While the exact specifications and procedures for tanning vary considerably, depending on the type of skin and its application, the basic processes are common to all tanning operations.
The skins and hides are received at the tannery in a cured form, which means they have been treated with salt to prevent rot before they leave the meat packing plant. The hides are then immediately soaked in water to soften them and to remove the remaining salt solution. The soaking period varies from two to forty eight hours.
The next step is fleshing. Machines equipped with a rubber roller and a shaft to which spiral knives are attached remove the flesh and tissues from the inner side of the skin. These knives leave a clean, uniform surface.
After fleshing, workers transfer the skin to a department of the tannery known as the beam house. Here unhairing is achieved by soaking the hides in a vat with a solution of lime and sodium sulfate. The hides are milled or kept in motion in the vat for several days.
The next operation involves removing the lime from the skins. After washing with cold water they are placed in a bating vat which contains an enzyme and a sulfate or chloride. Bating also softens the texture of the hide during this 3- 4 hour treatments.
At this stage, the hides enter one of two possible processes of tanning: Vegetable or Chrome tanning.
Leathers for shoe soles, heavy cases, harnesses and most upholstery applications are prepared by vegetable tanning. Many plants and barks contain a bitter ingredient called tannin. It has the property of combining with proteins to form a compound that will not rot or decompose easily. In this case, the protein is the hide and after tannin is added, the compound is leather. The principal sources of tannin are leaves, nuts, bark and woods of hemlock, oak, chestnuts and various other types of trees.
Chrome tanning is used for tanning the upper leather of shoes, handbags, wallets and garments. Prior to chrome tanning, the hides must be pickled after the bating step. Pickling involves soaking the hides in a solution of salt and acid for several hours to achieve a low pH level. This is necessary because the chrome tanning agents that are to follow are not soluble otherwise.
The pickled skins or hides are then placed in a tanning drum containing a solution of common salt, soda and acid. The chemical reaction of the compounds tan the hide and after 5-10 hours, the conversion to leather has been effected.
After the leather has been tanned by either process it is then split and shaved to a uniform thickness appropriate for the intended product.
Dyeing or coloring is achieved by placing the leather in another drum with a combination of coloring materials and chemicals to increase their penetration. This process may take several hours.
Fat Liquoring is the last step in the "wet" stage and requires about one hour. Here the leather is placed in a drum with a variety of oils and greases. This step and the combinations of oils employed, determine the pliability of the leather.
The leather is then dried to remove all excess moisture. A number of different methods are used, each having a different "de-hydration" level which influences the characteristics of the final product.
Often the leather is then buffed. All skins have natural healed scratches or blemishes which attest to the genuiness of leather. However, to improve its final appearance, it is often desirable to lightly sand or buff the grain surface. If the leather is not buffed, the leather is top or full grain leather.
Finishing involves the application of film-forming materials to provide abrasion and stain resistance and to enhance color. Pigments are also added when a more opaque or vivid coloring effect is required. However, with unbuffed or top grain leather, usually only a light, transparent coating is applied. This is known as aniline dyeing. Of the two, aniline finished leathers are the finest quality.
The final processing step to influence the appearance and feel of the leather is called plating. The plating operation is done on a press capable of exerting up to 300 tons per square inch. The plating smoothes the surface of the coating materials just applied and firmly affixes them into the grain. At the same time, the plate may be specially engraved to emboss a particular pattern on the leather.
As has been indicated several times throughout, the specifications and required characteristics of the tanned leather is determined by the end use of product for which it is intended. Tanneries produce to order for a wide variety of final goods manufactures; as with any sort of product, there are huge ranges in quality of both materials and workmanship in leatherwoods.
In 1995, total world leather production was approximately 7,000,000 metric tons of cowhide; 1,400,000 metric tons of lambskin/shearling/sheepskin; 800,000 metric tons of pigskin and 450,000 metric tons of goatskins. It has been estimated that more than 2,000,000 people in the world are employed in the various branches of the tanning and leather industry.
Today, the leather tanning industry stands out as perhaps the most productive by-product industry in the world. It is hard to imagine the environmental impact of the additional synthetic product manufacturing which would be required to replace all of the current applications for leather, suede and shearling. It is impossible to imagine how any of these synthetic substitutes could ever match the esthetic appeal and aura of genuine leather.
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