Jeffrey Aquilante, age 11, of Bridgeport, Conn., for his question:
How is leather treated?
Most of our leather shoes are made from the skins of cattle, but there is a big difference between a glossy black boot and the velvety coat of a cow. The difference involves a series of leather treatments, the most important of which is a process called leather tanning.
The most durable leathers are made from the hides of horses and cattle, sharks and camels. Softer leathers are made from the skin of a doe or chamois, sheep or goat. The work of changing the animal skin into lasting leather is done in a plant called a tannery. The actual tanning is just one operation in the treatment, and almost every tannery has its own methods and recipes for performing the various operations.
There are, however, certain basic chores that must be done in the treatment of all leathers. The skin, of course, must be removed from its original owner with as little damage to it as possible. Its hair, bristles and the bits of flesh clinging to it must be removed. The dried hides are then soaked in some kind of tanning solution that changes the skin into leather. The leather must then be softened and surfaced.
The fresh skins are cured in salt solutions to prevent decay and then tied in great bundles to be sent to the tannery. In the first step of the treatment, rollers or scrapers or whirling knives are used to remove the fleshy fat clinging to the underside of the hides. The next step requites a vat of lime solution with perhaps some dissolved sodium sulphide. Here the hides are soaked from three to seven days. This softens the hair, which then can be removed by blunt blades and scrapers. The skins are then washed to remove the lime, and new substances may be added to the wash water to neutralize the other hair softening chemicals.
The clean hides are then steeped in a vat of tanning chemicals. Many of these chemicals are tannins soaked from the wood, leaves and barks of various plants. Many tanneries also use tanning baths of mineral chemicals, such as alum and salty compounds of chromium. The tanning may take weeks, and when it is done the hides and skins have been transformed into durable, pliable, waterproof leather.
The leather hides must be softened with oils or grease, pounded with hammers and squeezed through rollers. Some will be given glossy surfaces, and others will be Scuffed with gritty rollers to give velvety suede finishes.
Leather tanning is an ancient art, and dozens of different methods have been used through the ages. Chemical salts were introduced in modern times, and in the past only natural tannin chemicals were used. Most plants contain the bitter, rot preventing chemicals called tannins. We soak them from the leaves and nuts, wood and bark of oak and hemlock, sumac and chestnut. In the tropics, tannin is soaked from the bark of the mangrove tree, and Australian leather is tanned with tannins from the acacia wattle.