How Is Leather Tanned? Stopping a Skin From Rotting for Good

Skin gets its strength from collagen, a protein arranged in triple-helical molecules bundled into fibres that are themselves woven into a three-dimensional network, which is why leather resists tearing in every direction rather than splitting along a grain. Untreated, that network is vulnerable in two ways. Enzymes and bacteria break the protein down, which is putrefaction. And if the hide simply dries, the fibres stick to each other and the result is rawhide, which is hard and brittle and turns floppy again when wet. Tanning solves both at once by introducing cross-links between collagen molecules, which stabilises the structure chemically so it resists enzymatic attack and physically so fibres cannot bond into a solid mass as water leaves. That is the whole definition. A material is tanned when its shrinkage temperature has been raised, meaning the temperature at which it suddenly contracts in hot water, and that measurement is the standard test for whether tanning has actually occurred.

The preparatory stages, collectively called beamhouse work, remove everything that is not wanted and take longer than tanning itself:

• •Curing with salt or chilling, to stop decay between the abattoir and the tannery
• •Soaking, to rehydrate and wash out salt, dirt and blood
• •Liming in an alkaline solution, which loosens hair, swells the fibre structure and saponifies fats
• •Unhairing and fleshing, removing hair, epidermis and the fatty flesh layer mechanically
• •Deliming and bating, lowering the pH and using enzymes to remove unwanted proteins so the hide becomes soft and relaxed
• •Pickling in acid and salt, bringing the hide to the acidity that the tanning agent requires

Vegetable tanning uses tannins extracted from bark, wood, leaves and fruit, historically oak, chestnut, mimosa and quebracho, which are large polyphenol molecules that bond to collagen at multiple points. It is slow, traditionally taking weeks to more than a year in a sequence of pits of increasing strength, and it produces a firm, dense leather that holds a shape, takes tooling, darkens with use and is used for saddlery, belts and shoe soles. Chrome tanning, introduced commercially in the late nineteenth century, uses basic chromium sulphate and completes in hours rather than months, producing a soft, supple, heat-resistant leather that takes dye readily and accounts for the large majority of world production. The environmental record differs: chrome tanning generates effluent containing chromium that requires proper treatment, and tanneries operating without it have caused serious contamination, while vegetable tanning uses large volumes of water and organic load. Other tannages exist, including aldehyde and oil tanning, the latter producing chamois.

After tanning the leather is retanned, dyed, fatliquored by introducing oils that lubricate the fibres and keep them from sticking, then dried, staked to soften and finished at the surface. Hides are usually split horizontally, and the terminology that follows is frequently misunderstood in retail. Full grain is the outer layer with its natural surface intact, which is the strongest and most durable because the fibres are densest there and it develops a patina. Top grain has that surface sanded to remove blemishes and is then coated, which looks uniform and wears less well. Genuine leather is a labelling term meaning only that the product contains leather, not a quality grade, and frequently denotes lower layers. Bonded leather is shredded scrap bound with adhesive onto a backing and behaves like a plastic sheet. Suede is the flesh-side split, raised by abrasion. Reading a label for which of these it says is more informative than reading the price.

Tanning cross-links the collagen in a hide so bacteria cannot digest it and so the fibres do not glue themselves together as it dries. The test is whether the shrinkage temperature has risen. Vegetable tannins take weeks to a year and give firm leather that tools and holds shape, while chromium salts take hours and give soft supple leather that dominates production. Full grain keeps the dense natural surface, and genuine leather is a labelling phrase rather than a grade.