Tanning is what turns a skin into leather, and it decides everything after

A hide is protein, and untreated protein rots

The raw material is skin, and skin is largely collagen, a fibrous protein arranged in an interwoven three-dimensional network. That network is what gives leather its strength and its characteristic refusal to tear in a straight line, and it’s also, untreated, an excellent food source for bacteria.

Tanning is the process of altering the collagen chemically so that it no longer supports decay and no longer turns hard and brittle when it dries. Every tanning method, from the oldest to the newest, is doing the same underlying thing: introducing cross-links between the protein chains so they hold their arrangement permanently.

Everything that happens before tanning matters too. The hide is soaked, the hair is removed, the flesh side is trimmed, and the whole thing is prepared chemically so that the tanning agent can penetrate evenly. A great deal of the variation between leathers is set in that preparation rather than in the tanning itself.

Vegetable and mineral tanning produce different materials

Vegetable tanning uses tannins extracted from bark, wood, leaves and fruit, and it works slowly, with hides moving through a series of pits or drums over weeks. The result is a firm, dense leather that holds a shape when moulded and wet, that darkens and develops a patina with light and use, and that can be tooled and burnished.

Mineral tanning, most commonly using chromium salts, works in a fraction of the time and produces a softer, more supple leather that is more resistant to heat and water and holds bright colour far better. It is the dominant method for clothing and for most footwear uppers, for reasons of speed, cost and consistency that aren’t hard to understand.

Neither is simply better. Vegetable-tanned leather is the material of choice where structure and moulding matter, such as shoe soles, belts and hard cases, and it is stiffer and less forgiving in a garment. Chrome-tanned leather makes a jacket that drapes and a boot that flexes, and it doesn’t develop the same character over years.

The label describes which layer of the hide you are getting

A hide has depth, and it is split horizontally into layers. The outermost layer, the grain, is the densest and finest part, with the fibre structure tightest at the surface, and it is what makes leather durable and water-resistant on the outside face.

Full grain means the grain surface is intact and unsanded, so the natural markings remain and the strongest fibre layer is doing the work. Top grain has had that surface lightly sanded to remove blemishes, then usually finished, which costs some strength and a good deal of character in exchange for uniform appearance. Corrected grain has been sanded more heavily and given an embossed artificial grain pattern.

Below the grain lies the corium, and a split taken from it has no grain layer at all. Suede is generally made from a split, which is why it is soft, fibrous on both faces and much more vulnerable to water and abrasion. Bonded leather, made from shredded scrap adhered to a backing, is a composite and behaves like one.

Alternatives divide into coated fabrics and plant composites

The long-established alternative is a woven or knitted textile coated with a polymer, usually polyurethane, embossed to resemble a grain. It is genuinely useful material: it is consistent, inexpensive, and takes colour beautifully. Its weakness is structural, because the polymer layer is a skin over a base cloth rather than an integrated fibre network, and skins eventually crack, peel and delaminate.

Newer alternatives use plant material — leaf fibre, fruit processing residue, cork, mycelium grown on a substrate — bound and often coated. The interesting ones share a problem: a material with genuine mechanical strength usually still needs a polymer binder or topcoat, and the proportion of that binder decides both the performance and how meaningful the plant content is.

It is worth resisting the temptation to score these against leather on a single axis. A coated fabric that lasts three years and a hide that lasts fifteen have different production impacts, and comparing them fairly means comparing whole lifespans rather than materials at the point of sale. That comparison is genuinely contested.

Reading leather with your hands and your eyes

Look at the cut edge, which tells you more than any surface. Full grain shows a dense, fine layer at the top face fading into a looser fibrous structure beneath. A coated fabric shows a distinct film sitting on an obvious textile backing, and the two are usually different colours.

Then look at the grain pattern. Natural grain is irregular, with pores of varying size and spacing, and it will differ across a single garment. A pattern that repeats identically every few centimetres has been embossed, which tells you it is either corrected grain or not leather at all.

Finally, press it. Real leather creases into fine, soft lines that partly recover, and it warms to the hand. That is not a laboratory test and it can be fooled, but combined with a cut edge and a grain pattern it will settle most questions in a shop without anyone needing to consult the label.