![Tanning in Fes Morocco there worked with different colors](https://upload.wikimedia.org/wikipedia/commons/1/1d/Tanning_in_Fes_Morocco_there_worked_with_different_colors.jpg)
The leather manufacturing process is divided into three sub-processes: preparatory stages,
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 artificial light in place of the sun
**Sunless tanning, application of a stain or dye t ...
and crusting. All true
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 hogs, ...
s 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.
Production management
The
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 hogs, ...
making
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
is in general restricted to
batch
Batch may refer to:
Food and drink
* Batch (alcohol), an alcoholic fruit beverage
* Batch loaf, a type of bread popular in Ireland
* A dialect term for a bread roll used in North Warwickshire, Nuneaton and Coventry, as well as on the Wirral ...
processing, but if the surface coating sub-process is added then some continuous processing can be included. The
operation
Operation or Operations may refer to:
Arts, entertainment and media
* ''Operation'' (game), a battery-operated board game that challenges dexterity
* Operation (music), a term used in musical set theory
* ''Operations'' (magazine), Multi-Ma ...
flow has to follow the preparatory → tanning → crusting → surface coating sub-process order without deviation, but some of the sub-processes can be omitted to make certain leathers (or partially tanned/ untanned products).
Preparatory stages
The preparatory stages are when the hide/skin is prepared for tanning. During the preparatory stages many of the unwanted raw skin components are removed. Many options for pretreatment of the skin exist. Not all of the options may be performed. Preparatory stages may include:
* preservation- the hide/skin is treated with a method which renders it temporarily imputrescible.
*
soaking - water for purposes of washing or rehydration is reintroduced.
*
liming - unwanted proteins and "opening up" is achieved.
*
unhairing - the majority of hair is removed.
*
fleshing - subcutaneous material is removed.
*
splitting
Splitting may refer to:
* Splitting (psychology)
* Lumpers and splitters, in classification or taxonomy
* Wood splitting
* Tongue splitting
* Splitting, railway operation
Mathematics
* Heegaard splitting
* Splitting field
* Splitting principle
* ...
- the hide/skin is cut into two or more horizontal layers.
*
reliming - the hide/skin is further treated to achieve more "opening up" or more protein removal.
*
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 i ...
- liming and unhairing chemicals are removed from the pelt.
*
bating - proteolytic proteins are introduced to the skin to remove further proteins and to assist with softening of the pelt.
*
degreasing
Degreasing, often called defatting or fat trimming, is the removal of fatty acids from an object. In culinary science, degreasing is done with the intention of reducing the fat content of a meal.
Degreasing food
Degreasing is often used by diete ...
- natural fats/oils are stripped or as much as is possible from the hide/skin.
*
frizing
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 o ...
- physical removal of the fat layer inside the skin. Also similar to Slicking.
*
bleaching
Bleach is the generic name for any chemical product that is used industrially or domestically to remove color (whitening) from a fabric or fiber or to clean or to remove stains in a process called bleaching. It often refers specifically, to ...
- chemical modification of dark pigments to yield a lighter coloured pelt.
*
pickling
Pickling is the process of preserving or extending the shelf life of food by either anaerobic fermentation in brine or immersion in vinegar. The pickling procedure typically affects the food's texture and flavor. The resulting food is called a ...
- lowering of the pH value to the acidic region. Must be done in the presence of salts. Pickling is normally done to help with the penetration of certain tanning agents, e.g., chromium (and other metals), aldehydic and some polymeric tanning agents
*
depickling - raising of the pH out of the acidic region to assist with penetration of certain tanning agents
Tanning
![Igualada-Museu de la Pell (7)](https://upload.wikimedia.org/wikipedia/commons/8/80/Igualada-Museu_de_la_Pell_%287%29.JPG)
Tanning is the process that converts the protein of the raw hide or skin into a stable material which will not putrefy and is suitable for a wide variety of end applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that can putrefy when re-wetted (''wetted back''), while tanned material dries out to a flexible form that does not become putrid when wetted back. A large number of different tanning methods and materials can be used; the choice is ultimately dependent on the end application of the leather. The most commonly used tanning material is
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
, which leaves the leather, once tanned, a pale blue colour (due to the chromium), this product is commonly called “wet blue”.
