Understanding tanning processes
What is leather?
First of all, leather is the skin of an animal. The men feed on, and the skins are retrieved in order to be used by the leather goods industry. The leather is a generic term that includes many species of animal skins (calf, bull, pig, goat, fish, crocodile, ostrich etc.) that have been tanned to be rot-proof, and usable. Behind this transformation, there is a process from several days to several months, which is a critical variable, as well as the initial quality of the skin chosen in order to have a perfect leather.
How to move from skin to leather ?
This succession of steps that allow the passage from animal skin to final leather is called tanning (generic term but also one of the process components). It consists of four key steps that are:
- river work ;
- tanning ;
- currying ;
- finishing ;
1) The river work
This job is made in order to prepare the skin for tanning. When skin comes from the slaughterhouse, it was salted to be preserved until the tannery. The first job is to rehydrate (the salt dried it), as well as clean the skin. This water will also help to provide a full skin, so to better absorb the tanning substances. This work will thus help to remove unwanted components from the skin such as hair and subcutaneous tissues, to come to work only the dermis which will become final leather.
There is not so long, the skins were attached and immersed in river water, that were adjacent to tanneries. That is why historical tanneries are located few meters from rivers. Today skins are placed in treads which are large barrels of 2 to 3 meters of diameter, with a wall of 0,3 ft thick made from a material near steel's density.
The main steps of this river work are:
- Depilation: this step aims to remove the epidermis and the epidermal productions such as hair or wool of animals ;
- Unhairing : here, we try to remove subcutaneous skin and scraps from adhering muscles ;
- Shaping: it allows to remove debris that remained in the hair follicles to obtain a smooth leather surface. This will allow to fix color pigments evenly;
- Deliming: this is the last step of the river work. It adjusts the pH of the skin so that the reaction of tanning is optimal.
2) The tanning
There are 3 main types of tanning :
- Vegetable tanning : This is the oldest method. The tanning materials are leaves, bark, fruits etc. The skins will then be put into tanks with these natural elements. This will make them rot-proof : we can therefore talk about leather. Historically the duration of operations could last about 17 months. For economic reasons, this slow tanning process is no longer used, and the method of rapid tanning has largely replaced it. It takes about one month now.
- Mineral tanning : Historically, we used alum which is the oldest mineral tanning. Today chromium salts are the most used. The idea is to interact chromium and collagen in the skin of the animal by hydrolysis. This process is faster than the previous one, and takes approximately 24 hours. It is more prevalent than the vegetable tanning method, and offers wider range of leathers.
- Organic tanning or chamoisage : It is an oils fish tanning, primarily from cod liver oil. The skins gain flexibility, a velvety appearance and are washable. The leather goods, footwear and glove industries use sometimes this process for their leather. Now that we have raw leather, we have to adjust it according to its destination, it will be the currying stage.
3) The currying
At this stage and depending on the type of leather (wet leather or dry leather) we will rework the thickness of the leather (slitting), water content (wringing), make more or less flexible (threshing), working its appearance (exposure to air, sanding) etc.
It is an important preparatory step to the last one: the finishing.
4) The finishing
The finishing consists of 4 major phases: dyeing, fooding, finishing and finally drying.
- Dyeing : this step will give its final color to the leather. Again a distinction has to be made in the method between vegetable or mineral tanned leather ;
- Feeding : it aims to imbue the leather of a variable amount of fat in order to maintain or enhance its flexibility and waterproofing ;
- Finishing : this operation consists in covering the leather with a thin layer of adherent and solid substance, which protects it from external agents and improves its appearance. It will partly determine the final appearance and characteristics of the leather : obtaining uniform color, making it more or less brilliant, improving its water resistance etc.
- Drying : this last step has important repercussions on the quality of leather. This tends to shrink as it dries. Therefore, tanneries have to be careful not to dry them too quickly and strongly. The best method is to suspend leather in an open and ventilated space.
Now, you know the different steps that bring animal skin into finished leather. This overall tanning step is as important as the selection of the skin itself. It makes the renown of some french tanneries such as tanneries Haas, tanneries Du Puy, tanneries Bodin-Joyeux etc.