How Leather Is Tanned: Chrome, Vegetable and Other Methods

Leather is made by cleaning a raw animal hide and then tanning it, a chemical treatment that stabilises the skin's collagen fibres so the material no longer rots and stays flexible once dried. Today most leather is tanned with chromium salts, a method that takes about a day, while the older vegetable method uses plant tannins and takes weeks.

What tanning actually does

An untreated hide decays quickly, and if it is simply dried it turns hard. According to Britannica, a tanning agent pushes water out of the spaces between the protein fibres and bonds those fibres together. The result is a material that resists heat, moisture and bacteria and does not putrefy when it gets wet again.

Britannica identifies three agents as the most widely used: vegetable tannin, mineral salts such as chromium sulfate, and fish or animal oils.

A short history of tanning

One of the best documented early finds is a shoe recovered from Areni-1 cave in Armenia. In a 2010 paper in PLOS ONE, researchers dated the leather to roughly 5,600 years ago and described a shoe cut from a single piece of hide, probably cowhide, laced with a leather thong.

Britannica traces vegetable tanning through many ancient cultures. The Hebrews used oak bark, the Egyptians used babul pods, and the Romans used bark, certain woods and berries. Arab tanners, who worked with bark and roots, carried their methods into medieval Europe by way of Spain. By the 18th century, oak bark, sumac, valonia and hemlock were the established tanning materials in Europe and North America.

The real break came at the end of the 19th century, when chromium salts were introduced. Britannica describes this as probably the first change in leather chemistry in at least 2,000 years. Accounts differ on which chemist or tanner deserves credit for the first workable process, so this article does not name one. What is not disputed is the outcome: a commentary in Materials Engineering Research (Ahmed and Maraz, 2021) states that more than 85 to 90 percent of leather is now chrome tanned.

Period	Development	Source
c. 3600 BC	Areni-1 shoe made from a single piece of hide	PLOS ONE
Antiquity	Oak bark, babul pods, woods and berries used as tanning materials	Britannica
Middle Ages	Arab vegetable tanning methods reach Europe through Spain	Britannica
18th century	Oak bark, sumac, valonia and hemlock established as standard tannins	Britannica
Late 19th century	Chromium salts introduced for tanning	Britannica
2005	Leather Working Group formed by brands and leather manufacturers	Leather Working Group
2021	Chrome tanning estimated at 85 to 90 percent of production	Ahmed and Maraz

The beamhouse: preparing the raw hide

Before any tanning agent is used, the hide goes through a series of wet preparation stages known collectively as the beamhouse. The environmental guidelines published by the World Bank Group's Multilateral Investment Guarantee Agency (MIGA) summarise the sequence as follows.

1. Curing. Hides are salted or chilled so bacteria cannot grow before they reach the tannery.
2. Soaking. The hides are rehydrated in water to wash out salt, blood and dirt.
3. Liming and unhairing. An alkaline bath, traditionally lime with sulfide, loosens hair and opens up the fibre structure.
4. Fleshing. Fat and connective tissue are removed from the underside of the hide.
5. Deliming and bating. The alkali is neutralised and enzymes are used to soften the hide and clean the grain.
6. Pickling. Salt and acid bring the hide to the low pH that most tanning agents need to penetrate evenly.

Thick cattle hides are often split horizontally at this point or after tanning. The outer layer carries the grain, the dense surface pattern of the skin, while the lower layers become splits that are often used for suede. Our guide to our leather explains how these layers relate to leather grades.

Chrome tanning

In chrome tanning, the pickled hides are tumbled in large rotating drums with a solution of basic chromium sulfate. The chromium compounds bond with the collagen and cross-link the fibres. Britannica describes two historical versions: a double-bath method, in which hides were soaked first in chromic acid and then in a second bath that formed the tanning salts on the fibres, and the single-bath method that became standard, in which drums of increasingly strong chromium sulfate solution do the work.

