What chemicals are used to tan leather?

What is leather tanning?

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.

What chemicals are used in tanning leather?

When buying a leather good or sourcing leather, it is most likely chrome tanned leather. Around 80% of all leathers are chrome tanned. This makes chromium the most used chemical to tan leather. Chrome tanning is not the only option. There are also other methods and chemicals available to tan leather. We will discuss four main tanning methods and chemicals:
  • Vegetable tanning
  • Chrome tanning
  • Chrome-free / aldehyde tanning
  • Zeology: zeolite-based 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.

Tanning with chrome

As the name suggests, chrome tanning involves adding a small amount of chromium (chromium III) in the form of chrome mineral salts. Chromium III is a safe substance and most people are not aware of the fact that chromium is even an essential part of our daily diet.. 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. 
Chrome tanning produces consistent leathers that can be used or worn, year after year, without any loss of properties. Chromium tannage produces a blue/grey color after tannage and this is usually re-coloured to produce the correct final color in the resulting leather.

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’ or ‘metal-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.

Aldehyde tanning gives leather a soft and a pale creamy colour, which can easily be dyed to pastel colors.

Tanning with zeolites

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.
It is important to know that zeolite as a unmodified raw material, sold by the kilogram, is not a suitable tanning agent. The only perceived advantage of these zeolite blends is the price per kilogram of the tanning agent. Conversely Zeology, with a modified zeolite (patented), is developed for optimal leather performance on any substrate with low environmental impact and unmatched production consistency.

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