HOME

TheInfoList




Tanning is the process of treating skins and hides of
animals Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...

animals
to produce
leather Leather is a strong, flexible and durable material obtained from the tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of arti ...

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 Decomposition is the process by which dead organic substance , CH4; is among the simplest organic compounds. In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds com ...
, and also possibly coloring it. 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 , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark ...
, an
acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
ic
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
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

The English word for tanning is from medieval Latin , derivative of (
oak bark Tanbark is the bark of certain species of trees. It is traditionally used for tanning hides into leather Leather is a durable and flexible material created by tanning animal rawhide and skins. The most common raw material is cattle hide ...

oak bark
), 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, German abbr. ) 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 ...
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 waterskins, bags, harnesses and tack, boats,
armour Armour (British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and usage a ...
,
quiver A quiver is a container for holding arrow s and nock. An arrow is a fin-stabilized projectile launched by a bow. A typical arrow usually consists of a long, stiff, straight ''shaft'' with a weighty (and usually sharp and pointed) ''arrowh ...

quiver
s,
scabbard A scabbard is a sheath for holding a sword A sword is a Edged and bladed weapons, bladed melee weapon intended for cutting or thrusting that is longer than a knife or dagger, consisting of a long blade attached to a hilt. The precise definiti ...
s,
boot A boot, plural boots, is a type of specific footwear Footwear refers to garments worn on the feet, which typically serves the purpose of protective clothing, protection against adversities of the environment such as ground textures and tempera ...

boot
s, and
sandal Sandals are an open type of footwear Footwear refers to garments worn on the feet, which originally serves to purpose of protective clothing, protection against adversities of the environment, usually regarding ground textures and temperature. ...
s. Tanning was being carried out by the inhabitants of
Mehrgarh Mehrgarh (; ur, ) is a Neolithic The Neolithic period is the final division of the Stone Age, with a wide-ranging set of developments that appear to have arisen independently in several parts of the world. It is first seen about 12,000 ye ...

Mehrgarh
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 25st century BC was a century A century is a period of 100 years. Centuries are numbered ordinally in English and many other languages. The word ''century'' comes from the Latin ''centum'', meaning ''one hundred''. ''Century'' is sometimes a ...
, the
Sumer Sumer ()The name is from Akkadian language, Akkadian '; Sumerian language, Sumerian ''kig̃ir'', written and ,approximately "land of the civilized kings" or "native land". means "native, local", iĝir NATIVE (7x: Old Babylonian)from ''The ...

Sumer
ians began using leather, affixed by
copper Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and electrical conductivity. A fre ...

copper
studs, on
chariot A chariot is a type of carriage A carriage is a private four-wheeled vehicle for people and is most commonly horse-drawn A horse-drawn vehicle is a mechanized piece of equipment pulled by one horse or by a team of horses. These vehicles ...

chariot
wheel File:Roue primitive.png, An early wheel made of a solid piece of wood A wheel is a circular component that is intended to rotate on an axle An axle or axletree is a central shaft for a rotating wheel or gear. On wheeled vehicles, the ...

wheel
s. 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 tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Tri ...

flesh
and
fat In nutrition Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided ...

fat
. Hair was removed by soaking the skin in
urine Urine is a liquid by-product A by-product or byproduct is a secondary product derived from a production process, process or ; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

urine
, painting it with an
alkaline In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of ato ...

alkaline
lime Lime 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 * Austra ...
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
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 A pine is any Pinophyta, conifer in the genus ''Pinus'' () of the family (biology), family Pinaceae. ''Pinus'' is the sole genu ...
,
alum An alum () is a type of chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

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
suture
suture
s 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

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 Brine is a high-concentration solution of salt (NaCl) in water (H2O). In different contexts, ''brine'' may refer to salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of solutions used for brining food ...

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),
delimingThe deliming operation in leather Leather is a durable and flexible material created by tanning animal rawhide and skins. The most common raw material is cattle hide. It can be produced at manufacturing scales ranging from artisan to modern ...
, 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,
biocide A biocide is defined in the European legislation as a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...
s, typically dithiocarbamates, may be used. Fungicides such as 2-thiocyanomethylthiobenzothiazole may also be added later in the process, to protect wet leathers from mold growth. After 1980, the use of
pentachlorophenol Pentachlorophenol (PCP) is an Organochloride, organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latte ...

pentachlorophenol
and
mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

mercury
-based biocides and their derivatives was forbidden.


Liming

After soaking, the hides are treated with
milk of lime Limewater is the common name for a dilute aqueous solution of calcium hydroxide. Calcium hydroxide, Ca(OH)2, is sparsely soluble at room temperature in water (1.5 g/L at 25 °C). "Pure" (i.e. less than or fully saturated) limewater is cl ...
(a basic agent) typically supplemented by "sharpening agents" (disulfide reducing agents) such as sodium
sulfide Sulfide (British English British English (BrE) is the standard dialect of the English language English is a West Germanic languages, West Germanic language first spoken in History of Anglo-Saxon England, early medieval England, whi ...

sulfide
,
cyanides A cyanide is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemi ...
,
amines In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. ...
, 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 Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

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 dimer form of the amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemica ...
, which is the characteristic of the
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
class of proteins that gives strength to hair and
wool Wool is the textile A textile is a flexible material made by creating an interlocking bundle of yarn Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitti ...
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 A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and take ...

covalent
disulfide bond In biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-Europe ...
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 A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the ...
of the collagen (a tissue-strengthening protein unrelated to keratin) in the hide is also shifted to around pH 4.7 due to liming. 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 Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

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 compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composit ...

