Not everybody knows that tannery materials come from the skins of animals bound for the slaughterhouse, making them a by-product of the food industry. In a way the production cycle of leather is one of the oldest examples of rescue and valorisation of a by-product.

We all know that a tannery is the place where skins are processed and turned into a material ready for manufacturing.
Yet, what does exactly happen in a tannery? What are the technologies involved in the many processing stages?
Here is a brief overview to better understand the complex world of the art of tanning. Enjoy your reading!

Tanning may be considered a real art, transforming a putrescible skin into an incorruptible and durable finished product.
The complexity of the transformation of skins can be synthesized into a number of steps involving chemical, physical and mechanical treatments which turn the raw material into a finished one. Do you want to know more? Read on.

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1. Green skin

It all starts from green skins, that after removal undergo the initial preservation treatments.

The skin is dried (if small and thin) or treated with sea salt (for larger specimens). In this way we obtain raw skins that may show many pre-exising defects, such as damage by parasites or horn rakes, brand marks, or flaying defects, i.e. cuts or holes.

2. Raw skins

Raw skins first go through the so called beam house operations, that precede tanning and prepare the skins to receive the tanning substances. Through the use of special machinery, the skins undego desalting operations and an initial trimming (to shape the skins).

The next steps are:

  • Soaking: Dirt is removed from the skin, by washing it with plenty of water and special chemicals at 25ºC in containers known as drums.
  • Liming: lime and other chemicals (like sodium sulfide and hydrosulfite) are used to increase the absorbency of the dermal tissue by loosening and relaxing its fibres.
  • Unhairing: Epidermis and hair are removed.
  • Fleshing: The skins are transferred from the drums to special machines to eliminate flesh and fat residues from the “fleshings”, the subcutaneous layer.
  • Deliming: Most of the lime residues from previous processes are removed, using strong and weak acids and other substances to reduce the pH.
  • Bating: A further loosening of the fibres is favoured by using enzymes which target the elastic proteins holding the fibres together.
  • Degreasing: This stage is only performed on very oily skins  (e.g. pork and sheep) in order to remove natural fats present on their surface.
  • Pickling: Acids and sodium cloride (common table salt) are used to bring deliming to completion and to stop the bating, bringing the skins to a pH suitable for the subsequent tanning process.

 

3. Pickled

The result is a semi-finished product that takes its name from the last of the steps here described: pickling.
The process continues with the following phases:

  • Tanning: It is no coincidence that the whole process gets its name from this phase; tanning, as a matter of fact, is the stage in which the skin is stabilized irreversibly, by forming stable cross-links between collagen chains and promoting the consolidation that will give the product its typical resistance to humidity, temperature, chemicals and general degradation as well as its mechanical strength.Tanning is categorised according to the tanning agent used, which may be inorganic (mineral tanning, in which the most commonly used metal is chromium) or organic (when using tannins, oil, aldehydes …).
    The steps described here belong to chrome tanning, yielding the so called Wet Blue, the product which is then processed by Conceria Stefania.
  • Samming: This is to press and stretch the skin, that goes through a system of rotary cylinders, as a result the excess liquid from the tanning process is removed.
  • Splitting: Heavy and thick skins are split  into two layers: “grain” (the dermis layer, thinner and uniform, meant for the finest products) and “split” (the inner layer, which will undergo a different process).
  • Shaving: Through a special shaving machine that ensures a uniform thickness all over the skin.

 

4. Wet-blue

At this point we’ve got an intermediate product, already marketable, which is no longer putrescible and that is called wet-blue, because of its bluish colour. It is from this point that Conceria Stefania enters the transformation process. Let’s take a closer look at each production step.

  • Re-tanning: The skin absorbs more tanning and filling substances to give the product the required softness, strength, etc.
  • Dyeing: The skins are given the desired colour in closed rotating drums containing a solution of dyestuff in warm water; there is a wide variety of dyestuffs that comprises the most varied chemical compositions.
  • Stuffing: Another phase performed in drums, in which the leather is given its characteristic softness and water rejection properties and is also improved mechanically; organic oils and fats (both animal and vegetal) are used as well as inorganic substances (synthetic and mineral oils).
  • Setting outand drying: Finally the skins undergo a drying phase comprising several methods; in Conceria Stefania the skins are first piled up to rest on special structures made of wood or steel called “horses”; then they are placed into a machine with cylinders that squeeze out excess water and then, one by one, they are hung to an overhead chain inside the so called drying tunnel, where they are uniformly dried and get ready for the next step.

 

5. Crust

We have thus obtained a product called crust, which has already many important physical properties like water repellency, gas permeability, heat and abrasion resistance, etc; we only need to improve its appearance, in order to give leather the required characteristics like colour, shine, suppleness and strength by means of finishing processes.

In short:

  • Staking, milling and buffing: The hide is softened andspread by squeezing it between two surfaces with opposing ridges and grooves and drums with internal projections; dry buffing renders the surface uniform by passing the hides between two cylinders (one with an abrasive surface).
  • Coating: The skins are coated with a special roller machine (i.e. the coating machine) with various kinds of substances that, after drying, form a film with the required strength, elasticity, transparency and breathability.
  • Spraying: By means of an automated spray-guns system, the paint is sprayed on the skins placed on conveyor belts inside special booths.
  • Ironing: The skins are placed into rotary presses to iron and soften them, and to spread out any folds.
  • Glazing: A glass cylinder rolls on the surface of the skin (grain side) to fix the finishing substances thanks to the high pressure exerted, so giving the product the required shine.

 

6 Finished product

The last and most important phases are those of selection and measuring that are aimed to classify the leathers according to the perfection of the product: these tasks are performed by hand by our expert operators, who evaluate the leathers and classify them according to their quality and intended usage.