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Leather Tanning

Leather tanning | Chemicals & processes

Leather tanning is the first step in the leather making process, after the hides come from the beamhouse and are cleaned from dirt, blood, salt and hair. To use a hide or skin for a leather product it needs to be processed. This process is called the tanning process. Tanning changes the chemistry inside the leather fibers to make it more difficult for the enzymes from bacteria and fungi to break them down – this makes the leather durable.

Leather tanning is the process to prepare skins and hides for leather production. Leather tanning is done in a tannery.

Leather tanning: from hide to leather

How does the tanning process work?

Leather tanning turns a hide into leather. Depending on the type of hide and intent of usage, these are the steps of leather tanning:

  • Soaking
    Soaking is done to remove any excess salt or dirt. All non-leather making components should be removed from the hide to prepare for the production a clean and flat pelts.
  • Liming
    The goal of liming is to split the fibre bundles and to remove any unwanted hair from the hide through the introduction of alkali. Hair saving or semi-hair saving systems yield particularly good results. Here, lime is typically added before alkaline swelling occurs from subsequent additions of sodium sulphide and sodium hydrosulphide. This enables the lime to penetrate and diffuse evenly through the pelts, for optimal fibre splitting and separation. From this stage the skin is often referred to as a pelt.
  • De-liming
    The pelt requires an efficient and complete removal of lime and the neutralization of residual alkalinity, after fleshing and/or splitting. This is done most effectively using proprietary organic acids and acid-salts which form highly soluble lime complexes. Additional care must be taken to adequately buffer the de-liming bath and avoid the precipitation of soluble proteins through a rapid drop in float pH.
  • Bating
    The network of elastin fibers, which are insoluble in the highly alkaline conditions of liming, should now be evenly relaxed and flattened through the extended action of suitable enzyme preparations.
  • Pickling
    Acids are added to slacken the pelt and enhance penetration of the tanning agent.
  • Tanning
    By using a tanning agent the collagen structure of a pelt is being altered and converted into a leather intermediate. It is now a durable material and protected against chemicals, moisture and microorganisms.

Once a hide or pelt has been tanned, it is considered leather. It is however not yet ready to be used for the production of a leather items, such as a shoe, a bag or a leather chair.

Leather making process: step by step

Fact: where does the name ‘tanning’ come from?

Traditionally, a chemical compound called tannin was used to process the skins to produce leather. Tannin was derived from the bark of specific trees. Alternative methods were developed in the 1800’s, when chrome tanning and chromium salts were used instead of natural tannins. Nowadays, there are more alternatives to chrome tanning, for example zeolite tanning – which is a better option for the environment.

Leather tanning chemicals

We can divide leather tanning chemicals in four groups:

  • Vegetable tanning
  • Chrome tanning
  • Chrome-free / aldehyde tanning
  • Zeolite tanning

Vegetable tanning

Vegetable tanning is the oldest tanning method. It uses extracts from wood, and nuts of trees and shrubs. Responsible suppliers ensure these come from a sustainable source. It usually takes longer to tan leather using this method, but the result is a leather with a distinctive aesthetic and handle, which ages beautifully.

Its naming, as well as the tanning materials used, make it seem as if vegetable tanned leather is more ‘eco-friendly’. However, we need to take the entire balance across the whole process into account, to make a more meaningful comparison with other methods. For example, vegetable tanning uses a few times the amount of tannins than for chrome-tanned leathers. The effluent produced also requires more treatment before it can be discharged. However, it has the benefit of using natural, sustainable and renewable raw materials.

Chrome tanning

About 75% of leather made today is chrome-tanned. The process uses trivalent chromium (Cr III), which is a safe substance. Chrome tanning produces consistent leathers that can be used or worn, year after year, without any loss of properties. It is sometimes suggested that hexavalent chrome, or chromium VI (Cr VI) is used for tanning leather, and that it is carcinogenic. Chromium VI is not used in the manufacturing of leather. The process of chrome tanning is constantly being upgraded, as its uptake is improved, less of it needs to be used, there is comprehensive recycling, reduced water consumption and careful management of waste. The chemicals used in chrome tanning do put a strain on the environment.

Chrome-free / aldehyde tanning

There is a number of other tanning methods, known by different labels. They are usually grouped and referred to as ‘chrome-free’. Chrome-free leathers are usually made for a specialized performance requirement, or often specified for automotive use. The most common is aldehyde tanning, which utilizes glutaraldehyde. Leathers made with this tanning agent require relatively more chemicals after tanning, to improve the leather properties. For this reason, the effluent of a glutaraldehyde based chrome-free process will require additional treatment, before it can be discharged.

Zeolite tanning

A new innovation in tanning chemicals is the use of zeolites. Zeolites have a unique property in that they can absorb or release water, depending on the temperature. Water absorption is an important part of leather comfort, and a tannage that allows water absorption (without swelling) is a desirable characteristic. Zeolite tanning is chrome-free, aldehyde-free and heavy metal-free and does not compromise on leather performance.

Be inspired and discover the science and chemistry behind Zeology, the new sustainable tanning agent based on zeolite:

Learn more about leather tanning:

It’s time to revolutionize leather tanning
The tanning processes
Tanning chemicals: what are the options?

Do you want to know more about Zeology?

Visit the University of Zeology