The acidity of hides once they have finished pickling will typically be between pH of 2.8-3.2. At this point the hides are loaded in a drum and immersed in a float containing the tanning liquor. The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be made to see the penetration by cutting the cross section of a hide and observing the degree of penetration. Once an even degree of penetration is observed, the pH of the float is slowly raised in a process calle
basification This basification process fixes the tanning material to the leather, and the more tanning material fixed, the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned would typically finish somewhere between 3.8 and 4.2.
Crusting
![Ledertrocknung in Suai](https://upload.wikimedia.org/wikipedia/commons/1/1c/Ledertrocknung_in_Suai.jpg)
Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations:
* wetting back- semi-processed leather is
rehydrated
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
.
*
sammying - 45-55%(m/m) water is squeezed out the leather.
*
splitting
Splitting may refer to:
* Splitting (psychology)
* Lumpers and splitters, in classification or taxonomy
* Wood splitting
* Tongue splitting
* Splitting, railway operation
Mathematics
* Heegaard splitting
* Splitting field
* Splitting principle
* ...
- the leather is split into one or more horizontal layers.
*
shaving
Shaving is the removal of hair, by using a razor or any other kind of bladed implement, to slice it down—to the level of the skin or otherwise. Shaving is most commonly practiced by men to remove their facial hair and by women to remove the ...
- the leather is thinned using a machine which cuts leather fibres off.
*
neutralisation - the pH of the leather is adjusted to a value between 4.5 and 6.5.
*
retanning - additional tanning agents are added to impart properties.
*
dyeing
Dyeing is the application of dyes or pigments on textile materials such as fibers, yarns, and fabrics with the goal of achieving color with desired color fastness. Dyeing is normally done in a special solution containing dyes and particular ...
- the leather is coloured.
*
fatliquoring - fats/oils and waxes are fixed to the leather fibres.
*
filling
Filling may refer to:
* a food mixture used for stuffing
* Icing (food), Frosting used between layers of a cake
* Dental restoration
* Symplectic filling, a kind of cobordism in mathematics
* Part of the leather crusting process
See also
* Fi ...
- heavy/dense chemicals that make the leather harder and heavier are added.
*
stuffing
Stuffing, filling, or dressing is an edible mixture, often composed of herbs and a starch such as bread, used to fill a cavity in the preparation of another food item. Many foods may be stuffed, including poultry, seafood, and vegetables. A ...
- fats/oils and waxes are added between the fibres.
*
stripping - superficially fixed tannins are removed.
*
whitening - the colour of the leather is lightened.
*
fixation - all unbound chemicals are chemically bonded/trapped or removed from the leather
*
setting
Setting may refer to:
* A location (geography) where something is set
* Set construction in theatrical scenery
* Setting (narrative), the place and time in a work of narrative, especially fiction
* Setting up to fail a manipulative technique to eng ...
- area, grain flatness are imparted and excess water removed.
*
drying
Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered ...
- the leather is dried to various moisture levels (commonly 14-25%).
*
conditioning Conditioning may refer to:
Science, computing, and technology
* Air conditioning, the removal of heat from indoor air for thermal comfort
** Automobile air conditioning, air conditioning in a vehicle
** Ice storage air conditioning, air condition ...
- water is added to the leather to a level of 18-28%.
*
softening
In physics and astronomy, an ''N''-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity (see ''n''-body problem for other applications). ''N''-body simulations ar ...
- physical softening of the leather by separating the leather fibres.
*
buffing
Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, ''polishing'' refers to processes that uses an abrasive that is glued to the work wheel, whil ...
- abrasion of the surfaces of the leather to reduce nap or grain defects.
Surface coating
For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include:
*
oiling
*
brushing
*
padding
Padding is thin cushioned material sometimes added to clothes. Padding may also be referred to as batting when used as a layer in lining quilts or as a packaging or stuffing material. When padding is used in clothes, it is often done in an attempt ...
*
impregnation
Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...
*
buffing
Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, ''polishing'' refers to processes that uses an abrasive that is glued to the work wheel, whil ...
*
spraying
*
roller coating
*
curtain coating Curtain coating is a process that creates an uninterrupted curtain of fluid that falls onto a substrate. The substrate is transported on a conveyor belt or calender rolls at a regulated speed through the curtain to ensure an even coat of the die. ...
