Leather

• 1. Introduction

  ›

  Leather has been made for more than 7,000 years, which makes it one of the oldest crafts still practiced at industrial scale. A single ton of hide can produce up to 80 cubic meters of waste water, laced with chromium and sulfide. That tension sits at the heart of the material. It is strong, flexible, and durable enough to last for decades, yet the process that creates it can poison rivers and the people who live beside them.

  The word reaches into corners you might not expect. There is a horse leather found only in an under layer that no other animal possesses. There is a tanning method that turns hides pale blue. There is a city on the Ganges that called itself the Leather City of the World, and a neighborhood in Dhaka where chemicals drain into the main river. Why does turning skin into a lasting material require such force, and why do cattle account for roughly 65 percent of all leather made today? The chapters ahead follow the hide from the drum to the wardrobe, from the chrome bath to the question of what comes after leather entirely.

• 2. From Skin To Hide

  ›

  Tanning is the chemical treatment that stabilizes the proteins of a raw hide, particularly collagen, so it resists heat, microbes, and decay. The difference it makes is stark. A raw hide dries into a hard, inflexible material, and if it gets wet again it will putrefy. A tanned hide dries flexible and stays sound even when rewetted.

  The full sequence breaks into three core subprocesses, with a fourth that is optional. The preparatory stages ready the hide and can involve soaking, hair removal, liming, deliming, bating, bleaching, and pickling. Then comes tanning itself, where the hide soaks in a tanning liquor. Tanners load hides into a drum and immerse them in a tank, letting the drum rotate slowly so the liquor penetrates the full thickness of the skin.

  Once penetration is even, workers raise the liquor's pH in a step called basification, which fixes the tanning material to the leather. The more material that fixes, the higher the leather's hydrothermal stability and shrinkage temperature resistance. After tanning comes crusting, which thins and lubricates the leather and often includes coloring. Crusting can involve splitting, shaving, dyeing, or whitening, and it ends with drying and softening.

  Not every hide goes further, but some receive a surface coating called finishing. Oiling, brushing, buffing, polishing, embossing, glazing, and tumbling all fall under that final step. Oiling does more than shine the surface. It improves water resistance by supplementing the natural oils that washing can strip away, and regular treatment with mink oil or neatsfoot oil keeps leather supple and dramatically extends its life.

• 3. The Chemistry Of Tanning

  ›

  Vegetable tanning is the oldest known method, using tannins drawn from plant matter such as tree bark prepared in bark mills. The result is supple and light brown, and the color tan takes its very name from the look of undyed vegetable-tanned leather. It has a weakness, though. It is not stable in water; it discolors, and if soaked and dried it shrinks and hardens. Traditional shoemaking actually exploits that shrinkage. In hot water the leather shrinks drastically, partly congeals, and turns rigid and brittle. Boiled leather is the extreme case, hardened in boiling water or wax, and history saw it used as armor and for book binding.

  Chrome-tanned leather, tanned with chromium sulfate and other chromium salts, took a different path. Tanners in the latter half of the 19th century wanted to speed the process and make leather more waterproof, and this method delivered. It usually finishes in about a day, suiting it to large-scale industry, and it is now the most common method in use. Undyed, it carries a pale blue color that earns it the nickname wet blue. It stays more supple and pliable than vegetable-tanned leather and holds its shape in water. The catch is chromium itself, a heavy metal; the trivalent form used in tanning is harmless, but other byproducts can contain toxic variants.

  Aldehyde-tanned leather, made with glutaraldehyde or oxazolidine compounds, is the main chrome-free leather and turns up in infant shoes and automobiles. Its pale cream color earns it the name wet white. Formaldehyde once served here too, but it is being phased out because it endangers workers and many people are sensitive to it. Chamois leather is an aldehyde-tanned form, porous and highly water-absorbent, traditionally made using cod oil that oxidizes to produce the tanning aldehydes.

  Two other approaches reach back to older traditions. Brain-tanned leathers use emulsified oils, often from the brains of deer, cattle, and buffalo, in a labor-intensive process; buckskin is an example, prized for softness and washability. Alum leather, by contrast, is not truly tanned at all. The process is called tawing, using aluminium salts with binders like flour and egg yolk, and the material reverts to rawhide if soaked long enough to wash the alum out.

