Traditional Leather Tanning Processes: Brain Tanning, Vegetable Tanning & More
A glimpse into the history of leather through the lens of traditional tanning methods
Leather is a highly coveted material in the fashion world, but do you know how it is really made? Humans have been tanning leather for thousands of years using such ancient methods as brain tanning, tawing and vegetable tanning.
Modern chrome tanning has largely displaced these traditional methods but there is still a niche for traditionally handmade leather products – “slow leather,” if you will.
In this post, we explore three of the most intriguing traditional leather tanning techniques, before examining the modern chrome leather production and some of its pros and cons – as well as the resurgence of vegetable tanned leather and its promise for a more eco-friendly future for sustainable leather.
History of Leather Tanning
The wearing of animal skins goes back to the beginning of human existence, even before humans mastered the art of weaving. Throughout the ages, there seem to have been as many different tanning processes as there were ethnic groups using them.
The beginnings of leather-making go back to the Stone Age, around 8000 BCE, when early humans sought a way to preserve animal hides and make them waterproof by rubbing them with fats. The oldest leather shoe ever discovered was found in Armenia, and has been dated to 3500 BCE.
Fast-forward another 3000 years, and we see the dawn of vegetable tanning in what is now the Middle East: paintings and artifacts recovered from Egyptian tombs show us that they were already using bark from the gum arabic tree (Acacia nilotica), sesame oil and alum to process leather (a process also apparently known to the Romans).
Some rudimentary tanning methods used by early man are also mentioned in several ancient texts, including the Bible, the Assyrian texts and Homer’s Iliad:
“The ox hide, which is soaked in fat, is pulled to and fro by men standing in a circle, thus stretching the skin and causing the fat to penetrate into the pores.”
But what exactly is the tanning of leather?
What is Leather Tanning?
Tanning is the process by which animal skin is preserved and transformed into leather. It changes the raw hide’s protein structure, stabilizing it to prevent putrefaction, decay and oxidization, and to prepare it for several end uses.
Traditionally, animal hides were processed using natural tannins, acidic compounds with tannic acid as their main ingredient – hence the name, tanning. It would not be an exaggeration to say that millions of raw hides would be lying unused and decomposing if not for the art of tanning that transforms them into usable leather.
Tanning is just one part in the whole leather production process, but it is the most important part. “Real” tanning happens when the tannins form an irreversible bond with the proteins in the skin, such as with chrome or vegetable tanning. Earlier tanning methods – like brain tanning and tawing – are considered “false” tanning, as the tanning process is unstable. Parchment is another example of false tanning.
Why is Leather Tanned?
Tanning of one type or another is essential in order to transform raw animal hides into a useable material. Tannins do a variety of things when used to treat skins:
Prevent decay – this is a crucial first step and the most obvious benefit to tanning hides.
Water-resistance – tanning leather prevents moisture from entering the cells of the skin, causing them to swell. Tanned leather is therefore naturally water-resistant and won’t shrink or change shape when wet.
Heat-resistance – Untanned hide will shrink in water heated to just 62°C; on the other hand, chrome-tanned leather can resist water temperatures as high as 100°C.
Preserve flexibility – Skin is about one third collagen. This is what gives skin its suppleness and flexibility. When animal skins are left to dry, these proteins harden, making the skin stiff and sometimes tacky to the touch. The tanning process preserves collagen’s flexibility, resulting in a stable but supple leather.
The Tanning Process
There are many different ways to tan leather. You can even often tell the type of tanning method employed by looking at the colour of the leather before finishing: vegetable leather comes in shades of brown; brain tanned leather is yellowish; tawed leather is white; and chrome-tanned leather is bluish grey. You can usually tell chrome tanned leather even after it’s dyed by looking at a cut edge, which will still show the bluish tinge.
The whole leather tanning process from start to finish includes a dizzying number of steps, many of which happen even before tanning actually begins. The pre-tanning process includes fleshing, curing, soaking, liming, dehairing, splitting, deliming, bating, degreasing, pickling and de-pickling. Phew!
And it doesn’t stop there. After tanning, the leather may be dried, trimmed and shaved, and then dyed, re-tanned, sanded, oiled, ironed, stamped and more.