*
polishing
Polishing is the process of creating a smooth and shiny surface by rubbing it or by applying a chemical treatment, leaving a clean surface with a significant specular reflection (still limited by the index of refraction of the material accordin ...
*
plating
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improv ...
*
embossing
*
ironing
Ironing is the use of a machine, usually a heated tool (an iron), to remove wrinkles and unwanted creases from fabric. The heating is commonly done to a temperature of 180–220 °Celsius (356-428 Fahrenheit), depending on the fabric. Ironing wor ...
*
combing (hair-on)
*
glazing
Environmental impact
In addition to the
other environmental impacts of leather, the production processes have a high environmental impact, most notably due to:
* the heavy use of polluting chemicals in the tanning process
* air pollution due to the transformation process (
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...
during dehairing and
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
during deliming, solvent vapours).
One tonne of hide or skin generally leads to the production of 20 to 80 m3 of turbid and foul-smelling wastewater, including chromium levels of 100–400 mg/L, sulfide levels of
200–800 mg/L and high levels of fat and other solid wastes, as well as notable pathogen contamination. Pesticides are also often added for hide conservation during transport.
With solid wastes representing up to 70% of the wet weight of the original hides, the tanning process comes at a considerable strain on water treatment installations.
Tanning is especially polluting in countries where environmental norms are lax, such as in India - the world's 3rd largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg.
VOC
VOC, VoC or voc may refer to:
Science and technology
* Open-circuit voltage (VOC), the voltage between two terminals when there is no external load connected
* Variant of concern, a category used during the assessment of a new variant of a virus
* ...
emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. Very clearly, the process remains highly polluting all the same. A review of the total pollution load decrease achievable according to the
United Nations Industrial Development Organization
The United Nations Industrial Development Organization (UNIDO) (French: Organisation des Nations unies pour le développement industriel; French/Spanish acronym: ONUDI) is a specialized agency of the United Nations that assists countries in e ...
posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that « Even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries. »
In
Kanpur
Kanpur or Cawnpore ( /kɑːnˈpʊər/ pronunciation (help·info)) is an industrial city in the central-western part of the state of Uttar Pradesh, India. Founded in 1207, Kanpur became one of the most important commercial and military stations o ...
, the self-proclaimed "Leather City of the World" and a city of 3 million people on the banks of the river
Ganges
The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...
, pollution levels were so high that, despite an industry crisis, the pollution control board has decided to seal 49 high-polluting tanneries out of 404 in July 2009. In 2003 for instance, the main tannery's effluent disposal unit was dumping 22 tonnes of chromium-laden solid waste per day in the open.
The higher cost associated to the treatment of effluents as compared to untreated effluent discharging leads to
environmental dumping
Environmental dumping is the practice of transfrontier shipment of waste (household waste, industrial/nuclear waste, etc.) from one country to another. The goal is to take the waste to a country that has less strict environmental laws, or environm ...
to reduce costs. For instance, in
Croatia
, image_flag = Flag of Croatia.svg
, image_coat = Coat of arms of Croatia.svg
, anthem = "Lijepa naša domovino"("Our Beautiful Homeland")
, image_map =
, map_caption =
, capit ...
in 2001, proper pollution abatement cost 70-100 USD/t of raw hides processed against US$43/t for irresponsible behaviour.
No general study seems to exist, but the current news is rife with documented examples of untreated effluent discharge. In November 2009 for instance, it was discovered that one of Uganda's main leather producing companies directly dumped its waste water in a wetland adjacent to
Lake Victoria
Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
.
Nevertheless, several researchers have developed cleaner leather processing methodologies to reduce the environmental impact of conventional leather processing and to lower the burden of end-of-pipe treatment. They include salt-free preservation systems, enzyme-assisted low-sulfide hair removal processes, chemical-free enzymatic fiber opening, ammonia-free
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 i ...
systems, pickle-free chrome
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 artificial light in place of the sun
**Sunless tanning, application of a stain or dye t ...
process,
waterless chrome tanning technology, chrome-free
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 artificial light in place of the sun
**Sunless tanning, application of a stain or dye t ...
methods, formaldehyde-free
syntans
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 solvent-free finishing systems.
Some of these processes have found commercial acceptance.
References
External links
* (includes several diagrams)
*
{{DEFAULTSORT:Leather Production Processes
Leathermaking