Tanning is the ancient process of making animal skins into leather. For best results the animal is skinned right after death, before losing body heat. Flaying can be done by the tanner, or skins are acquired from a slaughterhouse, farm or trader.
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Skins are often dried stiff and grimy. Ancient tanners soak the skins to clean and soften them. Next they pound, scrape and scour the hide to remove flesh and fat. Flint hide-scraping tools are often found at Neolithic sites.
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Historically the tanning process is by vegetable tanning. Cedar oil, alum (a sulfate salt), or tannin extracts are applied to the skin as tanning agents. Tanning uses the same materials for thousands of years. Chemicals like mercury and chromium become popular after 1840.
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After soaking, hides are treated with milk of lime and a mixture of sulfides and cyanides to remove hair. The process swells and splits fibers and conditions the collagen in the hide. It breaks down cystine, a compound strengthening feathers, hooves, horns and hair.
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Hair may be stripped off by soaking the skin in urine. Another method involves letting the skin putrefy for several months. The tanner then dips it in a salt solution. After the hair is loosened, the tanner scrapes it off with a knife or stone scraper.
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Before chemical discoveries, tanners use plant and animal matter. After denuding, the hides are cured with salt to prevent bacterial growth and collagen rot. Curing removes water from the hides and skins. Similar processes are used in ancient Egyptian mummification.
In wet-salting, hides are heavily salted, then pressed into packs for about 30 days. In brine-curing, the hides are agitated in a saltwater bath for about 16 hours. Curing can also be done by preserving hides and skins at low temperatures.
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During tanning, hides are subject to a series of processes including soaking, liming, removal of any extraneous tissues (unhairing, scudding and fleshing), deliming, bating or puering (treating with heated dog dung), drenching, and pickling.
Soaking involves immersing the hides in clean water to remove any salt left from curing, and to raise the moisture content to further treat the hide. To prevent damage of the skin by rot or bacterial growth during the soaking period, fungicides or other biocides are used.
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From ancient times to the 18th century, tanners use the chemical compound tannin, derived from tree bark, resin and leaves. Hides are stretched out on frames and immersed in vats with concentrated amounts of tannin.
Tannins bind to collagen proteins in the hide. Coating the proteins makes them water and bacteria resistant. At the same time, tannins help soften the hide and make it more flexible. Tannins include natural fungicides to repel rot.
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Once the hair is completely removed, the tanners soften or bate the hide by pounding dung into the skin, or soaking it in a solution of animal brains. Bating or puering is a process of fermentation using enzymes produced by bacteria in dog or pigeon dung.
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After the treatment breaks down hair proteins, any remaining hairs are manually scraped off. A layer called grain, the part of the skin with hair follicles, is also scraped off. Bacterial fermentation with dung isn't needed in modern tanning.
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A process called braining can also be used to soften and condition the hides. Fat and lecithin in mammalian brains create an effective tanning solution.
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Native North Americans are known for using animal-based tanning methods. Brains are a major ingredient, but egg yolks can be substituted if desired.
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In a solution of:
454 g (1 lb) brains, or a dozen egg yolks, well blended
5.5 liters (1.5 gal) hot water
60 ml (1/4 cup) vegetable oil or melted fat
... hides are soaked for fifteen minutes to twenty-four hours. The skin can be wrung out and re-soaked several times. Afterward the hides are stretched and given a wash of alum.
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The hides can also be pickled. Pickling just means tanning, or turning rawhide into leather. Hides and skins are soaked in a salt bath of hot water. When the water cools, 28 g (1 oz) of sulfuric acid is added.
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In the ancient world corrosive chemicals such as ammonium chloride (NH4Cl) and zinc chloride (ZnCl2) are known in the Bronze Age. Sulfuric acid is discovered in the 8th century CE. Before that, natural tannins are largely used.
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In the pickling mix, skins are soaked for a few days up to 2 months, depending on size, condition or intended use of the skins. In vegetable tanning, the hides soak in a solution of vegetable tannins, such as found in gallnuts, sumac leaves, and green shells of walnuts.
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Tannins for vegetable tanning occur naturally in plant bark and leaves. Primary tannin barks include chestnut, oak, redoul (Coriaria myrtifolia), tanoak, hemlock, quebracho, mangrove and acacia.
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In Ethiopia, the combined vegetable oils of Niger seed (Guizotia abyssinica) and flax seeds are used in treating the flesh side of the leather, as a means of tawing (tanning with alum), rather than vegetable tanning.
In ancient Yemen and Egypt, hides are tanned by soaking them in a bath of crushed leaves and bark of the Salam acacia (Acacia etbaica). Hides that have been stretched on frames are immersed for several weeks in vats of increasing concentrations of tannin.
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Vegetable tanning takes longer than mineral (chemical) to convert rawhides to leather. Mineral tanned leather is used for shoes, car seats and home upholstery. Vegetable tanned leather is best for leathercraft and for small items, such as wallets, handbags and clothes.
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