In these exceptional times, where the ecological impact of products and production methods has become a key element guiding buyers in their choices, the transformation of hide into leather during the tanning process is the focus of attention. Tanning methods are compared and contrasted in order to define a new ranking of best practices so it is of crucial importance to take a new look at them:
In simple terms, leather, a living material, is the end product of a number of stages of transformation. Tanning is the first stage after the wet work, and is followed by currying and finishing. A physical-chemical process, tanning transforms a putrescible material (the skin or hide) into a rot-resistant material (leather) by soaking it in drums with tanning solutions.
Leather can be tanned by a variety of methods. The tanner decides on the process to use according to the purpose of the finished product and their own know-how, as each tanning method has specific characteristics governing its implementation and use.
CHROME TANNING – a much-criticised leader
A technique refined at the end of the 19th century, chrome tanning rapidly gained ground over other pre-existing techniques as it is easy to perform, quick and requires little water.
It is multipurpose, and the softness and elasticity of the skin is obtained with precision. It can be dyed easily offering multiple possibilities of colour and finishes and is stable over time and when exposed to light. Today, it represents 80 to 85% of global leather production.
The discharge of industrial effluents rich in heavy metals from chrome tanning can have consequences on the environment. These risks are reduced by following practices that respect the strict government regulations, particularly in Europe.
Chrome sludge, the result of effluent processing, is sent to a centre for the storage of non-hazardous waste while the waste water is processed in a water treatment plant. Currently, initiatives to recycle chrome are multiplying, with the goal of removing the chrome from the sludge and reusing it in the tanning process.
Trivalent chromium and hexavalent chromium – they are not the same!
Mineral tanning with chrome uses chromium III sulphate.
Chrome-tanned leather is often criticised because chromium III can, under certain conditions, oxidise to chromium VI or chrome hexavalent. In leathers, the presence of chromium VI (classed as a toxic and carcinogenic chemical agent only if inhaled or ingested) can provoke a risk of skin allergy.
It is possible for a fraction of chromium III present in leather to oxidise and generate a few mg/kg of chromium VI but with good tanning practices this can be avoided.
According to the CTC*, there are many techniques available to avoid generating hexavalent chromium and this dynamic player in the sector accompanies tanneries to this end.
In Europe, the regulations set the limit of hexavalent chromium present in leather at 3mg/kg.
VEGETABLE TANNING – a natural attraction
The term “vegetable leather” is shorthand for describing leathers tanned using plant-based tanning agents: extracts of tree bark, leaves, fruits or roots. As each has different properties, they are chosen or combined according to the desired result.
Vegetable tanning represents around 10 to 15% of world leather production.
Vegetable leather is appreciated for its appearance, evoking authenticity as well as a leather without any coatings, a minimum of chemical treatments, and little covering from dyes or decorative finishes. Vegetable tanning makes each skin unique.
These qualities also mean there are limits to its usage. The leather tends to be dense with a firm handle and limited elasticity, becoming more attractive over time, according to its exposure to light, humidity and the general environment. Beautiful in natural colours, its response to dyeing makes it difficult to produce bright colours.
The tanning residues are biodegradable, generally transformed into agricultural fertilizer, while the wastewater can be directly treated by a municipal water treatment plant if it is suitable for biological treatment processes and sized accordingly.
However, the tanning process is long (it can take up to one year to tan leathers for shoe soles), requires a lot of water and vegetable tannins and this nuances the positive ecological footprint.
TANNING USING ALUN OR ALUMINIUM SALTS
This is an ancestral technique, the first known form of mineral tanning, which is growing in popularity again today as it is a chrome-free option with very good receptivity to dyeing.
It is mainly used for the tanning of small animal skins, such as goat and sheep, as well as for furry skins.
OTHER METAL-FREE TANNING PROCESSES
This refers to tanning that does not use chrome or any other metal, but instead uses synthetic tanning agents made from organic components (polymers, polyphenols, modified glutaraldehyde, triazine). There is no official formulation for this type of tanning, as each tanner has their own recipes, sometimes patented and associated with environmental standards.
This type of tanning gives the leather softness, sponginess, wear-resistance and washability. Its base colour is close to white so allows intense bright colours and luminous light shades to be produced.
There is growing interest in this method because of the absence of metals in the process, but in contrast to chrome tanning, this is a nascent technique and is still developing, meaning tanneries need to refine their procedures and recipes.
Mixed tanning means combining different types of tanning methods in variable proportions according to the desired appearances and properties. In reality, leathers are the result of combinations of tanning and re-tanning, specific to the expertise and know-how of each tanner.
Chrome tanning followed by re-tanning using plant-based agents provides texture, softness and elasticity from the chrome, while the vegetable tannins allow more specific surface treatments to be applied, creating waxed, patinated and authentic appearances.
Vegetable tanning coupled with mineral or synthetic tannins will produce a leather with a more multi-purpose usage.
By combining tanning methods, it is possible to take advantage of the strengths of the various processes.
Rich in unique know-how and constantly evolving, the various tanning methods are a resource for imagining new technical, aesthetic and sensorial developments.
It is complex to rank the various tanning methods. The technical and environmental advantages and disadvantages of each process need to be balanced, but it is important to take into account the entire production cycle and, above all, the raw material being used. This then raises the question of traceability and more generally of transparency regarding commitments and processes.
The result of the merger between the Professional Committee for Economic Development (CPDE) and the Industrial technical centre (CTI), CTC is the technical body for the leather, footwear, leather goods, glove and apparel sectors. CTC is tasked with a public service mission to assist French companies in the leather sector.