Leather is made from animal skin, which (on its own), doesn’t serve us much of a purpose as it will just rot.
Drying animal skin out leaves you with the same sort of rawhide material that is given to dogs as treats – this isn’t able to be fashioned into clothing in the way we expect leather to be used. Instead, animal skin is put through a series of processes in order to make sure that the leather stays usable and in good condition for years to come.
These processes are called tanning. Tanning is used to stabilize the molecular structure of the leather, improve the resistance of the leather to heat and prevent bacteria and decay.
The molecular structure of leather is mainly made up of collagen, but there are other materials within all of this that need to be removed, whilst also stabilizing the collagen in order to create leather.
Within this article, we will cover the processes undergone during pretanning, as well as some of the various methods that can be used to tan leather.
Pretanning
The skin may be preserved in salt before it is used – this can dry them out, but it prevents bacteria from eating away at it before tanning can occur.
Before tanning can take place, the skin needs to be put through a process called ‘pretanning’. When the skins are ready to be used, they are washed off with water, removing the salt and rehydrating them.
- Firstly, any excess fat, hair and other excess materials on the skin are moved
- Next, the skin is treated with an alkaline pH – this removes the rest of the hair.
- The skin is then treated with other chemicals which bring it back to neutral.
- Enzymes are then added to the skin, which remove it of all of the biological components that aren’t collagen.
- Finally, the skin is brought down to a more acidic pH – this is where further chemicals are added to complete the pretanning process
Methods Of Tanning
Tanning leather is done through the use of tannins, which are the chemicals used in wine.
These chemicals are responsible for the darker hue of leather, and are found in naturally occurring substances – this can be anything from vegetable matter to animal fat.
Leather makers can choose from a variety of methods when it comes to actually tanning the leather.
Chrome Tanning
Since its creation, chrome tanning has become one of the most common forms of tanning. This is because the chrome tanning process is a much faster process than the vegetable tanning process.
Chrome tanning uses chromium salts – these belong to the mineral tannins group. Chrome tanning is used for around 85% of leather production globally, with almost 100% of leather used for clothing being tanned using the chrome tanning method.
Chrome tanning has just under twice the tensile strength of leather tanned using the vegetable tanning method.
Despite this, leather tanned use the chrome tanning method actually weighs less than the vegetable-tanned leather.
This is because, within chrome tanning, the salts used do not get fully absorbed by the skin – only 4% of the leather is made up of tannin, whereas in vegetable tanning around 20% of the content of the leather is eventually tannin, due to the amount of tannin that gets absorbed in the process.
Chrome tanned leather can have a blue hue whilst it is still wet, and as a result is sometimes referred to as ‘wet blue’.
Vegetable Tanning
Vegetable tanning is the most ancient method of leather tanning. The vegetable tanning method uses the bark of oak and spruce trees, as well as olive leaves, mimosa, and rhubarb roots.
These materials contain polyphenols, which is a class of compounds that react with the collagen in the leather.
They are usually placed with the leather in a pit alongside the leather, where the tannins are drawn out and therefore tan the leather.
As mentioned previously, the tannin content of leather using this method is often at around 20%, and as a result, this leather is heavier. There are approximately 300 different plant species around the world that can be used for tanning.
Synthetic Tanning
Synthetic tanning uses tanning agents that are produced artificially and can’t be found in nature. Synthetic tanning agents can include formaldehyde, acrylates, phenols and glutaraldehyde, to name a few.
Synthetic tanning is more often used in combination with another type of tanning, usually with either chrome tanning or vegetable tanning. A downside of synthetic tanning when it is chrome-free is that leather tanned using this method tends to have a sensitivity to moisture and heat.
This means that when the leather is wet, and when exposed to heat such as the sun, the water trapped within the leather starts to boil, and as a result, the leather shrinks and hardens.
Tanning With Fats And Oils
Tanning using fats and oils is a very old method of tanning. This method uses animal substances that are rich in fat, such as fish oil, marrow or brain, however it can also be done when using claw oil, soap, yak butter or even egg yolk.
An example of leather that is made using this method would be chamois leather.
Tawing
One of the oldest methods of tanning is called tawing. Tawing is a type of mineral tanning. It uses salts, similar to chrome tanning, but more specifically tawing is done using aluminum salts, also known as ‘alum’.
These salts can be produced both naturally and artificially. Alum makes a white leather that is quite water sensitive, but also both firm and stiff – it is often greased and tumbled in order to make it softer.
Because of how water-sensitive alum leather is, it has been replaced with chrome tanning in most cases.
So,now that we’ve covered pretanning and the various methods of tanning, you should hopefully have a pretty good understanding of how leather is made!
It takes a lot of time and effort, from the animal that grew it to the people that make the leather itself. Next time you put on your leather coat, appreciate it… because a lot of time went into it!
