Leather

Leather. Material intended for the manufacture of clothing and the manufacture of various articles, has been produced since ancient times from appropriate animals. Whereas in ancient times leather was made by tanning the skins by means of fat , milk , flour , alum , gut nut extract , finally leaf extracts, from suitable tree bark. Today, synthetic products are used almost exclusively for tanning.

The obtaining of leather, constitutes the oldest of the applications of the textile industries, is always based on the need to protect the skin of animals from hardening and putrefaction. Leather was initially used for dresses and increasingly constituted a material without which life could not imagine. Leather for clothing, such as for shoes, gloves and similar kinds of leather objects, as well as other objects such as chairs, handbags, chests, would acquire more and more importance.

Summary

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  • 1 The aminal skin
  • 2 Tanning
  • 3 tanning agents
  • 4 Different types of leather and skins
    • 1 Bovine
    • 2 Goats
    • 3 Equines
    • 4 Sheep
    • 5 Calf
    • 6 Otters
    • 7 Chinchilla
    • 8 Reptile Skins
    • 9 Fish
    • 10 Deer, Fallow Deer, Reindeer and the like
  • 5 Sources
  • 6 External links

Aminal skin

Animal skin consists of the epidermis (1%), the dermis (85%) and the hypodermis (14%). Crude skin contains:

  • 60% – 70% water.
  • 1% – 2% of mineral substances.
  • 1% – 2% fat (more in some animals) the rest is made up of collagen, protein , keratin and elastin .

Tanning

The tanning of the skins is carried out with a preliminary mechanical treatment and then they are treated with chemicals that combine with the skin protein, resulting in insoluble compounds that also do not re-swell. Depending on the type of leather or the use to which it is to be put, the leather obtained can be hardened, made more flexible or softened by using special additives in tanning.

The epidermis with horsehair or bristles is separated from the dermis by depositing the previously desiccated skins in cement bags or drums containing full of lime and sodium sulfide in adequate proportions (for 100 kg of skin 5 kg -10 are required kg of CaO and 1 kg-3 kg of NA2 S).

Thus the epidermis is softened and can then be easily separated by machine, along with the hairs or mane housed in it. For its part, the hypodermis, fleshy and fat, is extracted with the cylinder separator. Finally, the dermis, without tanning (skin or leather in the gut) is declassified by treating it with acids (lactic acid, hydrochloric acid ) or ammonia salts , to rid them of the calcareous salts, then apply pancreatic ferment and cure according to the type of Leather to manufacture the tanning can be made with organic vegetable or synthetic products, with mineral salts or with appropriate fats.

Tanning agents

Vegetable tanning agents are mainly Quebracho , Chestnut , Oak or Mimosa bark and Pine bark . These contain phenols poly ( pyrocatechol , resorcinol , pyrogallol and 1,3,5 trioxibenzol ), condensed form as carbonic acid descendants, or a group of piránicos rings.

The fundamental types are gallic tannin , catechin , brasilina and hematoxilina :

  • Syntano:in aromatic synthetic tanning agents, a distinction is usually made between complete tanning agents (barter tanning agents), which can be applied in convenient quantities together with vegetable tanning agents, or alone to obtain leathers of good condition and the so-called “auxiliary tanning agents” already known long and used, to improve the solubility of natural tanning agents, or to accelerate the tanning process.

The main difficulty in obtaining synthetic tanning agents is rather an economic problem, because natural tanning extracts are very cheap. These tanning agents often provide very light colored and even white leathers. An important elaboration procedure consists of obtaining a resinous component followed by final condensation with sulfolignic acid for molecular thickening, making it possible to solubilize it by applying formaldehyde as the bonding medium. Sulfoligenic acid, which is presented in its purest form (iron and calcium free ) as ammonium salt only as a tanning aid and together with a resinous component forms the basis for a whole series of barter tanners.

