how is leather made?

Question

like do they to wait for the skin of the animal to dry or they made right after they kill the animal...

Answer 1

Before a piece of animal skin can become a workable piece of leather, it must be treated. Animal skins and hides ( skins refers specifically to the skin of a smaller animal, such as a pig, calf, or sheep, while the term hides refers to the skin of a larger animal, such as a cow or horse. Skins, though, can be used generically to describe all animal skin, as it will be used herein.) consist of three layers of body cells. The top layer is called the epidermis, and the fatty bottom layer is known as adipose or flesh. It is the middle layer of skin, the derma or corium, that is what the leathermaker seeks to retain for his leather. This layer is not composed of true cells, but is rather a network of collagen fibrils, which are produced by cells and bundled into filaments and fibers (Waterer, Leather, 18). In order to separate thecorium from the epidermis and the flesh so that the skin or hide may become a piece of useful leather, several steps are needed.

First, the piece of skin must be washed and cleaned. If the animal skin is fresh from slaughter, as most Roman skins likely were, then this process can be started immediately. In the event that the Romans needed to keep their animal skins from rotting before being tanned, they either applied salt to the skin or let the sun dry the skins out for them (Waterer, Roman, 179). Today, most animal hides must be shipped from slaughter houses to tanneries, and so they must almost always be cured after slaughtering to arrest putrefaction. Cleaning the skin involves soaking it in water for up to two days, to remove all impurities and, if necessary, the last remnants of curing agents, such as salt (Orthmann, 144).

Next, the top layer of cells, the epidermis and hair, must be removed from the skin. This was, and still is, accomplished by first immersing the skins in strong solutions of lime for several days to loosen and soften the hair roots and epidermis, and then scraping the loosened layer off. To scrape, the Romans would have simply spread the skin over a wooden beam and used a two-handled, curved knife to rub the epidermis off. The modern process of removing the epidermis can still be as simple as scraping the skin over a beam, but the beam is now metal instead of wood (Waterer, Leather, 19). Some modern tanneries, however, use machines with blunt blades to scrape away the hair and cells (Orthmann, 144). It was observed by the ancients that, as a happy byproduct of the lime soaking process, the skins were more receptive to the agents and dyes used later on in tanning processes (Waterer, Leather, 19).

The final cell layer to be removed is the flesh, and the process of removing it is appropriately called fleshing. While fleshing is a process requiring considerably more skill than removing the epidermis, the skin does not need to be removed from the scraping beam for this process and the general principle is the same as scraping. In the cutting away of the fatty tissue, a single slip of the sharp knife can render an entire skin useless. Roman fleshers needed to be quite skilled at their craft. Today, most skins are fleshed by machines with quickly revolving helical blades, and the possibility of human error in fleshing is gone (Waterer, Leather,19).

The last step in the pre-leathermaking process is shaving or splitting, when the skin is made the correct thickness for tanning. Cattle hide is normally about four and a half millimeters in thickness, but for some purposes, the leather needs to be half this thickness, or even less. Shaving, like fleshing, is a process requiring great precision and skill. It also requires very specific tools: the skin is placed over a specially shaped beam and the skin is cut with a currier's knife, which has a double edge. The currier's knife is held at a right angle to the flesh-side of the leather, and small pieces are shaved off until the skin is the correct thickness needed for the tanning process. This is called "skiving". Currently, many factories shave skins by pushing them against an ever-revolving band knife. The skin can be split by this process into two useful sheets, whereas the small pieces shaved off by the Roman technique were useless (Waterer, Leather, 19).

After shaving, the skins are given a final cleaning, to remove every last bit of dirt and hair possible. If the ancients wanted a skin to be particularly soft, this was the time to make it so. Infusions of hen or pigeon dung, followed by dog dung, were applied to the skin, setting off a fermentation of bacteria which reduced the skin to the desired softness. These processes often took several days, or even weeks. It was important to halt the reaction at the perfect time in order to keep the skin from being destroyed by fermentation, and the judgment of whether or not a skin was properly tempered was one made entirely by feel. Today, softening compounds are chemical rather than natural, and the last bath the skins are given is one of sulfuric acid and common salt, designed to remove the last traces of lime and to impart an overall degree of acidity to the skin, which is helpful in the tanning process to follow (Waterer, Leather 19.)




