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Leather manufacturing - part 1

The first of four articles on the manufacturing of leather provides a general background to those who buy or sell leather for products.

Background information on the manufacture of leather and its various properties and attributes will enhance product knowledge where it can make a difference – for example, in making a sale, understanding a problem or the limitations of the material.

Leather has been produced for thousands of years, ever since man discovered that animal skins in contact with other materials, extracted by water from bark, wood or leaves, preserved the skins from decay. The earliest known tanneries date back over 5,000 years according to evidence found during excavations in the Egyptian settlement of Ghebelen.

Leather industry worldwide

Today leather is made in an estimated 9,000 tanneries worldwide. The annual area of leather produced is 19,000 million square feet, an average of around 2 million square feet per tannery. The size and standard of tanneries vary massively. Some tanneries produce more than 10 million square feet a year, whilst other tanneries are very small. Many have been in family ownership for several years. 

Hides, skins and leather are traded internationally in a variety of conditions: fresh, wet salted, pickled, sun dried, crust, wet blue and finished.

Leather is a by-product of the meat and dairy industries and may be made from the skin of any animal. Figure 1 shows the amount of leather produced by area for each animal type per annum with bovine (ox, cow, calf and buffalo skin) being the most produced type at 64 per cent.

 

Figure 1: Percentage of leather from each animal type by area produced per annum

Although China produces more leather garments and footwear than any other country, it does not have enough of its own hides to produce all of the leather required to meet its huge share of the market and has to import from other countries.
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After China, India and Italy are the next largest producers of finished leather and the USA and Brazil supply a high quantity of hides for production in other countries.   

Structure of hides and skins

All hides and skins have a similar composition, (see table 1). The water content of the hide is reduced to an ideal 12-14 per cent in the finished leather. During tanning, the keratin (hair or wool) is removed, except for wool-on sheepskin or hair-on calf, as well as the elastin. The non-structural or interfibrilliary proteins are present in hides and skins as a watery jelly around the fibres, from which the structural proteins are formed. These must also be removed in processing. If left around the fibres, these non-structural proteins would make the leather hard and brittle. In cowhide processing, the fat content needs minimal control. Sheepskins, on the other hand, contain a much larger proportion of fat which must be reduced during the conversion to leather.

Table 1: Composition of untanned hide
Approximate composition of fresh hides Percentage of material present  
Water      64  
Protein
 
 
 
33

Structural     proteins

Collagen 29 per cent
Keratin 2 per cent
Elastin 0.3 per cent
Non-structural proteins Albumens, globulins 1 per cent
Mucins, mucoids 0.7 per cent
Fats   2  
Mineral salts 0.5  
Other substances 0.5  
Note: The proportions of the constituents above may vary, depending on the animal, especially the keratin (hair or wool) and fat.

The three-dimensional mass of interwoven collagen fibres (see figure 2) gives rise to properties such as high tensile and tear strength, excellent shape retention and the capacity to absorb and transmit moisture. The grain of the rawhide is seen in most consumer products and is part of the intrinsic desirability of leather. However, the grain can be damaged during the life of the animal by wire scratches, ectoparasites, disease and after slaughtering by putrefaction and poor processing.

 

Figure 2: Typical variation of collagen fibre structure through the thickness of a hide

Curing

The defini­­­tion of leather is a hide or skin with its original fibrous structure more or less intact, tanned to be imputrescible. Once the animal is slaughtered, it is vital to stop the possible putrefying of the hide or skin immediately by preserving it in a satisfactory state for tanning. This means rapidly reducing the temperature and preventing attack by bacteria and fungi.

There are many methods of doing this. Applying salt, sodium chloride, either dry or by brining, together with biocides preserves the hides and skins and also gives effective resistance to micro-organisms, which can destroy the collagen if not controlled quickly. Wet salted hides and skins may be stored in this condition for an extended period and transported worldwide. However, excessive use of salt can harm the environment through waste water. One way to combat this is to remove loose, dry salt from the hides by hand before processing. Other salts with similar properties, such as potassium chloride, have been tried as a replacement for sodium chloride. This is because potassium chloride is required for proper plant growth when applied to the soil, whereas sodium chloride has a negative effect on plant growth and is therefore harmful to the environment.

­­­Low salt application with additives such as magnesium oxide is under consideration. Irradiation using electron beams has also been proposed to eliminate micro-organisms, but the cost of the equipment is high and a system for long-term preservation is still required.

Close proximity of the slaughterhouse to the tannery will enable the hides to be transported fresh to start the first process of soaking.

Another method is chilling the hides for short term preservation, although it is impractical and expensive to store or send the hides and skins long distances using this method. In hot countries, the sun is used to dry out the hides, but again this is not a viable alternative to wet salting for most hide sources.

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