|
All About Leather
- By: Ultra Couture Design
Leather is created through the tanning of hides and skins,
converting the putrescible skin into a durable and long lasting
material. Leather is not leather until the skin or hide has been
tanned. The Leather Making Process British Standard BS 2780
Definition of Leather
“A general term for hide or skin with its original fibrous
structure more or less intact, tanned to be imputrescible. The
hair or wool may, or may not, have been removed. Leather is also
made from a hide or skin which has been split into layers or
segmented, either before or after tanning, but if the tanned
hide or skin is disintegrated mechanically without combination
of a binding agent, is made into sheets or other forms, such
sheets or forms are not leather. If the leather has a surface
coating, this surface layer, however applied, must not be
thicker than 0.15mm”
The actual process of making a hide or skin into leather is not
just one simple task but a number of processes. These different
processes can be broken down into four sections.
* Beamhouse Operations * Tanning * Post Tanning * Finishing
Beamhouse Operations
The beamhouse area of a tannery is for the removal of unwanted
materials from the hide, such as hair, non-structural proteins
and fleshy tissue.
Soaking: This process is done to remove dirt and blood from the
surface of leather. The hides are soaked in water kept at 20
degrees Celsius to avoid shrinkage and detergents can be added
with an alkaline chemical that aids the uptake of water. This
process also moisturises the hide.
Unhairing and Liming: Again the hides are soaked in alkaline
conditions, this is to loosen the hair ( unhair ) and to swell
the collagen fibre and break down non structural proteins that
would harden the final leather if not removed.
Fleshing: The hides are washed for fleshing. They are passed
through a machine that removes unwanted flesh, connective tissue
and fat.
Splitting: The hides can now be split into two. The top layer is
used in most upholstery as it contains the grain pattern or can
be corrected and have an embossed pattern applied to it. The
bottom layer (split) is then used for suede or can have a
pigment applied to it and becomes a finished split (commonly
used on the backs and sides of upholstery).
Deliming: The hide is prepared for tanning by reducing the
swollen and rigid fibres. This is done by removing the alkali
with acidic chemicals such as ammonium chloride or ammonium
sulphate.
Bating: This process gives the hide a smoother, flatter grain
appearance and it is also the stage that makes the final leather
softer and stretchy.
Pickling: This process is used to adjust the hides ready for
tanning. This is done by introducing an acid into the hide to
lower its pH. The mixture may also contain salt to further
reduce the swelling. Tanning
This is the process most people are familiar with, and is the
most important part of making leather. Remember the definition
of leather? If a hide or skin is not tanned it cannot be
classified as leather.
The objective of tanning is to convert the collagen protein of
the hide into a stable material, which will not putrefy and will
be stable under conditions of heat and moisture.
There are a good few different methods of tanning hides but for
upholstery there is only one main one you need to know about
�" Wet Blue Also known as chrome tanning. This is the name
given to leather that has been tanned using chromium salts. Why?
Because it is both wet and blue after being tanned!
The hides are processed in large drums (usually these drums can
take up to 300 hides at a time). The blue colour in the chromium
binds to the collagen in the hide making it blue in colour. The
hide is fully tanned when it is resistant to heat and will not
shrink at 100 degrees Celsius. The benefits of this kind of
tannage are good strength, light fastness and heat resistance.
It is these properties that make wet blue the most popular
method of upholstery tanning.
At this stage the hides must then dried to remove some moisture.
This is done by passing the hides through a machine with felt
pads that squeezes the moisture out. Post Tanning
Neutralise: Mild alkalis are added to the leather, which prepare
it for later chemical processes..
Dyeing: Simple �" the leather is dyed in drums to give it
colour! Anionic dyes are very common as they are negatively
charged and so latch onto the chrome leathers well.
Fatliquoring: This process uses fats and oils to lubricate and
soften the fibre structure of leather. The manner in which the
oils are introduced into the hide coat every fibre. This is
where leather gets its smell!
