![Tanned leather](https://upload.wikimedia.org/wikipedia/commons/7/74/Tanned_leather.JPG)
Tanning is the process of treating
skins and
hides of
animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
to produce
leather
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 hog ...
. 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
Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
and coloring.
Before tanning, the skins are dehaired, degreased, desalted and soaked in water over a period of six hours to two days. Historically this process was considered a noxious or "odoriferous trade" and relegated to the outskirts of town.
Historically, tanning used
tannin
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
The term ''tannin'' (from Anglo-Norman ''tanner'' ...
, an
acidic
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
from which the tanning process draws its name, derived from the bark of certain trees. An alternative method, developed in the 1800s, is chrome tanning, where chromium salts are used instead of natural tannins.
History
![Peeling hemlock bark for tanning](https://upload.wikimedia.org/wikipedia/commons/2/2f/Peeling_hemlock_bark_for_tanning.jpg)
The English word for tanning is from medieval Latin , derivative of (
oak bark
Tanbark is the bark of certain species of trees, traditionally used for tanning hides into leather.
The words "tannin", "tanning", "tan," and " tawny" are derived from the Medieval Latin ''tannare'', "to convert into leather."
Bark mills are ...
), from French (tanbark), from old-Cornish (red oak). These terms are related to the hypothetical Proto-Indo-European *' meaning 'fir tree'. (The same word is source for
Old High German
Old High German (OHG; german: Althochdeutsch (Ahd.)) is the earliest stage of the German language, conventionally covering the period from around 750 to 1050.
There is no standardised or supra-regional form of German at this period, and Old High ...
meaning 'fir', related to modern ''
Tannenbaum''). Despite the linguistic confusion between quite different conifers and oaks, the word ''tan'' referring to dyes and types of hide preservation is from the Gaulic use referencing the bark of oaks (the original source of tannin), and not fir trees.
Ancient civilizations used leather for
waterskin
A waterskin is a receptacle used to hold water. Normally made of a sheep or goat skin, it retains water naturally and therefore was very useful in desert crossings until the invention of the canteen, though waterskins are still used in some par ...
s, bags, harnesses and tack, boats,
armour,
quiver
A quiver is a container for holding arrows, bolts, ammo, projectiles, darts, or javelins. It can be carried on an archer's body, the bow, or the ground, depending on the type of shooting and the archer's personal preference. Quivers were trad ...
s,
scabbard
A scabbard is a sheath for holding a sword, knife, or other large blade. As well, rifles may be stored in a scabbard by horse riders. Military cavalry and cowboys had scabbards for their saddle ring carbine rifles and lever-action rifles on the ...
s,
boots, and
sandal
Sandals are an open type of footwear, consisting of a sole held to the wearer's foot by straps going over the instep and around the ankle. Sandals can also have a heel. While the distinction between sandals and other types of footwear can ...
s. Tanning was being carried out by the inhabitants of
Mehrgarh
Mehrgarh (; ur, ) is a Neolithic archaeological site (dated ) situated on the Kacchi Plain of Balochistan in Pakistan. It is located near the Bolan Pass, to the west of the Indus River and between the modern-day Pakistani cities of Quetta, ...
in Pakistan between 7000 and 3300 BCE.
[Possehl, Gregory L. (1996). ''Mehrgarh'' in ''Oxford Companion to Archaeology'', edited by Brian Fagan. Oxford University Press.] Around
2500 BCE
The 25th century BC was a century that lasted from the year 2500 BC to 2401 BC.
Events
* c. 2900–2334 BC: Mesopotamian wars of the Early Dynastic period.
* c. 2560 BC: Construction of the Great Pyramid of Giza is completed.
* c. 2500 BC: R ...
, the
Sumerians began using leather, affixed by
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
studs, on
chariot wheels.
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
Flesh is any aggregation of soft tissues of an organism. Various multicellular organisms have soft tissues that may be called "flesh". In mammals, including humans, ''flesh'' encompasses muscles, fats and other loose connective tissues, but ...
and
fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
. Hair was removed by soaking the skin in
urine
Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra.
Cellular ...
, painting it with an
alkaline lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
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.
