Technical craftsmanship of LEATHER and HIDE

Preserve and work the leather: The tanning process

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Category: Technical craftsmanship of leather and hide

Published on Tuesday, 01 December 2015 10:57

The leather that arrives in the tanners, besides having the characteristics of animal, it’s at its natural stage, and therefore, passing of the days of putrefaction.To be able to be worked and preserved raw leather it must necessarily be “tanned” with chemical and mechanical processes on the one hand and with the aid  of animal or vegetable origin substances on the other.The treatment of the leather is a procedure very ancient, which dates back to prehistoric times when leathers were used for shelter from weather elements.Still the tanning business is carried out in the traditional way, even though large industries and tanneries use large machines and more specific chemicals.

The tanning process could be synthesized in five phases: preservation, riviera, tanning, post tanning, finishing touch.

PRELIMINARY STAGE OR PRESERVATION

After the animal skinning, the raw leather it is treated with salt to preserve it from bacterial action, during the transport to the tannery. For the treatment of raw leather with salt, it’s used ordinary sea salt, that penetrates the leather to help eliminate some of the water.
In the preliminary phase to the salting, follows the drying, to eliminate the most water possible from the leather, so as to preserve it from the attack of microorganisms and bacteria.

Three main phases, characterize the tanning process:  

RIVIERA PHASE

This is a preparatory stage, where the leather is prepared for tanning.
The name of this phase it remained unchanged, whereas in ancient times, the preparatory activities were carried out for convenience on the river bank.
In Riviera processing is done: revive, liming, fleshing and splitting.

• Revive: The leather needs to recuperate all the water lost during the preservation phase, has to be cleaned of excess parts and         especially from impurities.
  This procedure is done to bring back the leather in its original state, they are put in barrels, big wooden containers that turn mechanically on themselves.
• Liming: The leather is dipped in lime.
  This procedure is done to prepare the leather to be able to absorb the tanning products and to eliminate excess parts. With liming you obtain, depilation, in which it dissolves the hair and excess protein. To make the leather treatment easier, the parts of the leather that is not transformed into hide it’s eliminated.
• Fleshing and trimming: With the first procedure, the leather is passed in a rolling cylinder to be refined, while with the trimming, it is eliminated the raw and excess parts from the leather that is being worked.
• Splitting: it’s one of the most important processes, in the leather processing, it consists in cutting horizontally the leather into two layers: The above one is the flower and the bottom one is the famous rind.
  
  TANNING PHASE

This phase is the core of the leather working. It's a chemical operation which transforms the leather in perishable into an unalterable material.

The tanning process consists of soaking the leather with chemical or vegetable substances that prevent the decomposition without altering the flexibility and the structure.This process is done by putting the leather in a wooden container that rolls.
Here the tanned products eliminate the raw fibres, making the product more durable, softer and especially more resistant for the following processes.

There are different types of tanning:

• Vegetable tanning: it’s one of the oldest techniques. In this type of tanning it’s used tanning substances (the tannins are extract from trees) that give the colour brown to the leather;
• Mineral tanning: it’s composed by minerals, especially, chromium salts, aluminium, sulphur or silica;
• Chrome tanning: it’s the trivalent chromium technique that gives the leather a light blue colour;                                                      
• Other types of tanning: oil, titanium and iron...

 POST TANNING PHASE:

 After tanning, is the following, all the treatments that allow improvement of the leather usable and saleable.

• Pressing: The leather is squeezed to eliminate the water that accumulated during the tanning.
• Trimming: The leather is brought to the wanted thickness. Trimming the material considered extra. This way makes the product that you are working a better quality.
• Re-tanning, dyeing and fattening: The re-tanning consists of filling the pores, to compact and to smooth the leather and to improve the leather’s appearance. They are used soft and flexible pastes, to give greater fullness to the leather.
• Drying: That is the process in which the leather thatwas treated it must be dried. There are three different kinds of drying: The Wind one (the leather is passed through two smooth heated plates), Nailing ( the leather is nailed to a drying rack) and at last the Chain (the leather is hung by a chain and put to dry naturally).
• Stretching process: The leather is softened by a machine, that takes a mechanical beating, this is because the leather is hard to the touch after drying.
• Dry milling: In this process are used wooden barrels where you can control the temperature and humidity values. The leather becomes softer, because in the barrels they are mechanically tossed from the turning of the barrels.
• In the process of processing, the leather undergoes once again a process of grinding, scraping the surface with specific paper, in order to make the leather writable or printable.

 FINISHING PHASE

At this stage the leather acquires the characteristics of brightness, colour and appearance you want and it can be mechanical or chemical type.
The mechanical it can be done in different ways:

• Ironing if you want a surface flat and smooth;
• Polishing of the surface with a velvet wheel, to get a smooth surface;
• Drumming where the leather is dried spinning rapidly;

This technique is used when you want a surface with movement and the flower is evident, this way the leather acquires softness and resistance.

The chemical finishing, instead, consists of coating the surface of the leather with a film, more or less thick, synthetic or natural  dyes or paints. These aspects, together with the definition of leather trimmings, are mastered during the ironing and finishing stage.
At this point, before the leather is delivered to the masters, in the tanneries, are done quality controls: for the characteristics of the surface, thickness, as well as physical-chemical test on the basis of national and international standards.

After this analysis of the treatment process and preservation of the leather, you just need to watch the video made by FeniceCareSystem, where you can see concretely which are the processes the leather undergoes, in order to be preserved and processed. GOOD VISION!

Video La lavorazione della pelle e del cuoio : https://www.youtube.com/watch?v=hJyOtb84ctI

* foto di Shafiqbrothers

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