Chromium(III) sulfate
() 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 Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is ...
, although the latter arises in inadequate waste treatment. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, r(H2O)6sup>3+, which at higher pH undergoes processes called
olationIn inorganic chemistry features unusual bonding B: Caesium chloride Caesium chloride or cesium chloride is the inorganic compound with the formula Caesium, CsChloride, Cl. This colorless salt is an important source of caesium ions in a variety of ...
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 A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * Chin ...
ing of the collagen subunits. The chemistry of 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 compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s, and masking agents. Masking agents are
carboxylic acid A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group. The general formula of a carboxylic acid is R−COOH or R−CO2H, with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
s, such as
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

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 Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
,
proline Proline (symbol Pro or P) is an organic acid classed as a proteinogenic amino acid Proteinogenic amino acids are amino acids that are incorporated biosynthetically into proteins during translation (biology), translation. The word "proteinogenic ...

proline
, and
hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5H9 O3 N), is an amino acid Amino acids are organic compounds that contain amino (–NH2) and Carboxylic acid, carboxyl (–COOH) functional groups, along with a Substituent, side chain ...

hydroxyproline
, 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 A hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of ...
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 , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark ...
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, were water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the mai ...
s and used in modern times are
chestnut The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. They are native to temperate climate, temperate regions of the Northern Hemisphere. The name also refers to the edible nut (fruit), nut ...

chestnut
,
oak An oak is a tree In botany, a tree is a perennial plant with an elongated Plant stem, stem, or trunk (botany), trunk, supporting branches and leaves in most species. In some usages, the definition of a tree may be narrower, including on ...

oak
, redoul,
tanoak ''Notholithocarpus densiflorus'', commonly known as the tanoak or tanbark-oak, is a broadleaf tree A broad-leaved, broad-leaf, or broadleaf tree is any tree within the diverse botanical group of angiosperms The flowering plants, also known as ...
,
hemlock Hemlock may refer to: Plants *Several poisonous plants in the family Apiaceae **''Cicuta'' (water hemlock) **''Conium'', four species, of which ''maculatum'' is the only endemic outside of southern Africa; in history given to poison and execute p ...
, quebracho,
mangrove A mangrove is a shrub A shrub (often called a bush) is a small- to medium-sized perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Pl ...

mangrove
, (acacia; see
catechol Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is a toxic organic compound with the molecular formula C6H4(OH)2. It is the ''ortho'' isomer In chemistry Chemistry is the scientific discipline involved with Chemical e ...

catechol
), 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 South Asia is the southern region of Asia, which is defined in both geography, geographical and culture, ethno-cultur ...

Terminalia chebula
''. In
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country A landlocked country is a country A country is a distinct territory, territorial body or political entity. It is often referred to as the ...

Ethiopia
, the combined vegetable oils of Niger seed () and flaxseeds were used in treating the flesh side of the leather. In
Yemen ) , image_map = Yemen on the globe (Yemen centered).svg , map_caption = , image_map2 = , capital = Sana'a Sanaa ( ar, صَنْعَاء, ' , Yemeni Arabic: ; Old South Arabian: 𐩮 ...

Yemen
and
Egypt Egypt ( ar, مِصر, Miṣr), officially the Arab Republic of Egypt, is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses. Generally identi ...

Egypt
, 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 Chemically, an aldehyde is a compound containing a functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same ...

aldehyde
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 mummification, mummified in a Bog, peat bog. Such bodies, sometimes known as bog people, are both geographically and chronologically widespread, having been dated to between and the Second Wor ...
.


Tawing

Tawing is a method that uses
alum An alum () is a type of chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

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 In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structu ...
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 Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its so ...
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 repor ...
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 León () is the second largest city in Nicaragua Nicaragua (; ), officially the Republic of Nicaragua (), is the largest country in the Central American isthmus, bordered by Honduras to the northwest, the Caribbean to the east, Costa Ric ...

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 Cancer is a group of diseases involving abnormal cell growth with the potential to invade or Metastasis, spread to other part ...
hexavalent chromium Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium Chromium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is ...
, or chromium(VI). This hexavalent chromium runoff and scraps are then consumed by animals, in the case of Bangladesh,
chickens The chicken (''Gallus gallus domesticus''), a subspecies of the red junglefowl, is a type of domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influen ...
(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 , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's a ...

Methylisothiazolinone
, 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 Benzene is an organic chemical compound A chemical compound is a chemical substance composed of many identical molecules (or ...

Anthracene
, 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 A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide A nuclide (or nucleide, from atomic nucleus, nucleus, also known as nuclear species) is a class of atoms characte ...
.
Formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
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, responsible management of waste products."


Alternatives

As an alternative to tanning, hides can be dried to produce rawhide rather than leather.


Associated processes

Leftover leather would historically be turned into
glue Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advantag ...
. 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 Image:Schuhmacher-1568.png, Woodcut of shoemakers from 1568. 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 ...

shoemaker
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