Prof. Dr. G. Reich
Leather - an Old Material with a Future
Leather production is one of humanity’s oldest trades, and leather one of man's oldest
cultural creations. Archeological finds prove that our prehistoric forebearers were using the
hides and skins of animals they had killed as both clothing and decoration. This dual role
of leather has persisted unchanged down to the present time: footwear, upholstered
furniture, car seat covers, fancy goods, or leather clothing all combine strict functionality
with fashionable aesthetic characteristics.
Given its perfect balance of comfort in use and wear, serviceability and aesthetic appeal,
small wonder that leather is in such demand for these
outlets. Why is leather such an outstanding material?
The factors which account for its popularity are the
interweaving of its constituent fibres in a threedimensional pattern and the associated porosity, plus
the chemical nature of collagen, the albuminous
substance basic to the skin fibre structure. These factors
are the source of such physiologically important
attributes for clothing as permeability to water vapour
('breathability'), the ability to absorb up to thirty percent water vapour without loss of dry
handle ('water vapour uptake') and a characteristic, largely temperature-independent
stress-strain performance enabling firmness and softness, elasticity and shape retentivity
to be matched to the envisaged end-use. Equally important is the aesthetic appeal of the
leather surface, the grain pattern specific to each type of skin and hide.
The tanner further enhances what nature has already provided: he can vary the surface
still more by buffing to produce nubuck and suede, by astringent tanning to give a
shrunken grain, and by embossing to create fancy grain effects. He can colour leather a
rich variety of shades and hues, and adjust within wide limits such important properties as
light fastness, water and/or oil repellency, resistance to abrasion and scratching, and
bending endurance. Leather is also noted for outstanding stability to ageing. But these
superlative properties alone do not explain why nowadays around the globe 1.5 thousand
million square metres of leather are converted into footwear, furniture, car upholstery, bags
and clothing, and 500.000 tonnes into soles, insoles and items for industrial applications.
There is also the fact that the cattle, sheep, goat and pig skins used as starting materials
are a by-product of human nutrition, the animals in question providing meat, milk and, in
the case of sheep, wool too. The herds and flocks and hence the numbers of skins arising
increase by about 1.2% a year in line with human population growth. The leather industry
thus processes a renewable resource and by the same token provides convenient
"disposal" of an abattoir waste product.
Conversion of skin into leather involves numerous processing stages, starting with
preservation in the abattoir and collecting centres where, by cooling or salting, the
putrescible skin is protected against post-mortem changes. At the actual leather
production stage processing starts with the operations in the beamhouse, where the skin is
prepared for tanning by dehairing, fleshing, thickness adjustment and opening up. The
tanning operation that follows triggers a chemical reaction with the tanning agent, and this
has the effect of converting the skin’s collagen into leather, which does not putrefy when
wet or crack when dry. Since time immemorial tanning has been practised with vegetable
substances ("vegetable tans") derived from bark, wood, leaves and roots, usually of
tropical or subtropical plants (mimosa, quebracho, chestnut, etc.) cultivated on plantations
or cut in strictly limited amounts consistent with the rate of renewal. These agents entail no
physiological hazard - we safely consume chemically comparable compounds in tea, for
example. Other important tanning agents which came into use a century ago are trivalent
chromium salts. The chrome yellow process is very efficient and cost-effective, yielding
leather with a broad swathe of properties. For that reason more than 80% of all leathers
made in the world today are chrome tanned. Their physiological safety is attested by
decades of close contact with human skin. Synthetic agents, which extend the
technological scope of
tanning, are of no little importance
and are also
physiologically safe. The ancient
procedures of tanning
with fat ("chamois tannage") and
with aluminium salts
("glacé tanning") are little
practised nowadays.
After chrome tanning, the leathers
are hydroextracted
mechanically and converted to
"wet blue" by shaving or
by cutting into layers. Wet blue is
a semi-finished product
and a major item of international
trade. It owes its name
to its residual moisture content of
about 50% and to its bluish green colour, which comes from the chrome tanning agent.
Chromium-free leathers, which are steadily gaining in popularity, have a paler colour and
are therefore logically called "wet-white".
Wet processing stage follows in which the leathers are retanned to increase their fullness,
fatliquored to achieve the desired softness, and dyed. Once they have been dried,
generally on modern vacuum dryers, the leathers are given a final surface treatment in the
form of a finish. This can be very light and transparent, allowing the leathers to retain their
natural appearance. Alternatively, however, it can range from an effect with more covering
power to a real coating. Generally speaking, the heavier the finish, the more even and the
more resistant to abrasion and moisture, etc. the leather surface is. Minor surface defects
do not impair serviceability. On the contrary, by reflecting the individual character of each
skin, they are a quality mark for the natural product "Real Leather". Another thing too,
unfinished or only sparingly finished leathers convey the warm handle typical of the
material more tangibly than heavily pigmented leathers. But these last are much cheaper
because of their higher yield, a telling consideration for example when it takes five skins to
produce an upholstered suite of furniture. Fortunately the purchaser can choose here
entirely according to his personal taste. Sorting, quality control, measurement of area and
packing complete the leather production cycle. By-products of leather production find a
multitude of uses. Solid and liquid waste products are disposed of in an environmentally
acceptable manner in compliance with the statutory requirements (limit values). In former
times it took 100 m3 of water to process 1 ton of raw material, compared with about 20 m3
today - a further environmental bonus. Leather - and this applies to all types - contains no
toxic substances. The kind to choose depends on personal inclination, end-use and price.
All leathers have one thing in common : they are a refined natural product. Leather
products will give long years of satisfactory service provided a number of rules for handling
and care are observed. If wet, such products must not be exposed to sources of direct
heat. They should only be dried at ordinary room temperature. Leather upholstered
furniture should not be exposed for too long at temperatures above 40°C nor positioned
too close to radiators, etc. Exposure to intense sunlight or other sources of strong light
such as ultraviolet lamps should be avoided. Fairly high relative air humidity (about 65%)
prevents leather from drying out and avoids possible embrittlement. Regular care is a
must. Everybody is familiar with the use of cremes and polishes for footwear. Leather
garments are best entrusted to competent drycleaning firms. Leather fancy goods hardly
require care. Upholstery leather however needs regular care: dust can be taken off with a
dry or moist cloth or, in the case of suede or nubuck, with the appropriate vacuum cleaner
tool. Light stains can be removed with a neutral detergent or cleaning foam. The leather
must never be made too wet, the solution or foam must be lukewarm at most, and the
wash water must not contain any harsh ingredients. Cleaning should never be done with
just any solvent. Shoe cremes are for shoes and not for leather furniture. To be absolutely
on the safe side, the chosen care product should first be tried out on an out-of-sight part of
the furniture item. Advice is available from the leather industry and its institutes.
Incidentally, a little patina enhances the aesthetic appeal of leather without any loss of
serviceability. As long as there are people who eat meat and consume milk and other dairy
products there will be skins and hence leather. So far no artificial leather can match the
natural material’s outstanding aesthetic or physiological properties and is scarcely likely to
do so in the future.
There is therefore good reason to claim that this old material, leather, is also one
with a future!