The leather production process
The tanning industry enables a by-product of the food industry to be recovered and made into something special and noble.
Leather tanning is without a doubt one of the oldest human activities. In the beginning, skins obtained from hunting and livestock breeding could be used for clothing as a protection from the atmospheric elements.
The tanning process has undergone many changes from prehistory to today, especially in the twentieth century, when industrialization and new machines have allowed development in the research for specific and less polluting chemicals and new methods of tanning and finishing.
Leather is produced traditionally even today. The skin, discard of the food industry, is "recycled" from the tanneries and processed with advanced machinery and vast research, in such a way to make it a “noble” and fashionable material. There is a large number of tanneries, but the uniqueness of the result is given by the ability and the skill of experienced craftsmen.
Immediately after killing the animal, in order to avoid degradation processes in the tissues, the skin is salted, dried or refrigerated before the production process of tanning starts. Skins are salted with common marine salt, which penetrates very quickly into the fibers, helping to a partial removal of water. This is a very efficient and economical process , easy to apply and widespread. The drying system is to eliminate as much water as possible from the skin, so to avoid the development of micro-organisms and bacteria. The drying system is more suitable for sheep and goat skins, while less to preserve cowhides.
Defrost is another method , but it is not common in Italian slaughterhouses: this system cools the skin at very low temperatures
The leather manufacturing process can be basically divided into three major phases: tanning, retanning and finishing.
All chemical tanning operations are used to stabilize the skin making it unputrescible and are carried out with the use of water in the drums.
The drum is the typical machine for tannery : it consists in a cylinder rotating around its own axis , filled with water, chemical reagents and skins.
The drum used to be a simple wooden cylinder and has now transformed into a complex computer-controlled machine that regulates the input of water, the temperature, the speed of rotation, and the release of chemical reagents, thus avoiding manual errors.
THE LEATHER PREPARATION PROCESS
The preparation for tanning, starting from preserved raw materials, may be divided as follows:
Leather soaking: Once cured, the skins are then soaked in water for several hours to several days. The water and surfactants help in the removal of salt, dirt, debris, blood and excess animal fats. Rehydration is also reintroduced.
Leather fleshing: subcutaneous material is removed.
Leather unharing: the majority of hair is removed.
Liming is used to loosen the fibers allowing the skin to absorb chemicals that will be used later in the tanning processes. Limed hides appear swollen and with an increased thickness, therefore can be easily split into two or more layers. The splitting is necessary for the heavy cow hides, which are too thick to be used in the production of manufactured items. The process is carried out with the splitting machine in which the limed skin is pushed by two cylinders against a band blade which cuts the skin in two layers parallel to its surface.
The top layer is the grain (typical design of the skin of the animal, the external part) this is the full grain part , while the lower layers are the flesh split. In general, the thinner skins (sheep, goats and calves) are skived but not split.
Deliming: this process brings to removal of alkali from the pelt with the consequent deswelling of the fibers and helps lowering of the pH to the values used in the bating process. it Is carried out with slightly acidic chemicals.
Bating : is an operation to complete the deliming process, by eliminating residues of other substances and loosen the fibers of the skin, in order to smooth the grain and achieve a soft and flexible leather.
Degreasing: is carried out to remove and to reduce the natural fatty acids from the skin, which could lead to difficulties in the absorption of chemicals. The degreasing process removes excess fat and distribute it evenly .
Tanning is the process which converts the protein of the raw hide or skin into a stable material which will not putrefy and it is suitable for a wide variety of end applications, the leather.
There are several types of tanning: chrome tanning is the most widespread. The duration of the Chromium tanning is around 2 or 3 hours for small and thin skins, up to a maximum of 24 hours for thicker ones. At the end of the tanning the skins appear blue-green. This is called wet-blue and at this stage it can be sold.
The vegetable tanning is the oldest, made with the use of tannins which give the vegetable tanned leather shades of brown, more or less intense.
There are two types of vegetable tanning:
- slow tanning in which the skins are immersed in tanks containing solutions of increasing concentrations of tannins. This method takes about 30 days and is meant to produce leather for soles, very thick and stiff leathers.
- rapid tanning , made ??in the drums which , due to the rotation, creates a more flexible leather suitable for any kind of productions. This process lasts about 36 to 48 hours.
Re tanning:
The tanned leather is not yet usable to produce articles.
It is still wet and even if dry , it would be too rigid and colored with the typical color of tanning used.
To turn it into a marketable product the leather must be further treated with chemical and mechanical processes in the drums.
Drying process: is a mechanical operation which eliminates most of the water soaked by the tanned leather, by pressing the skins between two cylinders , where the top one is covered with pewter.
Shaving : this operation smoothens the thickness of the whole surface of the skin and eliminates the residues of fleshing. This operation is carried out with a cylinder machine where the top one is provided with helical blades.
Splitting - the leather is split into one or more horizontal layers. This can be done after liming or after chrome tanning. The choice depends on the product we want to achieve.
Re tanning - additional tanning agents are added to impart properties and change the features of the main tanning : to achieve a thick and stiff leather, skins need to be re tanned with vegetable tannins- For a soft and flexible leather , re tanning needs to be with chrome .
Dyeing is the process which gives the leather the requested color. It Is made in the drums with dyes.
Stuffing - fats/oils and waxes are added between the fibers, thus giving the leather the flexibility and the softness needed for the various products.
Drying : all above mentioned processes are carried out with water so leather needs to be dried. This can be done to various moisture levels, by simple exposure to air , by temperature controlling or by vacuum. All drying systems aim to achieve a uniform drying and constant timing. The choice of the drying system depends not only on economic factors but also on the end-use material.
FINISHING:
It 's the final stage and the most complex process, which includes all operations to be carried out on dried skins, to change the surface effect, both for aesthetic and functional aims. Finishing can be mechanical or chemical.
Mechanical finishing operations may include:
POLISHING : to create a shiny surface by rubbing it with a velvety wheel
IRONING and PLATING: to obtain a flat and smooth surface
EMBOSSING :to obtain a three-dimensional print
TUMBLING by rotating the drum quickly to create a more evident grain and a smooth surface
Chemical Finishing involves the application of a film, natural or synthetic by using curtain coaters, roller coatings and spraying.