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Leather is a material
created through the tanning of hides, pelts and skins of animals,
primarily cows.
Leather is a very important clothing material with many other uses.
Together with wood, leather formed the basis of much ancient technology.
Leather with the fur still attached is simply called fur.
Forms of leather
There are a number of processes whereby the skin of a dead animal can be
formed into a supple, strong material commonly called leather.
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 flesh.
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 plasticize, becoming
rigid and eventually brittle.
Alum-tanned leather
is tanned using aluminium salts mixed with a variety of binders and
protein sources, such as flour, egg yolk, etc. Purists argue that
alum-tanned leather is technically "tawed" and not tanned, as the
resulting material will rot in water. Very light shades of leather are
possible using this process, but the resulting material is not as supple
as vegetable-tanned leather.
Rawhide is made by
scraping the skin thin, soaking it in lime, and then stretching it while
it dries. Like alum-tanning, rawhide is not technically "leather", but is
usually lumped in with the other forms. Rawhide is stiffer and more
brittle than other forms of leather, and is primarily found in uses such
as drum heads where it does not need to flex significantly; it is also cut
up into cords for use in lacing or stitching, or for making dog toys.
Boiled leather is a
hide product (vegetable-tanned leather) that has been hardened by being
immersed in hot water, or in boiled wax or similar substances.
Historically, it was used as armour due to its hardness and light weight,
but it has also been used for book binding.
Chrome-tanned leather,
invented in 1858, is tanned using chromium sulfate and other salts of
chromium. It is more supple and pliable than vegetable-tanned leather, and
does not discolor or lose shape as drastically in water as
vegetable-tanned. More esoteric colors are possible using chrome tanning.
Brain-tanned leathers
are exceptionally absorbent of water. They are made by a labor-intensive
process which uses emulsified oils (often those of animal brains) and
which has not been industralized. They are known for their exceptional
softness and their ability to be washed.
Leather—usually
vegetable-tanned leather—can be oiled to improve its water resistance.
This supplements the natural oils remaining in the leather itself, which
can be washed out through repeated exposure to water. Frequent oiling of
leather, with mink oil, neatsfoot oil or a similar material, keeps it
supple and improves its lifespan dramatically.
For further information on the production of leather see tanning.
Leather types
In general, leather is sold in three forms:
Full-Grain leather, made from the finest raw material, are clean
natural hides which have not been sanded to remove imperfections. Only the
hair has been removed. The grain remains in its natural state which will
allow the best fiber strength, resulting in greater durability. The
natural grain also has natural breathability, resulting in greater
comfort. The natural Full-Grain surface will wear better than other
leather. Rather than wearing out, it will develop a natural "Patina" and
grow more beautiful over time. The finest leather furniture and footwear
are made from Full-Grain leather.
Corrected-Grain leather, also known as Top-Grain leather, is fuzzy
on one side and smooth on the other. The smooth side is the side where the
hair and natural grain used to be. The hides, which are made from inferior
quality raw materials, have all of the natural grain sanded off and an
artificial grain applied. Top grain leather generally must be heavily
painted to cover up the sanding and stamping process.
Suede is an interior split of the hide. It is "fuzzy" on both
sides. Suede is less durable than top-grain. Suede is cheaper because many
pieces of suede can be split from a single thickness of hide, whereas only
one piece of top-grain can be made. However, manufacturers use a variety
of techniques to make suede appear to be full-grain. For example, in one
process, glue is mixed with one side of the suede, which is then pressed
through rollers; these flatten and even out one side of the material,
giving it the smooth appearance of full-grain. Latigo is one of the trade
names for this product.
Other less-common leathers include:
Buckskin or brained leather is a tanning process that uses animal
brains or other fatty materials to alter the leather. The resulting
supple, suede-like hide is usually smoked heavily to prevent it from
rotting.
Patent leather is leather that has been given a high gloss finish.
The original process was developed in Newark, New Jersey by inventor Seth
Boyden in 1818. Modern patent leather usually has a plastic coating.
Shagreen is a rough and grainy type of untanned leather, formerly
made from a horse's back, or that of a wild ass, and typically dyed green.
Shagreen is now commonly made of the skins of sharks and rays.
Vachetta leather is used in the trimmings of luggage and handbags,
popularized by Louis Vuitton. The leather is left untreated and is
therefore susceptible to water and stains. Sunlight will cause the natural
leather to darken in shade, called a Patina.
Slunk is leather made from the skin of unborn calves. It is
particularly soft, and is valued for use in making gloves.