Historically the actual tanning process used vegetable tanning. In some variations of the process,
cedar oil
Cedar oil, also known as cedarwood oil, is an essential oil derived from various types of conifers, most in the pine or cypress botanical families. It is produced from the foliage, and sometimes the wood, roots, and stumps left after logging of tr ...
,
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.
Preparation
![Rabbit pelt tanned](https://upload.wikimedia.org/wikipedia/commons/1/11/Rabbit_pelt_tanned.jpg)
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. This can be done by the tanner, or by obtaining a skin at a slaughterhouse, farm, or local fur trader.
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
The deliming operation in leather processing is a drum/paddle or pit based operation where two main objectives are met:
* Removal of alkali from the pelt and the consequent deswelling of the fibres.
* Lowering of the liquor pH to the values used ...
, bating or puering, drenching, and pickling.
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
dithiocarbamate
In organic chemistry, a dithiocarbamate is a functional group with the general formula and structure . It is the analog of a carbamate in which both oxygen atoms are replaced by sulfur atoms (when only 1 oxygen is replaced the result is thioca ...
s, 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.
Liming
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
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...
,
amines
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...
, etc.
This:
* Removes the hair and other keratinous matter
* Removes some of the interfibrillary soluble proteins such as mucins
* Swell up and split up the fibres 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
Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mec ...
, which is the characteristic of the
keratin
Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
class of proteins that gives strength to hair and
wool
Wool is the textile fibre obtained from sheep and other mammals, especially goats, rabbits, and camelids. The term may also refer to inorganic materials, such as mineral wool and glass wool, that have properties similar to animal wool. ...
s (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
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
disulfide bond
In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...
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
The isoelectric point (pI, pH(I), IEP), is the pH at which a molecule carries no net electrical charge or is electrically neutral in the statistical mean. The standard nomenclature to represent the isoelectric point is pH(I). However, pI is also u ...
of the collagen (a tissue-strengthening protein unrelated to keratin) in the hide is also shifted to around pH 4.7 due to liming.
![ManCleaningAnimalSkin](https://upload.wikimedia.org/wikipedia/commons/a/a2/ManCleaningAnimalSkin.jpg)
Any hairs remaining after liming are removed mechanically by scraping the skin with a dull knife, a process known as scudding.
Deliming and bating
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.
Pickling
Once bating is complete, the hides and skins are treated first with common salt (sodium chloride) 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 of 2.8-3.0 so as to facilitate the penetration of mineral tanning agent into the skin substrate. This process is known as pickling. The salt penetrates the hide twice as fast as the acid and suppresses the swelling effect of the sudden drop of pH.
Process
Chrome tanning
Chromium(III) sulfate
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 ei ...
() has long been regarded as the most efficient and effective tanning agent.
[ Chromium(III) compounds of the sort used in tanning are significantly less toxic than ]hexavalent chromium
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...
, although the latter arises in inadequate waste treatment. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, 2O)6">r(H2O)6sup>3+, which at higher pH undergoes processes called olation to give polychromium(III) compounds that are active in tanning, being the cross-link
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ing of the collagen subunits. The chemistry of 2O)6">r(H2O)6sup>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 acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s, 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
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid ( carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinog ...
, proline, and hydroxyproline
(2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank.
Structure and discovery
In 1902, Hermann Emil Fischer isolated hydroxyproline from hydrolyzed gelati ...
, usually in the repeat -gly-pro-hypro-gly-. These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows cross-linking by hydrogen bonding
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
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.[Gustavson, K.H. "The Chemistry of Tanning Processes" Academic Press Inc., New York, 1956.] The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.
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. 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.[ This efficiency is characterized by its increased hydrothermal stability of the skin, and its resistance to shrinkage in heated water.][Covington, A. "Modern Tanning Chemistry" Chemical Society Review 1997, volume 26, 111–126. ]
Vegetable tanning
Vegetable tanning uses tannin
Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids.
The term ''tannin'' (from Anglo-Norman ''tanner'' ...
s (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 mill
Bark mills, also known as Catskill's mills, are water, steam, horse, ox or wind-powered edge mills used to process the bark, roots, and branches of various tree species into a fine powder known as tanbark, used for tanning leather. This powder ...
s and used in modern times are chestnut, oak
An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...
