Leather has played an important role in the development of civilisation. From prehistoric times man has used the skins of animals to satisfy his basic needs. He has used hides to make clothing, shelter, carpets and even decorative attire. To the Egyptian lady, a fur piece was highly prized as her jewellery. From leather, man made footwear, belts, clothing, containers for liquids, boats and even armour. The principle protective armour of the Roman soldier was a heavy leather shirt.
In recorded history, pieces of leather dating from 1300 B.C. have been found in Egypt. Primitive societies in Europe, Asia and North America all developed the technique of turning skins into leather goods independently of one another. The Greeks were using leather garments in the age of the Homeric heroes ( about 1200 B.C. ), and the use of leather later spread throughout the Roman Empire. During the Middle Ages, the Chinese knew the art of making leather . The Indians of North America also had developed great skills in leather work before the coming of the white man.
At some time, by accident or by trial and error, man discovered methods of preserving and softening leather treating animal skins with such things as smoke, grease and bark extracts. The art of tanning leather using the bark of trees probably originated among the Hebrews. In primitive societies, the art was a closely guarded secret passed down from father to son. As civilisation developed in Europe, tanners and leather workers united in the trade guilds of the Middle Ages, as did the craftsmen in other fields. Royal charters or licences were issued permitting people to practice leather tanning. In the nineteenth century, vegetable tanning, i.e., tanning using the extracts from the bark of certain kinds of trees, was supplemented by chrome tanning. This process uses chemicals and today accounts for about eighty to ninety percent of all tanning done except for the leather used in the soles of shoes and tooling leathers.
How leather is sold
The most economical way to buy leather is to purchase it directly from the tannery. Leather is usually priced by the square foot and sold as a complete hide. The exact size, to the nearest 1/4 sq.ft., is measured on government approved measuring devices at the tannery. When you purchase leather this way, you are assured of getting value for money.
Leather comes from tanneries in various shapes and sizes depending on its intended use and the animal which it came from. One thing to always remember about leather - it is an animal’s skin that has been processed into finished leather, not a synthetic material. Because of this, the exact size and shape of the finished leather is dictated somewhat by the size and shape of the animal hide that the tannery received to process.
For ease of handling during tanning, large animal hides are usually cut into smaller sections ( sides, shoulders, bellies, etc. ) at the tannery. Skins of smaller animals such as calf, goat, pig and reptiles are tanned and sold in their original shape.
Although leather is sometimes cut into various shapes for the convenience of the customer, the price is always higher because of the additional costs for labour and waste.
Buying leather is much easier if you understand what the different types of leather are used for, how different leathers are tanned and how large skins are cut and sold. Here are a few of the basic terms used in leather craft with their explanations.
Leathercraft terms
Back. A side with the belly cut off, usually 15 - 18 sq.ft.
Belly. The lower part of a side, usually 4 - 8 sq.ft.
Kip. The skin of a large calf, usually 9 - 17 sq.ft.
Split. This refers to the undersection of a piece of leather that has been split into two or more thicknesses. Splits are usually embossed with a design or sueded.
Suede. Leather that has been sanded to produce a nap.
Grain. The epidermis or outer layer of animal skins.
Full Grain. Leather that is just as it was when taken off of the animal. Only the hair has been removed and the grain or epidermis is left on.
NOTE: ONLY FULL GRAIN, VEGETABLE TANNED LEATHER will absorb water and tool correctly. All leather carving and tooling must be done on full grain leather.
Top Grain. Top grain leather has often been sanded to remove scars and then sprayed or pasted to "cover up" the work. Top Grain IS NOT the same as "Full Grain" leather.
To make leather a uniform thickness, first the hides are run through a splitting machine. Since animal hides are not of uniform thickness, and since they are wet when they are put through the splitting machine, thethickness of the leather will not remain the same throughout the hide. There will always be slight variations and that is why leathers are usually shown with a range of thickness - such as 2 - 2.4mm., 3.2 - 3.6mm., etc.
Forms of leather
Several tanning processes transform hides and skins into leather:
* Vegetable-tanned leather is tanned using tannin and other ingredients found in vegetable matter, tree bark, and other such sources. It is supple and brown in color, with the exact shade depending on the mix of chemicals and the color of the skin. It is the only form of leather suitable for use in leather carving or stamping. Vegetable-tanned leather is not stable in water; it tends to discolor, and if left to soak and then dry it will shrink and become less supple and harder. In hot water, it will shrink drastically and partly gelatinize, becoming rigid and eventually brittle. Boiled leather is an example of this where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armor after hardening, and it has also been used for book binding.
* Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other salts of chromium. It is more supple and pliable than vegetable-tanned leather, and does not discolor or lose shape as drastically in water as vegetable-tanned. It is also known as wet-blue for its color derived from the chromium. More esoteric colors are possible using chrome tanning.
* Aldehyde-tanned leather is tanned using glutaraldehyde or oxazolidine compounds. This is the leather that most tanners refer to as wet-white leather due to its pale cream or white color. It is the main type of "chrome-free" leather, often seen in automobiles and shoes for infants.
o Formaldehyde tanning (being phased out due to its danger to workers and the sensitivity of many people to formaldehyde) is another method of aldehyde tanning. Brain-tanned leathers fall into this category and are exceptionally water absorbent.
+ Brain tanned leathers are made by a labor-intensive process which uses emulsified oils, often those of animal brains. They are known for their exceptional softness and their ability to be washed.
o Chamois leather also falls into the category of aldehyde tanning and like brain tanning produces a highly water absorbent leather. Chamois leather is made by using oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to make the fabric the color it is.
* Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types (syntans, contaction for synthetic tannins). This leather is white in color and was invented when vegetable tannins were in short supply during the Second World War. Melamine and other amino-functional resins fall into this category as well and they provide the filling that modern leathers often require. Urea-formaldehyde resins were also used in this tanning method until dissatisfaction about the formation of free formaldehyde was realized.
* Alum-tawed leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Purists argue that alum-tawed leather is technically not tanned, as the resulting material will rot in water. Very light shades of leather are possible using this process, but the resulting material is not as supple as vegetable-tanned leather.
* Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tawing, rawhide is not technically "leather", but is usually lumped in with the other forms. Rawhide is stiffer and more brittle than other forms of leather, and is primarily found in uses such as drum heads where it does not need to flex significantly; it is also cut up into cords for use in lacing or stitching, or for making many varieties of dog chews.
Less-common leathers include:
* Buckskin or brained leather is a tanning process that uses animal brains or other fatty materials to alter the leather. The resulting supple, suede-like hide is usually smoked heavily to prevent it from rotting.
* Patent leather is leather that has been given a high-gloss finish. The original process was developed in Newark, New Jersey, by inventor Seth Boyden in 1818. Patent leather usually has a plastic coating.
* Shagreen is also known as stingray skin/leather. Applications used in furniture production date as far back as the art deco period. The word "shagreen" originates from France.
* Vachetta leather is used in the trimmings of luggage and handbags. The leather is left untreated and is therefore susceptible to water and stains. Sunlight will cause the natural leather to darken in shade, called a patina.
* Slink is leather made from the skin of unborn calves. It is particularly soft, and is valued for use in making gloves.
* Deerskin is a tough leather, possibly due to the animal's adaptations to the its thorny and thicket-filled habitats.[citation needed] Deerskin has been used by many societies including indigenous Americans. Most modern deer skin is no longer procured from the wild, with deer farms breeding the animals specifically for the purpose of their skins. Large quantities are still tanned from wild deer hides in historic tanning towns such as Gloversville and Johnstown in upstate New York. Deerskin is used in jackets and overcoats, martial arts equipment such as kendo and bogu, as well as personal accessories like handbags and wallets.
* Nubuck is top-grain cattle hide leather that has been sanded or buffed on the grain side, or outside, to give a slight nap of short protein fibers, producing a velvet-like surface.
There are two other types of leather commonly used in specialty products, such as briefcases, wallets, and luggage:
* Belting leather is a full-grain leather that was originally used in driving pulley belts and other machinery. It is found on the surface of briefcases, portfolios, and wallets, and can be identified by its thick, firm feel and smooth finish. Belting leather is generally a heavy-weight of full-grain, vegetable-tanned leather.
* Nappa leather, or Napa leather, is chrome-tanned and is soft and supple. It is commonly found in wallets, toiletry kits, and other personal leather goods.
How much of a product should be leather?
