Tanning: the trade nobody wanted next door

An untreated animal skin does two things: it rots, and if dried it goes hard as a board. Tanning prevents both by chemically altering the collagen so it resists decay and stays flexible.

Every society that kept animals worked out how to do it, and the resulting material was the plastic of the pre-industrial world — footwear, harness, containers, armour, bellows, book bindings, buckets, hinges, and the drive belts that ran early machinery.

The stages

Curing. Salting or drying the raw hide to stop decay before processing.

Soaking and liming. Rehydrating and then treating with an alkali — traditionally lime, or in older practice urine or wood ash — which loosens hair and breaks down unwanted proteins.

Unhairing and fleshing. Physically scraping hair from one side and fat and flesh from the other, over a beam, with a curved knife. Hard, wet, unpleasant work.

Bating. Softening the hide with an enzymatic treatment. Historically this used animal dung — dog and pigeon dung were both standard — which supplied the enzymes. This is the stage most responsible for the tannery's reputation.

Tanning proper. The chemical step, which takes anywhere from weeks to more than a year depending on the method.

Finishing. Drying, stretching, oiling, shaving to thickness, dyeing, polishing.

The tanning methods

Vegetable tanning uses tannins extracted from bark, wood, leaves, nuts or fruit — oak, chestnut, mimosa, quebracho, sumac, myrobalan, depending on region. Hides are moved through a sequence of pits with increasing tannin concentration, over months.

The result is firm, thick and develops a patina with use. It is the leather of saddlery, of shoe soles, of bookbinding, and it is what most people mean by traditional leather.

Brain tanning, practised in various forms across North America, northern Eurasia and elsewhere, uses emulsified fats — traditionally from the animal's own brain — worked into the hide, which is then softened by continuous manipulation while drying and frequently smoked. Produces an exceptionally soft, washable material.

Alum tawing uses aluminium salts to produce a very white, soft leather. Technically not tanning, since the process is reversible in water.

Oil tanning uses fish or marine oils, worked in and oxidised, producing chamois.

Chrome tanning, from the late nineteenth century, uses chromium salts and takes a day rather than a year. It now accounts for the great majority of world leather production, and it is why leather stopped being expensive.

The smell and the location

Tanneries were placed at the edge of settlements, downwind and downstream, and tanners were frequently a socially marginal group — a pattern documented across many societies, with the marginality sometimes formalised in caste or occupational status.

The reason is straightforward. The process involves decaying organic matter, lime, dung and large volumes of water that leave heavily contaminated.

Water pollution from tanning is not a modern concern. Municipal regulations restricting where tanners could discharge waste appear in medieval records, which suggests the problem was noticed.

The traditional tanneries that remain

A handful of historic tanneries operate as working sites and as visitor attractions — the pit tanneries of Fez being the most photographed, with their grid of stone vats visible from surrounding terraces.

They are genuinely working operations rather than reconstructions, and the working conditions are as difficult as they look. The tourist economy around them has changed their finances without much changing the labour.

Elsewhere, vegetable tanneries survive supplying saddlery, bookbinding and high-end shoemaking, where the material properties genuinely differ from chrome leather and buyers will pay for it.

The environmental question now

Chrome tanning is efficient and it produces effluent containing chromium, which requires proper treatment. Where regulation is weak or enforcement absent, the consequences for local water and for tannery workers have been documented repeatedly and are serious.

Vegetable tanning avoids the chromium and is not automatically benign: it uses far more water, produces high-strength organic effluent, and requires bark or wood in quantity.

The genuinely lower-impact options are, unhelpfully, mostly about using less leather. Which is complicated by the fact that hides are a byproduct of meat production and would otherwise be waste.

Judging leather

For anyone buying: full grain retains the outer surface of the hide with its natural texture, and it is the strongest and the most durable. Top grain has been sanded to remove imperfections. Corrected grain has been sanded heavily and had a texture printed on. Bonded leather is shredded scrap adhered to a backing and is not leather in any useful sense.

Vegetable-tanned leather can be identified by its firmness, its smell and the way it darkens with use. Chrome-tanned leather is more uniform, more flexible and does not develop the same patina.

Neither is superior in the abstract. A vegetable-tanned sole and a chrome-tanned jacket are both the right material for the job, which is generally the most useful thing to know about any traditional craft material.