LEATHER FINISHING

LEATHER FINISHING

Post 350 ⇒   by Gautam Shah 

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First preparation for leather begins when raw hides are washed to clean the blood and other tissue materials. After the tanning, the leather is treated for specific surface qualities. The leather passes through several wet processes for colouring and oiling. After drying the hide is mechanically processed for specific look or feel.

Suede Leather

Commercially available leathers are of basic four classes.

1 Full-grain leather is a hide without sanding, buffing, etc., nominally employed to remove or level out the natural surface imperfections. The grain structure nearly remains intact allowing to breathe while retaining the strength and durability. Some furniture upholstery, shoe soles, leather shuttle buffers (such as in weaving looms), belting, etc. use the full-grain leather.

2 Top-grain leather Many of the hides are split and shaved (at certain sections) to level out the surface to even thickness, before and after tanning, but for better quality the process is redone. The repeat process makes the leather thin, soft and pliable. Its surface may have less breath-ability.

3 Imprinted grain leather is a product over which grains are embossed or pressed. The surface imperfections are corrected by splitting, sanding, and then grains are embossed. After these processes the leather is dyed. Heavy pigmentations also cover up micro irregularities of the surface.

4 Split leather is created by separating the layers (process similar to veneering of wood) of leather. The top layer, if too thin, the bottom is reinforced with synthetic materials, woven or knitted fabrics or other splits of leathers. The splits are also top layered with synthetic polymer products. Splits are also used to create suede leathers. By-cast leather is a split leather with a layer of polyurethane lamination that is then embossed. It is a leather in only look aspect (and not the feel); otherwise, its surface is completely layered in plastic.

Textures on Leather

LEATHER TREATMENTS

Heavy leathers are finished by coating the grain surface with a finishing compound, and finally by brushing it under a revolving, brush-covered cylinder. The grain surface of light leathers is buffed, or sandpapered, to correct imperfections in the skin. Buffing the flesh side of leather raises the nap and produces the popular leather known as suede. For smooth finishes, most light-leather is seasoned, or treated with a mixture of such materials as waxes, shellac or emulsified synthetic resins, dyes, and pigments. Pigments are used sparingly to avoid a painted look.

rawhide consisting of rough untanned skin

Leathers are surface treated in many different ways. Impregnation is achieved by padding, dipping, brushes coating, spraying, rolling, screen printing etc. Impregnation systems are water or solvent based, and help strengthen the crust while saturating the colouring and coating. Fillers are also water and solvent based, used to for achieving surface uniformity of colour and gloss while adjusting the feel. Auxiliaries are many different types, used to optimize the performance of the finishing systems and for special effects. Modifiers improvise the feel quality of leathers, while altering physical qualities of the leather. Dulling agents help adjust the gloss at the top coat. Topcoats are surface covering systems applied to leathers to impart specific transparent, translucent or opaque (solid) colour, shade, or decorative (metal, pearl, etc.) effects.

Book Binding with coloured embossed Morocco leather.

Leathers are surface altered by chemical and mechanical treatments. Chemical surface treatments include bleaching, staining, and other coatings. Leathers are infused with such materials as Epsom salts, oils, and then lubricated with hot emulsions of soap, grease, and sometimes wax. Mechanical surface treatments are: pressing -for levelling and compacting, rolling -for firmness and gloss, and embossing -to achieve granular or wrinkled textures.

Embossed leather

Leather Substitutes, are synthetic substances that look, feel or behave like a leather due to its one of the dominant qualitative advantage, cost, uniformity or easy availability. Replacement products are also used by people who do not wish to use an animal product. These synthetics include such plastics as polyvinyl chloride and non woven fiber impregnated with binders. These materials lack leather’s porous quality, pliable nature, and resilience.

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LEATHER – 2 ● Tanning

LEATHER – 2 ● Tanning

Post 239 ⇒   by Gautam Shah 

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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.

Partially processed leather

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
  • 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.

Leather processing Skinning

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.

Raw Leather Items -Pouches

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.

