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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, 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 called basification.
Vegetable-tanned leather is tanned using tannin (hence the name
"tanning") and other ingredients found in vegetable matter, tree bark, and other
such sources. It is supple and brown in color, with the exact shade depending on
the mix of chemicals and the color of the skin. Vegetable-tanned leather is not
stable in water; it tends to discolor, and if left to soak and then dry it will shrink
and become less supple and harder. In hot water, it will shrink drastically and
partly gelatinize, becoming rigid and eventually brittle. Boiled leather is an
example of this where the leather has been hardened by being immersed in hot
water, or in boiled wax or similar substances. Historically, it was occasionally
used as armour after hardening, and it has also been used for book binding. This
is the only form of leather suitable for use in leather carving or stamping.
Chrome-tanned leather, invented in 1858, is tanned using chromium
sulfate and other salts of chromium. It is more supple and pliable than vegetabletanned leather, and does not discolor or lose shape as drastically in water as
vegetable-tanned. It is also known as wet-blue for its color derived from the
chromium. More esoteric colors are possible using chrome tanning.
Tanning is especially polluting in countries where environmental norms are
lax, such as in India - the world's 3d largest producer and exporter of leather. To
give an example of an efficient pollution prevention system, chromium loads per
produced ton are generally abated from 8 kg to 1.5 kg.
One ton 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.
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In Kanpur, the self-proclaimed "Leather City of World" and a city of 3
million people on the banks of the river Ganges, 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.
[10]
In 2003 for instance, the main
tannery's effluent disposal unit was dumping 22 tons of chromium-laden solid
waste per day in the open.
Leather tanning is without a doubt one of the oldest human activities. In
the beginning, skins obtained from hunting and livestock breeding could be used
for clothing as a protection from the atmospheric elements.
The tanning process has undergone many changes from prehistory to today,
especially in the twentieth century, when industrialization and new machines
have allowed development in the research for specific and less polluting
chemicals and new methods of tanning and finishing.
Leather is produced traditionally even today. The skin, discard of the food
industry, is "recycled" from the tanneries and processed with advanced
machinery and vast research, in such a way to make it a ―noble‖ and fashionable
material. There is a large number of tanneries, but the uniqueness of the result is
given by the ability and the skill of experienced craftsmen.
Immediately after killing the animal, in order to avoid degradation
processes in the tissues, the skin is salted, dried or refrigerated before the
production process of tanning starts. Skins are salted with common marine salt,
which penetrates very quickly into the fibers, helping to a partial removal of
water. This is a very efficient and economical process, easy to apply and
widespread.
Tanning is the process which converts the protein of the raw hide or skin
into a stable material which will not putrefy and it is suitable for a wide variety of
end applications, the leather.
The leather manufacturing process can be basically divided into three
major phases: tanning, retanning and finishing.
All chemical tanning operations are used to stabilize the skin making it
unputrescible and are carried out with the use of water in the drums.
The drum is the typical machine for tannery: it consists in a cylinder rotating
around its own axis , filled with water, chemical reagents and skins.
The drum used to be a simple wooden cylinder and has now transformed
into a complex computer-controlled machine that regulates the input of water, the
temperature, the speed of rotation, and the release of chemical reagents, thus
avoiding manual errors.
The drying system is to eliminate as much water as possible from the skin,
so to avoid the development of micro-organisms and bacteria. The drying system
is more suitable for sheep and goat skins, while less to preserve cowhides.
Defrost is another method, but it is not common in Italian slaughterhouses:
this system cools the skin at very low temperatures.
The preparation for tanning, starting from preserved raw materials, may be
divided as follows:
Leather soaking: Once cured, the skins are then soaked in water for
several hours to several days. The water and surfactants help in the removal of
salt, dirt, debris, blood and excess animal fats. Rehydration is also reintroduced.
Leather fleshing: subcutaneous material is removed.
Leather unharing: the majority of hair is removed.
Liming is used to loosen the fibers allowing the skin to absorb chemicals
that will be used later in the tanning processes. Limed hides appear swollen and
with an increased thickness, therefore can be easily split into two or more layers.
The splitting is necessary for the heavy cow hides, which are too thick to be
used in the production of manufactured items. The process is carried out with the
splitting machine in which the limed skin is pushed by two cylinders against a
band blade which cuts the skin in two layers parallel to its surface.
