Tannery

      Tanning is the process of treating skins of animals to produce leather, which is more durable and less susceptible to decomposition. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name (tannin is in turn named for an old German word for oak or fir trees, which supplied it). Coloring may occur during tanning. A tannery is the term for a place where the skins are processed. Tanning leather involves a process which permanently alters the protein structure of skin. Making "rawhide" (untanned but worked hide) does not require the use of tannin. Rawhide is made by removing the flesh and fat and then the hair by use of an aqueous solution (this process is often called "liming" when using lime and water or "bucking" when using wood ash (lye) and water), then scraping over a beam with a somewhat dull knife, then drying. The two aforementioned solutions for removing the hair also act to clean the fiber network of the skin and allow penetration and action of the tanning agent, so that all the steps in preparation of rawhide except drying are often preludes to the more complex process of tanning and production of leather. 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,typically dithiocarbamates are used. Fungicides such as TCMBT, 2-(Thiocyanomethylthio) benzothiazole, are added later in the process to protect wet leathers from mould growth. After 1980 the use of pentachlorophenol and quicksilver (mercury base) biocides and their derivatives was forbidden
Vegetable tanning       Vegetable tanning uses tannin (this is the origin of the name of the process). The tannins (a class of polyphenol astringent chemical) occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them causing them to become less water-soluble, and more resistant to bacterial attack. The process also causes the hide to become more flexible. The primary barks, processed in Bark mills and used in modern times are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle (acacia; see catechu), and myrobalan[disambiguation needed ]. 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.

Preparatory steps prior to tanning
     
Curing
      Preparing hides begins by curing them with salt. Curing is employed to prevent putrefaction of the protein substance (collagen) from bacterial growth during the time lag that might occur from procuring the hide to when it is processed. Curing removes excess water from the hides and skins using a difference in osmotic pressure. The moisture content of hides and skins gets 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, curing substantially reduces the chance of spoilage by bacteria. Curing can also be done by preserving the hides and skins at a very low temperature.

Beamhouse operations
      The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They include, in order, soaking, liming, removal of extraneous tissues (unhairing, scudding, and fleshing), deliming, bating (including puering), drenching, and pickling

Soaking
      In the process known as soaking, the hides are soaked in clean water to remove the salt left over from curing and increase the moisture so that the hide or skin can be further treated.

Liming
      After soaking, the hides and skins are taken for liming: treatment with milk of lime (a basic agent) that may involve the addition of "sharpening agents" (disulfide reducing agents) like sodium sulfide, cyanides, amines etc. The objectives of this operation are mainly to:
  • Remove the hairs, nails and other keratinous matter
  • Remove some of the interfibrillary soluble proteins like mucins
  • Swell up and split up the fibres to the desired extent
  • Remove the natural grease and fats to some extent
  • Bring the collagen in the hide to a proper condition for satisfactory tannage
      The weakening of hair is dependent on the breakdown of the disulfide link of the amino acid called cystine, which is the characteristic of the keratin class of protein that gives strength to hair and wools (keratin typically makes up 90% of the dry weight of hair). The hydrogen atoms supplied by the sharpening agent weaken the cystine - cysteine molecular link, and the covalent disulfide bond links are ruptured, which weakens the keratin. To some extent, sharpening also contributes to unhairing, as it tends to break down the hair proteins.

Liming
      Unhairing agents used at this time are: Sodium sulfide, sodium hydroxide, sodium hydrosulfite, calcium hydrosulfide, dimethyl amine, and Sodium sulfhydrate. The majority of hair is then removed mechanically, initially with a machine and then by hand using a dull knife, a process known as scudding.

Deliming and bating
      The pH of the collagen is brought down to a lower level so that enzymes may act on it, in a process is known as deliming. Depending on the end use of the leather, hides may be treated with enzymes to soften them, a process called bating.

Pickling
      Once bating is complete, the hides and skins are treated with a mixture of common (table) salt and sulfuric acid, in case a mineral tanning is 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 (sodium chloride) penetrates the hide twice as fast as the acid and checks the ill effect of sudden drop of pH.

Tawing
      Tawing is a method that uses alum and aluminium salts, generally in conjunction with other products such as egg yolk, flour, and other salts. The leather becomes tawed by soaking in a warm potash alum and salts solution, between 20°C and 30°C; . The process increases the leather's pliability, stretchability, softness, and quality. Adding egg yolk and flour to the standard soaking solution further enhances its fine handling characteristics. Then, the leather is air dried ("crusted") for several weeks, which allows it to stabilize. Tawing is traditionally used on pigskins and goatskins to create the whitest colors. However, exposure and aging may cause slight yellowing over time and, if it remains in a wet condition, tawed leather will suffer from decay. Technically, tawing is not tanning
      Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suedes, nubucks etc. are finished by raising the nap of the leather by rolling with a rough surface
      The first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage, known as retanning, applies retanning agents and dyes to the material to provide the physical strength and properties desired depending on the end product. The fourth and final stage, known as finishing, is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired.

Crusting
      Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations. Crusting may include the following operations:
  • wetting back - semi-processed leather is rehydrated.
  • sammying - 45-55%(m/m) water is squeezed out the leather.
  • splitting - the leather is split into one or more horizontal layers.
  • shaving - the leather is thinned using a machine which cuts leather fibres off.
  • neutralisation - the pH of the leather is adjusted to a value between 4.5 and 6.5.
  • retanning - additional tanning agents are added to impart properties.
  • dyeing - the leather is coloured.
  • fatliquoring - fats/oils and waxes are fixed to the leather fibres.
  • filling - heavy/dense chemicals that make the leather harder and heavier are added.
  • stuffing - fats/oils and waxes are added between the fibres.
  • stripping - superficially fixed tannins are removed.
  • whitening - the colour of the leather is lightened.
  • fixation - all unbound chemicals are chemically bonded/trapped or removed from the leather
  • setting - area, grain flatness are imparted and excess water removed.
  • drying - the leather is dried to various moisture levels (commonly 14-25%).
  • conditioning - water is added to the leather to a level of 18-28%.
  • softening - physical softening of the leather by separating the leather fibres.
  • buffing - abrasion of the surfaces of the leather to reduce nap or grain defects.