• 4. Grades And Grain

  ›

  Top-grain leather keeps the outer layer of the hide, the grain, where finer, more densely packed fibers give strength and durability. Depending on thickness, it may carry some of the fibrous under layer called the corium. Within this category, full-grain leather holds the entire grain layer with nothing sanded off the surface. Rather than wearing out, it develops a patina over its life, which is why it is usually considered the highest quality, finished with a soluble aniline dye and chosen for furniture and footwear. Russia leather is one form of it.

  Corrected grain leather takes a different route, with the surface buffed or sanded to remove flaws, then dyed and embossed for a uniform look. Nubuck is top-grain leather sanded on the grain side to raise a slight nap of short protein fibers, producing a velvet-like surface. Split leather comes from the corium left behind once the top-grain is separated off, a piece known as the drop split; in thicker hides that split can divide again into a middle split and a flesh split. Suede is made from the underside of a split for a soft, napped finish, often from younger or smaller animals since adult skins tend to give a coarse, shaggy nap.

  Bicast leather is split leather coated with polyurethane or vinyl bearing an embossed texture, mimicking grain with a more consistent surface that runs slightly stiffer than top-grain. Patent leather dates to the late 1700s and won wide popularity after inventor Seth Boyden developed the first mass-production process in 1818, using a linseed-oil-based lacquer; modern versions are usually a form of bicast. Bonded leather, also called reconstituted leather, shreds leather scraps and binds them with polyurethane or latex onto a fiber mesh, with the leather fiber share ranging anywhere from 10 percent to 90 percent.

  The phrase genuine leather is the trap in the lineup, because it names no specific grade. It often points to heavily processed split leather, and some sources treat it as synonymous with bicast or with multiple splits glued and coated together. On a product label in some countries it can mean nothing more than contains leather, with regulations on its use varying by jurisdiction.

• 5. Beasts And Their Hides

  ›

  Cattle hides supply about 65 percent of all leather produced, with sheep at roughly 13 percent, goats around 11 percent, and pigs near 10 percent. Accurate global figures are hard to pin down, especially where skin is eaten, and every other animal makes up only a fraction of a percent. The regional history is woven into the names. Goat leather was once called Turkey or Morocco for its Middle Eastern ties, while pigskin was historically used most in Germany.

  Horse hides yield especially durable leathers, and shell cordovan is the prize among them. It is made not from the outer skin but from an under layer called the shell, found only in equine species, and it is valued for a mirror-like finish and resistance to creasing. Lamb and deerskin give the soft leather used in pricier apparel, and deerskin in particular fills work gloves and indoor shoes.

  Reptile skins from alligator, crocodile, and snake carry the distinct patterns of their scales, which has driven both hunting and farming for hides. The Argentine black and white tegu ranks among the most exploited reptile species in the leather trade, though it is not endangered, and trade stays legal and monitored across most South American countries. Kangaroo leather goes where strength and flexibility both matter; it is the usual choice for bullwhips, and some motorcyclists favor it for its light weight and abrasion resistance. It also appears in falconry jesses, soccer footwear such as the Adidas Copa Mundial, and boxing speed bags.

  Ostriches were first raised for feathers in the 19th century and are now valued for meat and leather, the hide marked by a goose bump look from the large follicles where feathers grew. In Thailand, stingray leather goes into wallets and belts, tough and durable, dyed black and studded with tiny round bumps that are then dyed white to highlight the pattern. Stingray rawhide also grips Chinese swords, Scottish basket hilted swords, and Japanese katanas, and the leather reinforces high-abrasion zones in motorcycle racing gear. For a given thickness, fish leather is typically much stronger, thanks to its criss-crossed fibers.

• 6. The Price Of Production

  ›

  Estimates of the carbon footprint of bovine leather run from 65 to 150 kg of CO2 equivalent per square meter, before counting the chemical and air pollution of the transformation itself. Hydrogen sulfide forms when acids are mixed in, ammonia is released during deliming, and solvent vapors escape into the air. The tanning bath draws on chromium, phthalate esters, nonyl phenol ethoxylate soaps, pentachlorophenol, and solvents.

