Leather

Leather is a tough, pliable, and long-lasting material made by chemically treating animal skins and hides to keep them from rotting. The most common sources are cattle, sheep, goats, horses, buffalo, pigs, ostriches, and aquatic animals like seals and alligators. It is used to produce clothing, shoes, bags, furniture, tools, and sports gear, and can remain usable for decades.

The production process has three main stages: preparation, tanning, and crusting, with an optional finishing step. Preparation includes soaking, removing hair, liming, deliming, bating, bleaching, and pickling. Tanning stabilizes the proteins in the hide, especially collagen, to make it resistant to heat, chemicals, and microbes. Untanned hides become hard and brittle when dried and will rot if rewetted, while tanned leather stays flexible and does not decay. In tanning, hides are loaded into a rotating drum and soaked in a tanning liquor. Once the liquor fully penetrates, the pH is raised to fix the tanning agents to the leather, which increases its heat stability. Crusting thins and lubricates the leather, often involving dyeing, and ends with drying and softening. Finishing can include oiling, brushing, buffing, coating, polishing, embossing, glazing, or tumbling. Oiling leather after tanning improves water resistance, as repeated wetting can wash out natural oils; regular application of mink oil, neatsfoot oil, or similar substances keeps it supple and extends its life.

Tanning methods differ by the chemicals used. Vegetable tanning uses tannins from plant matter like tree bark, making a supple, light brown leather that is not water-stable—it discolors, shrinks, and hardens when soaked and dried, and can become rigid and brittle in hot water. Boiled leather is a hardened version of this. Chrome tanning uses chromium salts, producing a pale blue "wet blue" leather in about a day, making it efficient for large-scale use. It is more supple and water-resistant than vegetable-tanned leather, but raises environmental concerns because chromium is a heavy metal; while the trivalent form used is harmless, byproducts may contain toxic variants. Aldehyde tanning uses glutaraldehyde or oxazolidine, resulting in a cream-colored "wet white" leather that is chrome-free and common in infant shoes and car interiors. Formaldehyde was once used but is being phased out due to health risks. Chamois leather is an aldehyde-tanned type made with oxidizing oils, making it porous and highly absorbent. Brain tanning uses emulsified animal brains (from deer, cattle, or buffalo) in a labor-intensive process, producing exceptionally soft, washable leather like buckskin. Alum leather is made with aluminum salts, flour, and egg yolk, but is not truly tanned. Critics of tanneries argue that their practices can be unsustainable and pose health hazards to nearby people and the environment. The largest leather producers today are China and India, and leather making has been practiced for over 7,000 years.