Often associated with luxury items, leather enjoys a reputation as a high-end, even precious, material. The focus tends to be on its aesthetic and sensorial qualities rather than its physical or mechanical capacities. However, this has not always been the case. Some of its historical applications made greater use of its solidity and strength than its refinement. And still today, as a result of the combination of this heritage and new innovations, leather can resist harsh conditions and also be an outdoor material. A return to its roots for this timeless material.
A testing past
We tend to forget this but leather was once appreciated more for its strength than for its delicateness. During the Bronze Age, leather was used to transport solid foodstuffs such as cereals, flour or salt, or liquids such as wine, beer and oil. It was cut into strips of varying widths to protect the wooden wheels of carts or for use as harnesses. These early users also knew how to waterproof it so that it could line the hulls of ships. In Ancient times, leather was used by soldiers, for shields as well as for their armour, which at the time was known as cuirass (a derivative of the French word “cuir” meaning leather). In the Middle Ages, leather was used as a lining, to reduce the contact between the metallic armour and the body. Harnesses for mounts also combined metal and leather. And to protect wooden fortresses from flames, they were covered in animal hides that were regularly dampened. These days, vegetable-tanned leather is the worthy descendant of these so-called “strong” leathers. When it is thick enough, it offers considerable resistance to traction and abrasion. Until very recently, it was used to make protective blacksmith aprons, and is still used to make shoe soles. Maxime Jouineau, a CTC consultant specialised in the wet stage of the tanning process, confirms that “shoe sole leather that has been vegetable-tanned in pits is very resistant.”
Today, innovations in tanning and finishing allow leather to support very challenging conditions and can make it into a truly all-purpose material, able to be used by professionals in a hostile environment or adopted for its practical rather than aesthetic capacities.
Under mechanical stress
The first quality required of leather is resistance to mechanical stress. The species of hide and its thickness are the first parameters used to adjust the choice of material to its purpose. It is obvious that a thick bovine leather is stronger than a thinly-shaved lamb leather. But between two similar leathers, it is also possible to render one more resistant by the processes used during its production.
The Italian tannery, Volpi, manufactures leathers for motorbike saddles and saddlebags. It uses a vegetable tanned (quebracho and chestnut) bovine leather, some 3.5mm to 4mm thick, which it then finishes with fluoropolymer resins. But it also “plays with the fatting of the leathers to extend their lifespans where possible.”
France Tanneries also has a rustic bovine article in its catalogue, with a 14-16 or 18-20 thickness that has been oiled when re-tanned, and is particularly recommended for footwear or rustic leather goods. “The oil lubricates the fibres and thus increases tear resistance. Consequently, it is also essential to regularly grease the oil in order to ensure it will last,” a technical manager from the tannery tells us.
According to Lionel Lautesse, a finishing specialist from CTC, the finish does not have an impact on the leather’s resistance per se, “but it has to have similar properties”. This means that the finish must be able to resist the same stresses as the leather, without cracking. “To improve the bend resistance of a coated article, we use a more elastic finish, which is less likely to break,” confirms the manager from Averpeaux.
Resistance to rubbing, soiling and sweat
Friction is a major cause of wear, and it is essential to protect the surface of the leather. “By increasing the hardness of the pigments that we apply as a finish to the leather surface, we improve its rub-resistance. But we have to be sure that it does not become brittle, particularly if the underlying leather is soft. Often, the finishing resin is a blend of polymers, combining for example a polyurethane with a harder polymer and a cross-linking agent that strengthens the bond between the various polymers,” explains Lionel Lautesse.
Resistance to soiling is linked to rub-resistance. Over time, dust becomes encrusted in the grain of the leather, particularly in outdoor environments. “To combat soiling, we add a texture agent – such as a silicon oil – to the finish, which makes the leather smoother and easier to clean. But the product must be used sparingly otherwise the resin will soften and lose its resistance. The risk of soiling is higher if the finish is softer,” adds the expert.
Sweat also plays a part in deteriorating leather. “The sebum eventually penetrates the finish. The more the leather is coated, the better it is protected against sweat. But it should also be made easier to clean via the addition of a smoothing product.” At Arnal, an item in vegetable-tanned bovine leather, destined for use as a watch strap, is treated against transpiration with an anti-bacterial agent, thus minimising the development of odours, and is tanned without chrome or heavy metals to ensure its harmlessness. In addition, the dye is colourfast to ensure it does not stain the skin.
Resistance to light and sunshine
Leather is not affected by daylight in general, but it does tend to fade under ultra-violet rays. This is particularly noticeable when exposed to bright sunlight, where its colour will tend to be bleached or altered. To counter this, Maxime Jouineau recommends “using metalliferous colours, which are more resistant than acid dyes.” Vegetable-tanned leather is particularly sensitive to light, and synthetically-tanned leathers are more photosensitive than chrome-tanned leather. “We can reduce this photosensitivity by adding certain synthetic tanning agents during the retanning stage. Certain fatting products will turn yellow more than others,” the consultant said.