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Tanning Leather: Three Methods

Once a skin has been prepared, there are three possible methods that can be used to turn it into leather, all of which result in very different materials, aesthetically and texturally. A skin can be made into leather by tanning, chamoising, or tawing, and while these processes all differ considerably, they are often lumped together under the umbrella term "tanning", just as the term "skin" has both a generic and a specific meaning. Only two methods of leathermaking, tanning and tawing, have been found to have been in use in ancient times. Chamoising, a process which results in a soft, pale yellow, oiled type of leather, is not believed to have been used by the Romans, despite the fact that it originates from a primitive practice of rubbing brain matter onto animal skins (Waterer, Leather, 20).

Tanning gets its name from the tannins that are present in all vegetable matter, and these tannins are used to effect a permanent chemical change in the collagen content present in raw skins. This is also why tanned leather is sometimes more specifically referred to as vegetable tanned leather. (Waterer, Leather, 17). Skins that were to be tanned were immersed in vats filled with water and a large amount of chopped bark-- the material from which leathermaking tannins are derived. Present tanning practices place skins in rotating barrels full of water and tannins, thereby reducing the ancient tanning times of several months to mere days. There are also many variations of tanning, each tailored to the specific products the leather being treated will eventually become. Shoe uppers, for example, have a slightly different tanning process than handbags, (Orthmann, 146). The Romans, however, almost certainly used only one general method of tanning for all their leather (Waterer, Roman, 180).

To finish tanned leather, the Romans would have used a process called currying. Currying involves working grease, perhaps cod oil mixed with tallow, directly into the fibers of the leather with a "slicker" of stone, metal, or glass, as a means of waterproofing the leather and ensuring flexibility and strength (Waterer, Roman, 180). While currying was used by the Romans, and is still used today, on some of the finest military items and harnesses, one drawback of the process is that waterproofing leather eliminates its breathability (Waterer, Leather, 18). Vegetable tanned leathers can also be given smooth, grained, bright, or dull finishes, all depending upon how the leather is hammered, folded, or rolled. Even waterproofing changes the final appearance of leather and was sometimes used as more of an aesthetic choice than a practical one (Waterer, Roman, 180).

The other method of leathermaking used by the Romans is called tawing, or mineral tannage, and the white, porous leather resulting from this process bears little resemblance to the sturdy, hefty leathers produced by chamoising or vegetable tanning. The main step in tawing is to soak the prepared skin in a mixture of alum and salt. To soften the salt-stiff skin when it is pulled from the alum solution, tawed leather is pulled while damp over a wooden or metal stake in a process called, appropriately, "staking." The resultant leather is so white and open-pored that if it were to be rinsed in warm water, the alum and salt would wash out, leaving the finished piece of material with a texture very similar to that of the untreated skin when it began the entire tanning process. In order to give tawed leather the sturdiness required for common use, it must be rubbed with supportive substances which can fill its large pores, such as fat, grease, flour, or egg yolks. It is a widely accepted hypothesis that the Romans used this tawing process often and were responsible for spreading its use throughout Europe (Waterer, Leather, 20).

Once leather was tanned, it needed to be dried. All leathers, and particularly vegetable tanned leathers, are very temperamental about how they are dried: if the process is completed too slowly, mold can develop and ruin the material. If done too quickly, however, tanned leather can become brittle and loose the rich color it received during the tanning process. The drying process was also impeded by exposure to the elements: sun and rain are two foes of leather that is to be dried. To combat these obstacles, the ancients built drying lofts, which were covered structures that were kept open enough to allow warm air to circulate around the pieces of drying leather. Midway through the drying process, if needed, a process called "laying the grain" could be applied to the pieces of leather: the hides were struck with a triangular, blunt-edged tool, which would smooth and even out the look of the leather's grain. To test a piece of leather to make sure it is dry, modern tanneries have machines which detect moisture. The ancients, however, had a more interesting method: to see if a piece of leather was properly dried, they held a small, very cold mirror about a half an inch away from the surface of the drying leather. When moisture no longer condensed on the surface of the glass, the leather was considered dry and ready for use (Waterer, Leather, 47).