Leather is a non-woven mass of collagen fibers, derived from the skin of animals, that has been rendered biorefractive.
WHAT IS LEATHER’S MOST IMPORTANT PROPERTY?
Leather’s most important property is that it will not rot under its normal conditions of use. This property of resistance to biological degradation is called biorefractiveness. Biorefractiveness is leather’s most important property because without it, leather would not be useful for anything.
WHAT OTHER IMPORTANT PROPERTIES DOES LEATHER HAVE?
Toughness and flexibility in combination are leather’s outstanding material properties. This combination of properties is the reason why leather is so useful. Because accepts dies and pigments readily, leather can be made in many colors. It is also formed into shapes easily with simple tools. Thus leather can be formed into objects that are both useful and beautiful.
HOW IS LEATHER MADE BIOREFRACTIVE?
Leather is made permanently resistant to biological attack through the tanning process. The chemicals used to tan leather bond with the collagen proteins that form the fibers in leather, and render the protein indigestible to bacteria. The thoroughness and completeness of this chemical reaction is key to making good leather.
WHERE DOES LEATHER COME FROM?
Leather is made from the skin and hide of animals. The raw skin consists of hair, follicles, sweat glands, and fibrous proteins. The longest and largest fibers are found on the flesh side of the skin and can be as large as 0.01 mm in diameter and several centimeters long. Finer fibers are located at near the surface of the skin, and these finer fibers are able to give leather a silky feel. Generally speaking, the smaller the animal, the finer the fibers. For example, calfskin leather is considered to be silkier than cattle hide leather.
WHAT KIND OF PROTEINS ARE FOUND IN ANIMAL SKINS?
Collagen, elastin, albumen, and keratin and the kinds of proteins found in animal skin. Raw animal hides also fats in the form of triglycerides and mixed esters. In chemical composition, raw cattle hides contain 5% - 15% fats, and the balance is protein. Raw hides commonly have blood, dirt, and manure adhering to them. Keratin is the primary protein of hair.
HOW ARE RAW HIDES PRESERVED IF THEY ARE NOT TANNED?
Salt is commonly used to preserve hides before they are tanned. Tanning is the chemical process in which preservation is made permanent. Salting hides preserves them but because salt is easily washed away, preservation by salting in not permanent.
In modern practice, animal hides, and in particular cattle hides, are cured in a bath of cold brine. Hides are first washed in cold water to remove dirt, manure, blood, and loosely adhering flesh. Non-useful parts of the skin such as ears and tail are trimmed from the hide, and then the hide is soaked in an agitated brine bath for 24 to 48 hours. This period of time is necessary to ensure that salt is thoroughly and completely absorbed in the mass of hides.
WHAT HAPPENS AT THE TANNERY?
At the tannery, the cured but untanned hides are first soaked in fresh water, and the salt is removed. In addition, loose fat, dirt, and manure are removed also. Then the hides are dehaired.
In modern practice, hair is removed from hides by a chemical process. Lime and sodium sulfide attack the sulfur-based crosslinks in the protein keratin at a pH of 12-12.5. Keratin is the protein that is the primary chemical constituent of hair. Over a period of 4 to 12 hours, the keratin protein is broken down under the attack of sodium sulfide in alkaline conditions. The hides are then washed for an additional 12 to 20 hours in lime-water. Osmotic pressure causes the hides to swell to about double their original size during this process. The waste products of dehairing are then removed by washing the hides with fresh water. After the removal of keratin, the proteins left in the hide are: elastin, albumin, and collagen.
At this stage the hides can be subject to their first purely physical modification. Hides are often thicker than the final leather product needs to be. Thus they need to be cut to the desired thickness, and this is done with a horizontal belt-knife. Hides can be cut horizontally to an accuracy of 0.1 mm. This process is called splitting.
