Article: Tanning

This page is about making hides into leather. For the natural darkening of human skin, see sun tanning.

Tanning is the process of making leather from skin. This is commonly done with the acidic compound tannin, which prevents normal decomposition and often imparts color.

Modern methods of tanning

Tanned leather in Marrakech

Tanneries of Marrakech

The process of dressing up animal skin/hide into leather consists of three stages. The first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage applies finishing to the surface.

Preparing hides begins by curing them with salt. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing the hides are agitated in a salt water bath for about 16 hours. The hides are then soaked in clean water to remove the salt and a lime/water solution to loosen the hair. The majority of hair is then removed using a machine with remaining hair being removed by hand using a dull knife, a process known as scudding. Depending on the end use of the leather, hides may be treated with enzymes to soften them.

Tanning can be performed with either vegetable or mineral methods. Before tanning, the skins are unhaired, degreased, dessalted and soaked in water during 6 hours to 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides, such as pentachlorophenol, are used.

Vegetable tanning uses tannin, from which tanning gets it name. Tannin occurs naturally in bark. The primary bark used in modern times is chestnut, oak, tanoak, hemlock, quebracho, mangrove, wattle, and myrobalan. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable tanned hide is flexible and is used for luggage and furniture.

Tanneries at Fez

Mineral tanning usually uses chrome. In the raw state chrome tanned skins are blue and therefore referred to as "wet blue". Chrome tanning is faster (less than a day for this part of the process) than vegetable tanning and produces a stretchable leather excellent for handbags and garments. (Encarta, 2003)

Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suedes, Nubucks, etc. are finished by raising the nap of the leather by rolling with a rough surface.

Ancient methods of tanning

In ancient history, tanning was considered a noxious trade and relegated to the outskirts of town, amongst the poor. The ancients used leather for waterskins, bags, harnesses, boats, armor, quivers, scabbards, boots, and sandals. Around 2500 BC, the Sumerians began using leather, affixed by copper studs, on chariot wheels.

Tanners would take an animal skin and soak it in water. Then they would pound and scour the skin to remove flesh and fat. Next, either they soaked the skin in urine to loosen hair fibers or they let the skin putrefy for several months, after which they dipped the skin in a salt solution. After the hair fibers were loosened, the tanners would scrape them off with a knife.

Once the hair was removed, tanners would bate the material by pounding dung into the skin or soaking the skin in a solution of animal brains. They would also take cedar oil, alum, or tannin and stretch the skin as it lost moisture and absorbed the tanning agent.

Leftover leather would be turned into glue. Tanners would place scraps of hides in a vat of water and let them deteriorate for months. The mixture would then be placed over a fire to boil off the water to produce hide glue.

Variations of these methods are still used by do-it-yourself outdoorsmen to tan hides. The use of brains and the idea that each animal has just enough brains for the tanning process have led to the saying, "Every animal has just enough brains to preserve its own hide, dead or alive."

Another use

The term tanning is also used metaphorically for a hiding in the sense of severe physical punishment which leaves clear marks (reddening, stripes, or even scars) on the beaten skin.