Article One

Leather is a durable and flexible material created by tanning animal rawhide and skin, often cattle hide. It can be produced at manufacturing scales ranging from cottage industry to heavy industry.

People use leather to make various goods—including clothing (e.g., shoes, hats, jackets, skirts, trousers, and belts), bookbinding, leather wallpaper, and as a furniture covering. It is produced in a wide variety of types and styles, decorated by a wide range of techniques.

Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other chromium salts. It is more supple and pliable than vegetable-tanned leather and does not discolor or lose shape as drastically in water as vegetable-tanned. It is also known as wet-blue for its color derived from the chromium. More exotic colors are possible when using chrome tanning. The chrome tanning method usually only takes a day to finish, and the ease and agility of this method make it a popular choice. It is reported that chrome-tanned leather adds up to 80% of the global leather supply.

Vegetable-tanned leather is tanned using tannins and other ingredients found in different vegetable matter, such as tree bark prepared in bark mills, wood, leaves, fruits, and roots. It is supple and brown in color, with the exact shade depending on the mix of chemicals and the color of the skin. It is the only form of leather suitable for use in leather carving or stamping. Vegetable-tanned leather is not stable in water; it tends to discolor, so if left to soak and then dried it shrinks and becomes harder. In hot water, it shrinks drastically and partly congeals—becoming rigid, and eventually brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armour after hardening, and it has also been used for book binding.



Article Two

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.

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.[citation needed] At this point, tannery workers load the hides into a drum and immerse it in a float that contain 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. Basification 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 3.8 and 4.2

Tanning is a process that stabilizes the protein of the raw hide or skin so it doesn't putrefy, making it suitable for a wide variety of end applications. The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that, when re-wetted (or wetted-back) putrefy, while tanned material dries to a flexible form that does not become putrid when wetted-back.

Tanning is especially polluting in countries where environmental regulations are lax, such as in India, the world's third-largest producer and exporter of leather. To give an example of an efficient pollution prevention system, chromium loads per produced tonne are generally abated from 8 kg to 1.5 kg. VOC emissions are typically reduced from 30 kg/t to 2 kg/t in a properly managed facility. A review of the total pollution load decrease achievable according to the United Nations Industrial Development Organization[11] posts precise data on the abatement achievable through industrially proven low-waste advanced methods, while noting that "even though the chrome pollution load can be decreased by 94% on introducing advanced technologies, the minimum residual load 0.15 kg/t raw hide can still cause difficulties when using landfills and composting sludge from wastewater treatment on account of the regulations currently in force in some countries."



Article Three

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