Collecting Barks for tanning of Leather

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.

Vellum / Parchment

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.

Leather -Rawhide drum tops

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LEATHER – 1

Post 235 — by Gautam Shah

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Leather Leather Harness for Horses

Leather has been used by primitive man to cover own-self, protect food items, carry water, form a place of dwelling, to tie materials and make footwear. Raw-hides or untreated skins have a tendency to putrefy very quickly in wet weather, causing foul smell and degeneration of the material itself. In dry conditions, or on loss of moisture untreated skins become very hard, rigid and brittle.

Leather_bucket_of_a_well

Leather processing is an ancient craft that has been practised for more than 7,000 years. The first lessons were how to turn raw-hides in to a stable product, Leather. For leather making raw-hides must be removed early, cleaned, washed and dried in the sun. There were two easy treatments for making leather into better lasting, soft and a wearable product. The first was to curtail leather’s reactivity with moisture by coating it with oils or wax. Other methods included, salting (to dehydrate) and smoking to retard bacterial activity, and treatment with bark extracts (for tannic acid), and alum.

Leather Boxer_of_quirinal_hands

The hides of mammals, the chief sources of leathers, are composed of three layers: epidermis, a thin outer layer, corium, or dermis, the thick central layer, and a subcutaneous fatty layer.

treasure_chest_chest_gems_box_open_decoration_jewellery_chains-1111687.jpg!d

The corium constitutes the main commercial leather, after the two sandwiching layers have been removed. Fresh hides contain between 60 and 70 percent waters by weight and 30 to 35 percent protein. About 85 percent of the protein is collagen, a fibrous protein held together by chemical bonds. Basically, leather making is the science of using acids, bases, salts, enzymes, and tannin to dissolve fats and non-fibrous proteins and strengthen the bonds between the collagen fibres.

Leather Homo SapiensThe term hide is used to designate skins of larger animals, e.g. cowhide or horse-hide, whereas skin refers to smaller animals (calfskin or kidskin). Fur is a hide or skin of furry or hairy animals. Use of furs precedes use of hides as several species of hominoids including Homo sapiens and Homo neanderthalensis used fur for clothing. Common animal sources for fur include fox, rabbit, mink, beaver, stoat (ermine), otter, sable, seals, cats, dogs, coyotes, chinchilla, and possum. Major sources of leather were hunted animals, but later domesticated cattle and skins of calves, goats, kids, sheep, and lambs, were used. Other sources were horse, mule, pig, kangaroo, zebra, deer, seal, walrus, and reptiles. Skins of ostrich, lizard, eel, and of aquatic creatures such as seal, walrus, whale, and alligator were used.

Leather shoes of Chalcolithic Areni 1 caveLeather cleaning included scrapping the back or underside of all residual tissues, blood, etc. and top side of hair or wool by stone or wooden scrapers. The processes for tanning of leathers were developed at many places, in stone age period. Tanning could take up two years for very thick hides. The dressing of leather involved paring or shaving it to achieve a level thickness. Leathers were surface treated with wax, oils and dissolved gums, often with colourants.

Leather Sandals from Egy

Sumerians in Mesopotamia, during the 5th and 3rd C. BC. used skins for long dresses and diadems for ladies. Egyptians also used leather for clothing including gloves arms, belts and as ornaments. Phoenicians are credited with forming water tubes of leather. Romans and many other in central Asia were using leather footwear, clothing, shields and riding gears such as harnesses. Assyrians used leather for containing oils, and as inflated floats for rafts. In India thinned raw-hides were used as cover for percussion instruments.

Leather bag Water carrier Bhist India

By 1st C Ad a vast variety of hides and skins were produced, many with unique characteristics due to local materials and protected knowledge base. Leathers for parchment or writing media were extremely thinned down and stabilized from raw hides. Skins used for gloves and footwear toppers were soft and supple. Patterned skins of reptiles formed pieces of ornaments. Leather stripes, entwined leather belts, were used for their flexibility and tensile strength. Furs, were prized items for wearing, furnishings and arms covers.

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