Modern Methods of Leather Tanning

The first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage applies retanning agents and dyes to the material to provide the physical strength and properties desired depending on the end product. This is known as Retanning. The fourth stage is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired. This final stage is known as finishing.

Preparing hides begins by curing them with salt. Curing is employed to check putrefaction of the protein substance (Collagen) because of the chance of bacterial infection due to the time lag that might occur from procuring it to processing it. It removes the excess water from the hides and skins where water flows from inside because of difference in osmotic pressure. Thus the moisture content of hides and skins get greatly reduced. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing the hides are agitated in a salt water bath for about 16 hours. Generally speaking, methods employed for curing greatly make the chance of bacterial growth unfavorable. Thus curing is also done by preserving the hides and skins at a very low temperature

The hides are then soaked in clean water to remove the salt and mainly to bring back the moisture content to a desirable level so that the hide or skin can be treated with chemicals in an aqueous medium. This process is known as "Soaking" and sometimes a hydrating agent is also employed along with water in a very low percentage for hides and skins which have become very dry.

Liming Process of Hides & Skins

After soaking, the hides and skins are taken for the next operation where these are treated with milk of lime with or without the addition of sharpening agents like sulfide, cyanides, amines etc. The objective of this operation is to; remove the hairs, nails and other keratinous matters. Remove some of the interfibrillary soluble proteins like mucins. Swell and split up the fibers to the desired extent. Remove some of the natural grease, fats and oils. Bring the collagen to a proper condition for satisfactory tannage.

The weakening of hair is dependent on the break down of the disulfide link of the amino acid called cystine, which is the characteristic of the keratin class of protein like hair and wools. The hydrogen atoms supplied by the sharpening agent reduce the cystine molecule to cystine and the covalent links are ruptured. Some of the “unhairing agents” used during liming are; Sodium Sulphide, Sodium Hydroxide,  Sodium Hydrosulfite,  Arsenic Sulphide,  Calcium Hydrosulfide,  Dimethyl Amine and Sodium Sulphydrate. The majority of hair is then removed using a machine, with remaining hair being removed by hand using a dull knife, a process known as scudding.

Depending on the end use of the leather, hides may be treated with enzymes to soften them in a process called "bating". But before bating, the pH of the collagen is brought down to a lower level so that enzymes might act on it. This process is known as "Deliming". Once bating is complete, the hides and skins are treated with a mixture of common salt and Sulphuric acid in case a mineral tanning to be done. This is done to bring down the pH of collagen to a very low level so as to facilitate the penetration of mineral tanning agent into the substance. This process is known as "Pickling". The common salt penetrates the substance twice as fast as the acid and checks the ill effect of sudden drop of pH.

Tanning can be performed with either vegetable or mineral methods. Before tanning, the skins are unhaired, degreased, desalted and soaked in water over a period of 6 hours to 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides, such as pentachlorophenol, are used.

Vegetable tanning uses tannin (this is where the name tanning comes from). Tannin occurs naturally in bark. The primary barks used in modern times are chestnut, oak, tanoak, hemlock, quebracho, mangrove, wattle and myrobalan. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable tanned hide is flexible and is used for luggage and furniture.

Mineral tanning usually uses chromium in the form of basic chromium sulfate. It is employed after picking. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This is known as Basification. In the raw state chrome tanned skins are blue and therefore referred to as "wet blue". Chrome tanning is faster than vegetable tanning (less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments.

Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suede, nubucks, etc. are finished by raising the nap of the leather by rolling with a rough surface.