The manufacture of leather


Introduction

Leather has always held an inimitable place in man's life. It was used widely in many primitive societies and was considered a necessity to survival itself. Early man discovered that the quality of leather could be enhanced by curing the skins of hides. He therefore rubbed them with fat and smoked them over wood fire and concocted the early form of tannic solution from rotting leaves and bark for soaking the hides.

>Of course today leather making has evolved into a fairly complex process. Different manufacturers process the leather in different ways to achieve their own unique brand of leather. Handbag leather falls into a category known in the leather industry as 'light leather'. They are usually made from calf, goat or sheepskins. 'Light leather' must meet specific demands with regard to thickness, suppleness, durability, and suitability for the various finishing processes. Consequently, special conditions apply to the bating process, tanning method and tanning ingredients used in its manufacture.

The Skin

All leathers consist of a network of fibers made up of proteins. Collagen is the most important protein occurring in leather. The skin of mammals consists of the epidermis, the dermis, and subcutaneous tissue. For the manufacture of leather only the dermis is important.

Epidermis

A protective, hard-wearing layer of keratinous cells, which, although of varying thickness over the body of the animal, is very thin compared with the underlying
Dermis.

Dermis

This layer consists of a dense fibrous connective tissue which supports blood and lymph vessels, sebaceous and sudoriferous glands, hair follicles and their associated muscles. In the grain layer of the dermis these fibers become very thin and tightly woven. Consequently, when the epidermis is carefully removed, a smooth layer is revealed, sometimes known as the hyaline layer, which gives leather its characteristic grain surface. Toward the center of the dermis (or corium) the fibers are coarser and stronger, and the predominant angle at which they are woven can indicate the properties the resultant leather will display. If the fibers are more upright and tightly woven, a firm, hard leather with little stretch can be expected, while if they are more horizontal and loosely woven, soft, elastic leather can be anticipated. The interior of the dermis is generally the strongest part of the skin. and is not used for the manufacture of leather.

Preparation of the Skin

To prevent the onset of decomposition, fresh skins must be preserved within about 2-3 hours after stripping. The dehydration process significantly slows down this decomposition and, consequently the skins become hard.
There are several methods for preserving skins such as drying, salting, or pickling. The skins are processed in large drums as shown in the diagram for a period of 14-16 hrs before they are ready for tanning.

Soaking, unhairing and liming

To install the softness back into the skins after the preservation process, they are soaked in water to restore their original water content.
Next the epidermis (with hair) is removed. Sodium sulphide is generally used to facilitate this process.

Liming is a process by which the bonds between the fibrils are partly broken, so that the fibers become a little loose. The lime also takes out part of the interfibrillar skin matter, so that the fibers become less fixed. Simultaneously liming loosens the subcutaneous layer so that it is more easily removed later on. This process takes about 14 hours after which the skin is very alkaline (pH 13-14).

Deliming, fleshing and scudding

To neutralize the alkaline skin, carbon dioxide and ammonium salts are used. This process is essential before the tanning process begins, otherwise the acidic tanning agents would harden up the grain fibers. The next phase in the manufacture of leather is fleshing here the subcutaneous layer of the skin is removed. Traditionally, this was carried out using a wooden beam and a fleshing knife, but today it is done by machines.
Scudding is the final trimming process, where excess and unwanted skin elements are removed using a blunt knife. This leaves the grain more porous to the tannin solutions during the tanning process.

Bating

To achieve the fine, smooth, and elastic texture, as required for handbags the skin is put through yet another process, Bating. This process will further delime the skin, loosen up the protein fibers to yield a more elastic material, and give the surface texture a fine smooth feel.Proteolytic enzymes are introduced to the skin which dissolves away any remnant of the epidermis and other skin substances. This process is carefully controlled for pH levels, temperature, and the duration of the process. The skin is finally washed thoroughly to prepare for tanning.

Tanning

The chemical procedure used to ready raw animal hides for use is called "tanning." A piece of hide or skin which has been tanned produces strong, flexible leather which is able to resist decay and spoilage.
Tannins are divided into four different groups:
* Vegetable tannins
* Mineral tannins
* Aldehyde tannins
* Synthetic tannins (syntans)
We shall outline here only two of the most commonly used tannins.

Vegetable tannins

The vegetable tannins solution is derived from wood, roots, bark, leaves, and fruits of various plants, such as oak, chestnut, hemlock, and willow trees. Skins are suspended into large pots or vessels containing the vegetable tannins. Periodically the skins are exposed to increasing concentration of the tannin solutions by transferring the hides from one pot to another containing a stronger solution.
Skins which have been treated with vegetable tannins generally yield tougher, but supple leather. By and large these hides would be intended for suitcases, belts, and hats.

Mineral tannins

Mineral or Chrome tannins are used in leather production by treating the skins with chromium sulfate. As in vegetable tanned leather the degree of control defines the quality and type of leather produced. Chrome tanned leather tends to be softer and more flexible than the vegetable tanned leather, and durable when exposed to water. However it deteriorates with contact to sweat or organic acids, and is difficult to emboss. Chrome tanned leather are best applied to manufacture of handbags, clothing, and shoes .

Dyeing, Fatliquoring or lubricating

The next stage is dyeing, where appropriate dye-stuffs are added for desired color.
Because of the acidic nature of the tannin solutions the leather is treated with some fat emulsion which coats the outer surfaces and allows the leather to be supple. Vegetable tanned leather can be lubricated with oil which resists oxidation (darkening) of the tannin solutions in the grain.

Drying

After tanning and dyeing in a bath, the moisture content of the leather is about 65-70%. This must then be lowered to around 15%. When leather is being dried care must be taken to allow the water molecules to evaporate while leaving the fat molecules in place.

Staking

After the hides are dried they become stiff and less flexible. Staking is a mechanical treatment to make the leather soft and supple after it has dried out.

Finishing

The very final step in leather manufacturing is the finishing. This is carried out by coating the hide with chemical compounds and then brushing it. Light leather is buffed and sandpapered to remove imperfections. Leather which has been buffed for a long period of time becomes suede. Different leather types require different finishing processes. Full grain leather has a polymer or wax finish applied to the grain surface. The finish applied determines how the end product looks in terms of shine, color, and texture.