  • Mineral tanning: itis carried out exclusively in vats. The duration of the tanning comes to be from 8 to 24 hours. Trivalent chromium salts are used in all cases. These are formed by reducing sodium or potassium bichromate , for example, with glucose or molasses in acidic solution. Thus, hydroxosalts are formed that give rise to so-called polynuclear “oily combinations” that are true high polymers . Chrome salt solutions reach a colloidal state with increasing basicity, which is a precondition for their tanning action.

The process is extraordinarily complicated. Chrome tanning agents are almost always supplied by the industry in conditions to be used, taking advantage of the bichromate primarily for oxidations and the corresponding chromium sulfate broth is manufactured and adjusted. Before chrome tanning you need to “pickel” with salt and hydrochloric acid . In a moderate way, aluminum salts and recently also zirconium salts have a certain role in tanning . Chrome leathers they are light in color, faint greenish gray, and especially suitable for footwear.

  • Tanning with fish fats and oils:for example for leather leggings, is an important improvement, impregnation takes place, in this case the greasy remains are chemically bonded and cannot be removed by washing. It can also be tanned only with aldehydes, for example formaldehyde . And also tanning with sulfoparaffin chlorides, where bare skin is treated in the presence of soda with sulfonamide-paraffins with basic groups of collagens.

Different types of leather and skins

For the manufacture of clothing and shoes, designers and companies use different types of leather:

  • Skin cow, leather is used caprine ( goats ), equine ( horse ), porcine ( pigs ) and some reptiles .
  • Also, otters, foxes , chinchillas , among others , are used in the fur industry .

Other clothing and footwear manufacturing companies use fish and animal skins as deer.

  • For example, the famous brand of sneakers Puma, launched a series of shoes made with fish skin, which were designed by the Argentine Martín Churba.

The hides and skins differ in their structure according to the animal’s life habits, the season of the year, the age, the sex and the upbringing that they have received.The constitution of the skin, in whatever state of conservation it is in, but without alterations, it is of great importance in the final result of the leather after tanning. A good leather comes from skins of uniform thickness, healthy and of good resistance, a thin skin, of weak and brittle conformation, gives a product that, once industrialized, has characteristics that relegate it to inferior destinations.

From animals of poorly selected breeds, sick or dead due to disease, skins are obtained which, when transformed into hides, detract from their natural property; on the other hand, from healthy animals, from selected crosses and slaughtered in suitable establishments, the hides, if the tanning treatments are adequate, will be resistant, soft and flexible.

Bovine

The skins that are most interesting for their slaughter volume are vaccines, both green and preserved. The tanner, as he receives the leathers in his establishment, selects the well-shaped ones with a thickness as uniform as possible over the entire surface, seeking that the differences in thickness in the different parts are minimal.

Poorly shaped skins, or poorly proportioned with appreciable thickness differences, cause problems in tanning absorption ; for this defect the tanning operations will be arduous and the leather is of regular quality. The leathers of both cows and heifers are made up of a fibrous and elastic fabric and, once industrialized, give a fine cut and grain, with good characteristics to be used in fine clothing.

On the other hand, young, bull and young bull hides are thicker than females, and the constituent fabric is less elastic, with a less fine cut and grain, but also of good quality. Young cattle, in general, always give higher leathers than older animals. The cattle whose exploitation is in the field, always have better skins than those raised in stable.

In the region, due to its good pastures and appropriate climate, cattle are raised in the wild, and only the broodstock, both male and female, are kept in stables. However, in European countries, breeding is intensive and the animals spend several months in sheds, fed with balanced rations. Diet is important in the quality of the leather since the animals whose diets are destined to create greater muscle mass and abundant fat , produce unfavorable skins and the leathers are never the best. In contrast, cattle that do not receive a rational diet, which is subjected to excessive work, give poorly shaped skins and little value.

Goats

They are the ones that supply the leather industry with very fine leather, and for this reason, once tanned, they are used to make high-quality footwear, gloves, bindings of the highest quality, etc.

