Leather production is consists of several stages. Preservation, Tanning, Selecting, Splitting/Shaving, Retanning and Finishing. Each of these processes is complex and requires many steps. Below are the key points of some of these steps.
Step 1: Preservation
It is a process that allows for the temporary transport and storage of raw material. This is common, but not necessary if a tannery is very close to a slaughterhouse. The hide begins to deteriorate rapidly after being swam from the animal. In order to prevent this, the post is protected by various methods. The most applied method is salting. With salting, the water level in the leather is reduced and bacterial activity is prevented.
Step 2: Tanning
Soaking is the first step of leather processing. The purpose of soaking; cleaning the hide from dirt, blood and manure, restoring the moisture and swelling of the skin before storage, and removing the protectors and disinfectants, if any.
One of the main step of leather production in the tannery is liming. Leather can be used both with (for furs) and without hair. The purpose of liming is to separate the fiber bundles and remove unwanted hair from the skin by adding alkali, removing meat and fat residues, removing proteins that will affect the quality of the leather, swelling the skin fibers, and opening the skin structure. At the moment, lime is generally added before alkaline swelling occurs from additions of sodium sulfide and sodium hydrosulfite in the next step.
After liming, lime or other alkali on the skin is no longer necessary and in most cases has a detrimental effect on subsequent tanning. Removal of alkali from the skin and accordingly swelling of the fibers and lowering the pH value are provided with deliming.
With the use of specific enzymes, the type of proteins that do not need in leather production are dissolved.
Acids are added to loosen the leather and increase the penetration of the product to be used in the tanning process.
Using a tanning material, the collagen structure of a hide is modified and converted into an intermediate product as a leather. It is now a more durable material and protected against chemicals, moisture and microorganisms.
There are different types of tanning. Such as chrome tanning, vegetable tanning, oil tanning, resin tanning, combined tanning. Chrome tanning is the most common. The leathers are soaked in baths containing acidic salts until they are ready for chrome tanning. The bath contains chrome tanning materials and the acidity of the bath is lowered until the surface of the leathers begins to absorb the chromium materials. The leather produced from this process is blue in color and is called “wet-blue”.
Chrome tanning is faster than other tanning types and can take up to a day with modern machines. After the first tanning process, the leather is splitting in various thicknesses according to the request of the producers. Because it is a highly automated process, this type of production is more popular and less expensive than other labor-intensive tanning methods.
Step 3: Selecting
It is the process of separating tanned leathers according to their skin conditions, natural features and, amounts and location of defects. After tanning, excess water is removed from the skin.
Step 4: Splitting / Shaving
The leather must be divided into layers, and each layer is suitable for the manufacture of different products. To achieve this, the skin is first divided in the splitting machine. Then the leather should reach the desired and equal thickness. A shaving machine shaves the skin until the appropriate thickness is achieved.
Step 5: Retanning
In the retanning process, the leather is given its final properties by the application of syntans, oils and dyes. Each leather product demands different retanning and mechanical processes.
Step 6: Finishing
The finishing, determines the final appearance and surface properties of the leather surface. These include coloring, waterproofing, polished coatings, as well as mechanical steps such as ironing or embossing the leather.