introduction of the basic chromium species in tanning, several steps are required to produce a tannable hide. The pH must be very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit between the fibers and residues of the collagen. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This step is known as basification. In the raw state, chrome-tanned skins are greyish-blue, so are referred to as wet blue. Chrome tanning is faster than vegetable tanning (taking less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments.
After application of the chromium agent, the bath is treated with sodium bicarbonate in the basification process to increase the pH to 3.8–4.0, inducing cross-linking between the chromium and the collagen. The pH increase is normally accompanied by a gradual temperature increase up to 40°C.[17] Chromium's ability to form such stable bridged bonds explains why it is considered one of the most effective tanning compounds. Chromium-tanned leather can contain between 4 and 5% of chromium.[16] This efficiency is characterized by its increased hydrothermal stability of the skin, and its resistance to shrinkage in heated water.[13]
Vegetable tanning
Vegetable tanning uses tannins (a class of polyphenol astringent chemicals), which occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them, causing them to become less water-soluble and more resistant to bacterial attack. The process also causes the hide to become more flexible. The primary barks processed in bark mills and used in modern times are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle (acacia; see catechol), and myrobalans from Terminalia spp., such as Terminalia chebula. In Ethiopia, the combined vegetable oils of Niger seed (Guizotia abyssinica) and flaxseeds were used in treating the flesh side of the leather, as a means of tawing, rather than of tanning. In Yemen and Egypt, hides were tanned by soaking them in a bath containing the crushed leaves and bark of the Salam acacia (Acacia etbaica; A. nilotica kraussiana).[18] Hides that have been stretched on frames are immersed for several weeks in vats of increasing concentrations of tannin. Vegetable-tanned hide is not very flexible. It is used for luggage, furniture, footwear, belts, and other clothing accessories.
Alternative chemicals
Wet white is a term used for leathers produced using alternative tanning methods that produce an off-white colored leather. Like wet blue, wet white is also a semifinished stage. Wet white can be produced using aldehydes, aluminum, zirconium, titanium, or iron salts, or a combination thereof. Concerns with the toxicity and environmental impact of any chromium (VI) that may form during the tanning process have led to increased research into more efficient wet white methods.
Natural tanning
The conditions present in bogs, including highly acidic water, low temperature, and a lack of oxygen, combine to preserve but severely tan the skin of bog bodies.
Tawing
Tawing is a method that uses alum and other aluminium salts, generally in conjunction with binders such as egg yolk, flour, or other salts. The hide is tawed by soaking in a warm potash alum and salts solution, between 20 and 30°C (68 and 86°F). The process increases the hide's pliability, stretchability, softness, and quality. Then, the hide is air dried (crusted) for several weeks, which allows it to stabilize.[19]
The use of alum alone for tanning rawhides is not recommended, as it shrinks the surface area of the skin, making it thicker and hard to the touch.[20] If alum is applied to the fur, it makes the fur dull and harsh.[20]
Post-tanning finishing
Depending on the finish desired, the leather may be waxed, rolled, lubricated, injected with oil, split, shaved, or dyed.
Health and environmental impact
Tanned fish skin of salmon
The tanning process involves chemical and organic compounds that can have a detrimental effect on the environment. Agents such as chromium, vegetable tannins, and aldehydes are used in the tanning step of the process.[21] Chemicals used in tanned leather production increase the levels of chemical oxygen demand and total dissolved solids in water when not disposed of responsibly. These processes also use large quantities of water and produce large amounts of pollutants.[22]
Boiling and sun drying can oxidize and convert the various chromium(III) compounds used in tanning into carcinogenichexavalent chromium, or chromium(VI). This hexavalent chromium runoff and scraps are then consumed by animals, in the case of Bangladesh, chickens (the nation's most common source of protein). Up to 25% of the chickens in Bangladesh contained harmful levels of hexavalent chromium, adding to the national health problem load.[23]
Chromium is not solely responsible for these diseases. Methylisothiazolinone, which is used for microbiological protection (fungal or bacterial growth), causes problems with the eyes and skin. Anthracene, which is used as a leather tanning agent, can cause problems in the kidneys and liver and is also considered a carcinogen. Formaldehyde and arsenic, which are used for leather finishing, cause health problems in the eyes, lungs, liver, kidneys, skin, and lymphatic system and are also considered carcinogens.[22] The waste from leather tanneries is detrimental to the environment and the people who live in it. The use of old technologies plays a large factor in how hazardous wastewater results in contaminating the environment. This is especially prominent in small and medium-sized tanneries in developing countries.[24]
The UN Leather Working Group (LWG) "provides an environmental audit protocol, designed to assess the facilities of leather manufacturers,"[25] for "traceability, energy conservation, [and] responsible management of waste products."[26]
Alternatives
Untanned hides can be dried and made pliable by rubbing and stretching the fibers with a hide stretcher, and fatting. However the hide will revert to rawhide if not periodically replenished with fat or oil, especially if it gets wet. Many Native Americans of the arid western regions wore clothing made by this process.
Smoke tanning is listed among the conventional methods like chrome tanning and vegetable tanning.[27] Impregnation of the hide's cells with formaldehyde (from smoke) offers some microbial and water resistance.[28]
Associated processes
Leftover leather would historically be turned into glue. Tanners would place scraps of hides in a vat of water and let them deteriorate for months. The mixture would then be placed over a fire to boil off the water to produce glue.