"Deer Skin" - This is probably the toughest leather in the world,
given that most wild deer are constantly getting in and out of thorny
thickets in the forests. Deerskin has always been prized across societies
- notably the North American Indians who used to treat it with lime and
other compounds to make the raw deer hide more supple, often "staking" it
out in different weather conditions etc. Modern deer skin is no longer
procured from the Wild as it were, with "deer farms" breeding the animals
specifically for the purpose of their skins. Such farmed deer skins are
usually procured from New Zealand and Australia in today's times. Deer
Skin is prized for use in Jackets and Overcoats as well as high quality
personal accessories like handbags and wallets. It commands a high price
owing to its relative rarity as well as its proven durability.
There are two other descriptions of leather commonly used in specialty
products, such as briefcases, wallets, and luggage.
Belting leather is a full grain leather that was originally used in
driving pulley belts and other machinery. It is often found on the surface
of briefcases, portfolios, and wallets, and can be identified by its
thick, firm feel and smooth finish. Belting leather is the only kind of
leather used in luxury products that can retain its shape without the need
for a separate frame; it is generally a heavy weight of full-grain,
vegetable-tanned leather.
Napa leather, or Nappa leather, is extremely soft and supple and is
commonly found in higher quality wallets, toiletry kits, and other
personal leather goods.
The following are not 'true' leathers, but contain leather material.
Bonded Leather , or "Reconstituted Leather", is not really a true
leather but a man-made material composed of 90% to 100% leather fibers
(often scrap from leather tanneries or leather workshops) bonded together
with latex binders to create a look and feel similar to that of genuine
leather at a fraction of the cost. Bonded leather is not as durable as
other leathers, and is recommended for use only if the product will be
used infrequently. One example of bonded leather use is in Bible covers.
Bicast leather is a man-made product that consists of a thick layer
of polyurethane applied to a substrate of low-grade or reconstituted
leather. Most of the strength of bicast leather comes from the
polyurethane coating, which allows this material to be used where strength
or durability are required.
Leather is sold in a variety of thicknesses. 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:
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 millimeters.
Leather from other animals
Today, most leather is made of cow hides, but many exceptions exist. Lamb
and deer skin are used for soft leather in more expensive apparels.
Kangaroo leather is used to make items which need to be strong but
flexible, such as motorcycle gloves. Kangaroo leather is favored by
motorcyclists specifically because of its lighter weight and higher
abrasion resistance as compared to cowhide. Leather made from more exotic
skins has at different times in history been considered very beautiful.
For this reason certain snakes and crocodiles have been hunted to near
extinction.
In the 1970s, farming of ostriches for their feathers became popular. As a
side product, ostrich leather became available. There are different
processes to produce different finishes for many applications i.e.
upholstery, footwear, automotive, accessories and clothing. Ostrich
leather is considered to be of the finest and most durable in the world
and is currently used by all the big fashion houses like Hermès, Prada,
Gucci, and Louis Vuitton. Ostrich leather has a characteristic "goose
bump" look because of the large follicles from which the feathers grew.
In Thailand, sting ray leather is used in wallets and belts in the same
way as regular cow leather. Sting ray leather is as tough and durable as
hard plastic. 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. Leather is also
used for the clothing of many Thailand.
Buffalo leather is also used in America. It is used for gloves, jackets
and some baseball gloves. It is rugged but supple and has a waxy feel.
Overall, leather comes from a variety of other sources, including the
skins of cows, pigs, goats, sheep, alligators, ostriches, kangaroos, dogs
and cats.
Leather production processes
There are many steps like 1. soaking 2. liming 3. dehairing 4. deliming 5.
degreasing 6. bating 7. pickling 8. tanning 9. painting 10 finishing
Role of enzymes in
leather production
Enzymes like proteases, lipases and amylases have important role in
soaking, dehairing, degreasing and bating processes of leather
manufacturing. Proteases are the most commonly used enzymes in leather
production. The criteria for selection of best protease is that it should
be non- collagenolytic and non- keratinolytic in nature. It has property
to hydrolyze casein,elastin,albumin and globuline like proteins.
Lipases are used in degreasing process to hydrolyze fat materials of skin/
hide.
Amylases are also used in bating of animal skins/ hide.
Protease based enzymes, when used in soaking hydrolyze all the non
structured proteins which are not essential for leather making.
Elastin, which is a non structured protein is the binding material between
the upper grain layer and the lower collagenetic substrate which is the
actual leather. Complete removal of the elastin will result in double
layer of grain and if not removed properly, the elastin when subjected to
liming will get immuned and hardened resulting in a loose grain.
Preservation and conditioning of leather
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 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.
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