Different types of leather

FULL GRAIN LEATHER 

Full Grain Leather is the top layer of the hide. It is treated with aniline dyes which are soluble
and do not mask the hide’s natural surface. Sometimes a semi-aniline dye is used which will give
the leather a thin protective top coat and prevent staining. The vertical fibres of this part of the
leather make it the strongest and most durable part of the hide, it will not crack or peel, tear or
puncture. The grain pattern is tight and consequently resistant to moisture. Part of the beauty of
Full Grain leather is its unique appearance: every piece tells the story of the animal. All markings
and irregularities are preserved: scars where the animal has brushed against a barbed wire fence
or cactus, insect bites, even brand marks or wrinkles. As anyone who has owned Full Grain
Leather will know, it becomes more beautiful with age, developing a rich patina as the years go
by. It is the only type of leather which ages in this way. It is in short, the best leather that money
can buy, which is why it is the main leather used by MAHI for the majority of our products.

 
Top Grain Leather

Rather confusingly, this is not the top layer of the hide but the second layer. The surface of the
leather has been removed by sanding and buffing in order to remove the ‘imperfections’. This
gives the leather a more uniform appearance but it also means that the leather is much less
durable than Full Grain leather and that it will deteriorate much faster. Top Grain leather is the
most commonly used leather for the production of quality goods because it is thinner, more
pliable and less expensive than Full Grain. A variant of Top Grain leather is ‘Corrected’
leather. This is Top Grain leather which has gone through an extensive process of sanding,
buffing, stamping and dyeing in order to achieve a uniform appearance in which all natural
markings have been removed.

Nubuck Leather

Originally made from the hide of deer or elk, Nubuck shoes were first made fashionable by the
visit of the Duke of Windsor to America in the 1930s. Nubuck is Top Grain Leather which has
not been processed but the surface has been brushed and polished so that the short protein fibres
produce a velvet surface. Similar to suede, but more expensive, the lush surface will change
shade when you run your hand over it. Stronger and thicker than suede it can be white or
coloured and dyed. It has a rougher appearance than suede and may retain natural markings.

Suede
Suede is leather made from the inner layer of the hide, the side originally in contact with the
flesh of the animal. The surface is sanded and buffed and the resulting leather is softer and more
flexible than Nubuck, but not nearly as tough. Its absorbent surface makes it vulnerable to
staining.

Faux or Synthetic Leather

A by-product of the chemical industry, synthetic leathers have been in production since the
1940's. Produced under a wide range of brand names, synthetic leathers fall into two main
groups: polyurethane (PU) and polyvinyl chloride (PVC, Vinyl).

Vinyl upholstery became a popular cheap option for car interiors and utility furniture such as
might be found in hospitals or budget restaurants. Durable and easy to maintain, vinyl becomes
sticky and unpleasant when in contact with human skin. Polyurethane fabric is softer, more
flexible and to a certain extent more breathable, it can be printed to create a plausible visual
imitation of leather although it is entirely different to the touch. It is more expensive than Vinyl
but still considerably cheaper than leather.

New synthetic leathers have been created which mimic the chemical structure of real leather. It is
claimed that these synthetic leathers are even stronger and more durable than real leather but
their production is expensive and environmentally problematic. They are produced
using petrochemicals which are both non-renewable and non-biodegradable. There are also
concerns that the microfibres used in the production of these synthetic leathers are starting to
enter the food chain. Items washed in washing machines are shedding non-degradable fibres
which are subsequently finding their way into rivers and lakes.
Leather production process
The leather manufacturing process is divided into three sub-processes: preparatory
stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-
process, surface coating may be added into the sequence.

