The manufacture of leather
Introduction
Leather has always held
an inimitable place in man's life. It was used widely in many primitive
societies and was considered a necessity to survival itself. Early man
discovered that the quality of leather could be enhanced by curing the
skins of hides. He therefore rubbed them with fat and smoked them over
wood fire and concocted the early form of tannic solution from rotting
leaves and bark for soaking the hides.
>Of course today leather
making has evolved into a fairly complex process. Different manufacturers
process the leather in different ways to achieve their own unique brand of
leather. Handbag leather falls into a category known in the leather
industry as 'light leather'. They are usually made from calf, goat or
sheepskins. 'Light leather' must meet specific demands with regard to
thickness, suppleness, durability, and suitability for the various
finishing processes. Consequently, special conditions apply to the bating
process, tanning method and tanning ingredients used in its
manufacture.
The Skin
All leathers consist of a network of fibers made up of
proteins. Collagen is the most important protein occurring in leather. The
skin of mammals consists of the epidermis, the dermis, and subcutaneous
tissue. For the manufacture of leather only the dermis is
important.
Epidermis
A protective, hard-wearing layer of
keratinous cells, which, although of varying thickness over the body of
the animal, is very thin compared with the underlying
Dermis.
Dermis
This layer consists of a dense fibrous
connective tissue which supports blood and lymph vessels, sebaceous and
sudoriferous glands, hair follicles and their associated muscles. In the
grain layer of the dermis these fibers become very thin and tightly woven.
Consequently, when the epidermis is carefully removed, a smooth layer is
revealed, sometimes known as the hyaline layer, which gives leather its
characteristic grain surface. Toward the center of the dermis (or corium)
the fibers are coarser and stronger, and the predominant angle at which
they are woven can indicate the properties the resultant leather will
display. If the fibers are more upright and tightly woven, a firm, hard
leather with little stretch can be expected, while if they are more
horizontal and loosely woven, soft, elastic leather can be anticipated.
The interior of the dermis is generally the strongest part of the skin.
and is not used for the manufacture of leather.
Preparation of the Skin
To prevent the onset of decomposition, fresh skins must be
preserved within about 2-3 hours after stripping. The dehydration process
significantly slows down this decomposition and, consequently the skins
become hard.
There are several methods for preserving skins such as
drying, salting, or pickling. The skins are processed in large drums as
shown in the diagram for a period of 14-16 hrs before they are ready for
tanning.
Soaking, unhairing and liming
To install the
softness back into the skins after the preservation process, they are
soaked in water to restore their original water content.
Next the
epidermis (with hair) is removed. Sodium sulphide is generally used to
facilitate this process.
Liming is a process by which the bonds
between the fibrils are partly broken, so that the fibers become a little
loose. The lime also takes out part of the interfibrillar skin matter, so
that the fibers become less fixed. Simultaneously liming loosens the
subcutaneous layer so that it is more easily removed later on. This
process takes about 14 hours after which the skin is very alkaline (pH
13-14).
Deliming, fleshing
and scudding
To neutralize the alkaline skin, carbon dioxide
and ammonium salts are used. This process is essential before the tanning
process begins, otherwise the acidic tanning agents would harden up the
grain fibers. The next phase in the manufacture of leather is fleshing
here the subcutaneous layer of the skin is removed. Traditionally, this
was carried out using a wooden beam and a fleshing knife, but today it is
done by machines.
Scudding is the final trimming process, where excess
and unwanted skin elements are removed using a blunt knife. This leaves
the grain more porous to the tannin solutions during the tanning
process.
Bating
To achieve the fine, smooth, and elastic
texture, as required for handbags the skin is put through yet another
process, Bating. This process will further delime the skin, loosen up the
protein fibers to yield a more elastic material, and give the surface
texture a fine smooth feel.Proteolytic enzymes are introduced to the skin
which dissolves away any remnant of the epidermis and other skin
substances. This process is carefully controlled for pH levels,
temperature, and the duration of the process. The skin is finally washed
thoroughly to prepare for tanning.
Tanning
The
chemical procedure used to ready raw animal hides for use is called
"tanning." A piece of hide or skin which has been tanned produces strong,
flexible leather which is able to resist decay and spoilage.
Tannins
are divided into four different groups:
* Vegetable tannins
*
Mineral tannins
* Aldehyde tannins
* Synthetic tannins (syntans)
We shall outline here only two of the most commonly used
tannins.
Vegetable tannins
The vegetable tannins solution is
derived from wood, roots, bark, leaves, and fruits of various plants, such
as oak, chestnut, hemlock, and willow trees. Skins are suspended into
large pots or vessels containing the vegetable tannins. Periodically the
skins are exposed to increasing concentration of the tannin solutions by
transferring the hides from one pot to another containing a stronger
solution.
Skins which have been treated with vegetable tannins
generally yield tougher, but supple leather. By and large these hides
would be intended for suitcases, belts, and hats.
Mineral
tannins
Mineral or Chrome tannins are used in leather
production by treating the skins with chromium sulfate. As in vegetable
tanned leather the degree of control defines the quality and type of
leather produced. Chrome tanned leather tends to be softer and more
flexible than the vegetable tanned leather, and durable when exposed to
water. However it deteriorates with contact to sweat or organic acids, and
is difficult to emboss. Chrome tanned leather are best applied to
manufacture of handbags, clothing, and shoes .
Dyeing,
Fatliquoring or lubricating
The next stage is dyeing, where
appropriate dye-stuffs are added for desired color.
Because of the
acidic nature of the tannin solutions the leather is treated with some fat
emulsion which coats the outer surfaces and allows the leather to be
supple. Vegetable tanned leather can be lubricated with oil which resists
oxidation (darkening) of the tannin solutions in the
grain.
Drying
After tanning and dyeing in a bath, the
moisture content of the leather is about 65-70%. This must then be lowered
to around 15%. When leather is being dried care must be taken to allow the
water molecules to evaporate while leaving the fat molecules in
place.
Staking
After the hides are dried they become
stiff and less flexible. Staking is a mechanical treatment to make the
leather soft and supple after it has dried out.
Finishing
The very final step in leather
manufacturing is the finishing. This is carried out by coating the hide
with chemical compounds and then brushing it. Light leather is buffed and
sandpapered to remove imperfections. Leather which has been buffed for a
long period of time becomes suede. Different leather types require
different finishing processes. Full grain leather has a polymer or wax
finish applied to the grain surface. The finish applied determines how the
end product looks in terms of shine, color, and texture.