The leather manufacturing process is divided into three fundamental sub processes: preparatory stages, tanning, and crusting. A further sub process, finishing, can be added into the leather process sequence, but not all leathers receive finishing.
The preparatory stages are when the hide is prepared for tanning. Preparatory stages may include: soaking, hair removal, liming, deliming, bating, bleaching, and pickling.
Tanning is a process that stabilizes the proteins, particularly collagen, of the raw hide to increase the thermal, chemical and microbiological stability of the hides and skins, making it suitable for a wide variety of end applications. The principal difference between raw and tanned hides is that raw hides dry out to form a hard, inflexible material that, when rewetted, will putrefy, while tanned material dries to a flexible form that does not become putrid when rewetted.
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 about its axis, 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 hydro thermal stability and shrinkage temperature resistance.
Crusting is 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 methods.
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 can 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. Frequent oiling of leather, with mink oil, neat foot, or a similar material keeps it supple and improves its lifespan dramatically.