From rawhide to leather
Leather production involves a multifaceted and complicated finishing process. A hide or skin has to pass through about 40 processing stages until the biological raw material has become the natural product, leather, that meets the requirements of the processors, as well as the diverse and fashion-related wishes of the consumers.
Freshly delivered raw material
Several weeks are still required to transform the perishable raw material into durable, saleable leather, despite modern tanning processes. Some special leathers, such as the purely vegetable-tanned sole leathers, are still produced using traditional pit tanning processes that take many months to meet the demands for high durability. Leather production is thus highly complex and requires much expertise and experience in tanning.
An elaborate manufacturing process in about 40 stages
Leather production begins with the preparation of the hide for tanning. The first step is the cleaning stage of the soiled animal skins, some of which are preserved with salt.
All parts that cannot be converted into leather are then removed from the hides. These are dehaired, the subcutaneous connective tissue is mechanically removed and the non-collagenous proteins and fatty substances are extracted in various treatment baths. All water-based processes take place in huge rotating drums.
Splitting of the hide into the hair-side “grain split” for smooth types of leather and the body-side “flesh split” for the rough types of suede also takes place during this preliminary work. At the end of these elaborate processes, the “naked” hide ready for tanning, known as the pelt, is obtained.
Chemically, the tannable pelt consists only of the main component of a hide that can be converted to leather, collagen: a fibrous connective tissue protein, the molecules of which are tightly interwoven to form an infinite three-dimensional fibre network that is ultimately decisive for the many natural properties of leather, especially its durability.
The tanning process
Based on the desired leather quality, the appropriate tanning agents are applied to this collagen in the subsequent actual tanning process. These react with the collagen in the aqueous environment to form leather which, unlike the original hide, is now resistant to rot and heat stress and can be stored and transported. The choice of tanning method is determined by the subsequent intended use of the leather, e.g. as shoe or furniture leather, for leather clothing or car interiors. While only vegetable tanning with plant extracts from chestnut, oak or mimosa is used for abrasion-resistant shoe soles, mineral chrome tanning is the tanning method of choice for upholstery, upper or fine leathers. Chrome-free tanned leathers are also available upon request.
The retannage (process)
In the subsequent processes of finishing, the final character of the leather is determined with the help of vegetable, synthetic or mineral retanning agents. This gives the leathers a better and more uniform fullness and makes further technical quality improvements.
This is followed by dyeing, fatliquoring for suppleness and mechanical loosening of the fibre structure to fix specific properties such as elasticity or softness and, if necessary, hydrophobic impregnation.
The finishing process
After all this preliminary work, the leathers enter the final manufacturing process, known as dry finishing. This is a collective term for a series of mechanical and chemical treatments of the leather surface. In particular, the visual and fashion characteristics of the leather are adjusted during this process. However, quality factors such as high lightfastness and rub fastness (colour fastness) can also be significantly optimized in the finishing process.
In the final finishing process, the different types of leather are also given their typical characteristics, such as those of the velvety soft, buffed nubuck leathers. Various patterns or attractive surfaces are created on the leather by printing or embossing. In short, the leather is given “its final face and the finishing touches” in the finishing process – as the tanner says.