Tanning Leather

THE LEATHER MAKING PROCESS

British Standard BS 2780 Definition of Leather:

“A general term for hide or skin with its original fibrous structure more or less intact, tanned to be imputrescible. The hair or wool may, or may not, have been removed. Leather is also made from a hide or skin which has been split into layers or segmented, either before or after tanning, but if the tanned hide or skin is disintegrated mechanically without combination of a binding agent, is made into sheets or other forms, such sheets or forms are not leather. If the leather has a surface coating, this surface layer, however applied, must not be thicker than 0.15mm”

The actual process of making a hide or skin into leather is not just one simple task but a number of processes. These different processes can be broken down into four sections:

• Beamhouse Operations
• Tanning
• Post Tanning
• Finishing

BEAMHOUSE OPERATIONS

The beamhouse area of a tannery is for the removal of unwanted materials from the hide, such as hair, non-structural proteins and fleshy tissue.

Soaking:

This process is done to remove dirt and blood from the surface of leather. The hides are soaked in water kept at 20 degrees Celsius to avoid shrinkage and detergents can be added with an alkaline chemical that aids the uptake of water. This process also moisturises the hide.

Unhairing & Liming:

Again the hides are soaked in alkaline conditions, this is to loosen the hair (unhair) and to swell the collagen fibre and break down non structural proteins that would harden the final leather if not removed.

Fleshing:

The hides are washed for fleshing. They are passed through a machine that removes unwanted flesh, connective tissue and fat.

Splitting:

The hides can now be split into two. The top layer is used in most upholstery as it contains the grain pattern or can be corrected and have an embossed pattern applied to it. The bottom layer (split) is then used for suede or can have a pigment applied to it and becomes a finished split (commonly used on the backs & sides of upholstery).

Deliming:

The hide is prepared for tanning by reducing the swollen and rigid fibres. This is done by removing the alkali with acidic chemicals such as ammonium chloride or ammonium sulphate.

Bating:

This process gives the hide a smoother, flatter grain appearance and it is also the stage that makes the final leather softer and stretchy.

Pickling:

This process is used to adjust the hides ready for tanning. This is done by introducing an acid into the hide to lower its pH. The mixture may also contain salt to further reduce the swelling.

TANNING

This is the process most people are familiar with, and is the most important part of making leather. Remember the definition of leather? If a hide or skin is not tanned it cannot be classified as leather.

The objective of tanning is to convert the collagen protein of the hide into a stable material, which will not putrefy and will be stable under conditions of heat and moisture.

There are a good few different methods of tanning hides but for upholstery there is only one main one you need to know about – Wet Blue Also known as chrome tanning. This is the name given to leather that has been tanned using chromium salts. Why? Because it is both wet and blue after being tanned!

The hides are processed in large drums (usually these drums can take up to 300 hides at a time). The blue colour in the chromium binds to the collagen in the hide making it blue in colour. The hide is fully tanned when it is resistant to heat and will not shrink at 100 degrees Celsius. The benefits of this kind of tannage are good strength, light fastness and heat resistance. It is these properties that make wet blue the most popular method of upholstery tanning.

At this stage the hides must then dried to remove some moisture. This is done by passing the hides through a machine with felt pads that squeezes the moisture out.

POST TANNING

Neutralise:

Mild alkalis are added to the leather, which prepare it for later chemical processes..

Dyeing:

Simple – the leather is dyed in drums to give it colour! Anionic dyes are very common as they are negatively charged and so latch onto the chrome leathers well.

Fatliquoring:

This process uses fats and oils to lubricate and soften the fibre structure of leather. The manner in which the oils are introduced into the hide coat every fibre. This is where leather gets its smell!

Drying:

The leather is then dried. This can be done by hanging it on pegs or laying it on a board and applying heat. This method is poplar as it can dry a hide in just a few minutes.

Staking:

This process softens the leather. The leather is passed over a series of blunt pins that pummel and flex the leather to soften it up.

FINISHING

Finishing leather is the process of applying coatings to the surface of leather, the benefits of finished leather are:

• Protective surface aids durability
• Easier to care and maintain
• Improves water resistance
• Masks defects
• Enhances colour and appearance

There are three processes involved in finishing leather: Buffing, applying a finish and embossing a pattern. Not all these processes occur, which varies the different types of leather.

BUFFING

If the grain surface of the leather is in a good condition, it may be left unfinished and is known as full grain leather. However there are so few hides good enough quality to be classed as full grain that the price is very expensive for such upholstery.

The hide (on a cow) can get scratched, bitten and cut. All these imperfections show on the grain and need to be repaired. The damages are filled using the 'leather repair compound' (a flexible filler), the leather is then left to dry and then the grain side of the hide is passed through a machine with abrasive material to buff away the imperfections. The type of leather is known as correct grain.

Buffing the hide also creates an excellent key for pigments to adhere to and so serves more than one purpose.

APPLYING FINISHES

On corrected grain leather a pigmented finish is applied to give a sound covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution, either acrylic or solvent based, which contain binders to adhere to the leather.

After applying the pigment a lacquer is then applied to seal the colour in, giving the leather more protection and durability – not only that but the lacquers are used to adjust the gloss level of the leather.

The pigment can be applied to leather in three ways.

• Sprayed on by hand using an airgun.
• Sprayed on by a machine on a conveyor belt.
• Pressed on with rollers. Often a combination of part 2 (base coat) and part 3 are used.

The lacquers are sprayed on by machine or by hand with an air gun.

EMBOSSING

After the application of finishes on leather an artificial grain pattern can be applied to the leather. This is done by placing a metal plate containing a grain pattern on top of the leather and pressing it down onto the surface under pressure and heat. Or, by running the leather through rollers with an etched grain pattern on them.