The leather manufacturing process is divided into four fundamental sub-processes:
The preparatory stages
Preparatory stages may include: liming, de-liming, hair removal, de-greasing, bleaching, and pickling.
Liming
Liming is a process used for parchment or leather processing, in which hides are soaked in an alkali solution. It is performed using a drum and paddle or a pit. Its objectives are:
Liming operations of cattle hides usually last 18 hours and are generally associated with the alkaline phase of beamhouse operations.
Tanning
Tanning is a process that converts animal skin to leather. It stabilizes the proteins of the raw hide so it does not putrefy. The principle difference between raw and tanned hides is that raw hides dry out to form a hard, inflexible material that, when re-hydrated, will putrefy, while tanned material dries to a flexible form that does not become putrid when re-hydrated. Many tanning methods and materials exist. The typical process sees tanners load the hides into a drum and immerse them in a tank that contains the tanning "liquor". The hides soak while the drum slowly rotates and the tanning liquor slowly penetrates through the full thickness of the hide. Once the process achieves even penetration, workers slowly raise the liquor's pH in a process called basification, which fixes the tanning material to the leather. The more tanning material fixed, the higher the leather's hydrothermal stability and shrinkage temperature resistance.
Crusting
A process that thins and lubricates leather. It often includes a coloring operation. Chemicals added during crusting must be fixed in place. Crusting culminates with a drying and softening operation, and may include splitting, shaving, dyeing, whitening or other processes.
Surface coating
Surface coating can be added into the leather process sequence, but not all leathers receive surface treatment.
For some leathers, tanners apply a surface coating, called "finishing". Finishing operations can include oiling, brushing, buffing, coating, polishing, embossing, glazing, or tumbling, among others.
Leather may be oiled to improve its water resistance. This currying process after tanning supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water.
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