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Tannin and Tannin Sources

"Tannins, generally yellow-white to brown, deepen in color when exposed to light....Because they transform proteins into insoluble products that are resistant to decomposition, tannins are used as tanning agents for leather."

Encarta Encyclopedia

Contents Intro & History Tannins & Tannin Sources Hide Preparation Making the Bark Solution Soaking Currying Softening & Finishing

Tannin is a large, astringent (meaning it tightens pores and draws liquids out), molecule found in plants that bonds readily with proteins. When you apply tannins
 to your skin you can instantly see the skin contract. Put them in your mouth and your cheeks pucker. Medicinally, tannins are used to draw irritants out of your skin such as the venom from bee stings or poison oak. Next time you get stung, pull some fresh bark off the twig of a nearby tree, chew it up and apply it to the sting. The irritation will go away within seconds. Tannins are also applied to burns to help the healing and to cuts to reduce bleeding. 

Another every day interaction with tannin is in tea (from the tea plant....not herb teas). The tradition of adding milk to tea has the added benefit of causing the tannins to bind to the proteins in the milk rather than to the proteins in your liver and kidneys. When you drink tea without milk, you are literally tanning your insides. 

Tannins occur in nearly every plant from all over the world, in all climates. It is found in almost any part of the plant, from root to leaves, bark to unripe fruit (ever bitten into an unripe persimmon?). Algae, fungi and mosses do not contain much tannin. Many plants don't contain a useful amount of tannin. Most trees contain plenty of tannin. It is concentrated in the bark layer where it forms a barrier against microorganisms such as fungi and bacteria (when hides are stuck into tannin baths the bacteria are also killed). 

The Two Types

There are two types of tannin: Catechol  and Pyrogallol.. By understanding when to blend these together, the expert tanner could reputedly create the appropriate leather for any need: hard and firm, mellow and soft, light or heavy. Until you are an expert and can even notice the differences, I wouldn't worry about it, but it is interesting to pay attention to as you tan.

Catechols (aka condensed) are more astringent and tan more quickly than the pyrogallols. They deposit a reddish sediment known as 'reds' or phlobaphenes. They make leathers of pink, red or dark brown hues, that are more 'solid'. They also create greenish-black spots on contact with iron. Mimosa, birch, hemlock, quebracho, alder and fir bark contain catechols. Oak bark contains both types.

Pyrogallols (aka hydrolysable) deposit a pale-colored sediment called 'bloom' (elegiac acid} which, if deposited in the leather, improves its solidarity, wearing properties and resistance to water. Hence they are favored for sole leather. They are also preferable for leathers intended for bookbinding, upholstery and other purposes where longevity is essential. The resultant leather is of pale color varying from creamy or yellowish to light brown. Pyrogallols make bluish-black spots on contact with iron and resist changes in pH value. Sumac, chestnut, oak galls and oak-wood contain pyrogallols.

Common Sources: 

Typical materials used for bark tanning include any of the oaks, fir, certain willows, chestnut, sumac leaves, oak galls, canaigre root, birch, alder, hemlock. Bearberry (leaves), heather, bloodroot, alfalfa, tea, sweet gale, pomegranate rinds, certain fern's rhizomes and wood-hops have also been used.  In fact, when you peruse the literature, you realize that an enormous amount of plants were at one time or another, in one country or another, important sources of tannin.

In modern times 80% of all commercial bark tanning is done with highly concentrated extracts of Quebracho, Chestnut or Mimosa. These extracts are typically 30% tannin or more, whereas naturally occurring tannin is closer to 10% to 12% of the material. Using these concentrated extracts speeds up the tanning times considerably, although many sources say the resultant leather is of a lower quality.

Collecting: All barks are best collected in the spring when the sap starts to rise in the trees, the leaves are just coming out and the bark will peel easily (a fortunate coincidence). This is when they are most concentrated and the easiest to peel, but you can use bark from any time of year. Tannin is usually concentrated in the inner bark (cambium layer). Supposedly, an older tree has more tannin than a younger one, and the lower parts of the tree contain a higher concentration than the top parts. One source says that fir trees should reach 30 years old before debarking and the best oak trees are between 15 and 30 years. Another source said oaks are best between 30 and 35 years...so I wouldn't get to caught up in it.

Shredded bark from sawmills sold as garden mulch is excellent for bark tanning (assuming it hasn't been left out in the rain a bunch). 

How Much: It really depends on the quality of your source. Mark Odle suggests that in general it takes about twice the weight of the hide in bark to effect a good tan.

Storing: Bark should be dried out and stored dry. Tannin is water soluble and will be leached out of wood or bark that has been left out in the rain.  If kept dry, it can be stored indefinitely without losing its effectiveness. Bark is easier to grind if its dry too. 
 

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