Preservation by drying, salting or freezing
If a fresh animal skin cannot be processed immediately in the tannery, it must be stored and preserved in order to prevent the decay process. This must be done quickly, because uncontrollable bacterial growth starts after approximately 2 hours following slaughter. The bacterial attack leads to the destruction of the skin (putrefaction) and thus to the unuseability of fur or leather. Optimal is an intermediate storage at 4 - 7 ° C. Also, the hanging to drain the blood is important.
Common methods of preservation are freezing, salting and drying.
Contents
Freezing - cooling
Freezing is the most straightforward method. The fresh skins are packed in plastic and stored in the cooling rooms. Apart from the necessary energy costs of cooling, this method has only advantages. The decomposition process is immediately stopped, the shelf life of the stored raw material is unlimited and, if necessary, the skins can be processed immediately - as just coming from the slaughter.
Cooled Rawhide.
Salting
The salt preservation primarily causes a drainage of the skin. The salt used for salt preservation (chemical: sodium chloride) must be unused. A salt which has already been used for preservation, contains too many microorganisms and therefore does not guarantee good preservation when reused. The preservation salt should not have impurities due to iron compounds. Likewise, only very small amounts of calcium or magnesium compounds may be present as impurities.
The method of salting is free of cooling, but must be carried out very conscientiously in order to avoid putrefaction of the skin. A sufficient amount of salt must be applied to the skin in such a way that it is completely saturated with it to stop any bacterial growth in the skin. For this reason, the rawhide has to be salted with 40 - 50% salt in relation to the skin weight. This is more than one centimetre layer of salt on the flesh side of the skin. Therefore, a skin with 40 - 50 kg requires approximately 20 - 25 kg of salt.
The salt preservation can be carried out by sprinkling the skin with solid salt (dry salting) or by treating the skin with salt solutions (wet salting).
Salted hides can be kept for several months and are resistant to temperature fluctuations. However, they must be stored in a certain manner after drying. Skin areas cannot touch or lie on each other. Salted skins are a favourite food for mice and rats. The hides are to be kept in too humid air. They soon become a breeding ground for insects which lay their eggs on them.
The process of seeding is carried out by hand, but also by machine.
Salted rawhide in the tannery.
Drying
The simplest and oldest preservation process is drying. The skin is thus stretched in the dry air in such a way that it can flow around the hide from all sides. The moisture required for the development of microorganisms is thereby removed from the skins. The skins should dry quickly, but in no case at too high temperatures (not above 30 ° C) and in no case in the sun or directly next to a radiator, since this leads to irreversible damage of the skin collagen! Dried leathers are hard.
The skin must not be exposed to moisture during drying (e.g. by sudden rain). Otherwise, the decomposition process is initiated, the skin starts to rot, and insects are being lured.
Dried stingray leather.
Additional information
Video about leather production
The leather production in a modern tannery.
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