From the youngest animals the finest and most valuable leathers are obtained. Goats are ideal animals for places where there is no adequate grazing land for sheep or cattle. Goatskin has a very compact fibrous structure, they do not produce wool, but hair, that is, they are medullated fibers in all their extension.

Equine

Due to their thickness and resistance, they are, once industrialized, of lower quality than cowhides, however, they play an important role in the tanning industry despite the fact that their volumes never become interesting.

Equine leathers can be divided into two parts:

  • The front section has a relatively light skin and despite the fairly thick growth of hair, the texture of this area is similar to that of some types of goat fur.
  • The upper part of the hindquarters, the skin is much thicker and shows a network that is a very compact fiber structure.

Sheep

Unlike what happens with the bovine cattle, the majority of the ovine races are raised mainly for their wool or to obtain meat like wool, with the least the breeds exclusively for meat. The highest quality sheep skins are provided by those breeds whose wool is of little value.

Young animals are the ones that supply the best fur industry, from old animals only regular quality leathers are obtained. The destination of these skins, whose slaughter volume makes them very interesting, are generally the manufacture of gloves, shoes, bags. Since the sheep is fundamentally protected by wool, the primary function of the skin is to help the growth of the fibers.

In general, it can be said that the skin of sheep is thin, flexible, extensible and pink in color, although the dark pigmentation of certain breeds is normal. In fine wool-producing breeds, such as Merinos, the skin is thinner and has a greater number of follicles and glands, both sweat and sebaceous, than in the meat breeds. Another distinct characteristic is found in Merinos, in which the skin forms folds or wrinkles in the neck, called ties or aprons, and in some these wrinkles are found in part or in the entire body surface.

  • The follicles are invaginations of the skin in which the hairy and woolly strands originate. Inside is the root of the strand with the piliferous bulb that surrounds the papilla that nourishes it and causes the growth of skin fibers.
  • Sweat secretions are tube-shaped and empty into a pore of the skin through an excretory duct.
  • The sebaceous glands appear as clusters whose excretory duct opens at the inside and top of the follicle, shortly before the fiber appears on the surface of the skin.
  • The glandular secretions of the skin bind, originating the fat of the wool, also called sweat, which lubricates it and protects it from external agents.

The wool fiber consists of two parts:

  • Internal or root included inside the follicle and an external one, which constitutes the wool fiber itself. At first glance, the wool fiber has a cylindrical shape with a circular or oval section and with a point only in lambs, since the wool of sheared animals continues its growth without a point.

Histologically, the wool fiber is made up of three different layers:

  • External, the cuticular layer.
  • Internal, the cortical layer.
  • Central or medullary layer.

The cells of the cuticular layer have the characteristic of being placed semi-overlapping in the form of scales , leaving a protruding free edge, and when viewed under [Microscope | microscope]], they have a serrated appearance. This superposition of the cuticular cells is typical of wool and some other animal fibers, but it is not possessed by vegetable fibers, or synthetic or artificial fibers.

The cortical layer constitutes the body of the fiber, and is made up of very thin, elongated cells , as well as if they were spindles that, due to their position parallel to the longitudinal axis of the fiber, give the wool resistance and elasticity. The black or brown strands are due to the existence of pigmentation in the cells of this cortical layer. Sometimes a third layer called medullary is found inside, especially in wool from poorly honed animals. It is an air-filled channel interrupted by a variable number of overlapping cells of different sizes. On microscopic observation, the medulla appears black as a consequence of light refraction.

The fundamental histological difference that makes it possible to differentiate wool from hair is the existence in the latter of the medullary layer . The presence of marrow fibers in the fleeces of most of the improved breeds of sheep is considered a lack of refinement, but we must bear in mind that some breeds normally produce a greater proportion of hair than wool, as in the Karakul , the Black Face , etc.