Additional finishing processes may be used to improve the leather’s appearance by minimizing blemishes, adding gloss or to give it an “antique” look; to add protective coatings or make it easier to clean; or give it a greater degree of softness and malleability.
Traditional Methods of Tanning Leather
Traditional tanning techniques have been handed down through generations to those willing to invest the time in this highly skilled, time-consuming and cumbersome process. Once upon a time, tanning was considered an odiferous trade. Tanneries were foul-smelling places, thanks to the stench of rotting flesh mixed with urine, animal feces and other materials which were used in the tanning liquors. As a result, they could usually be found on the outskirts of town.
Let’s look at some of the most important traditional leather tanning processes throughout history: brain tanning, alum tanning and vegetable tanning.
Brain tanning may be one of the oldest traditional methods of leather tanning. Also known as smoke tanning, this method has been practiced by Indigenous peoples around the globe. Most commonly associated with the cultures of the Plains First Nations across Canada and the United States, there are also accounts of brain tanning in South America, by the Zulus of southern Africa, the Chukchee of eastern Russia, and nomadic peoples across Asia, as well as the Scandinavian Sami.
A waste-free tanning method, the brain tanning process uses fats and oils derived from the animal itself – including its brains – in order to tan the hide. Every animal is said to have enough brains and fats to tan its own hide! Brain-tanned leather is often referred to as buckskin or chamois leather. Brain tanning can be used to tan the hides of just about any animal, but it is most commonly associated with tanning deer and moose hides as well as seal skin.
Brain tanning may be the oldest leather-making process, but it was still widely used even during the early colonial period in North America. Have you ever told someone, “Oh, it’s just a couple of bucks”? Where do you think that word came from? Buckskin. Buckskin was such a common form of exchange around the time of the American Revolution that the term “buck” became slang for a dollar.
The Brain Tanning Process
Most of what we know about traditional brain tanning comes from First Nations traditional knowledge, alongside records kept by anthropologists and explorers.
Brain tanning is an intimate and labour-intensive process that requires specialized skills, getting down and dirty with the animal carcass, carefully stretching and working the skin by hand for hours. It is not for the faint of heart!
Here is an overview of the process:
Skinning/Fleshing: This is the first step where the flesh, fat and membrane that covers the inside of the hide is scraped out and thoroughly cleaned. In Greenland and other parts of the Arctic, people used traditional ulu knives and stones to scrape the hair from seal skin, then beat it – and even chew it! – to make the skin more supple.
Soaking: The next step is to soak the skin, allowing the hair to loosen and to soften the grain layer of the hide.
Graining: This is the process where the hide is thoroughly scraped to remove all hair as well as the epidermis (outer layer of the skin).
Braining: The hide is then stretched on a frame and then the brain tanning solution is rubbed on the hide. In addition to brains, this method may involve adding other fatty animal substances such as fish oil, sebum or marrow and other products like egg yolk, fermented milk, yak butter, liver, sour milk, claw oil or soap to prepare the tanning solution. The fat proteins in the brain moisturize the hides, adding softness and strength to it, which prepares the hide to be worked into leather products.
Stretching: Once the hides are removed, the most strenuous part comes in, which is the drying and even stretching of the hide.
Smoking: In some cases, the stretched skin was smoked over an open fire. The smoke from the fire (often laid with slightly rotted wood) releases phenol, a tanning agent which works to lock in the work of the previous steps to soften the skin and make it workable. Smoking also acts as a preservative, and gives the resultant buckskin a pleasant, slightly smoky odour.
The Disappearance & Modern Revival of Brain Tanning
Like so many traditional cultural practices around the world, the art of brain tanning has been on the decline for a long time. Although it is said that brain tanned leather is one of the highest quality leathers out there, the arduous steps involved in the process may be one of the reasons that this traditional craft gave way to modern tanning methods as the global leather industry grew.
However, there are those who are dedicated to keeping the brain tanning tradition alive. Indigenous youth are making the effort to preserve their heritage by learning this ancestral skill, and some traditional tanners not only teach the practice but also sell brain-tanned leather products to consumers looking for more natural tanned leather options.
