The principal methods of making leather haven’t changed that much over the years, but that doesn’t mean it is easy. Tanners are highly trained in what they do, and it is a surprisingly complicated and lengthy process! BLC leather training courses can help your understand the processes.
To get from a salted hide to a piece of leather ready for use in a sofa takes 10 working days!
The steps below show you what tanneries have to do to turn hides into leather :-
Raw hides and skins must be preserved to stop them deteriorating before the leather-making process can begin. Methods of preservation include salting, chilling, freezing and the use of biocides.
Cured hides or skins are soaked in water for several hours to several days. This allows them to reabsorb any water they may have lost in the curing process or during transportation. It also helps to clean them of salt and dirt.
Painting is a method by which wool can be removed from sheepskins using a sulphide based mixture.
Liming removes the epidermis and hair. This also results in alkaline swelling of the pelt to cause a controlled breaking of some of the chemical crosslinks of the collagen .
After liming the pelt is passed through a machine to remove fleshy tissue from the flesh side. Hides may be split into layers at this stage or after tanning.
The principal action of deliming is to gradually neutralise the alkali in the pelt, avoiding rapid changes in pH which could lead to distortion or disruption of the tissues.
A long delime can significantly improve the removal of any remaining lime, scud (miscellaneous debris) and residual components broken down during liming. Bating – based on the use of enzymes – completes this process so that the pelt is flat, relaxed, clean and ready for pickling and tanning.
Weak acid and salt solutions are used to bring the pelt to the weakly acid state required for most tanning processes. Stronger pickling solutions are used to preserve pelts so that they can be stored or transported in a stable form over periods of several months.
Solvents or water-based systems can be used to remove excess grease before tanning.
Tanning converts the protein of the raw hide or skin into a stable material, which will not putrefy and is suitable for a wide variety of purposes. Tanning materials form crosslinks in the collagen structure and stabilise it against the effects of acids, alkalis, heat, water and the action of micro-organisms. The main types of tanning materials are :
Most leather is tanned using salts of chromium.
Aldehyde and oil tannages
Tanning with aldehydes and oils produce very soft leathers and this system can be used to produce drycleanable and washable fashion leathers and also chamois leather.
Various plant extracts produce brown coloured leathers which tend to be thick and firm. This type of tannage is used to produce stout sole leather, belting leather and leathers for shoe linings, bags and cases.
A splitting machine slices thicker leather into two layers. The layer without a grain surface can be turned into suede or have an artificial grain surface applied.
A uniform thickness is achieved by shaving the leather on the non-grain side using a machine with a helical blades mounted on a rotating cylinder.
Neutralising removes residual chemicals and prepares the leather for further processing and finishing.
Additional tanning material may be applied to give particular properties which are required in the finished leather.
The dyeing of leather into a wide variety of colours plays an important part in meeting fashion requirements. Some leathers are only surface dyed, while others need completely penetrated dyeings, as is the case with suede leathers.
Fatliquoring introduces oils to lubricate the fibres and keep the leather flexible and soft. Without these oils the leather will become hard and inflexible as it dries out.
This process reduces water content to about 55% and can be achieved by a number of machines, the commonest being like a large mangle with felt covered rollers.
The leather is stretched out and the grain side is smoothed. This process also reduces the water content to about 40%.
Leather is normally dried to 10-20% water content. This can be achieved in a number of ways and each method has a different effect on the finished leather:
Staking and dry drumming
A staking machine makes the leather softer and more flexible by massaging it to separate the fibres. To finish off the leather may be softened by the tumbling action inside a rotating drum.
Buffing and Brushing
The flesh surface is removed by mechanical abrasion to produce a suede effect or to reduce the thickness. In some cases the grain surface is buffed to produce a very fine nap, e.g. nubuck leathers. After buffing the leather is brushed to remove excess dust.
The aims of finishing are to level the colour, cover grain defects, control the gloss and provide a protective surface with good resistance to water, chemical attack and abrasion.
Leather will be graded before despatch to the customer. This grading may consider the colour intensity and uniformity, the feel of the leather, softness, visual appearance, thickness, design effects and natural defects such as scratches.
The area of each piece of leather is measured by machine. Nearly all leather is sold by area so accurate measurement is important.