When keratinization occurs only in the cells of the cuticular and cortical layers, while the cells of the medulla have not absorbed enough cystine, the marrowed fibers and hairs are produced.

In summary, we can establish the following differentiation between hair and wool:

  • HAIR: itis a fiber with a marrow of variable thickness, continuous or discontinuous, with a straight and opaque appearance.
  • KEMP: itis a strongly medulled fiber, of large diameter, of discontinuous growth, which is observed in lambs until a few months of life.
  • WOOL: itis a fiber that completely lacks a medullar layer, with a translucent appearance and more or less wavy.

The raw hides obtained in the refrigerators are the best quality due to the care given to them, whereas the field hides are of inferior quality both for their presentation and for their later qualities, especially when they come from animals killed by various diseases.

Calf

Calf skins come from male dairy calves that are slaughtered at the proper age to obtain reasonable meat yield.

In Europe, for example, they are slaughtered without castration at a younger age, while in America it is a question of fattening them after castrating them; consequently, European calf skins are smaller than American calves. The main difference from a structural point of view between calf hides and cowhides is the fineness of the grain. Although the number of hair follicles is the same in both types of animals, those of calves are much smaller and are much closer together, forming smaller collagen bundles . The result is that calfskin has a very fine structure compared to cowhides.

Otters

The coat of otters is made up of two layers of hairs: an inferior one or hair, which is a dense plush and is the one that grants true commercial value to the otter’s skin and an upper one formed by long hairs that serve as a shelter against the weather and cold, which is removed from the leather when shaving.

This long hair can reach up to 8 cm on the back and is reduced in length on the belly, being very short on the head and extremities, disappearing almost entirely on the inside of the thighs. The top layer of the coat has a varied color ranging from grayish brown to dark brown and black, through the entire range of reddish and brown tones, without missing the variety of white otters. the difference in color is a consequence in part of the environment in which animals thrive, and in part they are hereditary factors. The coat of the belly is lighter than that of the loin in its upper layer, although some animals have an even color throughout the skin, as in the reddish and black.

It should be noted that in almost all animals there is a greater or lesser number of bluish black hairs interspersed between the browns. the plush or hair is much more dense than the long coat, it is always somewhat lighter in the part closest to the epidermis and independent of the color of the rest, and those on the belly are lighter than those on the back that are usually almost black.

Chinchilla

Chinchillas that are bred in captivity for the production of fur receive special treatment, providing them with extremely clean environments to avoid stains caused by urine , which devalue the product on the market.

Animals are slaughtered in winter as those slaughtered in summer give skins of little commercial value. When the animals have their skin in a mature state, the epidermis is white, while if it is not yet, the epidermis is bluish. As the hair at the nape of the neck matures before that of the rest of the body and the area of ​​the haunches is the last to mature, when the skins are checked, it is blown and examined from the head to the tail and from the shoulders to the hips.

In regions where the environmental temperatures are high, the skins mature in refrigerated chambers or temperatures ranging between 2º and 8ºC with a humidity lower than 40%. This artificial maturation system allows obtaining the appropriate skins in 70 days of preparation.

Reptile Skins

The reptiles are animal blood cold and their skins do not have any function thermostatic being devoid of hair and sebaceous glands . The scales perform in reptiles the functions of the hairs in warm-blooded animals.

Crocodile , alligator , lizard and snake skins give very attractive and long-lasting tanning, but it is quite difficult to obtain raw hides in perfect conditions for tanning, since they come to the industry with cuts, cut marks and peeling, excessively desiccated for a long time. sun exposure , very damaged by an inadequate extension even by the action of weevils after drying the skins.

Fishes

The fish have a totally different skin structure and in the case of shark skins , the scales are very small with an inert outer layer intended to confer greater protection.

Deer, Fallow Deer, Reindeer and the like

These skins are industrialized for chamois and their commercial use is the manufacture of clothing, gloves.

 

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