A tannery may be associated with a grindery, originally a whetstone facility for sharpening knives and other sharp tools, but later could carry shoemakers' tools and materials for sale.[29]
There are several solid and waste water treatment methodologies currently being researched, such as anaerobic digestion of solid wastes and wastewater sludge.
Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hogs, and aquatic animals such as seals and alligators.
Tannins are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
Potassium dichromate, K2Cr2O7, is a common inorganic chemical reagent, most commonly used as an oxidizing agent in various laboratory and industrial applications. As with all hexavalent chromium compounds, it is acutely and chronically harmful to health. It is a crystalline ionic solid with a very bright, red-orange color. The salt is popular in laboratories because it is not deliquescent, in contrast to the more industrially relevant salt sodium dichromate.
Ostrich leather is the result of tanning skins taken from African ostriches farmed for their feathers, skin and meat. The leather is distinctive for its pattern of bumps or vacant quill follicles, ranged across a smooth field in varying densities. It requires an intricate, specialised and expensive production process making its aesthetic value costly.
Liming is a process used for parchment or leather processing, in which hides are soaked in an alkali solution. It is performed using a drum and paddle or a pit. Its objectives are:
Chromium(III) sulfate usually refers to the inorganic compounds with the formula Cr2(SO4)3.x(H2O), where x can range from 0 to 18. Additionally, ill-defined but commercially important "basic chromium sulfates" are known. These salts are usually either violet or green solids that are soluble in water. It is commonly used in tanning leather.
Tanbark is the bark of certain species of trees, traditionally used for tanning hides into leather.
The leather manufacturing process is divided into three sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating may be added into the sequence. The list of operations that leathers undergo vary with the type of leather.
In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important for understanding the relationship between metal aquo complexes and metal oxides, which are represented by many minerals.
Arthur Waldorf Thomas was a professor and chemist who specialized in colloid chemistry. He studied and taught at Columbia University for 50 years.
Chromium toxicity refers to any poisonous toxic effect in an organism or cell that results from exposure to specific forms of chromium—especially hexavalent chromium. Hexavalent chromium and its compounds are toxic when inhaled or ingested. Trivalent chromium is a trace mineral that is essential to human nutrition. There is a hypothetical risk of genotoxicity in humans if large amounts of trivalent chromium were somehow able to enter living cells, but normal metabolism and cell function prevent this.
S.B. Foot Tanning Company is a leather production facility located in Red Wing, Minnesota. It processes leather for use in the crafting of shoes, boots, belts, and leather accessories. In 2010, the factory processed nearly 6 million linear feet of hides. S.B. Foot Tanning Company is a wholly owned subsidiary of Red Wing Shoes Company, Inc. and is the principal supplier of leather to their shoe manufacturing plants. The company also supplies over 1.5 million linear feet of leather for use in the production of military footwear per year, extending a tradition that began with World War I, when the tannery supplied boot leathers for the construction of soldier boots.
The Tannery is a historic tannery building constructed by the colonial Moravians in Bethlehem, Northampton County, Pennsylvania. It is a limestone building built in 1761, and is part of the Bethlehem Colonial Industrial Quarter.
Humanity has used animal hides since the Paleolithic, for clothing as well as mobile shelters such as tipis and wigwams, and household items. Since ancient times, hides have also been used as a writing medium, in the form of parchment.
Russia leather is a particular form of bark-tanned cow leather. It is distinguished from other types of leather by a processing step that takes place after tanning, where birch oil is worked into the rear face of the leather. This produces a leather that is hard-wearing, flexible and resistant to water. The oil impregnation also deters insect damage. This leather was a major export good from Russia in the 17th and 18th centuries because of its high quality, its usefulness for a range of purposes, and the difficulty of replicating its manufacture elsewhere. It was an important item of trade for the Muscovy Company. In German-speaking countries, this leather was also known by the name Juchten or Juften.
The conservation and restoration of leather objects is the process of determining the causes of deterioration, followed by deciding the best course of action for preserving the leather objects for the future.
Detergent enzymes are biological enzymes that are used with detergents. They catalyze the reaction between stains and the water solution, thus aiding stain removal and improving efficiency. Laundry detergent enzymes are the largest application of industrial enzymes.
Leather is created when an animal skin or hide is chemically treated in a process called tanning to preserve them for long term use as material for clothing, handbags, footwear, furniture, sports equipment and tools. Alligator leather is also commonly used to create similar items as mentioned above.
Carl Erengisle Hyltén-Cavallius, born 19 February 1817 in Vislanda; died 29 October 1853 in Gothenburg, was a Swedish pharmacist and chemist; he invented the now dominating technology of tanning by using chromium salts.
Bating is a technical term used in the tanning industry to denote leather that has been treated with hen or pigeon manure, similar to puering where the leather has been treated with dog excrement, and which treatment, in both cases, was performed on the raw hide prior to tanning in order to render the skins, and the subsequent leather, soft and supple. Today, both practices are obsolete and have been replaced in the tanneries with other natural proteolytic enzymes.
References
↑ Possehl, Gregory L. (1996). Mehrgarh in Oxford Companion to Archaeology, edited by Brian Fagan. Oxford University Press.
↑ Lofrano, G., Meric, S., Balci, G., & Orhon, D. (2013). Chemical and biological treatment technologies for leather tannery chemicals and wastewaters: A review. Science of Total Environment, 461-462, 265-281.
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