Leather Tanning | Tanning
Tanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Traditionally, tanning used tannin, an acidic chemical compound. Coloring may occur during tanning. A tannery is the term for a place where these skins are processed.
Leather Tanning Process | Leather Tanning Methods
Tanning leather involves a process which permanently alters the protein structure of skin. Making rawhide does not require the use of tannin and is made simply by removing the flesh and fat and then the hair by way of soaking in an aqueous solution (often called liming when using lime and water or bucking when using wood ash (lye) and water), then scraping over a beam with a somewhat dull knife, and then leaving to dry, usually stretched on a frame so that it dries flat. The two aforementioned solutions for removing the hair also act to clean the fiber network of the skin and therefore allow penetration and action of the tanning agent.
Tanning can be performed with either vegetable or mineral methods. 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 [tmtcb - thio ciano thio methyl benzo thyazol]. After 1980, it was forbidden the use of, pentachlorophenol and quicksilver (mercury base) biocides, and its derivates.
Leather Tanning History | Tannery
The English word for tanning is from medieval Latin tannāre, deriv. of tannum (oak bark), related to Old High German tanna meaning oak or fir (related to modern Tannenbaum). This refers to use of the bark of oaks (the original source of tannin) in some kinds of hide preservation.
In ancient history, tanning was considered a noxious or odiferous trade and relegated to the outskirts of town, amongst the poor. Indeed, tanning by ancient methods is so foul smelling that tanneries are still isolated from those towns today where the old methods are used. Ancient civilizations used leather for waterskins, bags, harnesses, boats, armour, quivers, scabbards, boots and sandals. Tanning was being carried out by the South Asian inhabitants of Mehrgarh between 7000–3300 BC. Around 2500 BC, the Sumerians began using leather, affixed by copper studs, on chariot wheels.
Skins typically arrived at the tannery dried stiff and dirty with soil and gore. First, the ancient tanners would soak the skins in water to clean and soften them. Then they would pound and scour the skin to remove any remaining flesh and fat. Next, the tanner needed to remove the hair fibers from the skin. This was done by either soaking the skin in urine, painting it with an alkaline lime mixture, or simply letting the skin putrefy for several months then dipping it in a salt solution. After the hair fibers were loosened, the tanners scraped them off with a knife.
Once the hair was removed, the tanners would bate the material by pounding dung into the skin or soaking the skin in a solution of animal brains. Among the kinds of dung commonly used were that of dogs or pigeons. Sometimes the dung was mixed with water in a large vat, and the prepared skins were kneaded in the dung water until they became supple, but not too soft. The ancient tanner might use his bare feet to knead the skins in the dung water, and the kneading could last two or three hours.
It was this combination of urine, animal feces and decaying flesh that made ancient tanneries so odiferous.
Children employed as dung gatherers were a common sight in ancient cities. Also common were “piss-pots” located on street corners, where human urine could be collected for use in tanneries or by washerwomen. In some variations of the process, cedar oil, alum or tannin were applied to the skin as a tanning agent. As the skin was stretched, it would lose moisture and absorb the agent.
Leftover leather would be turned into glue. Tanners would place scraps of hides in a vat of water and let them deteriorate for months. The mixture would then be placed over a fire to boil off the water to produce hide glue.
A tannery may be associated with a grindery, originally a whetstone facility for sharpening knives and other sharp tools, but later could carry shoemakers’ tools and materials for sale.
Vegetable tanning uses tannin (this is the origin of the name of the process). The tannins (a class of polyphenol astringent chemical) occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them causing them to become less water-soluble, and more resistant to bacterial attack. The process also causes the hide to become more flexible. The primary barks, processed in Bark Mills and used in modern times are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle (acacia; see catechu), 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.
Prior to the introduction of the basic chromium species in tanning, several steps are required to produce a tannable hide. These steps include scudding, or removing the hair, liming, or the introduction of alkali agents such as sodium hydroxide, deliming, or restoring neutral pH, bating, or softening the skin with enzymes, and pickling, or lowering pH of the hide with salt and sulfuric acid. The pH is very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit in between the fibers and residues of the collagen. 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 step 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.
Chromium(III) sulfate ([Cr(H2O)6]2(SO4)3) is regarded as the most efficient and effective tanning agent.Chromium(III) compounds of the sort used in tanning are significantly less toxic than hexavalent chromium. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H2O)6]3+, which at higher pH undergoes processes called olation to give polychromium(III) compounds that are active in tanning,being the cross-linking of the collagen subunits. The chemistry of [Cr(H2O)6]3+ is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen’s carboxyl groups, amine groups from the side chains of the amino acids, as well as “masking agents.” Masking agents are carboxylic acids, such as acetic acid, used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen’s reactivity without inhibiting the penetration of the chromium(III) complexes.
Collagen is characterized by its high content of glycine, proline, and hydroxyproline, usually in the repeat -gly-pro-hypro-gly-.These residues give rise to collagen’s helical structure. Collagen’s high content of hydroxyproline allows for significant cross-linking by hydrogen bonding within the helical structure. Ionized carboxyl groups (RCO2-) are formed by hydrolysis of the collagen by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). The ionized carboxyl groups coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters.
Gustavson identified that pre-tanned collagen has spacing of 10 Å between protein chains whereas in tanned, swollen collagen the spacing is 17 Å. The difference could not be compensated by mononuclear chromium cross-linking. This evidence supports cross-linking polychromium species. A model for cross-linking is given.
Subsequent to application of the chromium agent, the pH is increased to 4.0-4.3 through addition of sodium bicarbonate. This increase induces cross-linking between the chromium and the collagen. The pH increase is normally accompanied by a gradual temperature increase up to 40 °C. Chromium’s ability to form such stable bridged bonds explains why it is considered one of the most efficient tanning compounds. Chromium-tanned leather can contain between 4 and 5% of chromium.This efficiency is characterized by its drastic increase in the hydrothermal stability of the skin, or its resistance to shrinkage in heated water.
Tawing is a method that uses alum and aluminium salts, generally in conjunction with other products such as egg yolk, flour, and other salts. The leather becomes tawed by soaking in a warm potash alum and salts solution, between 20°C and 30°C. The process increases the leather’s pliability, stretchability, softness, and quality. Adding egg yolk and flour to the standard soaking solution further enhances its fine handling characteristics. Then, the leather is air dried (“crusted”) for several weeks, which allows it to stabilize. Tawing is traditionally used on pigskins and goatskins to create the whitest colors. However, exposure and aging may cause slight yellowing over time and, if it remains in a wet condition, tawed leather will suffer from decay. Technically, tawing is not tanning.
Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suedes, nubucks etc. are finished by raising the nap of the leather by rolling with a rough surface.
The first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage, known as retanning, applies retanning agents and dyes to the material to provide the physical strength and properties desired depending on the end product. The fourth and final stage, known as finishing, is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired.