LEATHER – 2 ● Tanning
Post 239 ⇒ by Gautam Shah →
A hide of a dead or hunted animal needs to be removed and cleaned within hours, to prevent decadence, foul smell and attack by vultures and scavengers. In tropical and warm climates this period occurs even before the hunt has been cooked and served as meal. Detaching and cleaning of hides is very labourious and time-consuming activity. Raw hides must be washed with water and scrapped to remove fats, tissues and blood. The hides need several days of sun exposure, shaving of surface hair, scrapping for a surface thickness levelling and than most important process of tanning.
Raw hides, are processed by removing the flesh, fat and hair with a lime solution (liming), or by rubbing in wood ash (lye) (bucking). Rawhide once cleaned, are fairly stable, durable and stronger, than any processed (tanned) leather. The difference between a rawhide and a tanned leather is that the former dries to hard horny stuff, which on re-wetting begins to putrefy. Whereas, tanned leather remains flexible in spite of several cycles of wetting-drying, and will not putrefy.
Rawhides were used for tying straps, drum tops, and abrasion resistance (speed breaks). Now these are used for lampshades and chew toys for dogs. Rawhide is susceptible to water than leather, and it softens and stretches in wet state. Wet rawhide has been used as a means of torture.
Tanning permanently changes, the proteins in the hides, to make the perishable hide into a stable and non-decaying material, Leather. The tannic acid displaces water from the interstices of the hide’s protein fibres, and cements these fibres together.
In every region of the world the craft of tanning emerged, using local sources of natural tannic acid, such as the tree-barks. Wherever, tanning was not done perfectly well, other methods of preservation, such as smoking, liming, grease or oil dousing were used. Till middle ages Tanning processes were experimental, each type of hide was treated by several tanning processes. Many artisans closely guarded their secrets within the family or community. Leather tanning craft was protected and patronised by local powers with exclusive licences to practice leather tanning.
Traditionally, tanning was done, using a natural acidic chemical compound, the tannin. The English word tannin derives from an old German word tannum for oak or fir trees. Tanning has always been a noxious process, conducted beyond the precincts of the town. Leather was very important raw material to be ignored. Ancient civilizations used leather for water-skins, bags, harnesses, boats, armour, belts and harnesses, armour sheaths, boots and sandals.
Post middle ages two principal tanning processes became distinct: mineral, or chrome, tanning, and vegetable tanning. Chrome tanning was quicker and can often be completed in a day, whereas vegetable tanning can take weeks or months. Vegetable tanning offers a firm leather with greater water and stretch resistance. Chrome tanning can shrink the material but creates a longer lasting product. Sometimes, both the processes are applied.
The modern commercial leather-making involves three basic phases:
- preparation for tanning
- 3processing tanned leather
Processes before Tanning: The raw materials used by leather industry originate chiefly from the meat industry. After removing the skin from the animal, it is washed to remove the flesh-tissues, blood etc. Hides are then limed to soften the outer layer, epidermis, which facilitates removal of hair, remaining fat, flesh, etc.
Curing: The hides are cured by wet or dry salting, air-drying, or pickling with acids. Liming plumps up the hides, whereas air drying and salting help to reduce the moisture content, provide an anti bacterial treatment. In wet-salting, the skins are liberally salted and piled on top of one another until they form a pack. The pack is left for 30 days to allow the salt to penetrate the skin thoroughly. Brine-curing is a much quicker method, here the hides are placed in large vats called raceways that contain a disinfectant and brine. After about a day in the raceway, the skins are completely saturated with the salt. The hide is now left with fully convertible substance, the collagen.
Soaking and hair removing: The cured skins are soaked in clean water to eliminate salt, dried blood, and dirt, and also to replace moisture lost in the curing process. When the skins have been soaked, the flesh tissues are removed from the inner face. The hides are then immersed in a solution of lime and water to loosen the hair.
De-liming and Bating: The next operation removes the lime with a wash of aluminium phosphate and pickling by Acid. Bating with enzymes provides smoother grain and makes the hide soft and supple. Gloves, garments and shoes’ toppers are made from softer leathers then shoe-soles.
Vegetable Tanning: Sources of the tannin include the wood of the quebracho tree of South America, mangrove barks from the island of Borneo, wattle barks from South Africa, myrobalan fruit from India, and chestnut wood, oak bark, and hemlock bark from the United States. In vegetable tanning the hides are suspended from rocking frames in a series of vats containing increasingly stronger tannin liquors.
Mineral Tanning: The mineral tanning process is known as chrome tanning because the tanning agent, most frequently employed is a compound of chromium. Chrome-tanned leathers, stretch more than vegetable-tanned leathers, are suitable for handbags, shoe uppers, gloves, and garments.
Oil tanning: Oil tanning is an old method in which fish oil or other oils and fatty substances are stocked, or pounded, into a dried hide until they have replaced the natural moisture of the original skin. Oil tanning is used principally to make chamois leather, a soft, porous leather that can be repeatedly wetted and dried without damage.
Combination-tanned leather: The skin is first chrome-tanned and then re-tanned with a vegetable tannin. The modified applications of both processes produce leather with some of the advantages of each type.
Processes after tanning and Processing the tanned leather: After the basic tanning process is completed, the pelts are ready for processing, the final phase in leather production. These include removing the excess tanning material, drying or semi drying the leather depending on the subsequent processes, averaging the thickness, colouring, pressing, embossing, coating etc.
Draining and sammying (semi drying): Draining and sammying involves draining the excess tanning material and reach a semi dry state. Leathers which are to be further finished are re-tanned to improve penetration of dye and treatment chemicals.
Splitting: Splitting involves thickness levelling of leather. Most leathers have different thickness over various sections of the animal body. Back area (of the animal), which is located in the middle of the leather piece is split to reduce its excess thickness. Removed material if sufficiently thick and extensive is used for lining of shoe uppers or treated further to achieve suede finish leather. If skins are suitable for splitting, surfaces are shaved to adjust the thickness. Leathers are buffed to remove blemishes.