How Leather is Made
Most leather is tanned using chrome tanning salts. Only a small amount of shoe sole, saddle, and belt leathers are still tanned by the traditional vegetable tanning process which uses vegetable tannins extracted from tree bark as the tanning agent.
The following is a brief outline of how chrome tanned leather is made:
Soaking and Liming
The salt-cured (or more often nowadays, fresh or "green") hides are soaked in water in large, slowly turning drums to wash out the salt and return the hide to the same condition as it was on the living animal. After a minimum of four to six hours, the water is drained off and fresh water is added. Lime and sodium sulphide are added and the drum is run intermittently every hour for five minutes for the next fourteen to eighteen hours. This removes the hair and other unwanted protein in the hide and opens up the fibre structure.
The following morning the hides are scraped clean of excess fat on a "fleshing machine" and the limed pelts are then loaded into another drum for further processing. Upholstery hides at this stage are also split into two layers, a grain split and a drop or suede split.
Deliming, Bating, Pickling and Tanning
In the drum the hides are firstly washed and then in a fresh float, ammonium sulphate is used to remove excess lime from the pelt. An enzymatic bating agent is added to further open up the fibre structure and clean the grain and then the hides are washed again with cold water.
A small amount of fresh water is now run into the drum and common salt (sodium chloride) and sodium formate buffer are dissolved in it. Sulphuric acid is pumped into the moving drum through the axle and the drum is run for a further one and a half to three hours. The salt prevents the acid from damaging the hides by preventing swelling. Chrome tanning salt (a green powder) is added and the drum is run for 1-3 hours, after which magnesium oxide is added and the drum is run overnight to an end pH of 3.6-4.0 and an end temperature of about 40 degrees Celsius. The following morning the hides have been tanned and can now be called wet blue. Chrome-tanned (wet blue) leather has a characteristic bluish colour.
After being piled on "horses" or pallets for a day to drain, the hides are "sammed" (ie. squeezed through a machine like a giant mangle) to remove excess moisture, sorted visually into various grades, then split through a bandknife splitting machine and finally shaved to the correct thickness. The drop (suede) split is trimmed and shaved for either suede leather, finish coating or industrial gloving split leather.
Neutralising, Retanning, Dyeing and Fatliquoring
The shaved wet blue is now made up into lots for dyeing. Because the chrome tanning process is acidic, the leather is first neutralized with sodium bicarbonate and sodium formate or acetate, after which it is retanned with synthetic tanning agents called syntans, resins and natural tannins, which impart the desired properties to the leather being made. The leather is then dyed to the required shade and finally "fatliquored"-natural and synthetic oils are taken up by the leather to replace the natural greases removed in the preceding processes, so that the fibres will be lubricated when the leather is dried.
Drying, Conditioning, Staking and Buffing
The leather is again horsed up overnight to drain, set out on a setting machine to remove wrinkles and excess water, and then dried. Various methods are employed:
"Corrected Grain" sides are usually paste-dried. They are pasted onto glass or steel plates, which then pass through a drying tunnel.
Alternatively, they are vacuum-dried by slicking out on a smooth heated stainless steel table and then lowering a head onto the table and removing the moisture by vacuum suction at a temperature well below the boiling point of water.
"Full Grain" sides are generally toggle-dried - stretched out on frames by means of toggle clips before passing through a drying tunnel.
Hung up to dry with no stretching.
After drying, the sides are sprayed lightly on the back (flesh) side with water and piled under plastic to condition and even out the moisture content for a few hours. They are then staked, or softened, and the corrected grain sides are then buffed, or sandpapered, to remove, or at least minimize, surface defects. Full grain sides are not buffed, but are left with the natural grain pattern and are therefore made from top quality hides. Before being sent for finishing all leather is again sorted into various grades to ensure that the wet blue sorting was correct.
Finishing of leather may be likened to the paint shop in a motor assembly plant. Here various pigments, resins, binders, lacquers and dyes are applied by curtain-coater, pad, roller-coater and spray machines to give the leather its final colourful, scuff-resistant look and feel. Several coats of finish are applied to the leather surface.
Leather is often softened after finishing by milling in a milling drum for a few hours.
Finally, the area of the leather is measured and the leather is sent to the warehouse for final sorting and packing before dispatching.
Automotive and furniture upholstery leather is processed throughout as whole hides, while shoe upper leather is normally processed as sides (a hide is cut down the backbone into two sides).