You may have bought a pair of leather shoes or a leather wallet, and having looked at it closely wondered just how much of it is leather and how it can be sold as a 'genuine leather' article.
Manufacturers of leather articles are allowed to construct a product that is traded as 'genuine leather' with non-leather materials provided the incorporation on non-leather materials does not exceed specific levels.
The EC Directive 94/11 on Footwear Labelling states that a genuine leather shoe must contain the following proportions of leather:-
• Upper - 80% of surface area
• Lining/Sock - 80% of surface area
• Sole - 80% of volume
'If no one material accounts for at least 80%, information should be given on the two main materials used in the composition of the footwear.'
BLC Guidelines
The following are general guidelines which BLC Leather Technology Centre issue regarding the composition of leather articles :-
Watch Straps/Belts
Where both the outer layer and lining are leather, then it can be described as leather, genuine leather, or real leather provided no other materials comprise more than 50% of the surface area. However, if a strap or belt meets the previous definition of leather but at the same time clearly comprises less than 50% leather in total volume, then it should not be described as leather without further qualification as this may be misleading.
Upholstery
There are essentially two zones: "contact areas" i.e. seats, arm rests, vertical seat backs and rolls, and "non-contact areas" e.g. outside arms and back. Furniture should only be described as "leather" if both the "contact" and "non-contact" areas are leather.
Where only the "contact areas" are leather then the furniture should not be described as "leather" unless the description "leather chair with non leather areas" is used.
Sometimes the term "leather faced" is used under these circumstances.
Luggage, Bags, Leathergoods
At least 80% of the surface area of the main body should be leather (excluding internal dividers, pockets, pen holders etc.)
Areas
You'd be surprised the first time you see a full hide. Not only are they much larger than you think, they vary much more than you'd expect too.
Leather from different parts of the animal varies in its characteristics, and this has to be taken into account when using leather in products. The hide thickness varies all over the animal, and to get it to the right thickness it is usually split on a special cutting machine or buffed to an even thickness. The main parts of the hide are shown in the diagram below
Shoulder - the shoulder is thick and strong but tends to crease easily as this part of the hide is affected by movements of the head
Butt - the fibres in this part of the hide are tightly packed and hence the strongest part of the hide
Belly - this part of the hide is quite thin and has a much looser fibre structure than the back, and often stretches under stress.
Axillae - these are like the human armpits - they move a lot - so the fibre structure is quite loose, making it even more prone to loosening than the belly areas.
From Hide to Hair
Different parts of a hide have different properties in terms of strength, flexibility and durability. This makes some parts of a hide more suitable for use in sofa manufacturing than others.
Science
For a material that is so versatile, stylish and practical you could be fooled into thinking it is an extremely complicated material...far from it! There are basically just three main materials from which hides and skins are made :-
• Water 60-65%
• Protein 25-30%
• Fats 5-10%
The protein is mainly collagen (found in many cosmetics) and it is this collagen that is transformed into leather by the tanning process.
In good shape...
Raw hides and skins have four main parts - an epidermis, grain, corium and flesh - as shown in the diagram below :-
Two of these layers - the epidermis (which is a thin protective layer of cells during the life of an animal) and fleshy remains - are removed during tanning by a process called liming. This leaves just the grain and the corium, the interesting parts!
The grain layer is made of collagen and elastin protein fibres (found in many moisturisers and facial creams), and its structure varies quite a bit depending on the age, breed and lifestyle of the animal. The grain carries many distinctive marks such as insect bites, growth marks and wound scars giving the leather a unique appearance.
The corium is packed with collagen protein fibres, arranged in larger bundles and interwoven to give the structure great strength, excellent elasticity and durability.
The thickness of the corium increases with age which is why calfskins are thinner, smoother and softer than the hides of mature animals. Hides from cows are smoother, thinner and softer than the hides of mature male bull hides which are thick, tough, course grained and very strong.
Thick hides are often too thick for their end use and so they sometimes have to be split layerwise through the corium to give what we call a ‘grain split' - used for grain leather - and a ‘flesh split', used mainly for suede leather. Another little trick is to apply an artificial grain layer to the flesh split to make it look like grain leather! However the strength of these so called ‘finished split' leathers is reduced since the corium lacks the strength of the corium found in the grain layer.
How is leather made?
To get from a salted hide to a piece of leather ready for use in a sofa takes 10 working days!
