How is a leather jacket made?
Trimming and cleaning
The skins are trimmed and sorted according to size, weight, and thickness. It is necessary to remove any remaining proteins that could stimulate the growth of bacteria. To this end, the skins are soaked in revolving drums filled with water, bactericides, and detergents. Hair is removed with the application of chemical sprays or lime solutions. A scudding machine fitted with dull blades scrapes off any excess hair.
Another round of washing (de-liming) removes these chemicals before the hides are soaked in an acid solution and then bated, an enzyme treatment to further remove the skins' collagens. Finally, the hides are pickled with salt and sulfuric acid.
Tanning
The hides undergo one of three types of tanning. Vegetable tanning requires the most time with the skins soaked in progressively stronger tannic acid solutions for several weeks. However, some soft leathers, such as lambskin, may be soaked for as little as 12 hours.
Mineral tanning is significantly faster but can change the color of the leather. For this method, the skins are placed in alum salt-filled drums fitted with paddles that provide a constant agitating motion.
The third method, oil tanning, is the one that resembles the ancient methods most closely. Here, fish oil is sprayed onto the skins.
Washing and drying
After the skins have been tanned, they are washed once again and wrung out thoroughly. Then the skins are passed under a band knife, which cuts the skins to a uniform thickness, and placed on conveyer belts which carry them to drying tunnels. Usually, the skins are stretched on frames to prevent shrinkage during the drying stage. To combat the stiffness that drying produces, the hides are sprayed with water and soap and allowed to hang for a period of time.
After the skins have been conditioned, they are placed in machines designed to rhythmically manipulate the leather so that the fibers are further loosened and made more flexible. In the final drying period, the hides are hung in vacuum-drying cabinets.
When the skins are thoroughly dried, they are buffed with revolving steel cylinders covered with abrasive paper. Suede finishes are produced by passing the hides under high-speed emery wheels. At this point, glazes, dyes, and lacquers are applied. The skins are now ready to be sent to the garment factory.
The Chemical Reporter - How is Leather Produced?