Tanning hides is a by product of the process of killing animals. The meat is the first and principle product,but the skin is also used for a variety of products. Here is how it is done.
Animal skin is turned into leather via a number of complicated steps. First, the majority of animals are killed for the meat, so the skin is of secondary importance. To stop it from putrefying the skin must be preserved. Usually this is done at the slaughterhouse with arrangements made with a tannery. Since the slaughterhouse and the tannery are usually a distance away, the skins must be protected for as long as possible. This is done by simply adding common salt to the skin and then piling it 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. As you can imagine, the tannery is not a pleasant smelling place. This process is described in greater detail below.
The first step in the process is liming. The skin as it arrives at the tannery is not ready to be tanned. It usually has many ingredients that first have to be removed. The hair is removed by putting the skin in a rotating wooden 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 a high alkaline pH level which is why the lime is added. The mixture is combined with the skins in the drum; the tumbling and rotation continues until the hair is removed.
Once the hair has been removed, after about 16 hours, the pH concentration needs to be reduced. This process is called deliming. Here, carbon dioxide is blown into the drum while it is turning and because CO2 is an acidic gas it lowers the pH level to about 7. At this point enzymes are added to clean out the remaining proteins which are not wanted in the product.
The only thing that the tanner wants is collagen. The other skin proteins such as elastin and keratin have to be removed. The bating process does just that, it removes extra proteins present in the skin. The skin is clean and it is almost ready to be tanned.
After the bating process is complete the skin is pickled in the same way as you'd pickle onions, using salt and vinegar. In this case the ingredients are 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 forever. The main chemical used in tanning is chromium III salts. If you just left the skin in the salted state it would eventually dry out. But in this state it is unuseable for wear. However, once tanned the skin can be worn. Tanning is done in drums in an aqueous solution. These give the skin stability against heat, light, perspiration etc., and make the skin mouldable and useable. It is possible to use the skin, depending on its source, for anything from gloves to industrial belts, car upholstery to the finest shoes. The process is the same for all types of products. The product is hard but almost ready for the manufacturing process.
After tanning, which takes a few hours, the skin is a light blue color. It is then put through a wringer to remove excess water. It is then retanned by adding dye, fatliquored and finished. Retanning modifies the properties of the leather to suit the conditions of use; dyeing changes the color to whatever color is desired. You can have any color from white through black.
Fatliquoring or Oil Tanning
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.
Other types of tanning
Brain tanning is a form of oil tanning. The brain is rich in various lipids. The hides are soaked in the brain lipid, and this makes it soft. Most of the recipes involve an additional source of grease/fat, and sometimes soap. If done properly, it can indeed produce very beautiful leather. But it is a very labor-intensive process. Anyone who does traditional smoke/brain/buck tanning deserves to get a good price for the product.
Vegetable tanning is also a modern process, with some very important differences from how it was practiced in the middle ages. Saddle skirting, and many (but not all) of the vegetable tanned leathers commonly available today, are made to be firm, with little stretch. But they don't have to be that way. Vegetable tanned leather can be quite supple, with a lot more stretch than what one would expect.
This modern form of tanning was invented over the period 1860 - 1885, and it revolutionized the leather industry because leather could be tanned much more cheaply and quickly than in the past. The hides are immersed in a bath of water and chromium sulfate, agitated in a rotating drum, and are tanned in a few hours. Although chrome tanned leather lacks the body and moldability of vegetable leather, it is at least as abrasion resistant, more flexible, stronger, and better able to stand heat. And while it is universally called "chrome tanning," other materials are used. Zirconlure sulfate, for example, produces a white leather; also used are salts of aluminum, potassium and iron.
The grain is removed to facilitate penetration of the oil/grease, and the tannage takes place when the oils are oxidized. This oxidation is hastened by wood smoking in the Amerindian tradition.
Tanning is a complex process that takes a long time. The material has to be handled carefully, otherwise, it will be ruined. This process has evolved down through the centuries, but even in these days of technological sophistication, it is still a process of salts, chemicals, drum tumbling, and oiling.