The story of leather goes as far back as humanity itself. In prehistoric times, people used to wrap themselves in dried animal pelts. The fact that the skins turned stiff and rotted was a problem, but ways of softening and preserving the hides were discovered. This was the beginning of leather processing. As civilization advanced, preserving hides and tanning them into leather became an important industry.
The hides of cattle are the main source of leather. Leather is a by-product of the meat industry. Therefore no cow loses its life for its skin. The leather process converts these hides into a viable and useful material. Because of the by product nature of leather, it puts far less demand on the earth than synthetic leathers, which are produced from plastics and petroleum based chemicals. The leather process can be summarized in 5 basic steps.
After the hide has been removed, it is fleshed, removing any remaining meat tissue or fat. Freshly fleshed hides are shipped in refrigerated trucks to a tannery for immediate processing into leather. The hair is removed by chemical digestion using a solution of lime and sodium sulphide with occasional rotation. After the hair has been removed the hides are neutralized with acids and treated with enzymes to remove any deposits and to increase softness. The next operation is called pickling. which involves soaking the hides in a solution of water, salt and acid.
Tanning is the final process in turning hides and skins into leather. There are several methods of tanning but the most common are Chrome and Vegetable Tanning. Chrome is used most often. Most upholstery, shoe uppers, garments and bag leathers are chrome tanned. The process begins in rotating drums with a bath in a chemical containing trivalent chrome. It usually takes eight hours for the chrome to penetrate all the way into the hide. Once this has been achieved the chrome is fixed by adding an alkaline chemical such as sodium carbonate or bicarbonate. After this treatment the hide is considered tanned. Vegetable tanning is used for such various products as shoe soles, luggage, saddlery, belt leathers and some upholstery. The process is slower than chrome tanning and involves the chemical substance tannin, or tannic acid, which is extracted from the barks of trees. This process is normally performed in drums taking two to four days.
Splitting and Shaving
Splitting and shaving are the next steps following tanning. Depending on the end use of the leather product, the hides are split into sheets of required thickness and processed further through a shaving machine for added quality. After shaving, chrome tanned hides are again placed in rotating drums with hot water, dyes and synthetic tanning materials to obtain the desired colour. They are then lubricated with natural fat or synthetic fatty type chemicals, or a combination of both to obtain the softness required by the final product.
The next process is setting, which is the removal of excess water and spreading the hide out prior to drying. There are several methods of drying depending on the type of leather being produced. Upholstery leather is normally toggle dried (spreading the leather over expanding frames held by toggles or clips) Staking is the mechanical softening of the leather after drying. The hides may also be softened by milling which is dry tumbling with atomized moisture injected into the tumbler.
Finishing consists of placing a series of coatings on the surface of the leather. These coatings are designed to protect the leather and produce surface effects pleasing to the eye and hand. Finishing today reflects the latest technology in the use of coating materials. Various mechanical operations are necessary to obtain the desired effect. Hydraulic presses, printing, embossing machines, automatic spray applicators and vacuum driers are a few of the machines used in the finishing process. The end use of the leather determines the type of finish process to be applied. Each type requires different physical properties in the finish. Flexibility and resistance to water and wear are a few of the required properties in the finish.