, redoul, tanoak
''Notholithocarpus densiflorus'', commonly known as the tanoak or tanbark-oak, is a broadleaf tree in the family Fagaceae, and the type species of the genus ''Notholithocarpus''. It is native to the far western United States, particularly Oregon ...
, hemlock, quebracho, mangrove
A mangrove is a shrub or tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evolution in severa ...
, wattle (acacia; see catechol
Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula . It is the ''ortho'' isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amoun ...
), and myrobalans from ''Terminalia'' spp., such as ''Terminalia chebula
''Terminalia chebula'', commonly known as black- or chebulic myrobalan, is a species of '' Terminalia'', native to South Asia from India and Nepal east to southwest China (Yunnan), and south to Sri Lanka, Malaysia, and Vietnam.Flora of China''Te ...
''. In Ethiopia
Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...
, the combined vegetable oils of Niger seed (Guizotia abyssinica
''Guizotia abyssinica'' is an erect, stout, branched annual herb, grown for its edible oil and seed. Its cultivation originated in the Eritrean and Ethiopian highlands, and has spread to other parts of Ethiopia. Common names include noog/nug ( ...
) and flaxseeds were used in treating the flesh side of the leather. In Yemen
Yemen (; ar, ٱلْيَمَن, al-Yaman), officially the Republic of Yemen,, ) is a country in Western Asia. It is situated on the southern end of the Arabian Peninsula, and borders Saudi Arabia to the Saudi Arabia–Yemen border, north and ...
and Egypt
Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
, hides were cured by soaking them in a bath containing the crushed leaves and bark of the Salam acacia (Acacia etbaica; A. nilotica kraussiana). 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 aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s, 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
A bog body is a human cadaver that has been naturally mummified in a peat bog. Such bodies, sometimes known as bog people, are both geographically and chronologically widespread, having been dated to between and the Second World War. Fischer ...
.
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
Potash () includes various mined and manufactured salts that contain potassium in water-soluble form. alum and salts solution, between 20 and 30 °C. 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.
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
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. Chemicals used in tanned leather production increase the levels of chemical oxygen demand
In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution whic ...
and total dissolved solids
Total dissolved solids (TDS) is a measure of the dissolved combined content of all inorganic and organic substances present in a liquid in molecular, ionized, or micro-granular ( colloidal sol) suspended form. TDS concentrations are often report ...
in water when not disposed of responsibly. These processes also use large quantities of water and produce large amounts of pollutants.
The tannery in León, Nicaragua, has also been identified as a source of major river pollution.
Boiling and sun drying can oxidize and convert the various chromium(III) compounds used in tanning into carcinogenic
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
hexavalent chromium
Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...
, 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.
Chromium is not solely responsible for these diseases. Methylisothiazolinone
Methylisothiazolinone, MIT, or MI, is the organic compound with the formula S(CH)2C(O)NCH3. It is a white solid. Isothiazolinones, a class of heterocycles, are used as biocides in numerous personal care products and other industrial applications ...
, which is used for microbiological protection (fungal or bacterial growth), causes problems with the eyes and skin. Anthracene
Anthracene is a solid polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings. It is a component of coal tar. Anthracene is used in the production of the red dye alizarin and other dyes. Anthracene is col ...
, which is used as a leather tanning agent, can cause problems in the kidneys and liver and is also considered a carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
. Formaldehyde
Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
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. 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.
The UN Leather Working Group (LWG) "provides an environmental audit protocol, designed to assess the facilities of leather manufacturers," for " traceability, energy conservation, ndresponsible management of waste products."
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. Impregnation of the hide's cells with formaldehyde (from smoke) offers some microbial and water resistance.
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 shoemaker
Shoemaking is the process of making footwear.
Originally, shoes were made one at a time by hand, often by groups of shoemakers, or cobblers (also known as '' cordwainers''). In the 18th century, dozens or even hundreds of masters, journeymen ...
s' tools and materials for sale.[The Oxford English Dictionary, Second edition, Volume VI, entry: "grindery"]
There are several solid and waste water treatment methodologies currently being researched, such as anaerobic digestion of solid wastes and wastewater sludge.
See also
* Tanwater
References
External links
"Home Tanning of Leather and Small fur Skins" (pub. 1962)
hosted by th
UNT Government Documents Department
{{Authority control
Leathermaking
Manufacturing