Leather processing

In modern days as in the past, tanneries need to use large quantities of water.

This is why since the beginning, tanneries developed in areas with abundant water sites, particularly in the regions of Novara, Vicenza, Pisa and Avellino.

To give an idea why water is so important we shall consider that if we sum up the volume of hides, water and chemical products, water represents about 60% of the total, which means that the weight of water exceeds that of the hides!

Tanning processes can be divided into 3 main stages:

The first stage prepares the raw hides for tannage throuh the processes called banks works: the name was not given by chance because it refers to those operations that once were done along the bank of a river, the waste being thrown directly into the water.

The second stage includes the tannage, prepared with chrome salts or aldehydes (mineral tannage) or with tannins (vegetable tannage), followed by re-tannage, dyeing and fattening: the whole area where these operations are carried out is called the "wet area" due to the large quantities of sewage which once, but in smaller quantities even today, stagnate on the floor.

The last stage, called finishing, aims to ennoble, improve and protect the hide from external agents, and it is carried out, unlike the other stages, in dry areas.

Now we shall consider each single operation, starting from when the hide is flayed from the slaughtered animal.

Preservation - The raw hide, after being pulled off, is salted or brine salted in order to protect the hide from micro-organism degrading penetration and also to allow a longer warehouse storage.

Arrival in the tannery - Once the hides arrive in the tannery, experts inspect the goods and control if they correspond to the contract terms.

Soaking - The hides are loaded in drums with water and recovering products which allow to reset the normal swelling condition of the fibres of the hide, lost during storage, and also to clean the hide from the dirt accumulated during life and transportation.

Liming process - This operation is performed in drums filled up with lime and sulphide in order to remove the hair from the surface of the hides. It also softens the fibres, creating a partial soaping of the natural grease of the hides.

Fleshing - It helps to clean the flesh side of the hides from the grease and from the flesh which may be left after flaying. Nowadays, this operation is performed by automatic machines which eliminate the residual called "carniccio".

Splitting - This operation divides into two or more parts the section of the hide: one side is the grain (more valuable) and the other side is the split (less valuable as it is less thick and resistant). The splits are sold to companies which are specialised in their processing. When hides are particularly thick one can also obtain a third, even thinner, section, called second split. This operation can be also performed right before the shaving (see below).

Tanning - The purposes of this operation are the irreversible stabilization of the fiber of the skin, otherwise destined to putrefy, and the increasing of the strength characteristics.
There are two main types of tanning: vegetable and mineral, the latter itself mainly divided into chrome tanning and "white" tanning. All these types of tanning are performed in large drums of 3-4 m in length and diameter.
Vegetable tanning is one of the oldest tanning systems and it is still used, mainly for the production of footwear leather. This tanning system involves the use of organic substances which are contained in the wood, in the bark, leaves, etc. Alongside the natural tannins are artificial tanning substances, whose chemical composition is somewhat similar to the natural ones.
Chrome tanning has assumed considerable importance and it can be said that most of the skins are nowadays transformed in leathers by treatment with chromium salts. The skins so tanned present with a finer grain, a tighter fibrous tissue, a rubbery feel characteristic, all qualities that the tannins are not able to provide; also the tanning process is faster, simpler and better controlled. Chrome tanned leathers have a characteristic blue color, and are therefore called wet-blue. The chromium used is trivalent type and not harmful to health, not to be confused with the hexavalent chromium.
The "white" tanning may be carried out with aluminum salts or zirconium salts and produces a white color leather, both in surface and in section, thus called wet-white. This leather have similar physical and technical characteristics to the wet-blue.

Samming - The hide, after tanning, is completely full of water: the samming operation is used to squeeze the hide between two felt rollers in order to reach an optimum moisture content.

Selecting after tanning - It means selecting the hides according to their defects. It is a delicate operation which requires a person with great experience. We can easily say that sorting is  80% of the determining factor in the loss or profit of a tanner and the other 20% is a result of the other operations.

Shaving - This operation is carried out by a machinery, called shaving machine, consisting of a rolling cylinder with cutting blades which shave the hides to the desired thickness on the whole surface.

