In general, leather is sold in three forms:
- Full-Grain leather, made from the finest raw material, are clean natural hides which have not been sanded to remove imperfections. Only the hair has been removed. The grain remains in its natural state which will allow the best fibre strength, resulting in greater durability. The natural grain also has natural breathability, resulting in greater comfort for clothing. The natural Full-Grain surface will wear better than other types of leather. Rather than wearing out, it will develop a natural “Patina” and grow more beautiful over time.
The finest leather furniture and footwear are made from Full-Grain leather. Full grain leathers can mainly be bought as two finish types: aniline and semi-aniline.
- Corrected-Grain leather, also known as Top-Grain leather, is fuzzy on one side and smooth on the other. The smooth side is the side where the hair and natural grain used to be. The hides on this type of leather have all of the natural grain sanded off and an artificial grain applied. Top grain leather generally must be covered with dense colours painted to cover up the sanding and stamping operation. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.
- Suede is leather that has had the grain completely removed or is an interior split of the hide/skin. During the splitting operation the grain and drop split are separated. The drop split can be further split (thickness allowing) into a middle split or a flesh split. In very thick hides the middle split can be separated into multiple layers until the thickness prevents further splitting. The strongest suedes are usually made from grain splits (that have the grain completely removed) or from the flesh split that has been shaved to the correct thickness. Suede is “fuzzy” on both sides. Suede is less durable than top-grain. Suede is cheaper because many pieces of suede can be split from a single thickness of hide, whereas only one piece of top-grain can be made. However, manufacturers use a variety of techniques to make suede appear to be full-grain. For example, in one operation, glue is mixed with one side of the suede, which is then pressed through rollers; these flatten and even out one side of the material, giving it the smooth appearance of full-grain. Latigo is one of the trade names for this product. A reversed suede is a grained leather that has been designed into the leather article with the grain facing away from the visible surface. It is not a true form of suede.
Other 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 and is commonly confused with a shark skin and stingray skin combination.
- Vachetta leather is used in the trimmings of luggage and handbags, popularized by Louis Vuitton. 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.
- Deer Skin is one of the toughest leathers, partially due to adaptations to their thorny and thicket filled habitats. Deerskin has been prized in 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.
- Deer skin is used in jackets and overcoats, professional sporting equipment such as kendo bogu, as well as high quality personal accessories like handbags and wallets. It commands a high price due to its relative rarity and proven durability.
- 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 fibres, producing a velvet-like surface.
- There are two other descriptions 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 often found on the surface of briefcases, portfolios, and wallets, and can be identified by its thick, firm feel and smooth finish. Belting leather is the only kind of leather used in luxury products that can retain its shape without the need for a separate frame; it is generally a heavy-weight of full-grain, vegetable-tanned leather.
- Nappa leather, or Napa leather, is extremely soft and supple and is commonly found in higher quality wallets, toiletry kits, and other personal leather goods.
The following are not ‘true’ leathers, but contain leather material:
- Bonded Leather, or “Reconstituted Leather”, is not really a true leather but a man-made material composed of 90% to 100% leather fibres (often scrap from leather tanneries or leather workshops) bonded together with latex binders to create a look and feel similar to that of genuine leather at a fraction of the cost. Bonded leather is not as durable as other leathers, and is recommended for use only if the product will be used infrequently. One example of bonded leather use is in Book covers.
- Bicast leather is a man-made product that consists of a thick layer of polyurethane applied to a substrate of low-grade or reconstituted leather. Most of the strength of bicast leather comes from the polyurethane coating, which allows this material to be used where strength or durability are required.
The vast majority of leather is sold according to its area. The leather is placed through pin-wheel or electronic measuring machines and its surface area is determined. The unit of measurement is square metre, square decimetre or square foot. The thickness is also important, and this is measured using a thickness gauge (the unit of measurement is millimetres, e.g., 1.8 mm is a standard thickness for a school shoe).
In some parts of the world top-grain thicknesses are described using weight units of ounces. Although the statement is in ounces only, it is an abbreviation of ounces per square foot. The thickness value can be obtained by the conversion: 1 oz/ft² = 1/64 inch (0.4 mm) Hence, leather described as 7 to 8 oz is 7/64 to 8/64 inches (2.8 to 3.2 mm) thick.
The weight is usually given as a range because the inherent variability of the material makes ensuring a precise thickness very difficult. Other leather manufacturers state the thickness directly in millimetres.
Leather from other animals
Today, most leather is made of cattle skin, but many exceptions exist. Lamb and deer skin are used for soft leather in more expensive apparels.
Kangaroo skin is used to make items which need to be strong but flexible, it is the material most commonly used in high quality bullwhips. Kangaroo leather is favored by some motorcyclists for use in Motorcycle Leathers specifically because of its lighter weight and higher abrasion resistance compared to cowhide, thus providing greater protecting in case of a fall on the roadway. Kangaroo leather is also used for high performance soccer footwear.