Drying: The leather is then dried. This can be done by hanging
it on pegs or laying it on a board and applying heat. This
method is poplar as it can dry a hide in just a few minutes.
Staking: This process softens the leather. The leather is passed
over a series of blunt pins that pummel and flex the leather to
soften it up. Finishing
Finishing leather is the process of applying coatings to the
surface of leather, the benefits of finished leather are;
* Protective surface aids durability * Easier to care and
maintain * Improves water resistance * Masks defects * Enhances
colour and appearance
There are three processes involved in finishing leather:
Buffing, applying a finish and embossing a pattern. Not all
these processes occur, which varies the different types of
leather.
Buffing
If the grain surface of the leather is in a good condition, it
may be left unfinished and is known as full grain leather.
However there are so few hides good enough quality to be classed
as full grain that the price is very expensive for such
upholstery.
The hide (on a cow) can get scratched, bitten and cut. All these
imperfections show on the grain and need to be repaired. The
damages are filled using the 'leather repair compound' (a
flexible filler), the leather is then left to dry and then the
grain side of the hide is passed through a machine with abrasive
material to buff away the imperfections. The type of leather is
known as correct grain.
Buffing the hide also creates an excellent key for pigments to
adhere to and so serves more than one purpose.
Applying Finishes
On corrected grain leather a pigmented finish is applied to give
a sound covering of colour. A concentrated pigment would not
cover leather and so it is dispersed into a base solution,
either acrylic or solvent based, which contain binders to adhere
to the leather.
After applying the pigment a lacquer is then applied to seal the
colour in, giving the leather more protection and durability
�" not only that but the lacquers are used to adjust the gloss
level of the leather.
The pigment can be applied to leather in three ways.
1. Sprayed on by hand using an airgun. 2. Sprayed on by a
machine on a conveyor belt. 3. Pressed on with rollers. Often a
combination of part 2 (base coat) and part 3 are used.
The lacquers are sprayed on by machine or by hand with an air
gun.
Embossing After the application of finishes on leather an
artificial grain pattern can be applied to the leather. This is
done by placing a metal plate containing a grain pattern on top
of the leather and pressing it down onto the surface under
pressure and heat. Or, by running the leather through rollers
with an etched grain pattern on them.
The most expensive type of leather due to the exclusive
selection of hides. The leather is dyed in a dye bath and no
pigmented coating is applied, allowing the leather to breathe
naturally showing all its own unique markings and shade
variations. Because of this, no two aniline leathers are the
same and so when you purchase one you are buying an exclusive
item with its own story of nature to tell.
Pros: The most natural type of leather available, it is very
soft and delicate to the touch and bares all of natures own
markings on the hide. It stays warm in the winter and cool and
ventilated in the summer.
Cons: Because the leather contains no protective coating it is
very absorbent and so is prone to staining and fading. Semi
Aniline Leather
This type of leather is similar to aniline but with the added
advantage of a thin protective coating that helps resist stains.
Some natural markings may still be visible but not to the extent
of an aniline leather.
Pros: Easy to clean and maintain. It has a nice soft feel and
still has some natural markings visible, often with a two tone
appearance.
Cons: There isn't much room for complaint with semi aniline
leather except that it is slightly less natural than aniline
leather. Pigmented Leather
This type of leather is coated with a fine pigmented spray,
which gives the leather a sound covering of colour with no shade
variations. Sometimes known as corrected grain leather, this is
when an artificial grain pattern is embossed into the hide.
Pros: The process in which the leather is made makes it very
durable and hard wearing. It has high resistance to light and is
very easy to clean and maintain. The colour is uniform and all
defects are masked.
Cons: Reduced breath ability and the grain pattern is masked
making this type of leather less natural. Bi-Cast
This type of leather has a polyurethane film bonded to the
surface creating a very hard wearing and easy clean leather.
Pros: Extremely easy to clean and maintain, uniform colour and
good light fastness.
Cons: It has no natural grain surface and so is not very
natural. Pull Up
A very natural type of leather that is designed to distress as
it ages because the colour lightens when stretched or scratched.
