Leather

Leather is made from animal skins or hides that are chemically treated. This is called tanning. When prop­erly done, the resulting product is strong, flexible leather that is able to resist decay and spoilage.

The skins are typically cured, a process that involves either salting or drying the hide as soon as it is removed. In the so-called wet salting process, the skins are rubbed with salt, stacked, and bundled together. After about a month, most of the salt is absorbed into the skin. An alternative is to pack the skins in vats with a mixture of salt and disinfectants. This process can be completed in less than 24 hours and is called brine curing. After curing, the hides are allowed to soak in water to remove excess salt and debris. In the next step the preserved flesh is usually stripped away mechanically.

Skins are usually then moved to large vats and again soaked up to 2 weeks in a mixture of lime and water. This process loosens the hair and makes subse­quent removal easier. Bits of hair and fat that are missed by machinery are removed by scraping the skins by hand with a plastic scraper or dull knife. This is called scudding. Once the skins are cleaned, they pass to a vat containing acid. This acts to remove excess lime. The hides are then typically treated with enzymes to smoothen the grain of the leather and to make the skin softer and more flexible. The tanning process follows.

Hides can be subjected to a variety of steps at this point, depending on the desired product. For example, vegetable tanning produces leathers that are flexible but stiff. This material would be used in belts, luggage, or furniture. In these cases, the hides are often stretched onto frames and suspended in vats containing various tannins. These are agents found in bark, wood, and leaves. Tannins from oak, chestnut, or hemlock trees are frequently used. The hides are often transferred to multiple vats containing progressively stronger con­centrations of tannins. Mineral or chrome tanning is an alternative process used to produce leathers for use in shoes, gloves, and clothing. After curing, scudding, and lime removal, the hides are soaked in a chro- mium-sulfate solution. Depending on the desired product, the hides can then be dyed. This not only adds color as desired, it also adds moisture to increase softness and flexibility. Vegetable-tanned hides are usually bleached and then soaked with oils and soaps to increase flexibility.