Leather

Pubblicato 07/11/2016 15:18:15
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What is a leather? 

A brief guide on the leather world. 

Leather is a product resulting from animal skin and is obtained after an endless series of treatments aimed at making the rot hide. From a microscopic point of view, the skin can be seen as a tight intertwining of numerous collagen fibers.

In the Italian language we use the word "Cuoio" to describe the leather hide, although in reality in the language used in the tanning industry, the "leather" word is only used within the scope of the footwear (sole and insole).

In English in fact, there is no corresponding word of Italian "Cuoio" because the leather intended as skin obtained from the animal is usually translated as "leather" or "hide" (depending on the animal).

Why does this skin is one of the most known commodities, precious and used in the fashion world?

This skin has several qualities that make it suitable for use in clothing, and leather lining.

In fact, skin is a raw material:

  • Very resistant: it has a consistency suitable to wear. It varies depending on the type of animal from which it is formed, the thickness (in general it ranges from half a millimeter to several millimeters) and the finishing, which can give a greater resistance to manufactured leather article.

  • Hygiene: this quality is obtained only after various processes that make skin not subjected to the typical phenomenon of putrefaction.

  • Not prone to moisture: this feature makes this leather suitable for the production of shoes and clothing while maintaining a high level of wearability pleasure, given by the good perspiration of this raw material. Another curiosity, I want to add that here, is that skin is a good electrical conductor (so be careful!).

  • Thermal isolation: skin acts as an insulation between the body and external environment, for keeping the wearer at a very comfortable temperature. Not surprisingly, the first use of an animal to derive hides, took place in prehistoric times to protect themselves from cold during their coldest winters.

  • Pleasantness to the touch: characteristic that makes skin one of the most expensive and sophisticated materials to create products of fashion.Often several imitations are made with surrogates or completely different materials from leather that aesthetically can resemble, but to the touch and resistance, they can not be enforced compared.

  • Look nice to eyes: the last quality that we list, certainly not least, it is the aesthetics of the skin. Skin is a "living" material with unique and inimitable characteristics that make each different from the others. Each sign, wrinkle and nuance gives skin a distinctive appearance and often increases its economic value.

Thanks to these these characteristics that make skin a high quality product, rules are born in defense of the consumer to avoid the trade of imitation leather, realized with synthetic leathers or other production. In defense of the consumer is Article 1112 of the Civil Code (Italy) and the European Directive 94/11 / EC which provides for the mandatory labeling of footwear. Also, to assure leather quality, you can request to put the mark "real leather" or "genuine leather" on the finished product, with the consent of Unic (National Union Tanning industry). In this way we ensure the customer a leather product that contains no toxic and / or harmful substances, and at the same time guarantees Italian origin.

The scalp or skin can be classified into two main categories (based on processing status):

  • Husked or in crust: the skin is not finished. The only treatment that receives is the drying after tanning.

  • Finished: all leather marketed products are finished both for reasons of greater resistance, both for product quality reasons (aesthetics, feeling to touch, etc).

Another classification is usually done according to the part of the leather that is used. We have different sizes ranging from whole skins, half hides, double butts to square shoulders, hips and bellies. 

The history of leather

The use of leather goes back to prehistoric times. In addition to hunting to provide food, the animal was skinned and skin was also used as a tool to protect themselves from the cold during the winter seasons. The main problem, which the prehistoric men found themselves facing, was the preservation of leather to prevent that organic matter would not rot quickly. As time passed, they noticed that there were ways to avoid putrefaction and preserve leather for a longer-lasting use. Indeed, some of the principles of these ancient methods are still used. For example, if leather was exposed to smoke or leaves made fireplace, it lasted longer. The same technique was also applied by burning fresh timber. This type of treatment is considered the oldest type of "tanning" that takes the name of "all'aldeidi" tanning, name derived from using smoke.