Pros: Has a very natural and lived in look. All natural markings
are visible and has a soft and delicate touch.
Cons: It can be very difficult to clean and it stains quite
easily. Antique Finish
This is a pigmented leather with an additional top coat of a
darker colour sprayed on top. It is designed to give an antiqued
look and is very common with chesterfield style sofas.
Pros: Unique antique style differs from sofa to sofa as the
leather is enhanced through use. Protective coating helps resist
stains.
Cons: The top coat of colour may wear off after continued use
causing the leather to look severely distressed. Nubuck
This is leather buffed grain side to create a velvety surface.
Pros: Nubuck is very natural with a lovely soft and delicate
feel. Cons: It is very difficult to clean and maintain as it has
no protective coating. Light fastness is poor and the leather is
prone to staining.
Natural Markings
Leather is a natural product and as such, bares all hallmarks of
its origin. Unique and individual markings may appear on a hide
and should not be considered as defects. Instead, they give
leather a unique identity, which enhances its natural beauty and
characteristics.
Corrected Grain Leather
Correct grain is where the leathers surface has been corrected
to remove the above natural markings. This is done when there is
an abundance of marks that cause the hide to look damaged or
unsightly, and so the marks are removed.
The process in which this is done can slightly differ depending
upon the amount and depth of the marks. When the scuffs and
scratches aren't to severe the hide can be buffed (with a type
of sander) and then have a pigment applied to the surface. The
light buffing reduces the marks and then the pigmented coating
covers them. When the marks have some depth to them they need
filled before they can be concealed. This is done by spreading a
flexible paste (repair compound) over the surface of the leather
and then sanding it down. A pigmented coating is then applied to
the surface as before.
Corrected grain leather falls under the category of 'pigmented
leather' on our leather types web page.
NOTE: Because the surface is buffed the leather loses its
natural grain pattern and so, an artificial grain pattern is then
embossed into the leather. Full Grain Leather
Full grain leather is where the natural markings are minimal and
so the leather an be dyed or lightly pigmented without the need
for buffing. The natural markings on a hide should in no way
affect the strength and durability of the leather as they are
mainly superficial. The exclusive selection of hides for full
grain leather means that they have a premium price over other
leather types.
Full Grain Leather Types: Aniline, Semi-aniline, pull up,
oily/waxy pull up Selection of Hides and Price
The majority of leather used for upholstery in the car and home is
pigmented leather. This is because, in the field, a cow can get
caught on barbed wire, rub against thorn bushes, and get cut by
other animals horns. In addition to this, cows are often bitten
by insects, parasites, ticks, and lice, all of which result in
markings on the hide.
The majority of hides available for tanning are in such a
condition that the surface needs to be 'corrected' before it is
suitable or visually acceptable for upholstery. Only a small
percentage of hides have few marks and are in good enough
condition to be left untouched. So full grain leather, is
therefore more expensive. Simply, its a numbers game, there are
fewer full grain leathers available, and so, supply and demand
dictates that they be charged at a premium price Leather
Selector What type of leather do I have?
In order to properly determine a type of leather there are a few
tests that we can do.
1. Magnifying Glass Test: Looking at leather under a magnifying
glass will show us in detail the finish applied to the leather,
or if there is a finish on the leather. 2. Water Absorption
Test: This test helps us identify how easy your leather will be
to clean, and how effective the finish on it is. 3. Visual Test:
Looking at leather tells a lot, are there any natural markings
visible? what colour is it? etc etc 4. Touch Test: Feeling the
leather for softness, smoothness, grain pattern also helps
identify the leathers type.
Leather is created through the tanning of hides and skins,
converting the putrescible skin into a durable and long lasting
material. Leather is not leather until the skin or hide has been
tanned. The Leather Making Process British Standard BS 2780
Definition of Leather
http://www.ultracouturedesign.co.uk
http://www.ultracouturedesign.co.uk/sofa_info_center.html
|