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Leather Manufacturing - A Journey To Leather Making by Joyce Thomas
Leather is a material created through the process of tanning of hides and skins of animals. The process helped in converting putrefiable skin into durable, long-lasting and versatile natural material.
History of leather manufacturing
Leather making is one of the most primitive forms of activity men have involved in centuries ago. Leather is associated with animals, and they were hunted and killed for food, but before consuming, their skin was removed using sharp flints. This skin was then used to wrap around and protect from cold weather. But the skin soon started putrefying and decomposing. So, ancient men started drying the leather, a first step towards preserving leather. But this resulted in leather turning hard and inflexible losing its sheen and comfort.
Leather was then softened by rubbing fat on the skin. This made the skin to last longer and made it even more pliable and at the same time prevented it from getting wet. Later using water, barks, leaves and berries, a liquid with vegetable extracts was used for making leather supple soft and rot resistant. The active agents in this extract are called tannins and were probably the first method of tanning leather.
When leather manufacturing became more organized and systematic, tanneries were set up at special pockets with close source of raw materials like adequate supply of hides and skins, plenty of water - lime for softening and hair removing and ample plant extracts for tannins. Even the processes we use today are based on the ancestral methodology used thousands of years ago.
Leather Manufacturing Process
The leather manufacturing process involves many stages. All genuine leathers undergo all these processes going to the tannery. But the actual manufacturing stage starts from the tanning process. When the skins come at the tannery they are in their cured form. They are then examined, to check for any evident damages due to poor whipping or curing. Then the following steps are performed before the starting the tanning process.
Soaking - The skin is soaked in order to return the skin back to its original state and to remove the blood, dirt and other impurities. This process as other processes involves chemicals and is carried out in large wooden drums.
The skin is first soaked in cold water containing detergent, salt and biocide. The water is rarely changed, unless it is heavily contaminated. The process is finished when the water remains clean, and the skins are re-hydrated. The time taken for this process depends on the method of curing implemented at the abattoir. For example it takes longer for dry cured skins (approximately 72 hours) and shorter for wet salted skins (approximately 18 hours). The entire process can be expedited by using hot water and enzymes. But soaking should be done correctly; so that the leather can relax properly allowing the chemicals to penetrate properly in other processes else it can give a patchy and inconsistent leather look.
Liming - The second stage in leather processing is liming. In this stage, hair is removed from the cleaned skin. The hair needs to be removed from the roots and the epidermis so as to expose the grain layer. For this sodium sulphide or sodium hydrosulphide is added to the soaked skins leaving it for an hour. Almost after an hour, these chemicals penetrate deeply into the hair and the grains of the leather, causing the breakdown of the keratin, the main protein constituent of the hair and epidermis. Lime and caustic soda are strong alkalis that enhance the further breakdown of hair root and epidermis away. Beside this the liming process removes the inter fibrillary proteins. These are proteins similar to egg white which, if not removed, would harden like glue and make the leather as stiff.
Lime also produce two important physical effects - osmotic swelling and lyotropic plumping. Due to this the skin becomes swollen and engorged with water causing the fibre bundles to open up allowing the thorough penetration of the tanning materials at a later stage.
Washing - The process of liming is followed by washing, where in the skins are washed to remove excess lime, other chemicals and materials loosened during the previous stage.
Fleshing - This process is followed in order to remove any sticking fat and tissues on the underside of the skin which can be a major barrier to the penetration of subsequent chemicals. So these are to be removed, assisting the skin to relax as well helps in removing the remaining hair roots out of the skin. This procedure is carried out using a fleshing machine.
Thus, removal of this 'flesh' at an early stage, is highly desirable. The fleshing operation not only removes flesh, but assists in relaxing the skins and it also aids in the removal of any remaining hair roots out of the skin. This procedure is carried out using a fleshing machine.
De-liming - Since the liming process makes the skins more alkaline, it is required to lower the levels of alkalinity before the tanning operation. Ammonium chloride is used to neutralize the skin and make the skin normal. As a result, skin de-swells and the keratin proteins destroyed during liming process wash out from the skin.
Bating - Bating makes the skin supple and soft as it removes the muscle fibers using enzymes. After this treatment, the finished grain will be super smooth and relaxed. Initially animal dung - dogs and chicken were used as the source of enzymes, followed by enzymes extracted form cattle pancreas, but now it is replaced with bacterial enzymes.
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