Leather is considered a by-product because it comes from animals slaughtered for human consumption. The hides can be transformed into a wide variety of different consumer goods. The tanner recycles the hide and turns it into a product. Demand for the tanned hide depends on and fluctuates with fashion. The price is also a regulatory factor, the greater the scarcity of leather, the higher the price and this usually means more demand for synthetics.
In Spain there is no tradition of tanning pig skin, whereas there is a tradition of tanning beef cattle hides, the reasons for this are twofold. Firstly, part of the pig’s skin is sold as part of the meat, secondly, farm pigs that have been fattened up in captivity and have been fed pellets have very week skin which will not withstand the mechanical tanning process.
The preparation of hides in the slaughterhouse includes separating them from the carcass (dislocation), removal of the flesh, trimming, selection and classification, storage and transportation. Sometimes the removal of the flesh and trimming are done before preservation. Once this process is finished the hides are taken to the tannery where the tanning is done.
There are two different techniques for flaying cows, sheep and goats. There is the traditional separation using a knife, either conventional or automatic knives of compressed air. And there is the more modern technique of separation by traction.
Pigs are usually scalded in hot water at 57-71º until the hair is loosened. This heating causes the denaturation of proteins of the hair follicle, thus weakening the skin’s hold on it. Usually the scalding makes 50% of pig skins inadequate for producing high quality leather. Also scalded skins are 10% thinner and less resistant to tension.
Goats skins are more valuable than sheep skins because they are bigger and the leather lasts longer.