Usually, the animal is killed and skinned before the body heat leaves the tissues. The freshly removed skin or hide is immediately cured with salt to remove water. The skin is allowed to remain in the shade until it is completely dry. The cured skin or animal hide is then transported to the tanneries for further processing.
A. Preparatory Steps
The cured skin or animal hide needs to be prepared for the tanning process. Although there are various preparatory processes, the purpose of each process is to remove unwanted raw skin components. The tannery may not perform all of them, depending on the quality and type of the desired product.
At this stage, the cured hide is soaked in water for several hours to several days. This process not only restores the moisture lost during salting, but helps to remove dirt, debris, blood, and excess animal fats.
This process removes subcutaneous material from the flesh side. The pelt is passed through a machine to remove the fat, muscle, and flesh mechanically. Usually, this process takes place after slaughter, soaking, or liming. At this stage, or after tanning, hides may be split into different layers.
Hair is removed at this stage using mechanical instruments such as rollers and blades.
This process involves cleaning and soaking the rawhide in acids or salts to prevent decomposition. It helps the penetration of tanning agents such as chromium and aldehydes. Stronger pickling agents are used to preserve hides for several months.
The hide is soaked in sulfuric acid to lower the pH after pickling.
This process loosens the fibers and allows the skin to absorb various tanning chemicals. Usually, sodium sulfide and hydrated lime are used to treat the hide, as they remove keratinous material such as hair and wool. Fats get hydrolyzed as the pH increases. Water is absorbed into the skin fibers, resulting in a swollen skin structure.
In this process, the hide is washed with a mixture of water and ammonium chloride or ammonium sulfate. This removes water (along with any impurities) to reduce swelling.
Bating marks the end of the liming process. The flaccid skin is treated with proteolytic bating enzymes to remove non-fibrous proteins. It cleans the grain and makes the pelt smooth and silky.
Sometimes, water-based solutions and solvents are used to remove excess grease or natural fatty acids from the skin.
Chemical agents are used to making the hide colorless to add the desired color at a later stage.