Degreasing: is carried out to remove and to reduce the natural fatty acids from
the skin, which could lead to difficulties in the absorption of chemicals. The
degreasing process removes excess fat and distribute it evenly .
Retanning - additional tanning agents are added to impart properties and
change the features of the main tanning: to achieve a thick and stiff leather, skins
need to be re-tanned with vegetable tannins. For a soft and flexible leather, retanning needs to be with chrome.
The tanned leather is not yet usable to produce articles. It is still wet and
even if dry, it would be too rigid and colored with the typical color of tanning
used. To turn it into a marketable product the leather must be further treated with
chemical and mechanical processes in the drums.
Shaving : this operation smoothens the thickness of the whole surface of
the skin and eliminates the residues of fleshing. This operation is carried out
with a cylinder machine where the top one is provided with helical blades.
Dyeing is the process which gives the leather the requested color. It is
made in the drums with dyes.
Splitting - the leather is split into one or more horizontal layers. This can
be done after liming or after chrome tanning. The choice depends on the product
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we want to achieve.
Stuffing - fats/oils and waxes are added between the fibers, thus giving the
leather the flexibility and the softness needed for the various products.
The Tanning Process
Hides and skins are primarily composed of water, protein and fatty
materials. The most important protein in the production of leather is collagen,
which makes up approximately 29% of the mass of a freshly flayed hide. The
collagen desirable for tanning is found in the grain and reticular layers where it is
"intimately woven" in a three-dimensional mesh that is thin and tightly woven in
the grain and coarser and stronger in the reticular layer. After completion of the
operations intended to stabilize the organic matter the product is not yet ready for
use.
It needs finishing.
FINISHING
It is the final stage and the most complex process, which includes all
operations to be carried out on dried skins, to change the surface effect, both for
aesthetic and functional aims. Finishing can be mechanical or chemical.
Mechanical finishing operations may include:
POLISHING : to create a shiny surface by rubbing it with a velvety wheel
IRONING and PLATING: to obtain a flat and smooth surface
EMBOSSING : to obtain a three-dimensional print
TUMBLING by rotating the drum quickly to create a more evident grain
and a smooth surface
Chemical Finishing involves the application of a film, natural or synthetic
by using curtain coaters, roller coatings and spraying.
The use of hides and skins, tanned or untanned, as useful articles has been
with us for thousands of years. The Oxford Dictionary refers to leather as
"material made from the hide or skin of an animal by tanning". Tanning, in
simple terms, refers to the treatment of raw hides and skins with tanning
substances to render the material immune to bacterial attack, i.e. to produce
leather. Additional changes introduced in the process of tanning are secondary
and are related to the tanning and retanning chemicals which are used in the
processes mentioned.
There are hundreds of different leather types and tens of thousands of different
chemicals to choose from when producing these leathers. The most important
chemicals in the tanning process are the tanning agents as they define the process
of leather manufacture as a whole. In this modern day and age, tanners will
choose tanning chemicals based on price, convenience of use, environmental
issues, and by matching the physical and aesthetic properties introduced by the
tanning chemicals to the desired leather properties of the end product. A basic
knowledge of the general processes involved and chemicals used in the
production of leather are all factors which are important in leather manufacture.
In an age of plastics, metals and synthetics, leather has kept its place as a product
of superior quality. As a result, tanning remains an essential economic activity.
Leather processing can be done at the small-scale or large-scale level, all to
varying degrees of sophistication.
The tanning industry has been subject to important challenges and changes.
Foremost has been the introduction of processing technologies with less impact
on the environment. As the production of finished leather is concentrated in
developing countries, UNIDO, together with other partners, have provided
support to enhance tanning industry practices in developing countries.
Cleaner leather production technologies remain UNIDO‘s main focus in the
field of leather processing. Cleaner production applications include green hide
and skin processing (supply of raw material from slaughterhouses without
preservation, e.g. salting), water management (use minimum volume of process
water), recycling (e.g. in liming) and chromium recovery (after tanning), hair
saving (to reduce dissolved solids in effluent) and application of environmentally
friendly chemicals (e.g. enzymes). Special attention is also given to occupational
health and safety (OHS) in tanneries.
The natural fibers of leather will break down with the passage of time.
Acidic leathers are particularly vulnerable to red rot, which causes powdering of
the surface and a change in consistency. Damage from red rot is aggravated by
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high temperatures and relative humidities, and is irreversible.
