How Is Leather Made?
Leather is made from the skins or hides, of such animals as cows, sheep and goats. The skins are made into leather by a process called “tanning.” Before tanning, the raw skins are “cured”- softened and purified – in successive baths of salt water, fresh water and chemicals.
Workers remove hair and flesh from the skins with machines and by hand. The tanning process involves soaking the cured skins in vegetable oils and chemical solutions, to give the skins qualities desired of leather, such as softness and durability. Then the leather is oiled and dyed, and shaped into a final product.
The leather manufacturing process is divided into three fundamental sub processes: preparatory stages, tanning, and crusting. All true leathers undergo these sub processes. A further sub process, surface coating, can be added into the leather process sequence, but not all leathers receive surface treatment. Since many types of leather exist, it is difficult to create a list of operations that all leathers must undergo.
The preparatory stages are when the hide/skin is prepared for tanning. Preparatory stages may include: preservation, soaking, liming, unhairing, fleshing, splitting, reliming, deliming, bating, degreasing, frizing, bleaching, pickling, and depickling.
Tanning is a process that stabilizes the protein of the raw hide or skin so it does not putrefy, 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 (or wetted-back) putrefy, while tanned material dries to a flexible form that does not become putrid when wetted-back.
Many tanning methods and materials exist. The choice ultimately depends on the end application for the leather. The most commonly tanning material is chromium, which leaves the tanned leather a pale blue color (due to the chromium). This product is commonly called wet blue. The hides, when finished pickling, are typically between pH 2.8 and 3.2.
At this point, tannery workers load the hides into a drum and immerse them in a float 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.
Workers periodically cut a cross-section of a hide and observe the degree of penetration. Once the process achieves even penetration, workers slowly raise the float’s pH in a process called basification, which fixes the tanning material to the leather—and the more tanning material fixed, the higher the leather’s hydrothermal stability and shrinkage temperature resistance. Chrome-tanned leather pH is typically between pH 3.8 and 4.2.
Crusting is a process that thins, retans, and lubricates leather. It often includes a coloring operation. Chemicals added during crusting must be fixed in place. For some leathers, workers apply a surface coating. Tanners call this finishing.