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Tanning is the process where the hide is preserved. It removes all the impurities and converts all the proteins of the raw hide into stable molecules which means the pelt will not putrefy or degrade. Tanning a hide helps to create cross-links in the collagen structure: this helps to stabilise it against the effects of acids, alkalis, heat, water and the action of micro-organisms. In short, tanning protects the hide and ensures its longevity.
There are a number of ways of tanning a hide. Chromium salt mineral tannages are usually used in the preparation of leather. Aldehyde and oil tannages can also be used: this method of tanning produces very soft leathers but is understandably more expensive. Oil tanning is often used to produce dry-cleanable and washable fashion leathers as well as chamois leather. Vegetable tannages are used to produce tougher and stouter leathers that are mainly used for belts, shoe linings, bags and cases. Vegetable tanning uses various plant extracts which produce a distinctive brown colouring.
Once the hide is tanned, it is then ready for the next stage in the process. If the leather is thick or course, it can be split using a specialised splitting machine which will produce two different types of leather product. The topmost layer is of the highest quality: the under-layer tends to have no visible grain and is usually turned into suede. Grain can be added to this layer artificially if necessary. All quality hides are uniform in thickness. This is achieved by shaving the leather on the non-grain side with a specialised machine which uses helical blades mounted on a rotating cylinder. Once this process has been completed, the leather is given a final neutralising wash which removes all the chemicals and any residue so that the hide is ready for dyeing.
The methods of dyeing leather are as long and complex as the colour range of leather is wide and varied. Leather can in fact be dyed to any colour of choice, but the procedures vary depending on the nature of the dye used and the desired quality of the final piece. Many leathers are only surface dyed. Other leather products however, like suede, need penetrating dyes that completely soak through every strand and molecule of the hide to produce lasting colour.
Once the leather has been dyed to the colour of choice, oils need to be re-introduced into the material to stop it drying out and going rigid. The process is called fatliquoring. The process puts back the essential oils into the hide to keep the fibres lubricated and the leather soft and supple. The water content of the leather then has to be reduced so that the hide doesn’t shrink or warp. This process is done gradually in order to maintain the leather’s integrity.
First it is ‘sammed’ by passing it through a large roller with felt covered mangles: this reduces the water content to approximately 55%. It is then ‘set out’ and stretched. During this stage in the process the grain side of the leather is smoothed: this reduces the water content further to about 40%. The last stage in the process is the final drying. The leather ideally should have a final water content of between 10 and 20%. To achieve this, the hide is either passed through a staking and drying drum which tumbles the hide within a rotating drum, producing a softer and more-pliable leather, or by buffing and brushing where the grain surface is brushed to create a very fine nap, such as seen on nubuck leather.
After the final drying is completed, the leather is finished. This is a sort of quality check where colour levels are analysed and any grain defects are corrected. A protective gloss, or surface layer, is added to the leather to give it a good resistance to water, chemical attack and abrasion. The final stage in the process is grading where the overall quality of the product is checked for softness, uniformity and visual appeal. Any leather not deemed up to standard is disgarded.]]>