MadSci Network: Other

Re: How is animal skin made into leather(tanning)and how long does it take?

Area: Other
Posted By: Graham Lampard, Grad student Leather Technology, British School of Leather Technology
Date: Fri Nov 14 10:30:37 1997
Area of science: Other
ID: 878341436.Ot

Animal skin is turned into leather via number of quite complicated steps. First the majority of animals are killed for the meat, so the skin is of secondary importance. To stop it putrefying, ie falling apart, the skin must be preserved. This is done by adding common salt to the skin and piling them on a pallet. The salt kills the bacteria involved in the putrefying process.

Once preserved, the skin is taken to a tannery where the hair and any remaining flesh have to be removed. The skin is soaked in water to rehydrate it and remove the salt. It is then fleshed on a machine to remove lumps of flesh and dung etc.

The hair is removed by putting the skin in a drum and adding chemicals. These are usually sodium sulfide and calcium hydroxide (lime). The sulfide attacks the keratin in the hair and dissolves it. The reaction only occurs at alkaline pH which is why the lime is added. The process is called liming. Once the hair has been removed, after about 16 hours, the pH needs to be reduced. This is done by deliming. Carbon dioxide is blown into the drum while it is turning and because CO2 is an acidic gas it lowers the pH to about 7. At this point enzymes are added to clean out the remaining proteins whic are not required. All the tanner wants is collagen. The other skin proteins such as elastin and keratin have to be removed. This process, bating, does just that. After bating the skin is pickled, in the same way as you'd pickle onions, using salt and vinegar. In this case we use salt and sulfuric acid, but the principle is the same.

This does two things: it preserves the skin again and gets the collagen ready for tannage, the most important step. Tanning the collagen stops it from putrefying fo ever. If you just left the skin in the salted state it would, eventually, go off and, of course it is unuseable for wear. Once tanned the skin can be worn. Tanning is done in drums in an aqueous solution. The usual tanning agent is chromium(III) salts. These give the skin stability against heat, light, perspiration etc., and make the skin mouldable and useable. It is pssible to use the skin, depending on its source, for anything from gloves to industrial belts, car upholstery to the finest shoes; all are processed in the same way.

After tanning, which takes a few hours, the skin is a light blue colour. It is then put through a wringer to remove excess water. It is then retanned, dyed fatliquored and finished. Retanning modifies the properties of the leather to the suit the conditions of use; dyeing changes the colour to whatever colour is desired. You can have any colour from white through black. Fatliquoring adds fats and oils to the skin. In the previous processes all the natural fat is lost, so the skin, if dried, would dry hard. Fatliquoring makes the skin soft. The skin is then dried and finished by adding a protective coat to stop it getting dirty etc. The process from beginning to end takes about 2 weeks, if done continuously. Usually though the batch process is done in stages with the skin being left until needed for an order etc.

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