FactSheet # 1: Tannery Process Description
1.
Hide and skin curing
Hide and skin curing
a. Air drying, used in warm, dry climates such as India and Africa, consists of either
stretching the hides and skins on bamboo frames or by pegs or spreading on the ground
in mild sun. Dried hides are called “flints.” Arsenication of dried skins intended for
export takes place to prevent hide beetles. Naphthalene powder or flakes, DDT, zinc
chloride, chlorophenols and dichlorobenzenes are also used as insecticides, mostly for
goat and sheepskins before export.
b. Dry-salting involves rubbing the fleshing side with a solution of salt. This is performed
prior to exporting hides.
c. Wet-salting is used for short-term preservation.
d. Brine curing, introduced in the US and South America, is replacing salt curing. The
process involves submerging flayed, washed, cleaned, defatted, and trimmed hides in
brine (a saturated solution of sodium chloride usually with some naphthalene) for 12-24
hours.
e. Pickle curing is used for dewooled sheepskins in New Zealand and Australia to prepare
the skins for export. The process involves immersing the skin in a bath of sodium
chloride and mineral acid
Beamhouse
2. Receiving and storing
Receiving and storage
3. Cutting and trimming
Trimming hides
4. Stamping
5. Washing occurs in drums, soaking pits, baths, or vats with paddle stirrers. Soaking softens
the dry-salted and/or flint-dried hides and skins. Disinfectants such as bleaching powder,
chlorine, and sodium acid fluoride prevent putrefaction of hides. Caustic soda, sodium
sulfide, and wetting agents are added to accelerate soaking.
6. Liming involves immersing the hide in a milk of lime (calcium hydroxide) to loosen the
epidermis, hair roots, and to remove other unwanted soluble proteins and fats. Sodium
sulfide, sodium hydrogen sulfide, and dimethylamine are used to enhance and accelerate
the unhairing process. {Another method uses a depilatory paste of lime, sulfide and salt on
the fleshside of skins in order to save hair and wool}
Placing hides in lime pits
7. Washing involves cleaning the hides with warm water combined with wetting agents and
bactericides.
Washing the dehaired
hides
8. Unhairing and defleshing usually is completed by machines where fresh and limed hides pass
between several rollers while being pressed against a cylinder with spiral knives. This
operation can also be completed using hand tools.
9. Scudding is the removal of fine hair roots.
Tanning
10. Deliming and bating is the process where all lime is removed using weak acids or ammonium
salts (carbon dioxide ammonium sulfate and sodium bisulfate). Enzymes may be added to
digest residues of hair roots and non-structural proteins.
11. Pickling is performed prior to chrome tanning. The hides are soaked in a dilute solution of
sulfuric or hydrochloric acid and common salt. 2-naphthol or p-nitrophenol are sometimes
added to prevent mold growth.
12. Tanning converts the hides and skins to non-putrescible leather by stabilization of the
intermediate layer (derma) between epidermis and subcutaneous tissue. Tanning takes
place in rotating drums and may be followed by retanning. There are two general tanning
processes.
Chrome tanning is used for lighter leathers and can be performed in one bath containing
chromium III (chrome sulfate) as a bulk liquid. This process has largely replaced
the two-bath method that uses chromium VI. Chromium VI is now used only for
special purpose leathers.
Vegetable tanning is used for heavier leathers and involves the use of certain barks
(mimosa, acacia, quebracho and sumac) and nuts (myrobolam nuts used in India).
Synthetic tannery liquids (used mainly for retanning) include amino resins and
phenol and naphthalene derivatives.
Preparing vegetable tanning liquor
Vegetable Tanning Pits
Chrome tanning drums
Tanning drums
Finishing
13. Pressing is performed to remove excess water.
Tanning drums
14. Splitting divides the hide into two sheets.
15. Shaving removes excess thickness on the flesh side of the leather.
Shaving
16. Retanning may involve subsequent or simultaneously retanning, dyeing, and fat liquoring of
the hide. These processes define the properties of the finished leather.
17. Samming is used to remove excess water and oils and involves machine smoothing and
stretching of the hides
Pressing the hides
18. Drying is performed in one of three ways, vacuum drying, air-drying, and hot air circulation
drying.
19. Softening is performed to improve certain properties of the leather and involves sliding the
leather through a combination of folds, bumps, and stretches.
20. Dyeing is performed by soaking in vats or drums, or, more commonly, by color-spraying.
Dyeing and dressing
Hot Air Drying
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Packing
References
1.
Encyclopaedia of Occupational Health and Safety, Third Edition, Volume 2. L.
Parmeggiani, ed. Geneva: International Labor Organization (1983).
2.
Burgess WA: Recognition of Health Hazards in Industry. New York, John Wiley &
Sons
Photo Credits
Terry Dear, Celtic Leather
Holmes Halls Processors, Ltd.