Leather is basically treated animal skin, called hide. Most leather comes from cattle hide, but other animals are used as well. Horses, pigs, goats, etc. are also used to create leather. The process of turning raw hide into finished leather is called tanning, though the process is complex. It’s called tanning because of the most easily-recognizable part of the process, which uses the tannic acids in plant matter to alter the hide so it doesn’t rot. As you can imagine, if raw animal hide were used, it would not only be messy and smelly, but eventually the hide would putrefy.
Tanning leather is performed in plants called tanneries. The tanning procedure can be divided into two broad steps: wet blue processing and finishing. Animal hide, like human skin, has three layers. These layers are the epidermis at the top, the derma in the middle, and the adipose at the bottom. To make leather, only the strong and flexible derma is desired. The derma layer is strong and flexible because it is made up of a protein called collagen, which is used in connective tissue throughout the bodies of animal and humans. This makes it ideal for making leather, because it will make the finished product more durable, something leather is famous for.
In wet blue processing, the hide is first fleshed, which means the epidermis and adipose are removed. The hide is then washed and soaked, removing any remaining blood and dirt. Alkalinizing chemicals are added, which remove the hair. The fibers of the collagen start to swell and become thicker. When this stage is completed, the alkalinizing chemicals are neutralized to allow enzymatic cleaners to remove any additional organic debris. Next, the hide is pickled in an acidic solution, which prepares it for the actual tanning. Tanning agents are then added that bond with the collagen in the hide, which builds up a resistance to bacterial attack.
When the processing is done, the hide undergoes finishing. First the hide is split to the desired thickness. The precise thickness achieved depends on the intended use of the leather. Leather to be used in furniture is thicker; while leather used to make backpacks and purses is a little thinner. The leather is then retanned using different agents to make it either firmer or softer and to prepare it for dyeing. After the leather is dyed it is oiled so that it will remain flexible. Finally, the leather is dried and buffed to give it a smooth surface, and it is waxed or coated with a polymer to protect the surface of the leather.
