Leather Tanning

Tanning turns a raw animal hide, which soon rots when wet and dries hard as wood, into leather that stays supple and resists decay far better. In the pit method the hide is cleared of hair and flesh, then left for months in water and crushed oak bark, whose tannins bind to the fibres of the skin.

What it is

Leather in section, enlarged: the smooth grain on top, the tangle of collagen fibres below. Beside it, raw hide with the hair on.

A fresh hide is mostly water and collagen, a protein woven into a dense mat of fibres. In warm weather it starts to rot within days. If it dries first, it becomes rawhide, hard and horny, which softens in water and rots if it stays wet.

Leather is a hide whose fibres have been changed to resist both. It can get wet and dry again with far less risk of rotting or setting stiff, it can be cut, stitched and moulded, and kept dry it lasts for centuries. Shoes, harness, buckets and bellows were made from it. Hunters softened skins with fat and smoke, later craftsmen used alum, and most leather today is tanned with chromium salts. This plate follows the method that served Europe and the Mediterranean for some two thousand years: vegetable tanning with bark, in pits.

How it works

Many plants store tannins, bitter compounds that protect them against insects and fungi. Oak bark is rich in them, and so are chestnut wood, sumac leaves, oak galls and acacia pods. Crushed bark steeped in water gives a brown liquid that tanners called ooze, or tan liquor.

In this liquid, tannin molecules work their way between the collagen fibres of a cleaned hide and attach to them at many points. Coated in this way, the fibres no longer glue together as they dry, and bacteria break them down far more slowly. The hide becomes firm, brown leather.

Tannin moves in from the surface, and a strong liquor at the start would seal the outer layers and leave a raw core. Hides therefore started in weak, used liquor and moved to stronger ones over weeks and months.

The hide must first be opened up. Lime water or a lye made from wood ash makes it swell, loosens the hair roots and removes part of the fat and soft proteins. The hair and the top layer of skin can then be scraped off and the flesh side cleaned, so the tannin enters from both faces.

How to build

This outlines the traditional bark method for one cow hide as tanneries practised it; it is not a tested recipe. Count on most of a year. You need a hide from a healthy animal of known origin, watertight tubs or barrels, slaked lime or wood ash, plenty of dried oak bark, a wooden beam and a blunt two-handled knife.

Soak and lime. Soak a salted hide in clean water for a day or two until it is limp, then put it in water mixed with slaked lime or wood-ash lye and stir it daily. The hide is ready when the hair slips out under a thumb.

Dehair and flesh. Lay the hide over the sloping beam, hair side up, and push the blunt knife away from you to strip the hair. Turn it and scrape the flesh side clean of fat and membrane. Wash out as much lime as you can in several changes of water, because lime left in the hide spoils the tanning; the weak, slightly sour used liquor of the next stage takes up what remains.

Oak bark is peeled in spring, when the rising sap loosens it from the wood.

Prepare the bark. Peel bark from felled oaks in spring, dry it, crush it and steep it in water for several days.

Tan in the pits. Put the hide in weak liquor first and move it every day for a few weeks. Then lay it flat or folded in a tub between layers of crushed bark and cover it with stronger liquor. Tanneries stacked many hides this way in one pit. Renew bark and liquor every few months. At J & F J Baker in Devon, the last traditional oak-bark tannery in Britain, hides spend three months in liquor and nine more stacked with bark. Cut a sliver from the thickest part: the leather is done when the brown colour has gone right through, with no pale streak in the middle.

Dry and dress. Rinse, dry slowly in the shade, and work oil or tallow into the leather while it is still damp.

Safety. Lime and wood-ash lye are caustic and can burn skin and eyes. Wear waterproof gloves, eye protection and boots, and use slaked lime rather than quicklime, which heats violently with water. Wash a splash off the skin at once with plenty of water; if lime gets in an eye, keep rinsing it with clean water and get medical help immediately. Raw hides can carry anthrax spores, which enter through cuts or are breathed in with dust and cause serious illness; you cannot tell by looking whether a hide is contaminated. Use only hides from inspected animals of known origin, never from an animal found dead, cover cuts, do not shake or beat dry hides, and wash hands and clothes after work. If you think you have been exposed, or a sore, fever or breathing trouble follows work with hides, see a doctor at once and say that you handle hides. Lime and tan liquors pollute: keep them in watertight tubs well away from wells, springs and streams, and never pour spent liquor on the ground, into drains or into water; take it to a proper disposal point.

History and evolution

Long before tan pits, skins were stretched, scraped, worked with fat and smoked.

People prepared skins long before they tanned them. Hunters in many regions worked brains or other fats into a scraped skin and then smoked it, so it could get wet without turning stiff. Ötzi, who died in the Alps around 3300 BCE, wore goat, sheep, deer and calf skins, and a shoe from the Areni-1 cave in Armenia, dated to 3627–3377 BCE, seems to be cowhide. How either was treated is uncertain.

Dating vegetable tanning is difficult: only chemical analysis can tell how an ancient leather was treated, and specialists read the results differently. Carol van Driel-Murray notes that Bronze Age leathers were mostly cured with fat, oil or smoke. Yet analysis of Egyptian leathers at the Metropolitan Museum, published in 2017, found plant tannins in objects of the Middle or New Kingdom, between about 2000 and 1100 BCE. The Greek botanist Theophrastus wrote about tanning plants around 300 BCE, and in north-west Europe tanned leather arrives with the Roman conquest. At Pompeii a tannery with fifteen cylindrical tanks was working when the eruption of 79 CE buried it.

Alum tawing, with a mineral salt instead of tannin, gave a pale, soft leather prized for gloves, but water washes the alum out and the skin can dry hard again. Pit tanning changed little until the end of the nineteenth century, when chromium salts cut the work from months to days. Chrome tanning is an industrial process with toxic waste, not a home craft. Bark-tanned leather is still made for saddles, belts and soles.