Ancient methods of tanning
In ancient history, tanning was considered a noxious or "odiferous
trade" and relegated to the outskirts of town, amongst the
poor. Indeed, tanning by ancient methods is so foul smelling that
tanneries are still isolated from those towns today where the
old methods are used. The ancients used leather for waterskins,
bags, harnesses, boats, armor, quivers, scabbards, boots and sandals.
Around 2500 BC, the Sumerians began using leather, affixed by
copper studs, on chariot wheels.
Skins typically arrived at the tannery dried stiff and dirty
with soil and gore. First, the ancient tanners would soak the
skins in water to clean and soften them. Then they would pound
and scour the skin to remove any remaining flesh and fat. Next,
the tanner needed to remove the hair fibers from the skin. This
was done by either soaking the skin in urine, painting it with
an alkaline lime mixture, or simply letting the skin putrefy for
several months then dipping it in a salt solution. After the hair
fibers were loosened, the tanners scraped them off with a knife.
Once the hair was removed, the tanners would bate the material
(see below) by pounding dung into the skin or soaking the skin
in a solution of animal brains. Among the kinds of dung commonly
used were that of dogs or pigeons. Sometimes the dung was mixed
with water in a large vat, and the prepared skins were kneaded
in the dung water until they became supple, but not too soft.
The ancient tanner might use his bare feet to knead the skins
in the dung water, and the kneading could last two or three hours.
It was this combination of urine, animal feces and decaying flesh
that made ancient tanneries so odiferous.
Children employed as dung gatherers were a common sight in ancient
cities. Also common were "piss-pots" located on street
corners, where human urine could be collected for use in tanneries
or by washerwomen. In some variations of the process, cedar oil,
alum or tannin were applied to the skin as a tanning agent. As
the skin was stretched, it would lose moisture and absorb the
agent.
Leftover leather would be turned into glue. Tanners would place
scraps of hides in a vat of water and let them deteriorate for
months. The mixture would then be placed over a fire to boil off
the water to produce hide glue.
Variations of these methods are still used by do-it-yourself
outdoorsmen to tan hides. The use of brains and the idea that
each animal (except buffalo) has just enough brains for the tanning
process have led to the saying "Every animal has just enough
brains to preserve its own hide, dead or alive."
Modern methods of tanning
The first stage is the preparation for tanning. The second stage
is the actual tanning and other chemical treatment. The third
stage applies retanning agents and dyes to the material to provide
the physical strength and properties desired depending on the
end product. This is known as Retanning. The fourth stage is used
to apply finishing material to the surface or finish the surface
without the application of any chemicals if so desired. This final
stage is known as finishing.
Preparing hides begins by curing them with salt. Curing is employed
to check putrifaction of the protein substance (Collagen) because
of the chance of bacterial infection due to the time lag that
might occur from procuring it to processing it. It removes the
excess water from the hides and skins where water flows from inside
because of difference in osmotic pressure. Thus the moisture content
of hides and skins get greatly reduced.In wet-salting, the hides
are heavily salted, then pressed into packs for about 30 days.
In brine-curing the hides are agitated in a salt water bath for
about 16 hours. Generally speaking, methods employed for curing
greatly make the chance of bacterial growth unfavorable. Thus
curing is also done by preserving the hides and skins at a very
low temperature
The hides are then soaked in clean water to remove the salt and
mainly to bring back the moisture content to a desirable level
so that the hide or skin can be treated with chemicals in an aqueous
medium. This process is known as "Soaking" and sometimes
a hydrating agent is also employed along with water in a very
low percentage for hides and skins which have become very dry.
Liming Process of Hides & Skins
After soaking,the soaked hides and skins are taken for the next
operation where these are treated with milk of lime with or without
the addition of sharpening agents like sulfide,cyanides,amines
etc.The objective of this operation are mainly to
1) Remove the hairs, nails and other keratinous matters 2) Remove
some of the interfibrillary soluble proteins like mucins 3) Swell
up and split up the fibers to the desired extent 4) Remove the
natural grease and fats to some extent 5) Bring the collagen to
a proper condition for satisfactory tannage
The weakening of hair is dependent on the break down of the disulfide
link of the amino acid called cystine, which is the characteristic
of the keratin class of protein like hair and wools.The hydrogen
atoms supplied by the sharpening agent reduce the cystine molecule
to cystine and the covalent links are ruptured.
The iso electric point of the collagen is also shifted to around
4.7, due to liming which is more towards an acidic tannage.
Unhairing Agents used during liming are
1) Sodium Sulphide 2)Sodium Hydroxide 3) Sodium Hydrosulfite
4) Arsenic Sulphide 5) Calcium Hydrosulfide 6) Dimethyl Amine
7) Sodium Sulphydrate
The majority of hair is then removed using a machine, with remaining
hair being removed by hand using a dull knife, a process known
as scudding. Depending on the end use of the leather, hides may
be treated with enzymes to soften them in a process called "bating".
But before bating,the pH of the collagen is brought down to a
lower level so that enzymes might act on it. This process is known
as "Deliming". Once bating is complete, the hides and
skins are treated with a mixture of common salt and Sulphuric
acid in case a mineral tanning to be done.This is done to bring
down the pH of collagen to a very low level so as to facilitate
the penetration of mineral tanning agent into the substance. This
process is known as "Pickling". The common salt penetrates
the substance twice as fast as the acid and checks the ill effect
of sudden drop of pH.
Tanning can be performed with either vegetable or mineral methods.
Before tanning, the skins are unhaired, degreased, desalted and
soaked in water over a period of 6 hours to 2 days. To prevent
damage of the skin by bacterial growth during the soaking period,
biocides, such as pentachlorophenol, are used.
Vegetable tanning uses tannin (this is where the name tanning
comes from). Tannin occurs naturally in bark. The primary barks
used in modern times are chestnut, oak, tanoak, hemlock, quebracho,
mangrove, wattle and myrobalan. Hides are stretched on frames
and immersed for several weeks in vats of increasing concentrations
of tannin. Vegetable tanned hide is flexible and is used for luggage
and furniture.
Mineral tanning usually uses chromium in the form of basic chromium
sulfate. It is employed after picking. Once the desired level
of penetration of chrome into the substance is achieved,the pH
of the material is raised again to facilitate the process. This
is known as Basification. In the raw state chrome tanned skins
are blue and therefore referred to as "wet blue". Chrome
tanning is faster than vegetable tanning (less than a day for
this part of the process) and produces a stretchable leather which
is excellent for use in handbags and garments.
Depending on the finish desired, the hide may be waxed, rolled,
lubricated, injected with oil, split, shaved and, of course, dyed.
Suedes, nubucks, etc. are finished by raising the nap of the leather
by rolling with a rough surface.