Dressing / Dyeing - In this stage the final product is achieved: its softness, its hand, its grain structure and of course the basic dye. Many products are used during this stage but the main ones are fat liquors and tannins which, added in different percentages in the drums together with hot water, give to the hide the above mentioned features.

Dyeing is always performed in the same drum in batches added with metal-complex dyes, which colour the hide by piercing the dye throughout the hide section (passing-through dyeing).

Drying - All the hides before this stage are processed in humid/wet conditions: the next stage is the finishing, carried out strictly in dry conditions. In order to proceed with the finishing it is necessary to go through an intermediate stage: drying.

There are two methods for the drying: the nailing and the vacuum-packing.

During nailing hides are laid out on a reticular panel, fixed with pliers. They are then put into a big tunnel oven at temperatures around 60° C. After about four hours the hides will come out dry. The pliers are also called "nails": this is why a dyed hide which is dried with this procedure is called "nailed leather".

Vacuum-packing consists of laying the hides on a flat surface heated at 50/60° C. The hides are covered with a sort of bell chamber from which the air is extracted: after about 5/6 minutes the hides will come out with a residual humidity of 30/40% and will be further dried on air chains for 8/9 more hours.

With the nailing method the hides are stretched, so their surface will increase, thus compensating the cost of the drying. However they are weakened, and therefore they will show an irregular grain, especially on the flanks.

With the vacuum-packing method, the hides shrink since they are dried almost free of constraints. This is a very expensive method, which is balanced by the fact that the hides will be much more regular grained, smoother and softer.

Typically nailing will be used for cheaper hides, or with the ones with pronounced grain, natural or corrected. On the contrary, vacuum-packing will produce full grain hides, softer and more uniform: the more valuable ones.

Stackering - After drying, the hide looses some of its softness which is re-established by the stacker. This machinery separates the fibres left after the drying by "chewing" the hide through two toothed metallic plates. We have used the expression "chew" since the effect of this machinery is very similar to the mastication process of the eskimos, to soften the hide in the areas which have to be sewed.

Buffing - In order to prepare the hide to better absorb the finishing and to remove the grain defects, the leather is rubbed on its surface with thick emery paper depending on the needed article and on the defects to be removed.

"Buffed leather" can have different meanings: it can either be used to indicate corrected grain, clean grain, etc., or it can refer to a processing of the hide (the buffing) which does not say anything about its quality.

In fact there are hides which need to be deeply buffed in order to remove large, widespread defects which are then further covered with a heavy finishing.

On the contrary, there can be good quality hides which are buffed (less and with a thinner emery paper) only to remove small imperfections and make the surface levelled: for this kind of article, which is called polished grain, different expressions are used like "half grain" or "retouched grain" which technically have no meaning but they show that for that article the grain has been slightly corrected.

Finishing - This is the third main stage of leather processing and could be called "maquillage", the final touch of the artist-tanner who through the finishing will define the exact colouring, the touch, the print, the fashion effects, and the protection against wear, humidity and stains.

The most popular finishing is the spray finishing, where hides are laid on roller conveyors and sprayed with a mixture of pigments, resins, polymers and various additives blended with water which create a protective film that after being oven dried will make the leather ready for the final use. Spray finished leather have an optimal compromise of physical resistance and a pleasant touch.

Other types of finishing, which lead to different final effects, are the following.

Hand buffing: the hides undergo a manual process where the operator apply or remove colour with a sponge, creating a bi-colour effect.

Veiling: the colour is applied with a thin cascade of paint, which create a thicker finishing.

Foil: a roller applies a transfer paper, with pressure and heat, to the hides, to create a multitude of different effects 

Milling - The hides, once finished, need to be loaded in big dry drums for the milling operation. Here they will remain, rolling for eight/ten hours, in order to re-establish their softness and naturalness.

Final sorting / Trimming - Once the finishing is over, the hides are controlled and trimmed one at a time by skilled selectors who will measure, stacke and forward the hide to the final customer.

