The origins of vegetable tanning and full-grain vegetable tanned leather can be traced back to the earliest modern humans. The harsh environment and the need to travel great distances in search of food and shelter require tough and durable materials that can offer bodily protection and housing. Animal hides were the only suitable material available to meet those needs at that time and played a vital role in supplying materials for clothing, tents, hunting and weapons.
Our ancestors started out using raw hides but quickly found that they cannot stand up to damp or humid environments. The rapid putrefaction of raw hides inevitably forced them to find a solution, which led to the discovery of tanning, specifically bark or vegetable tanning. Legend has it that our ancestors may have stumbled upon raw hides soaking in tannic rich pools of still water and through centuries of trial and error perfected the art of vegetable tanning. 
The Earliest Form of Full Grain Leather Tanning
Ancient historical records clearly indicate there were at least 3 other ancient methods of tanning leather. But vegetable tanning eventually predominated the other methods, which gradually became insignificant through the ages. We will never know if Alum (Aluminum Salts) tanning, Oil tanning, or Brain tanning predate vegetable tanning and each other.
But we know the various reasons for the general decline of all 3 tanning methods.
Alum Tanning
Goat and sheep skins soaked in a solution of Alum and salts for 10 to 15 minutes, then removed and dried. This process, also known as Tawing, produced pure white leather that is somewhat stiff and resistant to microorganisms. Unfortunately, immersion in warm water reverses the process and cannot produce permanent leather. Hence, the leather starts to decay upon contact with water.
Oil Tanning and Brain Tanning
Both tanning methods were extremely labour intensive and time consuming, resulting in leather of exceptional quality and softness that remains pliable even when wet. But the limited quantities of hides produced using these methods meant they cannot take precedence over vegetable tanning.
Henceforth, vegetable tanning dominated virtually every aspect of society including agriculture, transportation, industry, and warfare for the next 2 millennia.
Decline of Vegetable Tanning
Towards the end of the 19th century, a chemical tanning process utilizing chromium salts started gaining ground rapidly. The invention of chrome tanning in the mid-19th century together with the introduction of fat-liquoring and synthetic dyes into the tanning process led to the eventual decline of vegetable tanning.
Vegetable tanning has 3 major disadvantages pitted against it by chrome tanning;
1) A much longer tanning time.
2) White & pastel colours cannot be achieved with pure vegetable tanning alone, unless a finish is applied to the surface, which poses a problem as many premium quality leather goods buyers want to see the natural grain.
3) Vegetable tanned leather is light sensitive and darkens over time to develop a nice patina when exposed to light but some leathers we do not want to age and need them to stay more consistent.
Resurgence of Vegetable Tanning
In the past 2 decades, vegetable tanning has seen renewed growth, fueled in part by a new found appreciation of the inherent qualities of the natural tanned leather, boasting a luxurious feel and sensuality to the touch that delights leather aficionados. Much sought after in the making of luxury furniture and fine leather goods such as luxury handbags, shoes, belts and wallets because of its ability to develop a wondrously beautiful patina over time. 
Vegetable tanned leather absorbs humidity better than other types of leather and does not contain toxic chemicals like chromium. It is ideal for making products having direct contact with the skin such as shoes and orthopedic aids.
It plays an important role in modern industry by supplying it with vital mechanical parts such as machinery belts because of its strength and ability to handle stress.
Further advancements in drum tanning enabled its timely introduction into the vegetable tanning process to cut short the tanning duration from days and months to mere hours, making veg tanned leather suppler and easier to handle. Tanneries reaped the benefits from the boom in leather goods production.
Tannins and their Role in the Vegetable Tanning Process
Tannins are naturally occurring plant polyphenols found in varying concentrations in tree barks, plants, leaves, roots and even fruits, giving rise to hundreds of possible tannins. Stored in plants as a chemical defence and deterrence against pests and predators, only released during stressful reactions of the plant (e.g. parasite attack). It is able to transform pelt to leather with minimal acidity. All vegetable tannins in use today are broadly classed as Pyrogallol or Catechol.
Pyrogallol tannins contain pyrogallic acid, notable examples include Chestnut, Myrobalan Extract, Oak and Sumac leaves. Leathers tanned with these tannins have creamy colours that do not easily discolour with light. Chestnut is the most well-known among this class of vegetable tannins.
