The leather tanning process

The leather tanning process, at its core, uses the hides and skins of different animals as its raw material: goats, sheep, cows and bulls, among others. It is important to note that these hides come from animals raised for food; none is slaughtered solely for its skin.

It is worth noting that hide is one of the oldest materials worked by humans in order to obtain leather for different uses. Through the leather tanning process, the animal's hide is protected from the hardening and rotting that would otherwise occur naturally.

Leather is, in fact, the end product of the hide tanning process, and in turn it is the raw material other industries use to create leather goods. Thanks to this process, the characteristics of the animal hide are improved so that it can be adapted to any kind of use.

Stages of the leather tanning process

To ensure the leather can be adapted to different commercial uses, it goes through several stages whose characteristics vary according to the type of hide (depending on the animal) and its intended use (the end product). For the leather tanning process, the following stages are generally required:

1. Trimming

This removes the excess parts of the hide that will not be part of the leather tanning process.

2. Curing

This is a treatment using salt or drying that slows down the decay of the hide.

3. Soaking

This stage restores the hide's moisture level.

4. Liming and unhairing

The hair attached to the hide is removed, leaving only the pelt.

5. Fleshing and trimming

This is the removal of fatty tissue from the hide.

6. Splitting

By cutting the hide horizontally, different layers and thicknesses are obtained according to the intended end use.

7. Deliming

A special process in which calcium hydroxide and sodium sulphide are removed from the hide.

8. Bating

An enzymatic action intended to break down certain fibres in the dermis.

9. Pickling

The pH of the hide is adjusted to make it suitable for the leather tanning process that follows.

10. Degreasing

This consists of removing excess grease from the hide.

11. Tanning (chrome or vegetable)

This stabilises the collagen fibres to prevent the hide from rotting. This stage can be carried out by chrome tanning or vegetable tanning (using tannins).

12. Shaving

The leather is shaved to obtain the desired thickness.

13. Neutralization

Unlike pickling, this stage adjusts the pH so that the retanning, dyeing and fatliquoring that follow can be carried out properly.

14. Retanning

A second treatment that gives the leather the commercial characteristics the customer requires.

15. Dyeing

This gives the leather its colour, as well as the elegance and fastness required by market specifications.

16. Fatliquoring

Gives the leather the desired softness and feel.

17. Drying

The aim of this stage is to obtain leather with a low moisture content, so it is exposed to heat for a certain period of time.

18. Mechanical operations

Operations carried out to make the finishing of the leather easier.

19. Finishing

The application of special products to coat the surface of the leather, which is then brushed.

20. Pressing

Heat and pressure are applied to the leather to obtain a uniform, smooth and compact surface that gives elegance and authenticity to the end product.

Types of leather tanning

Today there are two main leather tanning methods: chrome tanning and vegetable tanning. Here is a brief description of what each of these processes involves:

· Chrome tanning

This is the most widely used leather tanning process to date, thanks to its great versatility, lower cost and speed.

After liming and unhairing (removal of the hair), the mechanical action of the rotating drums fixes the chromium in the hide, giving it stability and preventing it from rotting.

· Vegetable tanning

Unlike chrome tanning, vegetable tanning is fully ecological (the leather can be recycled). It is less widely used, yet it is the oldest of the leather tanning processes in the world. The processing time with this technique is also similar to that of chrome tanning.

Vegetable tanning requires the use of natural tannins. During the process, the tannin (an organic substance) binds to the protein in the fibres of the hide during the leather tanning process.

In this way, the tannin makes the leather rot-proof: it does not decompose or degrade over time. It also improves other properties such as absorption and breathability, an added advantage when the leather is used for shoes, for example. It also gives a better appearance thanks to the colour it brings to the leather.

The leather resulting from this process is more valuable on today's market than chrome-tanned leather.

In conclusion

When we talk about the leather tanning process, it can be carried out in many ways, because the specifications for the end use must be taken into account. This depends on what the leather will be used for, the animal it comes from and the specific characteristics each process gives it to improve its properties and make it marketable.

There are currently two leather tanning processes: chrome and vegetable. Both share common stages; what changes is the tanning method (one uses chromium, the other tannin), whose main purpose is to prevent the leather from rotting naturally over time.