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Introduction
I remember a school trip to a Tannery
in Bermondsey, London, (more years ago than I care to remember). The smell
was "different" but the trip was fascinating. We saw leather
being tanned as well as furs being processed. Below some details about
this trade.
Nearly everyone uses leather in some
way each day. People all over the world wear shoes, coats, belts, and gloves and
carry handbags or wallets made out of leather. Cowboys wearing leather boots
ride on leather saddles, and industrial workers wear special safety work shoes
or boots made from leather to protect their feet. Industry depends on leather
products. The furniture and automobile industries, for example, use leather for
upholstery, and leather gaskets are still found in some engine blocks.
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Manufacturing
The hides and skins of domestic animals are the
main source of leather. The skins of large animals such as cattle and horses are
called hides. Those of smaller animals such as sheep, goats, and calves are
referred to as skins. The United States is the largest producer of hides and
skins with an annual supply of more than 1,100,000 tons (1,000,000 metric tons).
Hides and skins are removed from animals after they
are slaughtered at modern slaughterhouses. Electric knives and hide pullers that
are powered by compressed air are used.
After the hide is pulled off, hooves, tails, ears,
horns, and other parts unusable for leather are trimmed off and used in the
preparation of gelatine and glue. The hide is fleshed with modern machinery that
removes any remaining meat tissue or fat.
Fresh fleshed hides are shipped in refrigerated
trucks to a tannery for immediate processing into leather. If this is not
possible, the fleshed hides are cured, or preserved by immersion for at least 16
hours, in large pools called raceways filled with salty water, or brine. After
being cured, the hides can be stored for several months without rotting and can
be shipped to manufacturers throughout the world.
Cured hides arriving at a tannery are rehydrated,
or re-soaked, and washed in large, rotating wooden drums. Hair is removed by
chemical digestion; that is, by soaking the hide or skin for 10 to 12 hours in
drums containing a solution of lime and sodium sulphide and rotating the drums
occasionally. After the hair is removed, hides are de-limed, or neutralized,
with acids and treated with enzymes to remove any deposits and to increase
softness. The next operation is called pickling. The hides are soaked in a
solution of water, salt, and hydrochloric or sulphuric acid.
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Tanning
Is the final process in turning
hides and skins into finished leather. Properly tanned leather can pass the test
of being boiled in water for three minutes without shrinking. There are several
methods of tanning, but the two most common are chrome and vegetable tanning.
Chrome tanning is the most used. Most leather shoe uppers and garment,
upholstery, and bag leathers are chrome-tanned. The process begins in rotating
drums with a bath in a chemical containing trivalent chrome. It usually takes
eight hours for the chrome to soak all the way into the hide or skin. Once it
has penetrated, the chrome is "fixed" by adding to the tanning bath an
alkaline chemical such as sodium carbonate or bicarbonate. After this treatment
the hide is considered tanned.
Vegetable tanning is used for such various products
as shoe soles, luggage, saddlery, and belt leathers. The process is slower than
chrome tanning and involves the chemical substance tannin, or tannic acid, which
is extracted from the barks of trees. This process is performed in rotating
drums, and it takes from two to four days.
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Wringing, splitting,
and shaving
Following tanning.
Wringing lowers the moisture content of the hides in preparation for splitting.
Depending on the end use of the leather product, hides are split into sheets of
the required thickness and processed further through a shaving machine for added
quality. All three operations require specialized machinery run by highly
skilled operators.
After shaving, chrome-tanned hides are again placed
in rotating drums with water, dyes, and synthetic tanning materials at
temperatures from 120º to 140º F (49º to 60º C) to obtain the desired colour.
They are then lubricated with natural fat, synthetic fatty type chemicals, or a
combination of both to obtain the softness required by the final product.
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Finishing
Consists of placing a series of
coatings on the surface of the leather. These coatings are designed to protect
the leather and produce surface effects pleasing to look at and to touch.
Finishing today reflects the latest technology in the use of coating materials.
Some finishing processes apply plastics such as acrylic and urethane resins.
Others coat with vinyl, waxes, nitrocellulose, dyes, and many other materials.
Various mechanical operations are necessary to obtain the desired finish.
Hydraulic presses, printing machines, automatic spray applications, and vacuum
dryers are a few of the machines used in the finishing process.
The end use of the leather product determines the
type of finish process to be applied. Each type requires different physical
properties in the finish. Film flexibility and resistance to water and wear are
a few of the required properties in the finish. Much research and development
continues in the quest of improved surface coatings.
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The Leather Sources
Cowhide
The most useful leather, comes from cattle and is
tough and long wearing. It is used in shoe soles and some shoe uppers as well as
in machine beltings and harnesses. Split hides are made into luggage, gloves,
clothing, and many other articles.
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Calfskin
Has a fine grain. It is good for shoes because it
withstands scuffing and hard wear. It is also used for handbags, gloves, fine book bindings,
luggage, and garments.