Where can you buy brain tanned leather? A great first stop for all things buckskin is Traditional Tanners, a group who has been working to revitalize traditional, natural tanning methods since 1989.
Tawing, aka Alum Tanning
An ancestral tanning method that uses alum and other aluminum salts in combination with binders such as flour and egg yolk, alum tanning was likely invented in Mesopotamia and spread to Egypt by about 1600 BCE.
Through Arab traders and the Roman Empire, the production of alum leathers spread throughout the Mediterranean and reached Britain by the 9th century CE. In medieval Europe, tawed leather was in frequent use and was even developed into a specialized guild. Alum was mined for tanning skins to transform them into a supple distinctive white leather used for gloves, ladies’ shoes, fur skins, and covers, but was gradually replaced by vegetable tanned leather as printed books entered the market. Fine examples of medieval tawed leather are still present in museums and libraries around the world.
Tawing is mainly used for tanning small animal skins such as goat and sheep, pig and calf skins, rather than tanning cow hides. While chrome leather has no doubt replaced alum leather to a large extent, it is still used for bookbinding, high quality gloves, furs, and cricket balls.
Tawing: The Alum Tanning Process
Like other traditional tanning methods, tawing is an elaborate process. The skin is first dried or salted to preserve it, soaked in a solution made of warm alum and other salts, then neutralized in a sodium carbonate solution. Then the skin is unhaired in a lime (calcium hydroxide) pit which cleans, loosens, and swells the fiber structure. Then the skin is trimmed to remove any remaining dirt, lime, grease, and hair using an unhairing knife.
Once the skin is clean it is immersed in warm water with animal dung, a process known as puering. Sometimes instead of dung, cereal husks are used. After this, the skin is hung for weeks to age, during which time the alum salts become fixed in the skin. Finally, the resulting stiff, firm dried leather undergoes staking, where the skin is softened by stretching it over a blunt wooden or metal edge.
The smoothed leather is typically white in colour and can be dyed as desired. The whole process of tawing increases the softness, stretchability, and quality of the leather. Alum tawing is defined as a “semi-tannage” as the leather is water-sensitive unlike other forms of “true” tanned leather. To improve water resistance, tawed leather is often treated with fats and waxes.
Tawed Leather Today: Where to Buy Tawed Leather Products
The tawing process has not changed dramatically over the years, apart from the usual move from manual to mechanized processes, common across most trades.
Some companies such as Eden Workshops (for bookbinders), leather manufacturers J Hewit & Sons Ltd, Talas (conservation and archival supply), and German leather wholesaler Feinleder sell alum tawed leather products online for global customers; you can also find some tawed leather products on Amazon and eBay.
Until the late 19th Century, vegetable tanning was the most widespread tanning method in use. One of the oldest leather tanning methods, vegetable tanned leather (also known as bark tanning or veg tan) is said to have been first used in Sumer (modern day Iraq & Kuwait).
Records show that from as early as 400 BCE, Egyptians and Hebrews were using vegetable matter to tan leather. The Arab world was renowned during the Middle Ages for their leather-working expertise, a tradition that continues today in the Fez district of Morocco, and by the 15th Century, vegetable leather tanning was spreading throughout Europe. (Wemegah, 2014)
In the late 18th and 19th Centuries, there was a great demand for “Russia leather” produced in Eastern Europe created from tanning hides with birch or willow barks. This leather was primarily used for furniture upholstery and lavish coverings and was exported from Moscow to Western Europe.
The Vegetable Tanning Process
Veg tanning is another painstakingly slow, handmade process that requires a lot of patience and care. Skilled artisans soak hides in large baths of concentrated vegetable tannins repeatedly over several months. These vegetable tannins can come from hundreds of sources, including birch, catechu, chestnut, olive leaves, oak, spruce bark, quebracho, rhubarb roots, mimosa or tara seed extracts. Tanners would use whatever tannin source was locally available or most easily obtained.
