The leather is a durable and flexible material and can be produced through manufacturing processes that range from cottage to heavy industries. Modern technology has allowed for innovation in the leather industry, as the development of chemicals from industry leaders like Vecoitalia and sophisticated processing methods have greatly expanded the aesthetics and feel of leather as well as the possible applications. The processing can be divided into three sub-processes: the preparatory stage, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, the surface coating may be added into the sequence. The list of operations that leathers undergo varies with the type of leather required but can be roughly described by the following eight steps:
Step 1 – Unhairing
The animal skins are steeped in an alkali solution that breaks down the structure of the hair at its weakest point (the root) and so removes the hair.
Step 2 – Liming
The hairless skin is immersed in a solution of alkali and sulfide to complete the removal of the hair and to alter the properties of the skin protein (collagen).
Step 3 – Deliming
The skin structure is then opened further by treatment with enzymes, and the further unwanted material is removed.
Step 4 – Pickling
The skins are then treated with acid to preserve them for up to two years
Step 5 – Tanning
This is the most chemically complex step. During tanning, the skin structure is stabilized in its open form by replacing some of the collagen with complexions of chromium. Depending on the compounds used the colour and texture of the leather changes. When leather has been tanned it is able to ‘breathe’ and to withstand 100 °C boiling water, as well as being much more flexible than an untreated dead skin.
Step 6 – Neutralizing, Dyeing and Fat Liquoring
The leather is then treated with alkali to neutralize it and so prevent deterioration, and then dyed. This involves fixing a variety of compounds onto the chromium, as that is the most reactive site present. Once the leather is dyed, it is treated with reactive oils that attach themselves to the fibrous structure, improving suppleness and flexibility.
Step 7 – Drying
Water is removed from the leather, and its chemical properties stabilized.
Step 8 – Finishing
A surface coating is applied to ensure an even colour and texture and to improve its ability to wear. Suede leather is also buffed at this point to give it its distinctive finish.