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Goatskin and kidskin
Are used in women's fine shoes and gloves. Goatskin
is also used in garments. Kid is one of the sturdiest leathers and also one of
the softest and most pliable. It is an excellent material for suede, or leather
with a napped surface.
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Sheepskin and lambskin
Are good for shoe uppers and linings, gloves,
garments, handbags, chamois, parchments, textile-mill rollers, and piano parts.
Lambskins with wool are used for coats and boots.
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Pigskin
Comes mainly from the peccary, a wild hog found in
North and South America, and domestic pigs. When the bristles are removed, pores
are left that give it an unusual texture. Pigskin is used for gloves, saddles,
wallets, sport shoes, fine book bindings, upholstery, and razor strops.
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Buckskin
Is made from deer. Almost all buckskin sold in the
United States is imported from Latin America and Canada. It is used for
garments, gloves, and the uppers of high-quality shoes.
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Alligator
Skins, though the animals are carefully protected,
are legally available through controlled hunting and farming from Latin America,
Florida, and Louisiana. The beautifully textured skins are made into luxurious
shoes, handbags, luggage, and belts. These accessories are also made from the
skins of water snakes, lizards, pythons, and cobras.
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Kangaroo
Hide from Australia makes strong, flexible leather
for shoe uppers.
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Ostrich
Skin comes from the only bird that provides
leather. Its pinkish skin is used for fine handbags and wallets. Many unusual
leathers come from seals, sharks, and whales.
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Synthetics
Have been used as substitutes for leather in a wide
variety of products. These synthetics are mostly materials called polyvinyl
plastics.
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World Production
Total annual world leather production is about
7,275,000 tons (6,600,000 metric tons) of bovine, or cattle and domestic
buffalo, hides; 1,200,000 tons (1,100,000 metric tons) of sheepskins; and
419,000 tons (380,000 metric tons) of goatskins. Trade in bovine leather is
concentrated in industrialized countries. Argentina, Italy, Germany, the United
States, and Japan are the largest exporters. Germany, Italy, France, and the
United States are the largest importers. Footwear, clothing, and accessories
make up the largest groups of finished leather products with Italy and South
Korea the leading exporters.
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History
The story of leather is long and colourful. Many
years before recorded history people probably wrapped themselves in dried animal
pelts. The fact that the skins turned stiff and rotted were a problem, but ways
of softening and preserving them were discovered. This was the beginning of
leather processing. At first skins were probably dried in air and sunlight.
Later they may have been soaked in water and dried over a fire. Still later it
was discovered that certain twigs, barks, and leaves soaked with the hides in
water helped to preserve them.
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Sumeria and Egypt
Evidence exists for the use of
leather by the ancient Sumerians as far back as 6000 BC.
Preserved specimens of leather dating to 5000 BC
have been found. Egyptian stone carvings of about the same date show leather
workers. Egyptian leather sandals more than 3,300 years old and an Egyptian
queen's funeral tent of gazelle hides made in 1100 BC
are in museums.
The Israelites learned to make leather from the
Egyptians. A passage in the Old Testament reads, "Unto Adam and also unto
his wife did the Lord God make clothes of skins and clothe them." By New
Testament times tanning had become common.
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Greece and Rome
The Greeks and Romans left evidence
that their methods of tanning were highly developed. Both Herodotus and Homer
mention the use of leather. In Rome leather served as money, and leather shoes
of different types indicated the rank of the wearer. The English word pecuniary,
which means consisting of or measured in money, comes from the Latin pecus, meaning
"cattle." Pliny the Elder, writing in the first century AD,
gives the Roman recipe for tanning: "Hides were tanned with bark, and
gallnuts, sumac and lotus were used." Gallnuts are caused by insects laying
eggs on the leaves or buds of oak trees. These eggs produce a growth that yields
a high percentage of tannic acid. Gallnuts are still used in one type of
tanning.
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America
When the first settlers
arrived in America, they found that the Indians' tanning method was much like
the ancient shamoying, a method used by the Arabs and mentioned by Homer. The
Indians taught the pioneers how to make buckskin.
The colonists brought oak-bark tanning methods from
England. The first leather maker, named Experience Miller, arrived in Plymouth
in 1623. In 1629 two shoemakers arrived. By 1650 there were 51 tanners in
Massachusetts. The early leather makers simply dug holes in the ground and
walled them with planking. In these holes hides were covered with oak bark and
left for at least six months. This method was no more advanced than that used by
the ancient Hebrews.
In 1805 Sir Humphry Davy discovered that materials
from other trees--hemlock, mimosa, chestnut, and ash--could be used in tanning.
These trees were plentiful in the United States and helped make it the centre of
the leather trade.
Samuel Parker invented a machine in 1809 that could
split hides to any thickness. Until then it took one worker an entire day to
divide four hides. Now 100 could be split in the same length of time. From these
beginnings grew the scientific process of modern leather manufacture.
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