As time passes, the techniques of conservation of the leather evolved in step with the evolution of man. It was introduced vegetable tanning, named like this due to the use of substances derived in nature that allowed to avoid the leather rot. Vegetable tanning is made by placing the skin in contact with the tannins, substances derived from the bark of various types of trees (oak, chestnut etc). Depending on the type of tree used skin, it took on different colors and shades, from light brown to darker brown. The principles on which this tanning was based are those also used today, although the machinery and process have been introduced, they are much more evolved, both in terms of quality of the leather obtained both in amount of time required to produce. 

Another type of tanning was the one with aluminium and it still exists today.

One of the stages of leather processing is to shave the animal's skin to remove superfluous hair. Even in this case, since antiquity we are using techniques that are "alive" in current industry: the lime obtained thanks to the stove stones, then mixed with water, once in contact with the hair, it eradicate hair, making the skin ready for subsequent treatments.

As time passes, man began to use leather for purposes other than protection from the elements. The Arabs were the first to introduce leather decorations, promoting it from simple garment to an object of prestige and power. Slowly the appreciation of this raw material also spread to Europe where it was processed to produce luxury items, and to provide a material resistant to enemy's soldiers during the battles. The warriors were equipped with shields and armor made from thick leather, that diminished the impact of weapons in combat with enemy armies. Sometimes, even in the military, they were not spared decorations with carvings, reliefs and gold foils.

In Europe we must remember the famous leather of Cordova, where the leather was decorated by reliefs on a gold background, adding carvings and paintings. When Renaissance era came, the passion for leather also extended to Italy where were realized decorations for the wealthier classes, as bulinati, stamped, decorations for walls (see the Ducal Palace in Urbino), but also leather was manufactured for the less rich population like footwear and various tapestries. The processing techniques evolved in a short time, and depending on the geographical area and technique adopted, the leather took different names:

  • Russia leather or Bulgarian: vegetable tanned which has an ethereal smell arising from birch bark.

  • Boiled leather: cattle hides were heated together with wax, rubber, resin and glue. The final effect is of a rather soft skin, particularly suitable for cartons, liners and bags.

  • Maschereccio: obtained by heavy cattle hides, tanned or alum tallow (fatty oils). This process was suitable for saddlery products.

Only in the second half of the '800, the today more prevalent tanning was introduced : the chrome tanning. This processing is faster and more economic than the previous ones, not only for the technique used but also thanks to the mechanization of the processes introduced as a result of the industrial revolution.

Leather processing

What we call skin, is the key part (dermis) of the fabric covering every living being and separates it from the outside, it makes a whole and can be comparable in large-scale to membrane separating each cell from the outside and by extension, to electromagnetic force which makes every atom individually identifiable and enclosed in a whole thing.

The leather processing, also known as tanning, may provide various phases depending on which type of leather we wantto obtain at the end of process.


Talking in details about skin, we briefly list the following steps of leather processing:

  1. Determination of various animal hides
  2. Hide acquisition
  3. Conservation
  4. Beamhouse jobs
  5. Tanning
  6. Re-tanning, dyeing and fattening
  7. Finishing

 

Now, we are going to analyze in more detail these seven steps:

Determination of various animal hides

The determination is made on the basis of several parameters: genus, species, breed, variety, gender, age, habitat (ie usual environment where the animal was found in its existence including nutrition possible treatments to which the animal has been subject) vegetation , diseases, insects and pests.

Hide acquisition

A further definition of this set depends on the acquisition of the leather which can be of various nature and origin, such as hunting, fishing, domestication, breeding. A clarification should also be made regarding the impact on the environment and the necessary alterations that this acquisition entails; in other words identify and observe what species the material comes from and how intensively each material it comes. This problem will be with us in the further consideration of this discussion. Animal skin should be mainly considered , but not entirely, a byproduct of other human activities, such as essentially nutrition of communities other than other typical utilities of animal species, and their domestication to use them as a driving force or transportation or war, hunting, fertilizer etc.

