The conversion of animal skins to leather
Animal skins were most likely the clothing of choice for our ancestors from prehistoric times, being a by-product of their hunter-gatherer lifestyle. Warm and windproof, they performed the job of keeping primitive man protected from the elements, with only one drawback. After a period of time (from weeks to months), the skin would begin to decay, to the point where it was no longer fit to wear.
Probably the earliest method of preserving skins was to stretch them out and allow them to dry them thoroughly in the sun and wind. This produced a more durable though much stiffer piece of material, better suited to protection than comfort. Later on, however, methods were developed which provided not just durability, but also comfort.
Tanning involves some form of bating, or chemical treatment of the raw hide, which alters the nature of the skin. In its simplest form, a tanning agent removes moisture from between the protein fibres, and binds the fibres together, forming a barrier to water and the fungi and bacteria which would cause the leather to rot. Tanning agents are usually divided into three types: those of animal origin (brains and fish or animal oils), vegetable products (most often some form of tannin or tannic acid) and mineral salts (for example, chromium sulfate).
In all methods of preparing leather, the skins are prepared by scraping away subcutaneous fat and hair. The rawhide is then usually allowed to dry off before the appropriate methods are used. Finally, the leather is stretched and dried before use, although frequently, a dressing (often oil or lanolin-based) is applied.
Possibly the oldest form of preservation included the use of animal products including oils (lanolin) and brains (at one time the favoured method used by Native Americans to produce a pliable buckskin). In some parts of the world, notably the Americas, vegatable tanning methods took second place to animal tanning. Brain and oil tanning also produces soft leathers, such as chamois, whose major advantage is that it is highly resistant to water, and is capable of being wetted and dried without negative consequences.
Brain tanning involves taking the brains of the animal and rubbing it into the skin, pressing the material into the hide with a wide, blunt tool, rather like a dull axe. A similar process is used with animal or fish oils, pressing the tanning agent into the skin until it is well distributed. After this is complete, the skin is stretched and allowed to dry. It normally needs no further treatment than this, although for a highly resistant leather, a final topical application of oil or grease may be used.
Historically, leather making methods involved tannin, and hence vegetable products. The bark of many trees contain high levels of tannin, and the ancient Hebrews were using oak bark some two and a half thousand years ago. The Egyptians used babul pods (from the acacia tree Acacia Arabica), the Romans used a wide variety of barks and fruits, including cherry, plum and oak gall. These methods involve soaking the hide in vats containing the vegetable solution, often in several stages, involving increasingly stronger solutions. Finally, the treated hide would be coated with oil, hung up or slightly stretched, and allowed to dry naturally and slowly.
Vegetable tannins change the chemical structure of the gelatin in the hide into insoluble and water-resistant compounds, consolidating the dermal network of the hide into firmer and drier structures of improved thermal stability, durability, and water resistance.
The late 19th century saw a huge rise in demand for leather products, and traditional methods of tanning were too expensive, time-consuming and unpleasant. Commercial production of leather required that more effective, and above all, cheap means of leather making were developed. The most common method used is chromium salt tannage.
There are two different processes used. In the less common "double-bath method", the hides are soaked in mild chromic acid solution, and then in a second stage, added to sodium thiosulfate to produce basic chromium salts. These salts are deposited within the fibres, forming a tough and impermeable bond. In the more common single-bath method, the hides are placed in revolving drums, which are filled with a strong mineral solution, normally sulfates of chromium, aluminium or titanium. Finally, the leather is dried and an application of grease-based sealant is given as final protection.