Tanning is the method of preserving animal skin, with or without hair using tannins. These are acidic chemical compounds that stabilise the fibre structure of the skin and prevent it from decaying, decomposing and oxidising.
The tanning process involves many stages where the skins have to be treated first and, once tanned, depending on the application and specific customer requests, the leather is dyed, ironed, sanded, oiled etc. There are endless variations.
The main tanning methods are chrome tanning (most of the clothing leather and upper leather of shoes), vegetable tanning (most leather belts, sole leather, riding leather) and synthetic tanning. Another well-known kind of tanning is tanning with fats and oils; then fatty animal substances such as brain, fish oil or tallow are used.
Chrome tanning is the most common method. It is the most preferred tanning method by tanneries mainly due to the speed at which leather can be produced (a few days) and the simplicity of the working process.
For instance vegetable tanning requires 15 months or longer.
Globally, approximately 80% uses the chrome tanning method and about 10% the vegetable tanning process.
Often various tanning methods are combined in order to achieve certain properties of the final product. For example, the combination of synthetic tanning with chrome or vegetable tanning. This is known as combination tanning. Variants include, for example, vegetable with subsequent chrome tanning (semi-chrome leather) and chrome tanning followed by vegetable tanning (chrome re-tanning).
Leather tanning with chromium (III) salts is used in 95% of shoe upper leathers, 70% of leather upholstery (but decreasing in favour of chrome-free leather) and almost 100% of clothing leather.
Chromium (VI) is harmful and can occur in chrome tanned leather as a result of improper tanning conditions.
Many product recalls for leather consumer products are caused by chromium VI. Even the slightest amount of chromate (=chromium VI) is enough to cause inflammatory skin reactions, such as swelling, blisters, itchy red spots and peeling.
Vegetable tanning refers to leather that is tanned with oak and spruce bark. Also quebracho, tara pods, olive
leaves, rhubarb roots or mimosa are common. These substances are placed in a pit along with the skins and hides. As these tannins are derived from plants, the leather is called vegetable-tanned leather. Also the term "natural leather" is used.
Synthetic tanning agents (aromatic syntans) are produced industrially. Examples of these include formaldehyde, glutaraldehyde, phenols and acrylates. The synthetic tanning method is not employed as an isolated tanning process, but mostly as part of a combination tanning process with either chrome tanning or vegetable tanning.
Tanning skins and hides with fats and oils is a very old method. It involves using fat-rich animal substances such
as brain, fish oil, sebum or marrow. It can also be done with soap, claw oil, yak butter or egg yolk. In the past, animals were mainly hunted for food, clothing and shelter and using animal fats to preserve their skins was common practice. This method involves 100% natural products, so there is no need for additional "chemistry".
Globally, approximately 1% of all leather is tanned using this tanning method.
The tanning of animal skins with animal brain mass is an almost forgotten method and is rarely practised today. Historically, brain tanning was mainly done by North American Indians who also used smoke as a way of preserving the skins. This leather was also called "Indian leather" and "buckskin".
Tanning and re-tanning methods which include chrome and/or synthetic tanning might result in residues of restricted chemical substances in the processed leather. Especially chromium VI is a major concern.
Tanning of leather is necessary, make sure you know and manage the production processes.
For the chemical risk assessment of materials this is essential information from your supply chain.