Here is a very brief description of the steps that are used. Each of these steps is important and reacts with the follow steps.
When the animal is slaughtered it is immediately subject to decay. In order to stop bacterial damage the hide is subjected to various operations to put it into static sate. The most common method involve the use of salt as an agent to stop any microbial growth. One method involves using salt in the same weight as the hide and applying this to the flesh side. The leather is then stacked and can be stored for a period of time before processing.
The next method is similar but involves the use of a raceway curing system where the hides are placed in a pool in the shape of a raceway that is filled with water that has been saturated with salt. Paddles move the hides around the raceway ensuring adequate uptake of the salt solution. Once the hides are saturated they can be stored up to six months before further processing. Other Methods include air drying and freezing.
Hides are soaked in water that contains agents designed to facilitate wetting. After soaking, the hides are washed to remove the excess salt and other contaminants.
High pH (calcium hydroxide, sodium sulphate) chemicals are used to weaken and remove hair.
The hide is divided or split to ensure even thickness of the top grain. The bottom portion is referred to as the split. The top portion is the grain and may also be full grain.
Bating removes the chemicals used in the liming process. This is accomplished with neutralising agents and enzymes.
This steps controls the pH of the leather, adjusting it for optimum results in the tanning process. Chrome tanning will not perform well in alkaline conditions. Dilute sulphuric acid is used, buffered with salt to prevent swelling of the hide.
This is the actual transformation of the decaying material into an inert fabric. Chrome salts are the tanning chemical of choice for its speed and for the physical attributes it contributes to the leather. Vegetable tanning us used for heavier leather that is suitable for the shoe, equestrian or belting markets. Combination tanning processes are also common.
This is a mechanical step to remove excess water.
This ensures an uniform thickness of the hide. Variations up to 10% in thickness can occur, even on the same hide.
Re-tanning, Dyeing, Fat Liquoring
Considered as one step, the hides are re-tanned to give unique characteristics (white, or very soft leather). The leather is then dyed with basic or acid dyes to give the desired colour. Fat liquoring is the addition of oils to replace oils lost in the tanning process and will contribute to its handle or feel.
The leather is dried in rooms that allow moisture evaporation through the ventilation of the air. Vacuum drying is an alternate method used. The hides are dried under strict control of humidity and temperature to ensure it maintains the proper feel.
Some hides are buffed to remove minor imperfections in the hide and to give a surface that will readily accept the dolour or pigment coats.
Finishing (broad type definition):
Finished leather is any leather that has a film on the surface. The film is composed of a binder and pigment and can be as thing as human hair. The finish will develop micro cracks with use that are visible to the human eye. These hairline fractures will allow access of the cleaner into fibres of the leather.
Aniline leathers are dyed only, therefore porous and do not have any protective coating on the surface. This porous leather will allow immediate penetration of any liquid or cleaner into the fibres. If left untreated, they will show the history of their use in the ‘patina’ that they will develop. If materials spilled on the surface have any dyeing characteristics, the fibres will be stained or discoloured. Cleaning will not remove the majority of these stains. One answer to this problem is to treat the leather with a chemical to provide a ‘barrier coating’ similar to scotch-guard or Teflon fluro-chemicals protectors. This will act as a repellent and give you time to remove the soil by keeping suspended above the fibres.