Tanning

For other uses, see Tanning (disambiguation).
For human tanning, see Sun tanning.
"Tannery" redirects here. For other uses, see Tannery (disambiguation).
Tanned leather in Marrakesh
Two men pressing the leather near the end of the tanning process in an American tannery. circa: 1976
Tanneries of Marrakesh

Tanning is the process of treating skins of animals to produce leather, which is more durable and less susceptible to decomposition. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name (tannin is in turn named after an old German word for oak or fir trees, from which the compound was derived). Coloring may occur during tanning. A tannery is the term for a place where the skins are processed.

Tanning hide into leather involves a process which permanently alters the protein structure of skin. Making "rawhide" (untanned but worked hide) does not require the use of tannin. Rawhide is made by removing the flesh and fat and then the hair by use of an aqueous solution (this process is often called "liming" when using lime and water or "bucking" when using wood ash (lye) and water), then scraping over a beam with a somewhat dull knife, then drying. The two aforementioned solutions for removing the hair also act to clean the fiber network of the skin and allow penetration and action of the tanning agent, so that all the steps in preparation of rawhide except drying are often preludes to the more complex process of tanning and production of leather.

Tanning can be performed with either vegetable or mineral methods. Before tanning, the skins are unhaired, degreased, desalted and soaked in water over a period of 6 hours to 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides, typically dithiocarbamates, may be used. Fungicides such as TCMBT, 2-(Thiocyanomethylthio) benzothiazole, 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.

History

Further information: History of hide materials
Tannery at Fez
Tanning circa 1880
Tanned rabbit pelt, the fur has been left on apart from small patches exposing leather

The English word for tanning is from medieval Latin tannāre, deriv. of tannum (oak bark), related to Old High German tanna meaning oak or fir (related to modern Tannenbaum). This refers to use of the bark of oaks (the original source of tannin) in some kinds of hide preservation.[1]

In ancient history, tanning was considered a noxious or "odoriferous trade" and relegated to the outskirts of town, amongst 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. Ancient civilizations used leather for waterskins, bags, harnesses, boats, armour, quivers, scabbards, boots and sandals. Tanning was being carried out by the South Asian inhabitants of Mehrgarh between 7000–3300 BC.[2] Around 2500 BC, the Sumerians began using leather, affixed by copper studs, on chariot wheels.

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. Next, the tanner needed to remove the hair fibers from the skin. This was done by either soaking the skin in urine, painting it with an alkaline lime mixture, or simply allowing the skin to putrefy for several months then dipping it in a salt solution. After the hair fibers were loosened, the tanners scraped them 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 which relied on enzymes produced by bacteria found in the dung. Among the kinds of dung commonly used were that of dogs or pigeons.[3] Sometimes the dung was mixed with water in a large vat, and the prepared skins were kneaded in the dung water until they became supple from bacterial enzyme action, but not too soft. The ancient tanner might use his bare feet to knead the skins in the dung water, and the kneading could last two or three hours.

This combination of urine, animal feces and decaying flesh is what made ancient tanneries so malodorous.

Children employed as dung gatherers were a common sight in ancient cities. Also common were "piss-pots" located on street corners, where human urine could be collected for use in tanneries or by washerwomen. In some variations of the process, cedar oil, alum or tannin were applied to the skin as a tanning agent. As the skin was stretched, it would lose moisture and absorb the agent.

Leftover leather would 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 hide 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.[4]

Preparatory steps prior to tanning

Skinning

The actual tanning process begins with the obtaining of an animal skin. When an animal skin is to be tanned, the animal is killed and skinned before the body heat leaves the tissues. This can be done by the tanner, or by obtaining a skin at a slaughterhouse or farm.[5]

Curing

Preparing hides begins by curing them with salt. Curing is employed to prevent putrefaction of the protein substance (collagen) from bacterial growth during the time lag that might occur 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 salt water bath for about 16 hours. Curing can also be accomplished by preserving the hides and skins at a very low temperature.

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 (including puering), drenching, and pickling.[6][7]

Soaking

In the process known as 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.

Liming

Tanner, Nuremberg, 1609

After soaking, the hides and skins are taken for liming: treatment with milk of lime (a basic agent) that may involve the addition of "sharpening agents" (disulfide reducing agents) like sodium sulfide, cyanides, amines etc. The objectives of this operation are mainly to:

The weakening of hair is dependent on the breakdown of the disulfide link of the amino acid called cystine, which is the characteristic of the keratin class of protein 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 covalent disulfide 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 in the hide (this is a tissue strengthening protein unrelated to keratin) is also shifted to around 4.7 due to liming.

Unhairing and scudding

Main article: Unhairing

Unhairing agents used at this time are: Sodium sulfide, sodium hydroxide, sodium hydrosulfite, calcium hydrosulfide, dimethyl amine, and Sodium sulfhydrate. The majority of hair is then removed mechanically, initially with a machine and then by hand using a dull knife, a process known as scudding.

Deliming and bating

Main article: Deliming

The pH of the collagen is brought down to a lower level so that 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.

Pickling

Once bating is complete, the hides and skins are treated first with salt and then with sulfuric acid, in case a mineral tanning is to be done. This is done to bring down the pH of collagen to a very low level so as to facilitate the penetration of mineral tanning agent into the substance. This process is known as pickling. The common salt (sodium chloride) penetrates the hide twice as fast as the acid and checks the ill effect of sudden drop of pH.