Traditionally, this process used a series of pits with different concentrations of tannins, and skins would be rotated from one bath to another. Nowadays a combination of pits and large wooden or plastic drums is used. The rotation of the drums speeds the penetration of the tanning liquors into the hides, making it more efficient than pits; in addition, different tanning solutions can be poured into and later emptied out of the same drum, further increasing efficiency versus maintaining numerous stationary pits.
Here’s an overview of the basic steps involved in the traditional vegetable leather tanning process:
Curing: once the raw animal skin arrives, it must be immediately cured with salt to prevent bacteria from growing.
Liming: next, wool, hair and fat residues are removed by soaking the hides in large rotating drums filled with mild chemicals like milk of lime (calcium hydroxide, Ca(OH)₂).
De-liming: Once liming is done, the pH levels of the hides need to be lowered again and so they are soaked in an acidic solution. Common deliming agents include weak acids like boric acid, acetic acid, formic acid, lactic acid, phosphoric acid or carbonic acid.
Tanning: now the hide is prepared for tanning. They are placed into drums filled with tanning agents and water. Over the course of 30-60 days, the hides are moved from one drum to another, each filled with tanning solutions of varying concentrations. The combination of tannins is a closely guarded trade secret that each tanner passes down through the generations.
Drying: The damp hides are removed from the drums and dried over a few days.
Treatment: After losing all its natural fat, the leather needs to be lubricated. It is oiled and waxed which enhances its color concentration and improves durability. Then the oiled leather is stretched, trimmed, and measured.
The whole process can take as much as 15 months.
Veg Tanning Today
Very few tanneries today have the capability to produce vegetable leather (not to be confused with plant leather!). Take for instance the J&FJ Baker & Co. tannery in Devon, England – the only remaining tannery that uses oak bark. This tannery supplies veg tan leather to high end footwear brands such as Crockett & Jones and Purdey.
Other big tanneries like the Horween leather company in Chicago make use of both chrome and vegetable tanning, supplying leather to many heritage footwear brands including John Lofgren and Viberg.
Some areas in the world still specialize in veg tan leather. The Tuscan leather district of Santa Croce sull’Arno in Italy accounts for 35% of the national production of leather, of which 98% is veg tan leather.
Benefits of Vegetable Tanned Leather
Vegetable tanning uses natural materials, making it a more environmentally friendly leather option, safe to use and dispose of.
It is water-resistant, robust, relatively thick, and hard-wearing, durable and at the same time soft, flexible and heat resistant. It is also popular for its earthy tone and naturally rich look. Since there is no synthetic coating, vegetable leather is breathable and porous, absorbing oils and moisture over time to form a beautiful patina – an indicator of quality.
The few downsides of this type of leather are that, given the time it takes to produce and the quality of the final product, vegetable leather is quite expensive and its application is more limited. The sturdy yet malleable characteristics of veg tan leather make it ideal for goods like shoe soles, bags, and belts, but less suitable for thin and stretchy leather products like car seats and clothing.
Modern Leather Tanning Methods: Chrome Tanning
Following on the heels of the technological advances of the Industrial Revolution, the mid-19th Century was a golden age in chemistry. This period saw the development of a series of synthetic alternatives to natural products, from the first synthetic dye (Perkin’s mauve in 1856) and chrome tanning (invented in 1858), to early forms of synthetic rubber (1879).
Chrome tanning (or chromium tanning) revolutionized the leather industry, turning what used to be a months-long process into one that takes as little as one day. This modern process uses a solution of chemicals, acids and chromium salts instead of natural tannins. Today, at least 80% of the world’s global leather production is chrome tanned leather, with the other 20% being veg tan leather; 100% of the leather used in clothing is chrome-tanned.
Various tanning methods are sometimes combined in order to obtain certain desired characteristics in the finished product. Usually this means a combination of chrome tanning and vegetable tanning and goes by different names depending on the order of each process:
Semi-chrome leather: vegetable tanning followed by chrome tanning;
Chrome re-tanning: chrome tanning followed by vegetable tanning..
Re-tanning affects several leather characteristics, including the feel and its ability to take up dye.
It's important to note that many brands purporting to sell vegetable tanned leather today may actually be selling a combination tanned leather that uses both chrome and veg tanning techniques. Always contact the manufacturer to find out the true nature of each leather item they are selling.