Exposure to long periods of low relative humidities (below 40%) can cause
leather to become desiccated, irreversibly changing the fibrous structure of the
leather.
Various treatments are available such as conditioners, but these are not
recommended by conservators since they impregnate the structure of the leather
artifact with active chemicals, are sticky, and attract stains.
In some parts of the world top-grain thicknesses are described using
weight units of ounces. Although the statement is in ounces only, it is an
abbreviation of ounces per square foot. The thickness value can be obtained by
the conversion: 1 oz/ft² = 1/64 inch (0.4 mm) Hence, leather described as 7 to 8
oz is 7/64 to 8/64 inches (2.8 to 3.2 mm) thick. The weight is usually given as a
range because the inherent variability of the material makes ensuring a precise
thickness very difficult. Other leather manufacturers state the thickness directly in
millimetres.
Costs of Tannery Waste Treatment
Towards the end of the 20th century the tanning industry has made a
considerable progress in controlling the environmental pollution caused by its
activities, yet the situation varies from country to country and even from region
to region within some large countries. Some tanners in industrialized countries
hold the view that lax environmental regulations and poor enforcement account
for lower production costs, higher competitiveness and hence further expansion of
the tanning industry in developing countries.
This study compares the costs of treatment of tannery effluents, including
indicative investments costs in selected industrialized and developing countries.
While the figures concerning the investment and operational costs by now are
quite obsolete and technologies change, the comparisons of the cost structures are
still quite enlightening.
The Global Leather Coordinating Committee (GLCC) in 2013 sought to
identify real and perceived strengths, weaknesses, opportunities and threats of
importance to the leather industry. This paper sets down a mosaic of major issues
stemming from these considerations.
The report provides an overview of publications, standards and references
for the calculation of the Product Carbon Footprint (PCF) of the product Finished
Leather.
Introduction to Treatment of Tannery Effluents
In view of ever increasing legal and social pressures, no tanner can afford
to be unfamiliar with the main issues and principles of environmental protection
pertaining to tannery operations. Among these, preventing pollution and
promoting cleaner leather processing, which ultimately leads to lower treatment
costs, clearly remain a priority. The application of industrially proven low-waste
advanced methods makes it possible to decrease the pollution load of this
industry. Yet, despite all preventive measures, there is still a considerable amount
of pollution load to be dealt with.. This knowledge should make him better
equipped for communications with the factory‘s environmental unit,
environmental authorities and NGOs. To keep the manual short and concise, there
are many simplifications and omissions of details; for in-depth understanding of
the complexities of treatment of effluents and solid wastes (sludge) we
recommend you to consult extensive literature on this subject. Finally, and
contrary to the widespread misperception that vegetable tanning is
environmentally harmless, in reality its effluents have very high, difficult-to-treat
toxins. They are found in the combined chrome tanning (i.e. chrome tanning
supplemented by vegetable and synthetic tanning agents) because it is by far the
most prevailing leather tanning method.
Leather Goods
The leather-based industry especially leather products industry (footwear,
garment, leather goods) is highly fashion oriented. Moreover, articles made of
(genuine or simulated) leather are complementing clothing. Leather products
(shoes, garment, leather goods) is important export earner for many developing
countries. In many countries leather products export ranks within the first three
places in the total export. Especially the footwear industry‘s importance to the
national economies in developing countries is underlined by the fact that it is the
main contributor to the countries export and – being a labour intensive industry –
provides employment to the most vulnerable groups of the society (including a
large number of women) in towns and villages where other job opportunities are
very scarce. The most of the leather products and footwear industry is dominated
by small- and medium-scale operations. These SMEs lack of design information,
product development knowledge, information and educated personnel for
applying up-to-date quality assurance techniques and productive technology. The
institutional background is weak in providing necessary services, support and
professional training needed for becoming competitive and thus remaining in
business (providing/maintaining working opportunities). UNIDO with other
institutions and organizations through tailor made technical assistance assisted to
up-grade and enhance leather products sector.
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Today, most leather is made of cattle skin, but many exceptions exist.