  Water carries much of the burden. One ton of hide or skin generally produces 20 to 80 cubic meters of waste water, with chromium at 100 to 400 mg/L, sulfide at 200 to 800 mg/L, plus heavy loads of fat, solid waste, and pathogens. Solid wastes can reach up to 70 percent of the wet weight of the original hides, a serious strain on water treatment. There is one upside in the timeline of disposal. Leather biodegrades slowly, taking 25 to 40 years to decompose, while vinyl and petrochemical-derived materials need 500 years or more.

  The damage concentrates where regulations are lax. India, the world's third-largest producer and exporter of leather, offers the starkest cases. In Kanpur, which called itself the Leather City of the World, 10,000 tanneries operated as of 2011 in a city of three million on the banks of the Ganges. Pollution ran so high that the control board shut down 49 of the worst tanneries out of 404 in July 2009, and back in 2003 the main effluent unit was dumping 22 tonnes of chromium-laden solid waste a day in the open. In the Hazaribagh neighborhood of Dhaka, tannery chemicals reach the city's main river, and after roughly 15 years of ignored high court rulings the government shut down more than 100 tanneries the weekend of the 8th of April 2017.

  The math of cheating is plain. Treating effluent costs more than dumping it, so illegal discharge becomes a way to save money. In Croatia in 2001, proper pollution abatement ran 70 to 100 US dollars per ton of raw hides against 43 dollars for irresponsible behavior. In November 2009, one of Uganda's main leather companies was caught dumping waste water straight into a wetland next to Lake Victoria. The technology to do better exists; the United Nations Industrial Development Organization notes the chrome pollution load can be cut by 94 percent, yet warns the minimum residual load of 0.15 kg per ton of raw hide can still cause trouble for landfills under current rules.

• 7. Leather In The World

  ›

  A cowboy in leather chaps gave way to the leather-jacketed and leather-helmeted aviator, both drawing on the same trait: high resistance to abrasion and wind. When motorcycles arrived, riders pulled on heavy leather jackets against road rash and wind blast, some adding chaps or full leather pants for the lower body. The material's flexibility also lets it be formed into balls and protective gear, which is why baseball gloves, the cricket ball, and the gridiron football all rely on it.

  Many rock groups made leather a uniform, especially heavy metal and punk groups in the 1970s and 80s. Extreme metal and Goth rock acts lean on extensive black leather, while punk has worn less of it over the last three decades as some punks oppose its use on animal-rights grounds. Leather fetishism is the popular name for a fetishistic attraction to people wearing leather, or sometimes to the garments themselves. Cars and trucks meanwhile offer leather or leather-faced seating as standard or optional fittings.

  Religion shapes the trade in quiet but firm ways. Where populations observe faiths that restrict material choices, vendors clarify the source so a Muslim does not accidentally buy pigskin or a Hindu can avoid cattleskin, and such taboos raise demand for neutral leathers like ostrich and deer. Judaism forbids the comfort of leather shoes on Yom Kippur, Tisha B'Av, and during mourning. Jainism prohibits leather outright, since it comes from killing animals.

  Leather itself can fail with time, and conservators name its enemies precisely. Acidic leathers are vulnerable to red rot, which powders the surface and is worsened by heat and high humidity; the change is chemically irreversible, though treatments can add strength and slow disintegration. Long stretches below 40 percent relative humidity desiccate leather and permanently alter its fiber structure, while ultraviolet light, ozone, and acid from sulfurous and nitrous pollutants drive further chemical damage. Care is simpler than the chemistry suggests. A damp cloth rather than a wet one, conditioners, saddle soap for cleaning and softening, and shoe polish for shoes all keep the material alive.

• 8. After The Hide

  ›

  Pleather is a portmanteau of plastic leather, one of many names for the artificial substitutes built from polyurethane or vinyl coatings on a cloth backing, alongside brands such as Naugahyde. These imitate the look and feel of leather without an animal hide, though they are the same petrochemical-derived materials that take 500 years or more to break down.

  A newer path moves away from plastic entirely. Cultured leather is grown in a lab through cell-culture methods, while other approaches use mushroom-based materials or a gelatin-based textile made by upcycling meat industry waste. Leather made from fungi or mushroom-based materials is completely biodegradable, an answer to the disposal timelines that haunt both real leather and its plastic stand-ins. Whether grown from cells or cultivated from fungus, these materials reframe a 7,000-year-old craft as a question still being answered in the laboratory.