The Problem with Chrome Tanning
In addition to production speed (which also lowers cost), chrome tanning is favoured by leather manufacturers because the chrome tanned leather is especially water-resistant, soft and flexible, and takes dye easily. This opened up the number of applications that leather could be used for, from clothing to upholstery.
However, chrome tanning comes with a number of downsides. This synthetic tanning process uses sodium dichromate, a hexavalent chromium compound generated from ores containing chromium (iii). Hexavalent chromium is extremely toxic, a known carcinogen which can also affect the lungs, eyes, heart, kidneys and blood.
Less than 50% of the chromium employed in the chrome tanning process actually remains in the leather; the rest ends up in solid and liquid waste. If this waste winds up in the environment, it can contaminate water sources where it can lead to infertility and birth defects, among other problems. Chrome tanners are also at risk of inhalation of chromium-laden dust particles which can lead to chronic respiratory problems. If chrome-tanned leather is incinerated once thrown out, the burning leather can also release the toxic chromium (VI) into the atmosphere.
The amount of chromium in waste is strictly regulated in most countries and reducing agents are added to effluent to reduce any chromium (VI) that might be present to the less-harmful chromium (III). However those environmental laws are not always well enforced in some developing producer countries, where most production occurs. A 2000 report by the UN Industrial Development Organization looked at ways in which chromium pollution could be reduced, including through improved uptake of chromium by the leather, direct chrome recycling and other tanning techniques.
The Reemergence of Vegetable Tanned Leather
Due to the health and environmental toxicity issues associated with chrome tanning – and despite industrial innovations to create a more eco-friendly chrome tanned leather, from hybrid chrome tanning to wet blue and wet white tanning – there has recently been renewed interested in vegetable tanned leather.
Watch this fascinating video from the Saddleback Leather Co. of the making of their 100% veg tanned leather iPhone case. It’s surprisingly mesmerizing!
There are umpteen sustainable fashion brands out there consciously creating bags and other accessories made exclusively out of veg tanned leather. Silent Goods, for example, only uses organically farmed, veg-tanned leather and even allows you to trace the full supply chain of their products by simply scanning the digital tag on each of their bags.
ZAMT is another lifestyle brand that designs premium Italian veg tanned leather. Kmana makes gender-neutral bags out of 100% ethically sourced vegetable tanned leather, and Txture produces beautifully finished vegetable tanned shoes.
Where else can you buy vegetable tanned leather? Here’s a list of some of the brands selling premium veg tanned leather bags and accessories:
Couper et Coudre (C -ET- C)
In addition to vegetable tanned animal leather, there is an exciting new frontier in eco-friendly leather alternatives including plant-based leather.
There’s A Lot to Learn About Leather Tanning Techniques!
From brain tanning to vegetable tanned leather, there are some fascinating traditional leather tanning processes out there. These techniques embody thousands of years of traditional knowledge and hold promise for a more eco-friendly leather option.
As consumers, we have immense power to wield. The more we become aware of these traditional tanning processes and are willing to invest in them, the more fashion companies and leather industry giants – not to mention small-scale tanners and leather companies, too – will be willing to employ these ancient tanning methods.
About the Author
Vidya is an environmental lawyer from India, with special interests in biodiversity, climate change and sustainability issues. She is a fellow at the Guarini Center, NYU after graduating as a Vanderbilt scholar with a masters in environmental law at the NYU law school. Previously, she advised the UNDP, India, the Indian Ministry of Environment and worked with IDLO, Governments of Namibia and Vietnam on interesting environmental legal and policy issues. She enjoys travelling, bird watching and wildlife photography. In her free time, you can find her playing at the beach or convincing her friends to go vegan.
Edholm, S. and T. Wilder, Buckskin: The Ancient Art of Brain Tanning. (Paleotechnics): 2001.
Ludvik, J. “Chrome Management in the Tanyard,” United Nations Industrial Development Organization (US/RAS/92/120/11-51): 9 August, 2000.
Wemegah, R. “Vegetable Tanning in Bolgatanga: Challenges and the Way Forward,” Arts and Design Studies (Vol 16): 2014.