The process of leather production is a thorough one, divided into four parts: preparation, tanning, crusting and surface coating. Different grades of leather go through different kinds of operations to be made into the final material. Although all leathers go through these three processes, some of these processes can also get omitted for the production of certain types of leather, such as partial tanning or untanned products.
Preparation: The preparation stage of manufacturing leather involves preservation of the hide, removing unwanted components and pre-treatment operations for making the treatment of the leather easier.
Preservation of the leather hide is a very important part of leather processing, for halting the decay of the hide temporarily. This is done by freezing, salting & drying the leather. Freezing involves packing and storing the hide in big cooling rooms, which stop the decomposition of the leather almost immediately. The salting method includes using fresh and pure salt to completely saturate the hide so as to stop any growth of the bacteria. Although salted hides can be stored for several months, they must be stored with precautions related to humidity, the manner of placement of the hides, etc.
Drying method is the oldest and simplest method of preserving hide. The hide is stretched in way that air can come in contact with the hide on all parts. Thus the moisture required for the growth of bacteria goes away, preserving the leather.
Several operations are performed on the hide to eliminate the unwanted components of its surface as well as preparing the leather for further treatment. These operations depend on the type of leather. The hide is washed clean and rehydrated, the process called soaking. Then the hide goes through unhairing, which removes majority of the hair on the hide.
Liming and fleshing are processes in which unwanted proteins and subcutaneous material is removed respectively. At certain instances, the hide might have to go through liming twice, a process called reliming.
Proteolytic proteins are introduced to the hide to further remove the proteins and help in softening of the pelt of the hide. This is called bating. Then the hide undergoes degreasing and frizzing, the removal of natural fats and oils. Frizzing involves physically removing the fat layer inside the skin.
To make the pelt light-colored and easier to tan, the leather is bleached by chemical modification. Finally, the process of pickling lowers the pH value of the leather to acidic region to help with the penetration of certain tanning agents like chromium, aldehydic or polymeric tanning agents. This process is replaced by depickling for certain other tanning agents, which means raising the pH value out of the acidic region to assist the effect of tanning agents.
Tanning: The process of tanning plays a major role in the physical characteristics of the hide. It converts the protein on the hide into a material which will not experience decaying as well as making it suitable for variety of applications. There are many tanning methods available, depending on the end application of the leather, although the most common tanning material is Chromium.
After the pickling or depickling (depending on the tanning agent used), the hides are immersed into a drum filled with the tanning liquor. The drum slowly rotates and the hide is slowly penetrated by the tanning liquor. By regularly checking and observing the penetration by cutting a section of hide, once the degree of penetration is even across the leather, the pH inside the drum is slowly raised, the process called basification. Basification fixes the tanning material into to the leather and results in hydro-thermal stability and temperature resistance.
Crusting: The process of crusting includes thinning, re-tanning and lubricating the hide. The operations carried out under crusting ultimately decide the physical characteristics and grade of the leather. For example, splitting is an operation in which the leather is split horizontally into layers, which classify as the Split leather. Dyeing of the leather is also a part of crusting in which leather colored. On the contrary, the process of whitening lightens the color of the leather. Filling is the method of adding dense chemicals to the leather to make it harder and heavier. Other sub processes include various methods that result in drying and softening of the leather, such as drying, conditioning, softening, buffing, etc.
Surface coating: A surface coating is applied to some leathers which is referred to as finishing. Finishing operations give the leather its final look and feel, as well as making the leather resistant to abrasion and trying temperatures. Finishing includes operations such as oiling, brushing, buffing, padding, spraying, roller coating, polishing, embossing and glazing. The finish formula can comprise of casein, protein, wax, resins and lacquers. These are applied onto the leather by machine rollers or hand sprays.
Embossing is a method to achieve permanent printed effects on to the leather, which is done by putting the leather in contact with an engraved metal plate that carries the design under high heat and pressure. Brushing is used to get rid of fine leather dust that results from buffing of the leather.
After the complete processing of leather is over, it is then used for making into products such as accessories, clothing and footwear.