Preparatory stages
The preparatory stages are when the hide/skin is prepared for tanning. During the preparatory
stages many of the unwanted raw skin components are removed. Many options for pretreatment
of the skin exist. Not all of the options may be performed. Preparatory stages may include:

 preservation- the hide/skin is treated with a method which renders it temporarily

imputrescible.
 soaking - water for purposes of washing or rehydration is reintroduced.
 liming - unwanted proteins and "opening up" is achieved.
 unhairing - the majority of hair is removed.
 fleshing - subcutaneous material is removed.
 splitting - the hide/skin is cut into two or more horizontal layers.
 reliming - the hide/skin is further treated to achieve more "opening up" or more protein
removal.
 deliming - liming and unhairing chemicals are removed from the pelt.
 bating - proteolytic proteins are introduced to the skin to remove further proteins and to
assist with softening of the pelt.
 degreasing - natural fats/oils are stripped or as much as is possible from the hide/skin.
 frizing - physical removal of the fat layer inside the skin. Also similar to Slicking.
 bleaching - chemical modification of dark pigments to yield a lighter coloured pelt.
 pickling - lowering of the pH value to the acidic region. Must be done in the presence of
salts. Pickling is normally done to help with the penetration of certain tanning agents, e.g.,
chromium (and other metals), aldehydic and some polymeric tanning agents
 depickling - raising of the pH out of the acidic region to assist with penetration of certain
tanning agents

Tanning

Barrel for leather tanning, Igualada Leather Museum, Spain

Tanning is the process that converts the protein of the raw hide or skin into a stable material
which will not putrefy and is suitable for a wide variety of end applications. The principal
difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible
material that can putrefy when re-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. This basification process fixes the tanning material to the leather,
and the more tanning material fixed, the higher the hydrothermal stability and increased
shrinkage temperature resistance of the leather. The pH of the leather when chrome tanned
would typically finish somewhere between 3.8-4.2.

Crusting
Drying of leather in East Timor

Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is
included in the crusting sub-process. The chemicals added during crusting have to be fixed in
place. The culmination of the crusting sub-process is the drying and softening operations.
Crusting may include the following operations:

 wetting back - semi-processed leather is rehydrated.

 sammying - 45-55%(m/m) water is squeezed out the leather.
 splitting - the leather is split into one or more horizontal layers.
 shaving - the leather is thinned using a machine which cuts leather fibres off.
 neutralisation - the pH of the leather is adjusted to a value between 4.5 and 6.5.
 retanning - additional tanning agents are added to impart properties.
 dyeing - the leather is coloured.
 fatliquoring - fats/oils and waxes are fixed to the leather fibres.
 filling - heavy/dense chemicals that make the leather harder and heavier are added.
 stuffing - fats/oils and waxes are added between the fibres.
 stripping - superficially fixed tannins are removed.
 whitening - the colour of the leather is lightened.
 fixation - all unbound chemicals are chemically bonded/trapped or removed from the
leather
 setting - area, grain flatness are imparted and excess water removed.
 drying - the leather is dried to various moisture levels (commonly 14-25%).
 conditioning - water is added to the leather to a level of 18-28%.
 softening - physical softening of the leather by separating the leather fibres.
 buffing - abrasion of the surfaces of the leather to reduce nap or grain defects.

Surface coating
For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing
operations may include:

 oiling
 brushing
 padding
 impregnation
 buffing
 spraying
 roller coating
 curtain coating
 polishing
 plating
 embossing
 ironing
 combing (hair-on)
 glazing
How is leather made?
The principal methods of making leather haven't changed that much over the years, but that
doesn't mean it is easy. Tanners are highly trained in what they do, and it is a surprisingly
complicated and lengthy process!   BLC leather training courses can help your understand
the processes.

To get from a salted hide to a piece of leather ready for use in a sofa takes 10 working days!

The steps below show you what tanneries have to do to turn hides into leather :-

Curing

Raw hides and skins must be preserved to stop them deteriorating before the leather-making
process can begin. Methods of preservation include salting, chilling, freezing and the use of
biocides.

Soaking

Cured hides or skins are soaked in water for several hours to several days. This allows them
to reabsorb any water they may have lost in the curing process or during transportation. It
also helps to clean them of salt and dirt.

