Volume 1 - Spring 1996


The Manufacture of Leather - part 1

by David Lanning

Over the next few issues, our intention is to include several articles relating to the manufacture of leather, giving an insight into it's brief History, the sources of our raw material and the various processes involved, from pelt to finished leather.

What is Leather?

Once upon a time many thousands of years ago, primitive man began to make leather. It would have been one of the very first manufacturing industries .Animals were hunted and killed for food, but before they were eaten the skin had to be removed from the animals. Sharp flints were probably used to peel the skin away from the carcass. The skins would then have been worn for warmth and protection from the elements and probably wrapped around the feet; as the first type of shoes. But the skins left like this would soon begin to decompose and rot away. They realised that drying the skins would preserve them, but the result was a very hard, inflexible and uncomfortable material. The skins would have to be softened. This would have been done by rubbing with fats.

The fat also had another advantage, it helped prevent the skins getting wet. This way the skins lasted even longer and with more bending and flexing of the wearer, the softer and more pliable they would become. Then some old chap made a fantastic discovery. By using water, various barks, leaves and berries, you could make a liquid containing vegetable extracts. It was a powerful solution, as was discovered when the skins were immersed in it. They became rot resistant and considerably softer than the dried skins ever were. The active agents in this liquid are called tannins. This was probably one of the first methods of tanning leather.

This process of tanning skins spread and was improved upon. By Roman times, armour, water containers, belts, straps, tents, boats, etc. were regularly being made from leather. By the middle ages, things began to be very well organised. Tanneries were set up, mainly concentrated into special areas. These areas had to have good sources of materials: - A supply of hides and skins - Plenty of water - Lime for softening and assisting with hair removal - Plenty of trees for the extraction of tannins from the bark. The processes that we use today are based on those discovered by our ancestors all those thousands of years ago.


In Hewit's case all of the pelts we buy are a by-product of the meat industry. Taking the skins off of the animals is part of an efficient process that puts food on the table. Bleeding - The carcasses are bled - because blood discolours the meat and down grades the value of the hides. If it was left, it would putrify in the veins and this would accentuate the veins in the finished piece of leather. Cutting - The carcasses are hung and cut - a cut is made from throat, through the chest and belly and along to the tail. Right-angle cuts are made from this line across to the inside of the legs. It is worth noting that reptiles and camels are cut along their backs, because of their unusual shapes. Flaying - The skins are then pulled off - either by hand or by mechanical means. To flay cleanly by hand, a knife is used. If badly done by hand, flay marks would become prominent on the skins and if badly done mechanically, tearing and splitting can occur. In either case the value of the skins would drop dramatically. Furs are peeled away from the head without the need to cut.

The leather industry has also been known to use skins from animals which have died of natural causes. This source of skins tends to be inferior because if the animals have been lying around dead, the skins would have started to rot.

Methods of Preserving Skins Between Abattoir and Tannery

Whilst any animal is alive, it's body and skin are protected from putrefaction and rotting. As soon as the animal is dead, putrefaction sets in. Bacterial action starts immediately, causing a breakdown in the skin. Therefore, something has to be done as soon as the animal has been flayed, to prevent the skin from beginning to putrify. In an ideal world, we would take the flayed skins from the animals and immediately start tanning them. This would need to be done within minutes of the animals having been flayed. But we don't live in such an ideal world, since the abattoirs and tanneries can be halfway around the world from each other. The process which is required to protect the skins is known as curing, and there are several ways in which this can be done.

Chilling - Skins can be chilled and transported in refrigerated trucks. Common in Australia; this process is expensive but eliminates the need to use chemical preservatives.

Drying- Skins are laid out on stones in the sun to dry, this method is the oldest form of curing. With this method, there is very little control on speed of drying due to the unevenness of the stones and it is therefore common for the skins to dry unevenly. This could cause serious problems later on during the tanning process. One way around this would be to stretch the skins across wooden frames. These frames could then be hung in the shade until dry. This would make a better cured skin than the 'laying over stones' method. However, as a rule sun-dried skins do not generally make good quality leather. These methods are extremely cheap and are still extensively used in the third world. However, most of the skins would go into local production, since they would not normally be up to the standard required by tanneries in the industrialised world.

Brining - Washed skins are put into a salt solution in large containers and kept moving constantly, until the brine penetrates the skins. The concentration of salt is kept high, by continually adding salt to the brine solution. The skins are then taken out and dried ready for shipping. This is a very good method for preserving skins, but quite expensive.

Wet salting - A flayed skin is laid out flat. Salt is sprinkled on top, and a second skin is placed on top of the first. This too is sprinkled with salt and the process continues until there are about 50 skins in the pile. The salt absorbs the moisture from the skins and the resulting brine penetrates the skins killing off the bacteria. Once the hides are in this state, they will keep for a very long while, as long as they do not get wet or warm. This process is very common in temperate climates and most of the pig and calf arriving at the tannery in Edinburgh is cured this way. There can be one problem with this method. Some bacteria can survive in salt solutions. These can produce red stains on the flesh known as 'red heat'. However, this problem is not insurmountable. It can be solved by adding small quantities of naphthalene and soda ash to the salt or by avoiding the re-use of salt.

Dry salting - This involves wet salting as a first stage. The hides are then hung up to dry. This has two advantages: i) It reduces the chance of heat damage and ii) it brings down transport costs by removing the weight of any moisture in the skins.

Pickling - This is a method most commonly used with sheepskins. The skins are kept damp and cool (below 20 c) in a salt and acid solution.

The Problem of Damage to Skins

Damage can occur to the skins in three ways; during the life of the animal, after the animal dies and before tanning starts, or at some stage during the tanning and finishing process.

1 - No hide coming from a living animal is ever perfect. During their lives animals may aquire numerous scars from one or more of the following: barbed wire, thorn damage, in-fighting among male members, branding by farmers and ranchers, parasite induced holes in particular the warble fly which lays it's egg below the surface of the skin, vaccinations and if the animal was unlucky enough to be hunted, from bullet holes and trapping injuries.

It then follows that the older the animal the more chance there is during it's lifetime to accumulate an assortment of scar tissue. It is worth noting that older animals have a tendency to excess weight. This can lead to growth marks in the neck and belly, not unlike humans, and horny i.e. rough and hard areas along the spine.

2 - After the animal has been killed, correct bleeding, hanging and curing must be implemented. Damage which can occur due to poor practises include: Vein marks due to insufficient bleeding, Flay marks and cuts if the animals have been poorly skinned, Poor curing resulting in localised rot setting in, 'Red heat' damage caused by the use of contaminated salt. Mined salt only should be used, since sea salt contains harmful bacteria.

3 - In the tannery, the skins undergo numerous processes to turn them from pelts into finished leather. There are obviously many opportunities for damage to occur, but these factors are within our control and unlike the previous factors we can keep damage to an absolute minimum.

In the next issue we will cover the processes for preparing the pelts for tanning.


Skin Deep - Volume 1 - Spring 1996

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