Which plants are suitable for a fully and eco-friendly vegetable tanning?

Dr. Guilhem Mansion
Mon Dec 18 2017

The 100% natural Cervo Volante leather derives from wild deer rawhides devoid of any synthetic chemicals and therefore harmless to both human health and Mother Nature. Cervo Volante has indeed developed a “fully vegetable” tanning procedure, based upon herbal compounds called “tannins”, instead of toxic chromium-sulfate used by ninety percent of the tanneries worldwide! With regard to pelt treatment, tannins are not as stringent as chrome… They are, however, completely innocuous for the local workforce and far less damaging for the environment! Since all tannin sources are not equally eco-friendly, Cervo Volante has chosen to only use resources that match a series of strict criteria in terms of sustainability and nature conservation. Before exploring all those benchmarks in more details, it seems important to first define the central actors of this story – the “tannins” – and to get acquainted with the main plants that produce and accumulate them.

What are tannins?

The word “tannin”, proposed in the late 18th Century, designates a set of natural chemicals that have the predisposition to interact with nitrogen-bearing molecules like alkaloids or proteins. This unusual property is of great importance in agriculture (humic acid formation, upgrade of fodder…), ecology (plant defence…), food technology (brewery and oenology…), pharmacy (drugs against diarrhoea, dermatitis, infection, wounds…) or tannery (transformation of rawhides into resistant and durable leather). Etymologically, the term may derive from the Celtic word “tan” meaning “oak” thereby denoting the long-standing, traditional use of this tree in the tanning industry.

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Valonia oak (Quercus macrolepis) acorn cups are handpicked in Turkey.

For the chemist, the terminology “tannin” simply refers to phenolic compounds that are small enough to be water-soluble but sufficiently large to link with chains of proteins like collagen… Tannins are actually a set of complex secondary metabolites that often bear barbaric names – “hexahydroxydiphenoyl ester”, “proanthocyanidin”, “phloroglucinol”, to cite but a few – and whose classification has greatly changed over the past fifty years.

Where do tannins come from?

Tannins naturally occur in a vast number of plant species and mainly accumulate in the roots, barks, leaves, fruits and seeds, to reach an average concentration of 2-5% of fresh weight in the given tissues. Under stressful conditions like herbivory or insect egg deposition, plants can further amass polyphenols locally or induce the formation of special, extremely tannin-rich swelling named galls. In the Chinese gallnut of the sumac tree (Rhus chinensis) for instance, concentrations sometimes reach 70%! From a phytochemical point of view, the so-called “condensed tannins” are found in most plant groups, including ferns, gymnosperms or angiosperms, whereas “hydrolysable tannins” solely occur in some families of the dicotyledons.

Nowadays, the vegetable tanning industry mainly exploits the barks of various trees. Indeed, both types of tannins tend to accumulate in this organ to act as a chemical barrier against intruding microorganisms. The picking generally takes place in spring, when the polyphenols are the most concentrated and the bark easy to peel. Depending on the species, ten to thirty years are necessary to allow the trees to produce a satisfactory concentration in tannins.

The Cervo Volante eco-friendly sources of tannin

Today, the rare tanneries preferring vegetable tanning over the chrome technique generally depend on commercial extracts deriving from a dozen of woody species. The choice of such ready-to-use products is commonly driven by the price, the final polyphenol concentration or the esthetical impact of the tannin type (i.e. condensed or hydrolysable) on the finished leather. Still, it hardly relies on any environmental considerations!

At Cervo Volante, our criteria of choice differ considerably… Nature first! Before choosing a source of tannin, we think about any potential ecological burden inherent to its production. We want to avoid any impoverishment or eradication of native floras, soil erosion, local pollution, acclimatisation of invading species, etc. Let’s have a look at some examples!

Classical tannin-rich plants available in leather industry include quebrachos (Schinopsis balansae, S. lorentzii) and acacias (Acacia mearnsii, A. decurrens, A. saligna). Tanneries are mostly experienced with only these plants. That’s why part of the leather for our first shoe-collection is tanned with these plants. We are, however, reluctant to use any of them further! Quebracho wood extracts, for instance, derive from a massive and unsustainable exploitation of Argentinian forests. The result is a dramatic decline of natural populations through logging or clearing that we cannot support!

The problem with acacias is radically different. Most species favoured by the tanners originate from Australia and are not vulnerable at all… On the contrary! They have been introduced for high-scale cultivation in many countries and some of them have quickly become invasive. For any nature lover, invasiveness signifies tremendous consequences for the native ecosystems, including overconsumption of water resources or destruction of indigenous biodiversity. This is evidently in contradiction with our philosophy!

Using noxious pests like Acacia mearnsii as a source of tannins or firewood could be, of course, a provisory way to fight against their present dramatic propagation. Yet, we are opposed to such short-term, unsustainable solutions that imply the import of the timbers from the Southern Hemisphere… It would be an ecological non-sense!

 

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Valonea powder that is being used for the majority of Cervo Volante leather.

After considering the pros and cons of each potential tannin source, we finally reach the conclusion that only native European species with renewable tannin-rich parts (e.g. fruits or leaves) perfectly meet our standards. One such candidates include the Sicilian sumac (Rhus coriaria), a Mediterranean shrub that accumulates gallotannins in the leaflets. Another outstanding choice is the Valonia oak (Quercus macrolepis) whose acorn cups constitute an abundant and ecological source of hydrolysable tannins.

If we had to select only one, our preference would obviously go for the oak. By giving the tannins their appellative, “tan” is per se the perfect symbol of vegetable tanning… Without forgetting that pristine oak forests are of crucial importance for Lucanus cervus, our beetle logo!


Also published on Medium.