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The Tanning Process

Hides and skins are primarily composed of water, protein and fatty materials. The most important protein in the production of leather is collagen, which makes up approximately 29% of the mass of a freshly flayed hide. The collagen desirable for tanning is found in the grain and reticular layers where it is "intimately woven" in a three-dimensional mesh that is think and tightly woven in the grain and coarser and stronger in the reticular layer.

After slaughter, hides and skins must be temporarily preserved for shipment and storage prior to tanning. The most common commercial method of preservation is to cure the hides and skins using salt to produce a dehydrated wet-salted or dry-salted material.

The majority of leather (almost 90%) is tanned with chromium and is therefore commonly used as the basic format when trying to understand leather production. Other methods of tanning will have significantly different production flow although if the material to be produced is without hair, wool or scales, the preparation and isolation of the collagen in the beamhouse processes will remain largely similar. The use of drums as process vessels but paddles, pits and mixers are also common. Leather manufacture is usually divided into three or four zones but this may vary slightly according to processing and the perspective from which it is viewed.

Preparation of the Raw Pelt for Tanning

Tanning primarily involves the reaction of tanning chemicals with collagen, raw hides contain a number of undesirable components, which are commonly removed prior to tanning. Approximately 75% of the mass purchased as raw material is removed as polluting solid or liquid waste. Preparation of the raw pelt includes the processes from soaking to pickling.

Soaking

Soaking is usually the first process encountered in the tannery, as the previously dehydrated raw material must be carefully rehydrated before it can be subjected to extended mechanical action. Water is also the vehicle for both chemical penetration and removal, and is a necessary prerequisite for most of the processes to follow. Soaking also assists in the removal of curing agents, non-structural proteins and fats. The flesh layer is removed mechanically to aid an even and full penetration of the chemicals to follow. Fleshing is commonly done after slaughter, after soaking, or after liming.

Unhairing and Liming

The majority of leathers produced are treated in order to remove the hair or wool to leave the characteristic pattern of the grain surface (analogous to wood grain). Conventionally, the hides or skins are treated with sodium sulphide and hydrated lime to destroy the keratinous material of the epidermis and hair or wool. Fats are hydrolysed due to the increased pH and the skin structure swells as water is drawn into the fibre network to form a turgid, open-structured, translucent, jelly-like material.

Deliming and Bating

Weak acids are used to lower the pH and to reduce swelling which causes the water to flush out any impurities with it. The skin becomes flaccid and is treated with proteolytic bating enzymes to clean the grain and make the pelt smooth and silky.

Pickling

The bated pelts are finally treated with acid (commonly sulphuric and/or formic acid) to obtain the desired pH for optimal penetration of the tanning agent, and with salt, to suppress swelling when the acid is added. At this stage, the isolated collagen, termed the pickled pelt, is ready for a pretannage or man tannage.

Pretanning, Tanning and Retanning

This includes processes from pretanning and tanning through to retaining via samming, splitting, shaving and neutralisation as a preparation for the dyehouse.

Pretanning

The pickled pelt may be lightly tanned prior to the main tannage to improve the penetration and distribution of the tanning chemicals to follow, to add specific properties into the leather or to stabilise it for mechanical operations such as shaving.

Tanning

The main tannage has the primary function of producing an utilisable material resistant to microbial attack. The most common chemical used in tanning are chromium tanning salts, vegetable tannins and more recently glutaraldehyde. After the tanning agent has penetrated the collagen structure and distributed satisfactorily, it must be irreversibly bound to the collagen (eg a process called basification, in which the pH is raided, is used to bind chromium and glutaraldeyde tanning agents to the collagen).

Mechanical Operations

At some point during production, the leather is split longitudinally to yield an upper grain split and a lower flesh split of desired thickness. In the production of chromium tanned leathers, this process is most commonly performed after basification and samming. The material is then shaved to give a more accurate and even thickness depending on the requirements for the end products eg shoe upper (1.8 mm), garment (0.9 mm) or upholstery leathers (1.1 mm).

Neutralisation

The mechanical operations generally squeeze water out of the leather, so prior to further treatment a wetting back and washing process is used to rehydrate the leather and to remove dirt, shavings or grease that may have been picked up. The majority of the chemicals still to be added to the leather are anionic in nature, whereas the tanned collagen at low pH tends to be cationic in nature. Neutralisation is a process in which the pH is raised and chemicals are added to reduce the stringency of the leather to anionic chemicals such as retanning agents, dyes and fatliquors.

Retanning

The tanned leather is subjected to additional tannages with similar or new tanning materials. These agents may be used to lighten the colour of the leather, to produce a feeling of fullness and to aid in the penetration of dyes. The choice of pretanning, tanning and retanning chemicals is dependent on the properties desired in the final leather, and therefore, on the properties required in the final leather product.

Dyeing, Fatliquoring and Drying

This stage includes preparing the retanned material for finishing by processing through to dried crust.

Dyeing

Chromium tanned leather is blue in colour and must be dyed to obtain the desired colour. The dye acts as a base colour for finishing, and the depth of dye penetration and leather colour are of great importance.

Fatliquoring

Chromium tanned material dries out hard and crusty and is unsuitable for most purposes. Small quantities of oil, present as emulsions known as fatliquors, make a significant difference to the handle, ie the fullness, softness and flexibility, among other factors.

Drying

The retanning, dyeing and fatliquoring chemicals are allowed to penetrate and distribute within the collagen fibre structure before the pH is lowered and the astringency causes them to "fix" to the tanned material. The final binding of chemicals is encouraged by the drying process. Batches of leather are commonly toggle dried on frames in heated tunnels for four to six hours or are vacuum dried individually for two to ten minutes. Drying is usually followed by buffing, conditioning and staking or milling. The resultant curst material is resistant to microbial attack and contains all the leathering properties desired of leather and is ready for finishing.

Finishing

A finish process and finishing chemical must be carefully designed and "married" with the production of the curst to ensure compatibility. The finish may be required to hide defects, to contribute to the leather beauty and properties and to provide fashion effects. Resins, pigments, dyes, handle modifiers, fillers, dullers and other chemicals are added in layers to the surface of the leather by spraying, roller-coating, curtain-coating or by hand. Heated hydraulic or roller presses are used to produce smooth or patterned leathers, depending on customer requirements. Finishing finally completes the leather manufacturing process and the area is then measured and the leather sent for dispatch to a product manufacturer to be turned into shoes, clothing or upholstery.