Leather and animal hide tanning permanently changes the collagen fibers in the leather and makes it resistant to rot and the effects of water. Hair may be locked in during the process or intentionally shed depending upon whether you want fur or leather, and the color and texture of the leather can also be adjusted during the process. Tanning leather is an ancient skill and many cultures have unique variations on the basic process.
Skins and hides must be completely dried and cleaned before they can be tanned. Fresh hides are trimmed, then stretched and salted on the flesh side and air dried for 10 to 14 days or until completely dried out. Scrape off any remaining fat or muscle tissue with a scraping device such as a dull hacksaw blade or a dull knife. Remove the salt and the remaining flesh and grease from the leather. Soak the leather in a non-metallic container using several changes of fresh water to remove the salt and to soften the leather, which will be stiff from the drying process. Work the leather by scraping it over the edge of a wooden table and resoaking, then scraping, with a dull knife. This will break down any residual tissue on the hide and finish preparing the leather for the tanning process.
Removing the hair is done at this point if desired. The tanning process locks the hair into the hide and prevents shedding. Hair can be intentionally shed by soaking the hide in a solution of lime and water. Soak the hides for several days or until the hair slides off easily with hand pressure. Remove from the lime and scrape off the remaining hair. Scud (scrape) both sides of the leather with a dull knife to remove any foreign materials and follicles.
The action of the lime must now be arrested. Soak the leather in a solution of lactic acid or vinegar and water for 24 hours.
The leather is now either soaked in or pasted with the tanning compound. Ammonia alum or potash alum is the reactive agent in the process. Leather without hair may be soaked in an alum/ water solution for up to five days -- thicker leather will take the full five days and thinner leather will require less time.
Hides with hair or fur may be pasted with the tanning agent. Frequently, the hair or fur will react with the tanning agent. Mix some water and flour with the potash to make a semi-firm paste. Apply a coating and let stand for 24 hours. Scrape off the paste and reapply fresh paste. Cycle the coats in 24-hour intervals except the last, which is left on for 48 to 60 hours. Scrape off and rinse well.