Tannins and Tanning

In the build up to the Bushcraft Show in May, where we are having a display of bark derived crafts, we are doing a series of blogs on tree bark and it’s uses.  The first one is about one of the chemical components of tree bark, that is tannin.

Tannins are a group of high molecular weight, water soluble, phenolic compounds that tend to form strong bonds with proteins and other large organic molecules. They are widely distributed throughout the whole plant kingdom and can be found in most plant tissues but tend to be found in high concentrations in the bark, particularly tree bark.

Their exact role within the trees is uncertain but it is thought that they inhibit microbial activity, act as a chemical barrier and prevent colonisation by pathogens, prevent freezing by acting as an anti-freeze and, probably most importantly for the tree,  their bitter, astringent qualities prevent browsing by herbivorous animals and insects. It is  tannins that make acorns and sloes incredibly bitter and unpleasant to eat.

However, in smaller concentrations, tannins are present in many plant based foods, tea and wine are the best known examples.  Too many tannins in the diet are considered harmful and they have been shown to be carcinogenic with long term consumption, but don’t rush to give up your morning cuppa, by adding milk to tea the tannins present in it bind to the proteins in the milk rather than inside your digestive tract, effectively rendering them harmless.

Tree tannins have been utilised by humans for centuries. Because of their astringent (tightening pores and drawing water out) properties they have a long history of being used medicinally for a variety of medical complaints including; tonsillitis, pharyngitis, haemorrhoids, and skin eruptions as well as thorns and stings. Tannins also effectively bind form insoluble precipitates with some notable plant toxins such as alkaloids and glycosides as well as heavy metals and has been used as an antidote in cases of poisoning.

Tannins have been reported to produce other physiological effects including sppeding up blood clotting, lowering blood pressure, lower blood lipid levels, and modulate immune responses.

Different tannins react with iron salts to form coloured insoluble precipitates which range from dark blues and greens through to blacks and in this form have been used to make a range of dyes and inks that have been utilised in everything from old manuscripts through to furniture making.

Other uses of tannins include clarifying wines and beers, preventing lime scale formation in water boilers and even as viscosity adjusters when trying to drill through mud!

Perhaps the longest and most well-known use of tannins and one with which they are linked by name is in tanning leathers. Generally known as vegetable tanning these days to differentiate it from other tanning methods using chemicals, oils etc. it utilises tannins extracted from the different parts of  various tree species e.g. Chestnut wood (Castanea sativa), Quebracho wood (Schinopsis lorentzii), Tara pods (Caesalpinia spinosa), Catechu (Acacia catechu), Chinese gallnut (Rhustyphina semialata), Gambier (Uncaria gambir), Mimosa bark (Acacia meamsii), Oak wood (Quercus sp), Sumac (Rhustyphina coriaria), Turkish gallnut (Quercus infectoria) and Valonia Oak (Quercus macrolepis).


 Birch, willow and sweet chestnut bark ready for processing

Historically a wide range of tree barks have been used in various parts of the world so in addition to the species mentioned above;- birch (Betula sp.), Alder (Alnus sp.), Willow (Salix sp.) and in North America Western Hemlock (Tsuga heterophylla) as well as Sumac have been commonly used.  Although tannins are often now extracted using solvents and then used in their pure form to tan leathers, in the past barks or other sources where simply boiled in water to extract the tannins and make effectively a strong tannin tea.

2Boiling up willow bark to make a strong tannin tea

3A strong oak bark tea all ready for tanning

To make veg tanned leather, essentially all that happens is that fleshed, membraned  animal skins (generally de-haired by soaking in a strong alkali solution) are soaked in a tannin solution.  This process can take several months depending on the size and thickness of the hide. To start with the prepared skins are immersed in a weak solution which over a period of time is gradually topped up to increase the tannin concentration, this allows the tannins to penetrate the skin and prevent something called case hardening, where too strong a solution at the beginning can result in the outer layers of the skin tanning and then preventing the tannins to penetrate the skin and tan it all the way through.

The tannins bind to the proteins in the collagen fibres of the skin, coating them and at the same time causing the hide to tighten. This creates the characteristic properties of veg tanned leather which is different from other leathers in that it can be tooled and also wet-formed to make things like knife sheaths etc.

4A small bottle and fire steel pouch made from wet formed, oak bark tanned deer skin

Un dyed veg tanned leather generally will be varying shades of brown or reddish brown dependent on the source of the tannins. Generally tannins fall into two groups condensed (catechol type) and hydrolysable (or pyrogallol based) which will give different properties and colours to leather.  The former, tans skins more quickly and produces leather of red, pinks and dark brown colours,  whereas the hydrolysable tannins give a paler tan but produce a harder wearing leather with more water resistant qualities.  The skill of the ancient tanners was using appropriate sources of tree tannins to give the specific qualities to the leather being produced. Birch, alder and hemlock contain more condensed tannins whereas chestnut and sumac contain more hydrolysable tannins. Oak bark has both sorts.

For more information about tannins themselves go to http://www.ansci.cornell.edu/plants/toxicagents/tannin.html

And if you fancy trying out vegetable tanning for yourself one of the best resources on line is https://braintan.com/barktan/1basics.htm

Kev Palmer


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