Once tanned, the hide has a pale greyish blue colour. In this semi-finished state it is called "wet blue", as Wikipedia's overview of leather notes. The tanning stage itself usually takes about a day.

Why chrome dominates

• Speed. A day in the drum compares with several weeks for traditional vegetable tanning.
• Softness and stretch. Chrome leather is supple and elastic, which suits garments and handbags.
• Stability. It copes with water better than vegetable-tanned leather, which Wikipedia describes as not stable in water.

Vegetable tanning

Vegetable tanning relies on tannins, natural compounds found in bark, wood, leaves and fruit. Britannica notes that the method is essentially unchanged: hides are moved through a series of vats or pits filled with liquors of steadily increasing strength, so the tannin penetrates gradually without hardening the surface first.

The tannin sources listed in Wikipedia's article on tanning include the barks of chestnut, oak, hemlock, quebracho, mangrove and wattle (acacia), along with sumac leaves, gallnuts and myrobalans.

Vegetable-tanned leather is firmer and less stretchy than chrome leather, which is why it is widely used for luggage, belts, footwear, wallets and other leather goods. Its drawbacks are the time it takes, its lower stability in water, and a stiffer hand that is less suited to soft jackets.

Other tanning methods

Chrome and vegetable systems are not the only options. Wikipedia's overview of leather lists several others:

• Aldehyde tanning. Uses glutaraldehyde or oxazolidine compounds. The intermediate is pale and called "wet white". It is the main chrome-free route, and chamois leather belongs to this family.
• Oil tanning. Britannica describes fish oil being pounded into the skin, which is then hung in warm ovens so the oxidised oil bonds to the fibres. The result is soft leather, such as chamois and deerskin, that survives repeated wetting and drying.
• Tawing with alum. Aluminium salts produce a white, soft material, but it is not a true tan: soaked long enough, it reverts towards rawhide.

From crust to finished leather

After tanning, the leather is shaved to an even thickness and enters a stage often called crusting. According to MIGA's guidelines, this is where retanning, dyeing and fatliquoring take place. Fatliquoring means adding emulsified oils that coat the fibres so they slide past each other, which is what keeps a jacket soft rather than papery. The leather is then dried and mechanically softened. Undyed, unfinished leather at this point is known as crust.

Finishing gives leather its final look and performance. Options listed in Wikipedia's overview include oiling, buffing, coating, polishing, embossing, glazing and tumbling. Some leathers receive little more than dye, so the natural grain stays visible, while others are given a coloured coating for a more uniform surface. Suede and nubuck are buffed rather than coated to raise a nap, and embossing presses a pattern, such as a crocodile print, into the surface.

Chrome vs vegetable tanning compared

Feature	Chrome tanning	Vegetable tanning
Tanning agent	Basic chromium(III) sulfate	Plant tannins from bark, wood, leaves and fruit
Typical time in the tanning stage	About one day	Several weeks in traditional pit tanning
Share of world production	Estimated 85 to 90 percent	A small minority, alongside other methods
Colour after tanning	Pale grey-blue ("wet blue")	Tan to light brown
Feel	Soft, supple, stretchy	Firm, less flexible
Behaviour in water	More stable	Less stable
Typical uses	Jackets, handbags, garments, upholstery	Luggage, belts, footwear, wallets, furniture
Main environmental point	Chromium in wastewater if treatment is poor	Effluent still needs treatment; Leather UK says overall impacts of the main methods are broadly similar

Which is better for a jacket?

For clothing, softness and drape usually matter most, which is why most garment leather is chrome tanned, and why it is the usual choice for leather jackets. Vegetable-tanned leather makes excellent belts and bags, but a full jacket in it would feel stiff. Neither method is automatically better: hide quality and finishing matter at least as much as the tanning agent.