Peeling bark for the tannery in Prattsville, New York, during the 1840s, when it was the largest in the world.

Vegetable tanning

Vegetable tanning uses tannin. The tannins (a class of polyphenol astringent chemical) 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 catechu), and myrobalans from Terminalia spp., such as Terminalia chebula. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable tanned hide is flexible and is used for luggage and furniture.

Chrome tanning

Prior to the introduction of the basic chromium species in tanning, several steps are required to produce a tannable hide. These steps (all of them already noted above) include: scudding (removing the hair), liming (the introduction of alkali agents such as sodium hydroxide), deliming (restoring neutral pH), bating (softening the skin with enzymes), and pickling (lowering pH of the hide with salt and sulfuric acid).

The pH must be very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit in 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 blue and therefore referred to as "wet blue." Chrome tanning is faster than vegetable tanning (less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments.

Chemistry of chrome tanning

Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) has long been regarded as the most efficient and effective tanning agent.[8][9] Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium. 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,[10] 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, as well as "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-.[11] These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows for significant cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2−) are formed by hydrolysis of the collagen by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). The ionized carboxyl groups 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.[12] The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.

Possible Chromium(III) Tanning Mechanisms

Subsequent to application of the chromium agent, the bath is treated with sodium bicarbonate to increase the pH to 4.0–4.3. This increase induces cross-linking between the chromium and the collagen. The pH increase is normally accompanied by a gradual temperature increase up to 40 °C.[13] Chromium's ability to form such stable bridged bonds explains why it is considered one of the most efficient tanning compounds. Chromium-tanned leather can contain between 4 and 5% of chromium.[12] This efficiency is characterized by its increased hydrothermal stability of the skin, and its resistance to shrinkage in heated water.[9]

A 2013 study by the University of Dhaka found that, boiling and sun-drying can oxidize and convert the various chromium (III) compounds used in tanning into carcinogenic hexavalent 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.[14]

Tanning with other minerals

As chrome tanned hides and skins are called wet blue, other forms of tanning like the ones based on alum, zirconium, titanium, iron salts, or a combination thereof, lead to skins known as wet white. Wet white is also a semi-finished stage like wet blue, but is much more eco friendly. The shrinkage temperature of wet white varies from 70 to 85 degree Celsius, while that of wet blue varies from 95 to 100 degree Celsius.

Tawing with alum

Tawing is a method that uses alum and aluminium salts, generally in conjunction with other products such as egg yolk, flour, and other salts. The leather becomes tawed by soaking in a warm potash alum and salts solution, between 20 °C and 30 °C. The process increases the leather's pliability, stretchability, softness, and quality. Adding egg yolk and flour to the standard soaking solution further enhances its fine handling characteristics. Then, the leather is air dried ("crusted") for several weeks, which allows it to stabilize. Tawing is traditionally used on pigskins and goatskins to create the whitest colors. However, exposure and aging may cause slight yellowing over time and, if it remains in a wet condition, tawed leather will suffer from decay. Technically, tawing is not tanning.[15]

Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suedes, nubucks, etc. are finished by raising the nap of the leather by rolling with a rough surface.

The first stage is the preparation for tawing. The second stage is the actual tawing and other chemical treatment. The third stage, known as retawing, applies retawing agents and dyes to the material to provide the physical strength and properties desired depending on the end product. The fourth and final stage, known as finishing, is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired.

See also

References

  1. ↑ Reference dictionary, accessed July 22, 2009.
  2. ↑ Possehl, Gregory L. (1996). Mehrgarh in Oxford Companion to Archaeology, edited by Brian Fagan. Oxford University Press.
  3. ↑ Johnson, Steven (2006). The Ghost Map. New York: Riverhead Books. pp. 4,263. ISBN 978-1-59448-269-4.
  4. ↑ The Oxford English Dictionary, Second edition, Volume VI, ISBN 0-19-861218-4 entry: "grindery"
  5. ↑ James E. Churchill, The Complete Book of Tanning Skins and Furs, pages 1–2
  6. ↑ "Etherington and Roberts Dictionary". Foundation of the American Institute for Conservation. 2011-03-10. Retrieved 2011-10-14.
  7. ↑ "3. Tanneries, Description of the Tanning Process". Food and Agriculture Organization. Retrieved 2011-10-14.
  8. ↑ Wilson, J.A. The Chemistry of Leather Manufacture. The Chemical Catalog Company, Inc. New York 1923.
  9. ↑ 9.0 9.1 Covington, A. "Modern Tanning Chemistry" Chemical Society Review 1997, volume 26, 111–126. doi:10.1039/CS9972600111
  10. ↑ Harlan, J.; Feairheller, S.; Adv. Exp. Med. Biol. 1977, 86A, 425.
  11. ↑ Heidemann, E.; J. Soc. Leather Technol. Chem., 1982, 66, 21.
  12. ↑ 12.0 12.1 Gustavson, K.H. "The Chemistry of Tanning Processes" Academic Press Inc., New York, 1956.
  13. ↑ Heidemann, E.; Leather. Ullmann's Encyclopedia of Industrial Chemistry,2005. doi:10.1002/14356007.a15_259
  14. ↑ http://www.gulf-times.com/bangladesh/245/details/398475/toxic-poultry-feed-threatens-bangladesh’s-poor
  15. ↑ Dictionary of Descriptive Terminology: tawing.

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