Painting

Painting is a method by which wool can be removed from sheepskins using a sulphide based
mixture.

Liming

Liming removes the epidermis and hair. This also results in alkaline swelling of the pelt to
cause a controlled breaking of some of the chemical crosslinks of the collagen .

Fleshing

After liming the pelt is passed through a machine to remove fleshy tissue from the flesh side.
Hides may be split into layers at this stage or after tanning.

Deliming

The principal action of deliming is to gradually neutralise the alkali in the pelt, avoiding rapid
changes in pH which could lead to distortion or disruption of the tissues.

Bating
A long delime can significantly improve the removal of any remaining lime, scud
(miscellaneous debris) and residual components broken down during liming. Bating - based
on the use of enzymes - completes this process so that the pelt is flat, relaxed, clean and
ready for pickling and tanning.

Pickling

Weak acid and salt solutions are used to bring the pelt to the weakly acid state required for
most tanning processes. Stronger pickling solutions are used to preserve pelts so that they
can be stored or transported in a stable form over periods of several months.

Degreasing

Solvents or water-based systems can be used to remove excess grease before tanning.

Tanning

Tanning converts the protein of the raw hide or skin into a stable material, which will not
putrefy and is suitable for a wide variety of purposes. Tanning materials form crosslinks in
the collagen structure and stabilise it against the effects of acids, alkalis, heat, water and the
action of micro-organisms. The main types of tanning materials are :

Mineral tannages

Most leather is tanned using salts of chromium.

Aldehyde and oil tannages

Tanning with aldehydes and oils produce very soft leathers and this system can be used to
produce drycleanable and washable fashion leathers and also chamois leather.

Vegetable tannages

Various plant extracts produce brown coloured leathers which tend to be thick and firm. This
type of tannage is used to produce stout sole leather, belting leather and leathers for shoe
linings, bags and cases.

Splitting

A splitting machine slices thicker leather into two layers. The layer without a grain surface
can be turned into suede or have an artificial grain surface applied.

Shaving

A uniform thickness is achieved by shaving the leather on the non-grain side using a machine
with a helical blades mounted on a rotating cylinder.

Neutralisation
Neutralising removes residual chemicals and prepares the leather for further processing and
finishing.

Additional tanning material may be applied to give particular properties which are required in
the finished leather.

Dyeing

The dyeing of leather into a wide variety of colours plays an important part in meeting
fashion requirements. Some leathers are only surface dyed, while others need completely
penetrated dyeings, as is the case with suede leathers.

Fatliquoring

Fatliquoring introduces oils to lubricate the fibres and keep the leather flexible and soft.
Without these oils the leather will become hard and inflexible as it dries out.

Samming

This process reduces water content to about 55% and can be achieved by a number of
machines, the commonest being like a large mangle with felt covered rollers.

Setting out

The leather is stretched out and the grain side is smoothed. This process also reduces the
water content to about 40%.

Final drying

Leather is normally dried to 10-20% water content. This can be achieved in a number of
ways and each method has a different effect on the finished leather:

Staking and dry drumming

A staking machine makes the leather softer and more flexible by massaging it to separate the
fibres. To finish off the leather may be softened by the tumbling action inside a rotating
drum.

Buffing and Brushing

The flesh surface is removed by mechanical abrasion to produce a suede effect or to reduce
the thickness. In some cases the grain surface is buffed to produce a very fine nap, e.g.
nubuck leathers. After buffing the leather is brushed to remove excess dust.

Finishing
The aims of finishing are to level the colour, cover grain defects, control the gloss and
provide a protective surface with good resistance to water, chemical attack and abrasion.

Final grading

Leather will be graded before despatch to the customer. This grading may consider the colour
intensity and uniformity, the feel of the leather, softness, visual appearance, thickness,
design effects and natural defects such as scratches.

Measurement

The area of each piece of leather is measured by machine. Nearly all leather is sold by area
so accurate measurement is important.