Lamb and deerskin are used for soft leather in more expensive apparel. Deer and
elkskin are widely used in work gloves and indoor shoes. Pigskin is used in
apparel and on seats of saddles. Buffalo, goat, alligator, snake, ostrich, kangaroo,
ox, and yak skins may also be used for leather. Kangaroo leather is used to make
items that must be strong but flexible. It is the material most commonly used
in bullwhips. Some motorcyclists favor kangaroo leather for motorcycle
leathers because of its light weight and abrasion resistance. Kangaroo leather is
also used for falconry jesses, soccer footwear, and boxing speed bags. At different
times in history, leather made from more exotic skins has been considered
desirable. For this reason, certain species of snakes and crocodiles have been
hunted. Although originally raised for their feathers in the 19th
century, ostriches are now more popular for both meat and leather. Different
processes produce different finishes for many applications, i.e., upholstery,
footwear, automotive products, accessories, and clothing. Ostrich leather is
currently used by many major fashion houses such as Hermès, Prada, Gucci,
and Louis Vuitton. Ostrich leather has a characteristic "goose bump" look
because of the large follicles where the feathers grew.
In Thailand, stingray leather is used in wallets and belts. Sting ray leather is
tough and durable. The leather is often dyed black and covered with tiny round
bumps in the natural pattern of the back ridge of an animal. These bumps are then
usually dyed white to highlight the decoration. Stingray rawhide is also used as
grips on Chinese swords, Scottish basket hilted swords, and Japanese katanas.
Stingray leather is also used for high abrasion areas in motorcycle racing leathers
(especially in gloves, where its high abrasion resistance helps prevent wear
through in the event of an accident.)
Future Trends in the World Leather Products Industry
This survey prepared and presented during the 17th UNIDO Leather
Panel is a follow-up to Worldwide Study of the Leather and Leather Products
Industry, which was the outcome of an exhaustive survey carried out by UNIDO
in the 1970s. It is intended to assist the Organization in the formulation of future
assistance programmes and in detecting areas where further study of various
kinds may be useful. This report assesses the worldwide prospects of the leather
and leather products industry in the coming decade. It examines the major
underlying trends of recent years and how they are expected to evolve in the short
to medium term. In its attempt to provide a thorough picture of the leather sector,
the report covers its various aspects: the availability of raw material, the tanning
industry, and the manufacture of footwear and other leather products. The basic
intention is to help discern prevailing trends in global trade and to support efforts
to design an effective role for organizations in the industrial development arena.
The findings and forecasts published here are meant to be indicative rather than
definitive and to form a basis for further surveys and studies. The need to compile
this report arose out of the 16th session of the UNIDO Leather and Leather
Products Industry Panel held in Brazil in May 2007. The panel recommended that
UNIDO undertake a comprehensive study on the future development of the world
leather and leather products industry, a study that would cover demand,
technology, production, and trade. Consequently, the UNIDO study provides an
analysis of the contemporary demand for leather products (footwear, leather
goods, gloves, leather garments, sports goods, upholstery, etc.) vis-à-vis the
availability of resources (raw hides and skins, manufacturing capacities, skilled
labour, knowledge, support industries, and services). It also contains information
on other important aspects of the leather industry: trade statistics, the geographic
distribution of production, technology developments, physical infrastructure,
environmental conditions, and social aspects involved in the production of
leather.
Leather Production and Environment
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 during dehairing and ammonia 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 3d 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 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 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
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cause difficulties when using landfills and composting sludge from wastewater
treatment on account of the regulations currently in force in some countries. »
In Kanpur, the self-proclaimed "Leather City of World" and a city of 3
million people on the banks of the river Ganges, 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 to reduce costs.
For instance, in Croatia in 2001, proper pollution abatement cost 70-100 USD/t of
raw hides processed against 43 USD/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.
The leather making process is in general restricted to batch processing, but
if the surface coating sub-process is added then some continuous processing can
be included. The operation 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).
Cleaner production is an efficient method of preventing or minimizing
pollution caused by industrial activities. UNIDO continues to place an emphasis on
the introduction of cleaner leather processing technology in developing
countries.The primary task of all cleaner technologies is to reduce the amount
pollution emissions; and where possible change the nature of pollution emissions
to reduce the pressure and costs of end-of-pipe treatment. The expected results
primarily include:
Lower water consumption – better preservation of rapidly
diminishing water resources.
Lower total dissolved solids (TDS) content (including salinity) –
lower risk of affecting the usability of the receiving water bodies for irrigation and
livestock watering.
Avoidance of use in processing and/or presence in leather of
substances from the Restricted Substances Lists (RSL) promulgated by national or
regional legislation, leading (multinational) brands and/or ecolabel due to their
proven negative impact on human health and eco-systems.
Proportionally higher volume of solid wastes suitable for processing
into saleable by-products.
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