The main ingredient of leather is collagen, a tough, fibrous protein from the dermis, the layer of skin under the surface. Tanning makes that collagen stable so it won’t decay. Finished leather also contains tanning agents, oils, dyes and finishes.
Collagen: the fibre behind leather
Collagen is the structural protein that gives skin, tendons and bone much of their strength. Each collagen molecule is a triple helix of three chains. About a third of its amino acids are glycine, and proline and hydroxyproline make up about a fifth. Hydroxyproline is used as a marker to measure how much collagen a sample contains.
The dermis is mostly type I collagen, with some type III. Young skin has more type III.
Which part of the skin becomes leather
Only one layer of skin becomes leather. The dermis, also called the corium, has two parts: the grain (papillary) layer near the surface and the reticular layer below it. The outer epidermis, the hair and the fatty flesh layer underneath are removed during processing. Hair and epidermis are made of keratin, not collagen, so keratin is not part of the finished leather.
Raw hide compared with finished leather
| Raw hide | Finished leather | |
|---|---|---|
| Water | About 60–65% | About 12–18% moisture |
| Protein | About a third, mostly collagen | Mostly collagen fibres, now stabilised |
| Fat | About 2–10% | Fatliquor oils added, about 3–15% |
| Tanning agents | None | Chromium (about 3–5% as chromium oxide in chrome leather), or plant tannins in vegetable leather |
| Will it rot? | Yes, especially when re-wetted | Much less likely |
How tanning changes collagen
Tanning agents bind to the collagen fibres and crosslink them. This makes the structure stable, so it resists rot and heat. Shrinkage temperature shows the change. Raw collagen shrinks at about 60°C to 65°C in water. Vegetable-tanned leather shrinks at about 75°C to 85°C, and chrome-tanned leather at about 100°C to 120°C. The higher the number, the more heat the leather can take before it shrinks. The same heat sensitivity is why hardening leather by baking needs careful temperatures.
The tanning process itself is described in how leather is made. Plant tannins are covered in traditional leather making, and chromium in is chrome-tanned leather toxic?.
Rawhide and hide glue
Rawhide is the same protein, but it is untanned. It is dried hide that stays stiff and can rot again when wet. Heating collagen turns it into gelatin, which is why raw hide scraps are used to make hide glue.
What else is in finished leather
Finished leather isn’t pure collagen. Tanning agents, fatliquor oils, dyes and surface finishes are all part of the material. The collagen network gives leather its strength and flexibility, while the other ingredients tune its feel, colour and wear.
Common misconceptions
- Leather is made of keratin. It isn’t. Keratin is in the hair and epidermis, which are removed.
- Leather is skin cells. It isn’t. It is the fibre network of the dermis.
- Leather is pure collagen. It isn’t. Tanning agents, oils, dyes and finishes are part of it.
- Rawhide is leather. It isn’t. Rawhide is untanned.
For how the outer layers of hide differ in thickness and size, see hide vs skin. For how genuine leather uses the split layer, see how genuine leather is made.