Leather made from more exotic skins has at different times in history been considered very beautiful. For this reason certain snakes and crocodiles have been hunted to near extinction.
In the 1970s, ostrich farming for their feathers became popular, and ostrich leather became available as a side product. There are different processes to produce different finishes for many applications, i.e., upholstery, footwear, automotive products, accessories and clothing.
Ostrich leather is considered one of the finest and most durable in the world and is currently used by many major fashion houses such as Hermès, Prada, Gucci, and Louis Vuitton. Ostrich leather has a characteristic “goose bump” look because of the large follicles from which the feathers grew.
In Thailand, sting ray leather is used in wallets and belts in the same way as regular bovine leather. Sting ray leather is as tough and durable as hard plastic. The leather is often dyed black and covered with tiny round bumps in the natural pattern of the back ridge of an animal. These bumps are then usually dyed white to highlight the decoration. Leather clothing is also popular in Thailand.
In the United States, bison leather has become popular. It is used for gloves, jackets and some baseball gloves. It is rugged but supple and has a waxy feel.
Overall, leather comes from a variety of other sources, including the skins of Cattle, Pigs, Goats, Sheep, Crocodiles & Alligators, Ostriches, Kangaroos, Yak.
Leather production processes
The leather manufacturing process is divided into 3 fundamental sub-processes: preparatory stages, tanning and crusting. All true leathers will undergo these sub-processes. A further sub-process, surface coating can be added into the leather process sequence but not all leathers receive surface treatment. It’s difficult to have a list of operations that all leathers must undergo, as there are so many types of leather.
The preparatory stages are when the hide/skin is prepared for tanning. Preparatory stages may include: preservation, soaking, liming, unhairing, fleshing, splitting, reliming, deliming, bating, degreasing, frizing, bleaching, pickling and depickling.
Tanning is the process 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 end applications.
The principal difference between raw hides and tanned hides is that raw hides dry out to form a hard inflexible material that when re-wetted (or wetted back) putrefy, whilst tanned material dries out to a flexible form that does not become putrid when wetted back.
There are a large number of different tanning methods and materials that can be used, the choice is ultimately dependant on the end application of the leather.
The most commonly used tanning material is chromium, which leaves the leather once tanned a pale blue colour (due to the chromium), this product is commonly called “wet blue”. The hides once they have finished pickling will typically be between pH of 2.8-3.2. At this point the hides would be loaded in a drum and immersed in a float containing the tanning liquor.
The hides are allowed to soak (while the drum slowly rotates about its axle) and the tanning liquor slowly penetrates through the full substance of the hide. Regular checks will be made to see the penetration by cutting the cross section of a hide and observing the degree of penetration.
Once a good even degree of penetration exists, the pH of the float is slowly raised in a process called basification. This basification process fixes the tanning material to the leather and the more tanning material fixed the higher the hydrothermal stability and increased shrinkage temperature resistance of the leather.
The pH of the leather when chrome tanned would typically finish somewhere between 3.8 – 4.2.
Crusting is when the hide/skin is thinned, retanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying and softening operations.
Crusting may include the following operations: wetting back, sammying, splitting, shaving, rechroming neutralisation, retanning, dyeing, fatliquoring, filling, stuffing, stripping, whitening, fixation, setting, drying, conditioning, milling, staking and buffing.
For some leathers a surface coating is applied. Tanners refer to this as finishing. Finishing operations may include: oiling, brushing, padding, impregnation, buffing, spraying, roller coating, curtain coating, polishing, plating, embossing, ironing, ironing/combing(for hair-on)and glazing.
Role of enzymes in leather production
Enzymes like proteases, lipases and amylases have important role in soaking, dehairing, degreasing and bating operations of leather manufacturing. Proteases are the most commonly used enzymes in leather production. The criteria for selection of best protease is that it should be non- collagenolytic and non- keratinolytic in nature. It has property to hydrolyze casein,elastin,albumin and globuline like proteins.
Lipases are used in degreasing operation to hydrolyze fat materials of skin/ hide.
Amylases are also used in bating of animal skins/ hide.
Protease based enzymes, when used in soaking hydrolyze all the non structured proteins which are not essential for leather making.
Elastin, which is a non structured protein is the binding material between the upper grain layer and the lower collagenetic substrate which is the actual leather. Complete removal of the elastin will result in double layer of grain and if not removed properly, the elastin when subjected to liming will get immuned and hardened resulting in a loose grain.
Preservation and conditioning of leather
The natural fibres of leather will break down with the passage of time. Acidic leathers are particularly vulnerable to red rot, which causes powdering of the surface and a change in consistency. Damage from red rot is aggravated by high temperatures and relative humidities, and is irreversible.
Exposure to long periods of low relative humidities (below 40%) can cause leather to become desiccated, irreversibly changing the fibrous structure of the leather.
Various treatments are available such as conditioners, but these are not recommended by conservators since they impregnate the structure of the leather artifact with active chemicals, are sticky, and attract stains.