Conservation

The conservation essentially for the first stabilization of the collected material that prevents the rotting of the skin, can occur in different ways:

 

  • Drying the leather, trying to extract as much water as possible from skin (dehydration) and it may take place in the sun (traditionally used in very dry tropical areas like Africa and South America Uruguay Paraguay and Argentina). This form of drying, being usually too rapid (sun dryied), results in a poor preservation that substantially dry the outside while the inside too quickly deteriorates which does not dry completely. Or it can be in the shade (air dryied) which alleviates above problems but does not solve them altogether because hides are often not lie flat, therefore theytook an irregular shape and folds. A third way of drying is to dry skin after they have been drawn with timber through frames and put into suspension under huts for the time needed to a total drying. As a general remark, in reference to the three points above said, it is necessary to make a clarification on the specific weight of the material. The skins sun dryied are those with the higher specific weight because they retain a greater percentage of moisture. Imagine to have a 100 weight leather (green weight) just extracted and skinned: the ones dried in the sun will have a weight of 85, those dried in the shade will have 80 and those dried on the frames will have 70.

  • The second way of conservation is the use of salt, typically sodium chloride.

Beamhouse jobs

Beamhouse jobs are the first treatments which raw hides are subjected and they are needed to prepare it to the next stages. Of these we can list a few:

Greening: The goal of this phase is to restore leather to pre-conservation. In a nutshell, salts used to keep skin preserved, are removed, leather was cleaned from all unwanted debris and it was plunged into water to replenish the moisture lost during the previous phase. It is mainly used water together with surfactants, alkalis, sodium chloride, bactericides, and proteolytic enzymes.
Calcination and depilation: the calcination allows to loosen the fabric to make it suitable for the absorption of products used for tanning. Hair removal instead, uproots the hair and epidermis. These two operations are carried out simultaneously.
Fleshing and splitting: fleshing, as well as the name implies, removes last residues of meat and fat of leather, while the splitting makes the whole surface of the skin with the same thickness. With the splitting, which can also be carried out subsequently or after tanning, we get two sides of a piece of leather: the flower (top layer) and the crust (lower layer).
Decalcination: at this stage we remove lime from the skin, derived by liming. The skin is deflated and is prepared for maceration.
Maceration: last step of the Riviera jobs, where the dermal tissue of leather is loosened further by the use of proteolytic enzymes.

Tanning

In this case, we also have to distinguish three phases:

Pickel: it ends of the skin-liming process began in Riviera jobs. The pH of the fabric returns to a suitable threshold needed for the next operation.
Tanning: in this stage the leather is treated using different materials: chrome, tannins (vegetable tanning), aluminum, zirconium, aldehydes, fatty etc. The objective is to make leather resistant to traction, to atmospheric agents such as moisture and temperature and to chemical agents.
Crushing, splitting and shaving: in the first step leather is compressed and dried. The splitting is made for thicker skins and heavy or if it had not been accomplished during the Riviera jobs. Finally, shaving reduces skin thickness further, by acting on the part of the meat (crust), using crossing sharp blades that make the entire surface homogeneous in thickness.
 

Re-tanning, dyeing and fattening

Re-tanning, dyeing and greasing, unlike the tanning we have just described, aims to improve the skin from an "aesthetic" point of view :

Retanning: this process returns leather with desired degree of smoothness, softness or fullness through further absorption of tanning substances such as tannins and chromium.
Dyeing: thanks to the dyeing, you can choose which color you want for leather. The choice of colors depends on the type of tanning made previously.
Fattening: the skin is "greased" with oils of animal, vegetable, marine, synthetic or mineral. The hide will look thicker, softer and hydrophobic.
Drying, staking and nailing down: leather is dried, further reducing humidity. The staking further softens skin and finally the nailing down dries the leather again. At the end of this operation the skin surface is more extended than before, gaining something in terms of size.

Finishing

This is the last phase of the long-tanning process. The skins are now dry and ready for the latest treatments that aim to improve resistance and the appearance.

There are several different possible finishes which we can not list entirely. In general it is further modified through color pigments and tinctures or we can modify its surface texture which can be made more "wrinkled" or more smooth or with desired patterns (called prints). We can also apply more surface layers to increase strength and / or cover their defects.


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