The Vegetable Tanning Process

Vegetable tanning was the major tanning method until the 20th century and is now growing in importance once again.

Uses of veg tan leather

It uses extracts of barks, leaves, fruits, berries, roots and wood. In the 20th century it was mostly used for:

1. Shoe Soles
2. Belts
3. Saddlery
4. Straps
5. Industrial Leathers
6. Classical Upholstery
7. Shoe Linings
8. Bookbinding
   and many types of leather goods.

All these uses but the versatility of modern vegetable tanning and the quality of the leather in terms of its touch, smell, look and the way it handles noise means that new uses in interior design such as flooring and walls are being added along with many types of footwear and sandal uppers.

What are tannins and how are they used

There are hundreds of possible tannins. They are feebly acidic and will convert pelt to leather. All vegetable tannages are derivatives of phenol which contains a six-membered aromatic ring, bonded directly to a hydroxyl group (-OH). The tannins we use are classed as either Pyrogallol or (pyro) Catechol.

Pyrogallol tannins include CHESTNUT WOOD, MYROBALAN FRUIT, SUMAC LEAVES. They give a creamy leather that does not discolour with light.

Catechol Tannins are MIMOSA BARK, QUEBRACHO WOOD, GAMBIER and they turn more reddish in the light, sometimes in a dramatic way. The leather colour is pinkish to a rich tan.

Historically each country used the vegetable tannin that was available locally. In Europe Chestnut was common, the UK used oak (which was legally specified in certain centuries), the US hemlock while Russia was famous for the use of willow and birch bark. The significance of Russian leather comes from the distinctive odour of birch bark.
The way that different tans look, age with handling and in the light is appreciated by designers when choosing the leathers for different end use. Liquors can be blended. Most tanners today have a house blend mixing Mimosa, Quebracho and Chestnut or others depending on the type of leather they are making.

Originally vegetable tannins were extracted from the vegetable material (barks, wood etc) using cold water. After 1890 this was done by boiling; hot water will always extract the tan more quickly than cold. If you buy liquid extract expect about 30% tan contact and with powdered extract (most used, indeed almost only used these days) 60-65%. Some historic oak bark tanneries around the world still grind their own bark to produce the extract.

The tanning process

Before tanning the hair or wool is removed from the pelt, usually by the application of selected chemicals to eat away at the hair roots. Treatment with lime is done to open up the fibres and remove unwanted material from between the fibres. Other processes will be carried out to specifically remove any hair roots and adjust the stretch characteristics and some physical processes will remove the flesh and thicker hides will be split to get to the correct thickness.

Historically vegetable tanning was done in a series of pits with different concentrations of tannins. Today many tanners make fine leather using systems of drums and pits and some work with drums alone, which can greatly reduce the time involved as the drum action speeds the penetration of the tan into the leather.

Tanning starts with a penetration process leaving the combination (with the hide) until the tan is well through the leather. The first tan to reach the leather must be weak in concentration and mellow to prevent tanning the surface early on. So a setup is created that moves from a low tan:non tan ratio to a high tan: non tan ratio.

With pits a counter current system is normal where the hides move through a series of pits in one direction while the liquors move the other way. They hides go through three sets of pits which are the Suspenders, the Handlers and finally the Layers (cold method) or the Hot Pits (more common today).

In the layers each hide is sprinkled with solid tan material. Fermentation and deposition in the hide gives solid leather in 3 months. Traditional oak bark leather is required to be in the tan pits for a minimum of one year. The strong liquors from the final pits are gradually moved down the system while the hides move up from weak to strong. At the end the leather is piled for a few days to completely fix the tan.

Once tanned..

After tanning many varied processes follow which are often classified as Currying or Shedding vegetable tanned leather. Activity may involve treating the leather so the veg tan does not migrate to the surface, rolling on when the leather is quite moist, rolling off when it is a little drier. Drying is done in a stove, with the hides or skins usually hanging from hooks. Wooden slatted windows may be used to crudely adjust temperature and provide ventilation.

Rolling helps to control softness. Rolling wet makes the leather hard and rolling dry makes it softer. Heavy rolling machines of different weights are used depending on the type of leather. Sole leather is given the heaviest rolling as a dense, strong, well consolidated leather is needed.

If you have any questions about the veg tan leather tanning process, feel free to contact us.