In this last part we have only marginally talked about measuring, but it is necessary to dedicate further space to such an important operation, since the finished hides (this operation can also be carried out in the same way for crust) are paid to the supplier according to the surface extension, and therefore it is important to understand which unit of measurement has been adopted and, furthermore, one has to be sure that the measurement has been taken properly.

Until not many years ago, the unit of measurement commonly used was the square foot. To talk about a common choice could lead one to think of a clear and definite unit of measurement, equal for everyone: absolutely false! In the adoption of the square foot the only real common thing is the name of the measure unit of measurement, but not its dimension. In fact, there are the English foot, the Italian foot, the Russian foot, the French foot, all of them different from the others. The "right" foot should be the English one, which is 12 inches long i.e. 30.48 cm. and therefore one square foot comes out to 30.48x30.48 square centimetres. We have used the conditional because the unlimited Italian imagination means that several different interpretations exist regarding the dimension of one foot: if in Veneto it is quite confirmed that it is 30.48 cm. long, in Tuscany it could be 30.30 cm. while in Campania it could come down to 30 cm. It is obvious that with such a confusion, the obliged adoption all of the metric system throughout the EU has been very useful not only in order to fix one unit of measurement for everyone, but also in order to clear away any personal interpretation, since there is only one meter and we all know how long it is. Being the square foot equal to 30.48x30.48 sq.cm., in one square meter there are exactly 10.764 sq.ft.. Should we should use a square foot of 30x30 sq.cm., we would discover that one square meter comes out to 11.111 sq.ft.: it seems a small difference but actually it is a 3.2% difference. This means that while adopting a square foot of 30x30 sq.cm. the tannery can practice a lower price of 3.2%, while obtaining from one hide the same total amount: it is therefore a totally fictitious price difference. Despite all these efforts of unification, there is still an area where measuring is something open to question: the sole leather sector. Sole leather is not actually measured by its surface but according to its weight: one croupons weighs so-many kilos, which correspond to how many sq. mt.? It is not clearly specified, it depends.

It depends on the compactness: the higher the compactness the heavier it is, with the same surface measurement.

It follows that also for upholstery high compactness hides are defined as "heavy hides" and consequently for  finished leather compactness and weight are now synonymous.

It depends on the types of tanning: if the tanning has a strong compactness effect, it will tend to make the hide shrink and therefore the leather will surface have a smaller surface area with the same weight.

The unit of measurement defined, we can now see which equipment is used for the measuring. Until the 50's the square was used. It was a rectangular wooden loom which supported a squared-meshed net delimited with iron wire. Every square measured a quarter of a foot. The squares not fully covered by the leather were calculated... approximately.

The smallest tanneries used another system. They covered the surface with carton squares: each carton square measured a square foot. A great improvement in the precision of measuring came with the introduction of the mechanical measurer. It was made of two cylindrical structures with parallel axes: a single lower cylinder which supports and drags the hide to measure and above a series of wheels side by side on the same axis, each with radial pegs. The pegs are retractable: during their rotation when they meet the leather below them, they are pushed inwards, giving in that way the signal to the machine of the length of the leather strip that each wheel has measured. Being aware of the width of every wheel, the multiplication between the width of the single wheel and the length of the leather strip gives the area of the single strip: the sum of each strip gives the total surface of the leather. In mathematical terms, we can say that this is an integral measurement. Modern electronic machines of measurement are based on the same principle but instead of pegs the wheels have laser readers. With all the above described types of machine, in order to have a correct measurement, the leather has to be stretched tightly when entering the machine as leather is a product with a certain elasticity. There are also some electronic machines in which the leather is laid upon a tapis roulant, and so it passes through without physical contact, under a bar with laser readers: in this case the leather is only laid out and not stretched, so this type of machine is best indicated for stiff leathers.

Because leather is an elastic product and subject to a natural loss, if we check the leather measurements some days after from the measurement in the tannery, we may find some differences.

International customs allow a tolerance between the measuring made by the tannery and stamped on the back of the hide and the further checking measurement: this tolerance is +/- 3% for soft hides and +/- 2% for stiffer hides.