Leather Manufacturing is an age old industry that is talked about as long ago as when the bible was written. Animal hides are turned into leather via a process called tanning. Tanning involves treating the hide or skin with dyes and other substance to provide the skin or hide with a resistance to bacterial attacks. Tens of thousands of years ago tanning would have been a slow process compared to the manufacturing industry that produces leather today on an industrial scale.
There are many stages of the leather tanning process and factories use a continuous system through the use of conveyors and other systems to create a quick efficient process. The first stage is the shipment process where the hides are distributed to the factory where tanning occurs. After the hides arrive at the tannery they are prepared through soaking which involves the rehydration of the previously dehydrated hide. Soaking is important as it removes curing agents, unwanted fats, and non-structurally required proteins. The flesh layer is also removed during the soaking process.
Unhairing and Liming is the second stage that occurs in the tannery. The pelts are transferred from the soaking buckets to the unhairing and liming section via a rotary system. The removal of the hair or wool leaves the characteristic grainy surface of the leather. The hides are treated with sodium sulphide and subsequently hydrated with lime to degrade the epidermis and any remaining wool or hair. The Lime, by increasing the pH, aids in the hydrolisation of fats.
Following the Unhairing and Liming stage is the De-liming and Bating stage. The Lime that was used to treat the hides in the previous steps needs to be removed. The process is simple. Weak acids with pH’s close to that of neutral water are used to reduce the swelling causing the water molecules to be released and take the impurities with them. This process causes the hide to become flaccid. The hide becomes rough and so it is important for proteolytic enzymes to be used to clean the grain to regain the silky, smoothness of the pelt.
The final treatment of the preparation phase uses acid. Sulphuric and formic acid are the most commonly used acids for this part of the process which is known as pickling. They provide the optimum pH for the tanning agent to penetrate the hide. Salt is used to prevent swelling as it cause water to diffuse out through osmosis.
After the numerous preparation phases the pickled pelt is tanned, but only lightly, to again improve the penetration power of the tanning agents. The process stabilises the leather and prepares it for the main tanning where chromium tanning salts are added along with salts and vegetable tannins. The tanning agents bind to the collagen in an un-reversible process.
The leather is then transferred into the mechanical splitting area of the tannery where the leather is split to the required thickness which can be set on the splitting machine. After the splitting a mechanical blade shaves the leather to ensure that it has been refined to the required thickness. These operations remove water out of the leather so after the splitting and shaving has ceased the leather is transferred into a neutralisation tank where the leather is rehydrated and the pH is restored to a neutral 7. The process then splits into multiple different pathways as the leather is sent to different areas of the tannery for a specification process. The leather is finally organized into groups for specific dyeing and finishing before it is sent to the respective sub sections of the tannery where it will be turned into the required product such as clothing, seating, and upholstery.