Catechol Tannins contain organic phenolic compounds. The most notable examples are the barks of the Mimosa, Hemlock, Quebracho, Willow, and Gambier.
Catechol tannins’ colours range from Mimosa’s lightish yellow, to Quebracho’s rich reddish colour, which comes from the tannin rich bark of the Quebracho tree in Latin America. They produce leathers that are more heat and rot resistant than pyrogallols.
Availability
Before the advent of modern transportation, travel and trade was arduous and uncommon. Each tannery only used locally available vegetable tannins. In Europe, especially France and Southern Italy, Chestnut was commonly in use. The United Kingdom used Oak as it was widely available. The U.S favoured Hemlock while Russia’s preference for willow and especially birch gave her leathers their distinctive odor.
Method of Extraction
Historically, most tanneries extracted vegetable tannins from the vegetable material (barks, plants & roots etc.) using cold water. Towards the end of the 19th century, extraction by boiling became the industry standard as hot water always extracts the tannins more quickly than cold.
How it is Sold to Tanneries
Vegetable Tannins are available in both liquid and powdered form. Powdered extracts contain at least double the concentrations of tannins (approx. 65%) as compared with liquid extracts (approx. 30%). Hence, their sales and availability far exceed the liquid versions.
A few historic oak bark tanneries around the world, a prime example, J & FJ Baker, Great Britain’s only remaining traditional oak bark tannery, still grind their own bark to produce the extract.
Vegetable Tanning Process
An Overview
Leather’s character is greatly influenced by the tanning materials used. Required for a very wide range of uses, the tanner faces the constant challenge of selecting the correct tanning materials to produce a particular type of leather intended for a particular use.
Fortunately, today’s Technology has solved the problem. It is now possible to precisely adjust extracts to varying levels of acidity and buffer salt capacity to create entirely different specific effects in the tanned leather. Extracts have gifted tannery production with a new level of versatility that had previously placed much emphasis on the individual tanner’s skill and experience in blending extracts.
Designers take into much consideration how different tans look, or age with time, handling, and even under natural or artificial light when choosing the leathers for the different end uses. Tanners employ various secret recipes of proprietary house blends mixing Mimosa, Chestnut, Quebracho or others with equally secret blends of leather fat liquors depending on the type of leather they are making.
Traditional Vegetable Tanning Method
Tanning Pits Only
First Stage (Hide Preparatory Stage)
Oak bark tanning has a long association with the traditional vegetable tanning process. The raw hides come in from the local abattoirs preserved with salt and still with hairs. The first process is to de-hair them with lime and sulfide. They are then taken to the Tan Yard where the salt, lime and sulfide are washed out from the surface so that you have a clean surface.
How the Tanning Liquid is Created & Stored
The tanning process consists of oak barks stripped from trees in Spring and Summer, dried and stored for 2-3 years to dry them out completely. Grind down into small pieces about 2-3 inches long, the Tan (tannins) is soaked out of them, much like making tea except that cold water is used. 
When the Tan is concentrated enough, the liquid is pumped into the pits in the Tan Yard. There can be as many as over 70 individual pits in the Tan Yard each containing tans of varying concentrations and acidity.
The first rows of pits at the top of the Yard have the strongest Tans and liquors with the weakest ones at the bottom rows of pits or the bottom of the yard.
Second Stage – First Soak
The new skins are first soaked in the bottom-most pit with the weakest Tans and mildest acids at the bottom-most row. Then they moved from pit to pit and row to row and so on and so forth till they work their way up to the top pits with the strongest Tans and liquors in about 3 months. Suspending them on sticks enable the Tans to work all the way around the hides for a nice even tan and colour.
Third Stage – Second Soak
After 3 months, they are taken off the sticks to be layered. They are laid flat, one on top of each other into deeper pits where handfuls of barks are thrown in between each skin, just like making an enormous sandwich. There they remained for another 9 months so the whole process takes a total of 12 months. At the end, the raw hides convert into leather and the process irreversible.
Fourth Stage – Drying, Dyeing, Oiling & Waxing
The tanned hides exit the pits and it takes 2-3 days to drain away all the remaining tanning liquids before hand staining them with vegetable dyes to give them an assortment of natural colours. This is followed by the application of a proprietary ancient blend of oils and waxes to the leather surface to give it a luxurious, beautiful and smooth surface.