The steps below show you what tanneries have to do to turn hides into leather :-
Curing
Raw hides and skins must be preserved to stop them deteriorating before the leather-making process can begin. Methods of preservation include salting, chilling, freezing and the use of biocides.
Soaking
Cured hides or skins are soaked in water for several hours to several days. This allows them to reabsorb any water they may have lost in the curing process or during transportation. It also helps to clean them of salt and dirt.
Painting
Painting is a method by which wool can be removed from sheepskins using a sulphide based mixture.
Liming
Liming removes the epidermis and hair. This also results in alkaline swelling of the pelt to cause a controlled breaking of some of the chemical crosslinks of the collagen .
Fleshing
After liming the pelt is passed through a machine to remove fleshy tissue from the flesh side. Hides may be split into layers at this stage or after tanning.
Deliming
The principal action of deliming is to gradually neutralise the alkali in the pelt, avoiding rapid changes in pH which could lead to distortion or disruption of the tissues.
Bating
A long delime can significantly improve the removal of any remaining lime, scud (miscellaneous debris) and residual components broken down during liming. Bating - based on the use of enzymes - completes this process so that the pelt is flat, relaxed, clean and ready for pickling and tanning.
Pickling
Weak acid and salt solutions are used to bring the pelt to the weakly acid state required for most tanning processes. Stronger pickling solutions are used to preserve pelts so that they can be stored or transported in a stable form over periods of several months.
Degreasing
Solvents or water-based systems can be used to remove excess grease before tanning.
Tanning
Tanning converts the protein of the raw hide or skin into a stable material, which will not putrefy and is suitable for a wide variety of purposes. Tanning materials form crosslinks in the collagen structure and stabilise it against the effects of acids, alkalis, heat, water and the action of micro-organisms. The main types of tanning materials are :
Mineral tannages
Most leather is tanned using salts of chromium.
Aldehyde and oil tannages
Tanning with aldehydes and oils produce very soft leathers and this system can be used to produce drycleanable and washable fashion leathers and also chamois leather.
Vegetable tannages
Various plant extracts produce brown coloured leathers which tend to be thick and firm. This type of tannage is used to produce stout sole leather, belting leather and leathers for shoe linings, bags and cases.
Splitting
A splitting machine slices thicker leather into two layers. The layer without a grain surface can be turned into suede or have an artificial grain surface applied.
Shaving
A uniform thickness is achieved by shaving the leather on the non-grain side using a machine with a helical blades mounted on a rotating cylinder.
Neutralisation
Neutralising removes residual chemicals and prepares the leather for further processing and finishing.
Additional tanning material may be applied to give particular properties which are required in the finished leather.
Dyeing
The dyeing of leather into a wide variety of colours plays an important part in meeting fashion requirements. Some leathers are only surface dyed, while others need completely penetrated dyeings, as is the case with suede leathers.
Fatliquoring
Fatliquoring introduces oils to lubricate the fibres and keep the leather flexible and soft. Without these oils the leather will become hard and inflexible as it dries out.
Samming
This process reduces water content to about 55% and can be achieved by a number of machines, the commonest being like a large mangle with felt covered rollers.
Setting out
The leather is stretched out and the grain side is smoothed. This process also reduces the water content to about 40%.
Final drying
Leather is normally dried to 10-20% water content. This can be achieved in a number of ways and each method has a different effect on the finished leather:
Staking and dry drumming
A staking machine makes the leather softer and more flexible by massaging it to separate the fibres. To finish off the leather may be softened by the tumbling action inside a rotating drum.
Buffing and Brushing
The flesh surface is removed by mechanical abrasion to produce a suede effect or to reduce the thickness. In some cases the grain surface is buffed to produce a very fine nap, e.g. nubuck leathers. After buffing the leather is brushed to remove excess dust.
Finishing
The aims of finishing are to level the colour, cover grain defects, control the gloss and provide a protective surface with good resistance to water, chemical attack and abrasion.
Final grading
Leather will be graded before despatch to the customer. This grading may consider the colour intensity and uniformity, the feel of the leather, softness, visual appearance, thickness, design effects and natural defects such as scratches.
Measurement
The area of each piece of leather is measured by machine. Nearly all leather is sold by area so accurate measurement is important.
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