Bark Tanning: page 2
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."
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
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).
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.
Stats on various tannin sources
Oak bark averages 10% tannin. Oak wood = 6%. Oak
leather is considered mellow and tight, with a yellow-brown color. There
are so many varieties that this surely varies.
Fir bark has as much as
11% tannin and yields a yellow/brown leather.
willows are considered excellent, yielding a soft and supple leather. It can have 10% tannin.
Rahme says that "birch
bark yields a somewhat fragile leather, probably because it dissolves out
the hide's natural greases." Average tannin equals 12%. It is usually used
in combo with other materials and is sought for its high sugar content.
Gives a light red-brown color.
Alder makes a hard and fragile leather and is
often used just to color
finished leather. It gives a rust orange to red/brown. The brightest color
comes from the bark collected just after the first hard freeze.
Hemlock bark contains about 10% tannin.
are bright red and full of acid-forming sugars. Good for both heavy sole as
well as lighter fancy leather.
Chestnut oak also called rock oak is classified
as a white oak and is high in tannin (10%), as well as acid-forming sugars. It is among the most desirable of barks for tanning.
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
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.
bark from sawmills sold as garden mulch is
excellent for bark tanning (assuming it hasn't been left out in the rain a
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.
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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