Leather - The Tanning Process

Tanning refers to the procedure of chemically treating raw hide to make it stronger, more flexible and resistant to decay. In the leather manufacturing industry, the term ‘hide' refers to skins from larger animals, while ‘skin' refers to that of smaller animals. However, skin is skin regardless of coming from dead cattle or a dead lamb, so will be referred to as such.

The tanning process consists in strengthening the protein structure of the putrescible skin by creating a bond between the peptide chains. The skin is made up of three layers being the epidermis, dermis and subcutaneous layers. The dermis consists between 30-35% protein, which is mostly collagen, with the remainder being fat and water. It is the dermis that is used to make the leather after the other layers have been removed chemically or mechanically. The whole tanning process using acids, alkalis, salts, enzymes and tanning agents to dissolve the fats and non-fibrous proteins, as well as chemically bond the collagen fibres. The tanning process also produces a lot of waste which has now become an environmental concern.

The generic tanning process involves several steps, depending on the type of skin used and its desired end product:

  1. Curing: this process involves salting and/or drying the skin once it's been stripped from the dead animals. This is an immediate step upon removal and takes place inside the meat-packing industry or a nearby factory. Skins can be cured in one of two ways:
    1. Wet salting: salting the skin and then piling many skins together until a moist bunch is formed. They are then left for a month allowing the salt to be completely absorbed into the skin.
    2. Brine curing: more common than wet-salting since it is consider as being faster and easier. During brine-curing, skins are placed in vats and smothered with a mixture of salt and disinfectant. This process takes up to 16 hours with the skins being completely cured to proceed to the next stage.
  2. Soaking: Once cured, the skins are then soaked in water for several hours to several days. The water aids in the removal of salt, dirt, debris, blood and excess animal fats.
  3. Hair Removal: At this stage the animal hair is still present. The skins are transported to another large vat and immersed in a mixture of lime and water, which loosens the hair from the skin. After soaking from 1 to 10 days, the hair is then mechanically removed from the skin.
  4. Scudding: Any stray hairs and fat which were missed are removed with a plastic tool or dull knife, by hand.
  5. Deliming: After the hair, debris and excess fats has been cleaned from the skin, the skins are delimes in a vat of acid. After the lime has been pulled from the skin, skins are then treated with enzymes, which smooth the grain of the leather allowing for a product that is soft and flexible.
  6. The treatment of skins can be repeated several times during the tanning process. The type of tanning procedure depends on the type of skin and the resulting product intended. Wet-blue comes from "hides", wool skins, dewoolled or fell mongered pickled-pelts from sheepskins, finished leather or sole leather.
    • Vegetable tanning: skins are tanned with a vegetable tanning agent solution to produce flexible, but stiff leather. This process involves stringing the skins on large frames, situated in large vats, and exposing them to tannin, a natural product found in wood, bark, leaves and fruits from oak, chestnut and hemlock trees. skins are treated repeatedly with each step involving the skins being soaked in a stronger solution of tanning every time
      This type of leather is used in luggage, furniture, leashes, belts, hats and harnesses.
    • Mineral tanning (Chromium III): The most common tanning type in the world. Mineral or chrome tanning is carried out on skins needed for softer, stretchier leathers. skins are pickled first in an acid and salt mixture then soaked into a chromium-sulphate solution. A faster process than vegetable tanning since it is usually takes 1 day
      This type of leather is found in purses, bags, briefcases, gloves, shoes, boots, pants, jackets, and sandals.
  7. Dyeing process: Depending on the desired product, the skins go through a dyeing process which involves replacing moisture back into the skin. skins that have been vegetable tanned are bleached and then soaked with oils, soaps, greases and waxes to make them more pliable.
  8. Rolling: This involves running the skins through a machine to produce leather that is firm and stronger.
  9. After the rolling process, the leather is stretched and dried out in a heat controlled room.
  10. Finishing compound: This is the final step in the tanning process. The process involves covering the grain surface with a chemical compound and then brushing it. Light leathers are buffed and sandpapered to cover any imperfections. Leathers that have been buffed for long periods of time become suede.

To make the leather more appealing to the buyer, waxes, pigments, dyes, oils, glazes and other solutions are added to the leather.

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