Tanning

Tanning refines: Rawhides become leather

Recourse to millennia-old tanning methods serves health and environment

Vegetable-tanned leather is the basis of a holistic ecological approach towards shoe production.

From a waste product to a valuable raw material

The people of early times had to extract food, tools and materials from the harsh natural environment with the greatest privation and constant effort in order to survive. Hunting prey was therefore utilised as completely as possible: From the hides and skins of the animals, which were actually waste at first, our ancestors learned to make valuable as well as useful clothing and equipment. To do this, they had to develop methods to make the quickly perishable fresh skin of slaughtered animals, which was threatened by rotting processes, durable and usable – tanning is what turns rawhides into sought-after leather that can be used in a variety of options.
Obviously, people first learned that merely drying the skin does not help, but only leads to a hard, brittle consistency. Smoking and chewing the raw material was then a first successful approach to preservation – and at some point it was discovered that vegetable substances could be used for methodical tanning with better quality. This eventually gave rise to a craft: Tanners in antiquity and later in the Middle Ages used pits in the earth into which the hides were sunk together with a decoction of fats and suitable vegetable substances for a long time, often many months. The finished leather then served as a valuable raw material for other crafts to make equipment, accessories, clothing or shoes.

Natural leather goods based on vegetable tanning

Natural goods manufacturers not only rely predominantly on vegetable tanning for leather products, but also take a holistic approach to material extraction, processing, use and disposal. An essential approach is the shortening of transport routes in order to avoid torturous delivery conditions for the animals bound to be slaughtered and to be able to dispense with the temporary preservation of the rawhides.
For this purpose, cooperation is sought with farms that are committed to species-appropriate husbandry. Beyond ethical considerations, there are also very pragmatic reasons to avoid factory farming and improper animal transport, because these leave traces in the animal skin: The quality of the leather suffers from extensive wounds. In contrast, the skin of species-appropriate animals shows other, specific characteristics such as scars from thorns or insects, which makes this skin unmistakable and gives the products made from it an individual touch.
When resorting to traditional tanning methods, barrels or pits containing plant extracts are used, which should come from renewable sources – such as the fruits of the Peruvian tara tree or the Turkish valonea oak. In both cases, the trees remain after the fruit has been harvested. The tanning process, which can take 20 to 30 months, is based on the release of tannins, which have a preservative effect and are also antiviral and antibacterial. The leathers resulting from such tanning have the typical tart smell and feel soft.

Chemical industry: increase in efficiency – but also in risks

With the emergence of the chemical industry in the middle of the 19th century, the craft of tanning also became an industry in its own right, aimed at efficiency. The use of chromium salts brought with it an unimagined increase in productivity: No longer did the tanning process take weeks, months or even years, but within just a few hours a foul-smelling raw material threatened by decomposition was transformed into a valuable material. This progress almost caused the old vegetable-based tanning methods to be forgotten. Today, it is estimated that less than 20 percent of the leather used to make clothing still comes from tanning processes using purely vegetable substances.
Progress has brought advantages but also problems – above all, chrome tanning, which dominates the world market today, can cause risks for humans and the environment along the entire process chain. For example, the waste water from such tanneries is enriched with hazardous substances, including salts and heavy metals that can kill microorganisms, but also larger creatures.

Chromium III or chromium VI – an existential difference!

Chrome III salts are used for chrome tanning, which are comparatively harmless – at least in comparison to chrome IV (so-called chromate), which is considered to be a thousand times more toxic. If chrome tanning is not carefully carried out and monitored, the deliberately used chrome III can turn into the undesirable and very dangerous chrome VI, to which allergenic and carcinogenic effects are attributed.
The disposal of the waste produced during processing and of the leather products after the end of their useful life is also considered problematic. In Germany, in particular, tens of thousands of tonnes of old shoes, jackets and upholstered furniture made of leather containing chromium have to be disposed of every year in such a way that the environment is not harmed as far as possible – because chromate can also be produced under certain circumstances during incineration.

Strict environmental protection in Germany – tanning abroad to avoid it

In Germany, tanneries have to comply with legally defined limits – so environmental damage from chrome tanning can be limited in this country with the help of sophisticated wastewater technology. However, the strict local environmental protection combined with relatively high labour costs have an impact on the price of the finished leather, which is why tanneries abroad, especially in the so-called Third World, are used for the most part. Thus, most of the rawhides produced in German slaughterhouses are exported – the finished leather is then imported again.
In the leather-producing countries of the Third World or in the so-called newly industrialised countries, there are usually no sewage treatment plants, so that toxic wastewater is discharged unfiltered into water bodies and threatens to poison the groundwater. Wells can thus become permanently unusable: Agriculture and the people living and working in the area suffer damage, as does their environment. Furthermore, there are hardly any labour protection regulations there – even children have to work long, hard and for little money, often under undignified and dangerous conditions. Even elementary protective equipment, such as breathing masks or gloves, is usually missing. Many therefore suffer allergies, metabolic, respiratory or skin diseases.

Rawhide export and customer expectations: additional use of harmful substances

In order to protect raw hides from decomposition, especially as a result of mould and rotting during longer transports, preservatives have to be added to them in advance. Within Europe, salt or brine is usually used to bind the water of the still damp skins, but this must then be washed out again. The over-salination of the waste water in the regions concerned then also poses a serious ecological challenge.
For shipments to distant destinations, preservatives that are considered dangerous, such as trichlorophenol or tetrachlorophenol, are used. Once they arrive at the tannery, the rawhides must first be cleaned of hair and meat residues, which in turn requires the use of many different chemicals and soaps.
The high expectations of the customers regarding the colour and structure of the leather additionally require the use of further chemicals for the surface treatment – also for the coating. In this way, however, the leather, which is actually breathable, is sealed with the help of substances that are harmful to the environment and health. Examples include polyurethane, acrylic resins and fluorocarbon compounds.