Many consumers own a variety of leather products, ranging from shoes to jackets, and the versatile material has been used for thousands of years in a variety of applications. The process for making leather includes a complex set of steps from skin to finished product. These steps are often referred to as “tanning” due to the most distinctive part of the process, which uses the tannic acids present in plant matter to chemically alter the leather so that it will not putrefy or rot.

If used without any treatment, animal hides would quickly break down and begin to stink. For this reason, the hide needs to be treated to form leather. The process can be broken down into two basic stages: wet blue processing and finishing, both of which are accomplished in large commercial spaces known as tanneries. While it is possible to make leather at home, it is a malodorous and complicated process, and most people prefer to send out their hides for tanning.

Skin has three layers beginning with the epidermis, proceeding to the derma, and ending in the adipose, which is also known as flesh. When making leather, only the strong and flexible derma, which is a layer of collagen fibers, is desired. The collagen is exuded by cells, and forms a network of useful filaments. When leather is processed, these fibers are retained while everything else is stripped away.

First, the skin is fleshed, the flesh being cleaned off of the hide, and the excess adipose, or animal fat, is removed. Then the hides are washed and soaked, which restores moisture to dried hides while loosening blood and dirt. Chemicals are added to turn the water alkaline, which will eat away at the hair and epidermis. The collagen fibers begin to swell, while excess proteins precipitate out.

When this process is complete, the water is brought to a stable pH and the swelling goes down, so that enzymatic cleaners can be added to remove any remaining organic debris. Then the hide is pickled in a highly acidic solution, which prepares it for tanning. Tanning agents form bonds with the collagen in the hide, causing it to resist bacterial attack, and the hides are removed from the wet blue processing tank for finishing, after being run through rollers to remove excess water.

When the hides emerge from the wet blue stage, they are split to the desired thickness. Depending on the intended use of the leather, this thickness may vary. Then the leather is re-tanned, with different materials depending on whether it needs to be firm or soft, and dyed. After dyeing, the leather is oiled so that it will remain flexible and soft. Then the leather is dried, mechanically treated to soften it, and buffed so that it will have a smooth and attractive surface. Some leathers are embossed with patterns before the final stage, which is the application of a finishing coat of polymer or wax to protect the surface of the leather.

Leather treatment used to be a highly polluting industry, but tanneries responded to public protest about odor and pollution issues. Tanneries now recycle the liquids used in the leather making process and refrain from chemical dumping. Tanning still carries a strong odor, but is no longer accompanied by environmentally unfriendly business practices in most parts of the world.