In modern practice, the hides at this stage are delimed and bated. Ammonium salts and acids are used to reduce the pH in the hide and to remove the lime. Bating refers to a proteolytic enzyme that destroys the elastin. The presence of elastin in the final product would make it too stiff, and so it has to be removed.
The final step prior to tanning is called pickling. In this step, the albumin is removed leaving the collagen behind. The bated hides are soaked in a solution containing 1%-2% sulfuric acid and 3% to 4% sodium chloride. The purpose of the sodium chloride is to prevent further osmotic swelling. The sulfuric acid solubilizes the albumin.
With the albumin removed, the hide, now comprised primarily of collagen fibers, is ready for tanning.
TANNING
Tanning is the chemical process which renders the hide biorefractive and changes hide into leather. There are two principle processes of tanning: the chrome process and the vegetable process. Because of its speed and efficiency, the chrome process is the primary method by which leather is tanned. Vegetable tanning was for centuries the only method of tanning, and it is still used when flexibility and workability in the leather is needed. Even here, leather partly tanned by the chrome process is the raw material of the vegetable tanning process.
The hide from which the albumin has been removed by the salt/sulfuric acid solution is treated with a solution of basic chromium sulfate. The pH of the chromium solution being at 2.5 and is gradually raised through the addition of sodium bicarbonate to 4.0. The tanning reaction proceeds rapidly at pH 3.0, and the pH is kept below that at the beginning to ensure that the hide is thoroughly saturated with chromium solution before the reaction is driven to completion. (In all these processes, from brine preservation to tanning the rate determining step has been mass transport through the hides.)
In the tanning reaction, the basic chromium sulfate complex reacts with the carboxyl groups of adjacent protein molecules in the hide, forming a crosslink. At a pH of 3.5 to 4.0, the crosslinking reaction is made irreversible through the loss of an acidic hydrogen. Formic acid is used to help eliminate side reactions, and the use of formic acid substantially reduces the time of processing.
When the reaction is complete, the chrome tanning solution is wrung from the hide and, if necessary, the hide is shaved to the desired thickness. At this point the wet, tanned hide is blue in color and does not possess a leather feel. That is imparted by the vegetable process.
Vegetable tanning employs polyphenolic compounds that are obtained from certain plants. Pyrogallol, pyrocatechol and catechin are important active ingredients. These compounds are or contain polyhydric benzene rings and they work by replacing bound water through a reaction of hydrogen bonds with peptide bonds of the protein. Vegetable tanning does not significantly crosslink proteins, making the leather easy to shape, emboss, or tool. Vegetable tanning leather is the choice of hand-craft work.
DRYING, COLORING, FATLIQUORING
Freshly tanned leather is ready for processing into the final form desired. It is rinsed of the tanning solution and dried. To prevent shrinkage while drying, it is held or stretched by any of several means while it is slowly dried with dry air and gentle heat. Here again, mass transport is an important consideration. Too rapid a dry can result in undesirable secondary actions.
Leather readily accepts pigments and dies, and the chromium present in the leather is often used as a mordant. Dies can be used to deeply penetrate the leather in the cases of leather that might easily be scuffed. Alternatively, it can be given a surface treatment that shows leather grain, a polymeric surface, or a high polish.
To prevent cracking, to remain pliable and to lubricate the fibers, the leather is stuffed with oils and greases in a process known as fatliquoring. Sulfated or sulfonated animal fats or fish oils are commonly used. These oils are able to penetrate the leather and be adsorbed onto the fibers because of a strong wetting action.
The leather may be subjected to a mechanical flexing process to soften it up. It may also be buffed to improve it appearance and remove flaws, such as scars.
FINAL FINISHING
Leather that has been processed up to through the fatliquoring stage and partly cut to shape can be purchased on the wholesale market by craftsmen who make finished leather objects. Final shaping and coating is done by the craftsman. The craftsman cuts, shapes, tools, decorates, and otherwise forms the leather into the final leather object, and puts on a final finish that imparts the color and shine.