But it is also possible to intervene at the finish stage to make the leather more photo-resistant. The manager from Averpeaux recommends “using a finish that makes the skin more opaque so that the UV rays do not reach the tannins and alter the colour.”
“The finish is the most effective protection against UV rays,” adds Lionel Lautesse. Forming an opaque layer on the surface of the skin, the finish protects it from the action of the sun’s rays. The sun test, which is used to test resistance to light, consists of exposing a sample of leather to UV light for 24 or 48 hours and measuring the deterioration in the colour against a scale.
When exposed to heat and fire
The experts interviewed for this article say that leather is not affected by the cold. Only one of them, a finishing specialist, recommended the use of special acrylic and polyurethane resins if the leather was to be exposed to low temperatures. When lined with a warm material like wool or fur, leather is a good defence against wintery weather. However, if it is to protect against the heat, then action needs to be taken at the earlier stages of production, “during the tanning and retanning stages, with the addition of chrome,” said Maxime Jouineau. “The dryer the leather, the more heat-resistant it becomes, which is why it is important to include a waterproof product at the fatting stage.” “Synthetic tanning provides greater heat-resistance than vegetable-tanning,” adds the manager of the TMM tannery. The Pechdo tannery, which produces technical leathers for gloves for firefighters and motorcyclists, is very familiar with this question. “We have to limit the sensation of heat caused by the leather absorbing infrared rays. To do this, we carry out a deep treatment during the tanning process,” declares the director of production, Céline de Montigny, without revealing what the treatment is. The leather thus becomes heat-resistant and reduces the sensation of heat when in close proximity to a heat source. “Carbon black is the most heat-resistant dye. That is why these technical leathers generally tend to be black,” concludes Lionel Lautesse.
Fire-resistance is also literally vital for certain professions or for circumstances that mean there is the presence of fire. The Sovos Grosjean tannery, which supplies public buildings, as well as the hotel and aviation industries, fireproofs its leathers through the use of a flame retardant during the drum tanning stage and during the finish, spraying it onto the flesh side so that the fireproofing product does not interfere with the action of the other finishing products applied on the grain side.
There are two levels of protection against the rain. A water-repellent finish means the water slides over the surface of the leather. To achieve this, at the end of the cycle a specific treatment is applied, made up of hydrophobic substances “based on sulphonated products,” explains Lionel Lautesse. This is what some describe as a Scotchgard treatment.
But over time, this treatment wears off and can eventually allow water to penetrate. France Tanneries, which produces a water-repellent bovine nubuck – which by definition does not receive any finish – for leather goods, “incorporates an oil in the wet phase, during the drum retanning and before the buffing, which is done at the end.” The same approach is used by Alran for its goat nubuck, whose texture remains perfectly natural.
A superior level of water-resistance is waterproofing. More longer lasting than a water-repellent finish, it consists of stopping water from penetrating the leather. To waterproof leather, action is required at the heart of the material, “during the dressing and fatting stages. We replace the classic oils with waterproof silicon-based fatting products.” For gardening gloves for example, made from bovine belly or goat leather that has been deep-treated during the tanning stage, the Pechdo tannery guarantees that the leather will resist water penetration for up to 180 minutes.
For its item in thick bovine nubuck (18 – 20) used for safety shoes, the TMM tannery “intensively fattens the leather with oils in the drum then applies a Scotchgard treatment during retanning and very lightly during the finish, so as not to lose the moiré pattern,” the manager tells us.
At Volpi, the rustic bovine leather for saddle bags, motorbike saddles and other outdoor uses is more or less coated at the finish, according to its final purpose, but is intensively fattened.
“Nautical” articles in full grain bull leather, destined for the interior decoration of boats from the Sovos Grosjean tannery, are “treated with chemical agents throughout the production process, until the finishing stage, with products that reinforce the hold of the finishing resins. The treatment is even more intensive than for a leather that is simply water-repellent or waterproof, because seawater is more aggressive. But we refuse to “plasticise” the leather and manage to preserve its authenticity,” a spokesperson for the company told us. “It is not easy to combine water-repellence and waterproofing, as the fattening oils hinder the hold of the finishing resins,” says Lionel Lautesse.
In short, leather is not necessarily the fragile and delicate material that we imagine, and it can naturally resist challenging conditions of use. Thanks to the progress made by tanneries, which are constantly seeking to improve their items, leathers can be strengthened to resist climatic or mechanical stresses. When tested by the trials and tribulations of modern life, leather also rises to the challenge of riskier, even regressive uses. And proves to be a true companion for adventurers too.
PREMIERE VISION LEATHER / HALL 3