Other finishing processes may include drum rolling, where a heavy rolling machine uses different weights or pressure depending on the type of leather to impart certain characteristics and quality to the leather. For example, the heaviest roller weight is often used on dense and strong leather needed for making soles.
Advantages & Disadvantages of Traditional Tanning Method
The major disadvantage of the traditional method is the long time it takes to tan the hides. However, it produces a very strong tensile strength and hard wearing leather, often used for making the soles and innersoles of luxury shoes as it is also lightweight.
Traditional vegetable tanned leathers are highly sought after in the top end of the leather goods production for these said qualities. Ultra-luxury leather goods manufacturers and their clients are willing to pay a premium for them. They are the very best leathers money can buy.
Modern Vegetable Tanning Method
Nowadays, most tanneries employ either a combination of pits and drums, or drums only, to speed up the vegetable tanning process considerably.
Tanning Pits and Drums
First Stage (Hide Preparatory Stage)
As with the traditional method, the first operation the tanner needs to perform is soak the hides back to their original water content by removing the salt. The remaining hairs removed using a solution of lime and sulphides. The lime has the additional benefit of plumping the collagen fibres by raising the PH and making it easier for the collagen fibres to accept the tannins later.
Next, the limed hides undergo fleshing to remove fatty tissue, trimmed, weighed and sorted by size and type. A shaving machinery shaves certain hides, or split them thinly on a band knife splitter to make them level, or also to split the leather into 2 hides, top grain leather and the bottom layer (also known as split leather). Other tanneries may choose to split only fully tanned hides.
Removing the lime from the hides gradually lowers the PH back to neutral. Next, the hides undergo cleaning in a solution containing a special enzyme developed for the tanning industry which renders them clean and safe enough for even making rawhide dog bones when dried.
Second Stage – Tanning
The hides go into pits containing tanning solutions for up to 6 weeks to gradually develop the leather characteristics and qualities associated with vegetable tanned leather. The hides and the tanning solution carefully monitored daily for acidity, PH, tannin strength and purity.
Third Stage – to further alter hide characteristics
For the final chemical process, the hides go from the pits into rotating wooden drums for washing, bleaching, dyeing if required, and fat liquoring with special oils to penetrate and bind with the collagen fibres.
Fourth Stage – Drying & Smoothing
A Felt covered roller machine wrung dry the wet leathers. A setting machine then smooths out leather stretch marks and creases using a similar process to ironing without heat. After which, the leathers are dried on frames in humidity and temperature controlled rooms over 3 days.
Fifth Stage – Finishing
The dried leathers go to the finishing department for custom finishes depending on the individual customer’s requirements.
Tanning Drums Only
Vegetable drum tanning process only, without the use of pits. The entire tanning process now takes place in rotating wooden oak drums or vats, where the tannins are able to penetrate the raw hides much quicker because of the rotating drum action.
The hides next undergo a similar dyeing process in other drums containing various hands mixed vegetable dyes supplied to the tannery in powder form, to obtain the desired color effect. For the finishing stage, the skins go to another drum with oils and waxes added for the finishing touch.
The Future of Vegetable Tanned Leather
Vegetable tanning uses little chemicals, if any. Natural dyes made from vegetable extracts create the desired colors or waterproofing. Leather tanning uses a lot of water but the waste water from vegetable tanning is easy to purify and safely returned to the environment. Vegetable tanned leather is a natural product that is fully biodegradable and harmless to the environment.
Despite the revival of interest in vegetable tanned leather, chrome tanning still commands an overwhelming 80+% share of the market and is unlikely to lose their market dominance in the near or distant future. This does not bode well for the environment as chrome’s toxicity is an environmental and human health hazard globally.
Strict Environmental laws in Europe and the U.S made it costly for tanneries to pursue chrome tanning or even vegetable tanning in the long term. On the other hand, Tanneries in Asia are able to tan leather cheaply by blatantly disregarding the non-existent or weak environmental laws of their respective countries, thus wreaking havoc on their environments .
Despite the bleak outlook, global consumers are well-aware the leathers coming out of Asia are not of the best quality and pale in comparison with those from the West. Well-heeled high-end Leather goods consumers undoubtedly continue to look towards the West for quality assurance and superb craftsmanship. In this regard, tanneries in the West should focus on servicing this segment of consumers to ensure their long term survival and growth.