Environmental considerations

Tanning is a wet, chemical-intensive industry, and its record is mixed. The MIGA guidelines estimate that untreated tannery wastewater runs to roughly 20 to 80 cubic metres per tonne of hide, containing about 100 to 400 mg of chromium and 200 to 800 mg of sulfide per litre. The same document sets out how that load can be reduced: cutting water use by 30 to 50 percent, recycling lime and sulfide liquors, using only trivalent chromium, and recovering and reusing chrome.

The distinction between the two forms of chromium matters. Tanning uses chromium(III), which is much less toxic than chromium(VI). Wikipedia's article on tanning, citing chemistry literature, notes that hexavalent chromium can form when waste is handled badly. Where regulation and treatment are weak, the damage can be severe. A 2012 Human Rights Watch report on the Hazaribagh district of Dhaka cited a government estimate of about 21,000 cubic metres of untreated effluent discharged every day and documented skin and respiratory illness among workers and residents.

Certification and the by-product argument

The best-known industry response is the Leather Working Group (LWG), formed in 2005 by brands including adidas, Clarks, IKEA, Nike, Marks and Spencer, New Balance and Timberland together with leather manufacturers. Its audit for leather manufacturers has 17 sections, covering areas such as water and energy use, chemical management, effluent treatment, health and safety, and traceability back to the slaughterhouse. Tanneries are rated Gold, Silver, Bronze or Audited depending on their lowest section score. LWG itself notes the limits: the audit does not assess farms or slaughterhouses, so it says little about the cattle supply chain.

Industry bodies also stress that leather uses a by-product. Leather UK states that over 99 percent of leather comes from animals reared for meat and estimates that up to 40 percent of hides are still discarded as waste each year. The same body says the main tanning methods have broadly similar overall impacts. These figures come from a trade association with an interest in the subject, so they are best read alongside independent data such as the MIGA guidelines. Leather can make use of a waste stream, but only tanneries that treat their water properly make that case convincingly.

For anyone buying leather, the practical questions are where it was tanned, whether the tannery is audited, and how long the product will last. A well-made jacket that is cared for, using guidance such as our leather care page, spreads its footprint over many years of wear.

Sources

1. Britannica – Tanning
2. PLOS ONE – First Direct Evidence of Chalcolithic Footwear from the Near Eastern Highlands (Pinhasi et al., 2010)
3. Materials Engineering Research – Benefits and problems of chrome tanning in leather processing (Ahmed and Maraz, 2021)
4. MIGA (World Bank Group) – Environmental Guidelines for Tanning and Leather Finishing
5. Wikipedia – Leather
6. Wikipedia – Tanning (leather)
7. Human Rights Watch – Bangladesh: Tanneries Harm Workers, Poison Communities
8. Leather Working Group – What we do
9. Leather Working Group – Certification
10. Leather UK – Leather and the environment

FAQs

What is the difference between chrome-tanned and vegetable-tanned leather?

Chrome-tanned leather is treated with chromium(III) salts in rotating drums, takes about a day to tan and comes out soft and supple. Vegetable-tanned leather is treated with plant tannins over several weeks and is firmer and less flexible, which suits belts, luggage and small leather goods more than soft jackets.

Is chrome-tanned leather safe to wear?

Tanning uses trivalent chromium, which is far less toxic than the hexavalent form. The main risks arise in tanneries with poor waste treatment, where hexavalent chromium can form, rather than in the finished garment. Buying from audited supply chains reduces the chance that leather comes from badly run plants.

What does "wet blue" mean?

Wet blue is the name for hides that have just been chrome tanned. The chromium gives them a pale grey-blue colour. In this semi-finished state they still need shaving, retanning, dyeing, fatliquoring, drying and finishing before they become usable leather.

Does Leather Working Group certification mean leather is sustainable?

LWG certification shows that a tannery has been audited on areas such as water, energy, chemicals, effluent treatment and traceability, and rated Gold, Silver, Bronze or Audited. It does not assess farms or slaughterhouses, so it is a useful measure of tannery practice rather than a complete verdict on a product's impact.