Difference between revisions of "Leather production"

Latest revision as of 18:22, 13 July 2023

The leather production process is lengthy and involves several steps which have to be carried out before the actual tanning is done. The most important stages in converting the raw animal hide to leather are listed below. According to a tanner, furniture leather undergoes between 35 to 55 different processes, depending on the desired leather type. Each step is very important and regular checks need to be carried out to ensure the best quality of leather is produced.

Preservation by drying, salting or freezing

Rawhides have to be conserved after the animals have been slaughtered and before they reach the tannery. In some cases, they can be stored for a while. These rawhides are checked for quality and also sorted by weight before arriving at the tannery.

Additional information to this processing step

• Preservation by drying, salting or freezing
• Rawhide dealers - Skin collectors

Beamhouse Operations

The process of preparing the hides for tanning is known as beamhouse operations (wet production). This involves all stages mentioned below between preservation and tanning.

All skins and hides which are completed as finished products, without undergoing any tanning, retain the characteristics of untanned skin including parchment. One common by-product derived from rawhide is dog chews.

Soaking

The hides are soaked in clean water to get rid of any dirt and mainly to remove the salt applied in the earlier stage. Curing (salt conservation treatment) also removes water from the hides and soaking them makes the hides swell and return to their original softness. The hides can be treated for several days, depending on the amount of salt and the dryness of the skins. Salted bovine hides need several hours and dried hides need several days.

Additional information to this processing step

• Soaking

Liming

One of the most important steps in beamhouse operations is ‘liming’. The hides are soaked in liming drums which contain a solution made of lime and sulphur compounds. The main purpose of this process is to separate the hair from the hides. The net result is hairless hides which are very greasy, mainly because they still have a high fat content.

After liming the hide is hairless.

Additional information to this processing step

• Liming

Fleshing

By this stage, the hides are still in a soaked yet softened condition. Traditionally, using a sharp but curved blade, any excess flesh was removed by placing them on a beam. However, this process is now done by machine. Any debris that is left over is referred to as ‘glue stock’.

Additional information to this processing step

• Fleshing

Splitting

To get a uniform thickness, the hide/skin is cut into two or more horizontal layers. The top layer, or grain side will produce a fine, smooth grain leather. The bottom is used for suede or split leather for other uses.

Additional information to this processing step

• Splitting

Pickling

The Pickling process makes the fibres of the hides and skins more receptive to tanning.

Additional information to this processing step

• Pickling

Tanning

In the tanning process tannins are absorbed by the hides and skins and turn them into leather. By tanning the skin is irreversibly chemically preserved and converted to the material leather.

Additional information to this processing step

• Tanning leather

Neutralising

Leftover acids from the tanning are neutralised.

Additional information to this processing step

• Neutralising

Withering - Samming

Residual water from wet skins is pressed out by forcing it through metal rolls. There are various methods to dry the leather in the intermediate steps of the leather production in the tannery. Depending on the requirements, the splitting of the leather can only be carried out after the samming process.

Water from wet leather is drained by squeezing.

Additional information to this processing step

• Drying leather

Sorting

The leather then undergoes a quality check and is sorted according to the quality. Hides which do not have any blemishes or marks are best quality and can be processed to aniline leather. The ones which have marks may be processed into corrected grain.

Aniline leather must be flawless. Even the smallest damages in the main surface are sorted out.

Shaving

In the shaving process the leather is thinned and equalized using a leather shaving machine which cuts off leather fibres from the flesh side.

Additional information to this processing step

• Shaving

Dyeing - Fatliquoring - Re-tanning

Depending on the type of leather, the leathers are coloured using aniline dyes (through coloring by drum dyeing), the softness of the leather is controlled by fatliquoring (regreasing) and the retanning improved the desired properties of the leather.

Drying

For the drying of leather in the individual steps in the tannery there are many different methods. Vacuum drying and suspended drying in ovens are the most common.

Vacuum drying - Suspended drying - Stretching and drying frame in ovens

Additional information to this processing step

• The various methods of drying in leather production

Softening

After drying, the leather is further softened on different machines (tumbling, stacking).

Tumbling - Stacking machine

Additional information to this processing step

• Softening by tumbling and stacking

Finish - Surface colouration and surface processing

Depending on the required specifications, the surface of the leather has to be treated further. Pigmentation, pressing, ironing, embossing, adjusting of gloss level etc.

Additional information to this processing step

• Finish - Surface colouration and surface processing
• Leather colour
• Ironing leather
• Embossing leather

Final check

Final re-check of product quality according to the specifications of the type of leather and/or the customer and size measurement of the skins are carried out. Then the leather is stored or packed and sent to the customer.

Final inspection (photo by LEATHER NATURALLY) and size measurement of the skin area.

Videos about the leather production

The leather production in a modern tannery.

The brain tanning process.

The leather production with tannins from oak.

Chamois leather production in Germany.

Additional information

• Tannery
• Tanner
• Leather chemistry
• Wastewater from tanneries

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