Before tanning, the skins are unhaired, degreased, desalted and soaked in water over a period of 6 hours to 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides, such as pentachlorophenol, are used. Vegetable tanning uses tannin (this is where the name tanning comes from). Tannin occurs naturally in bark. The primary barks used in modern times are chestnut, oak, tanoak, hemlock, quebracho, mangrove, wattle and myrobalan. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable tanned hide is flexible and is used for luggage and furniture. Mineral tanning usually uses chromium in the form of basic chromium sulfate. It is employed after picking. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This is known as Basification. In the raw state chrome tanned skins are blue and therefore referred to as “wet blue”. Chrome tanning is faster than vegetable tanning (less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments. Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suede, nubucks, etc. are finished by raising the nap of the leather by rolling with a rough surface.
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