Leather is a natural, durable and flexible material created by the tanning of animal rawhide and skin, often cattle hide. It can be produced through manufacturing processes ranging from cottage industry to heavy industry.
Leather is used for various purposes including clothing, bookbinding, leather wallpaper, furniture covering, horseback-riding equipment such as bridles, reins and saddles and many more things. It is produced in a wide variety of types and styles and is decorated by a wide range of techniques.
Forms and variations
Several tanning processes transform hides and skins into leather:
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 exotic colors are possible when using chrome tanning. The chrome tanning method usually only takes a day to finish, and the ease and agility of this method make it a popular choice. It is reported that chrome-tanned leather adds up to 80% of the global leather supply. Can be used for horse collars, liners, pouches and bags, but is also very common in the automotive industry.
Chrome oil tanned leather is simply chrome-tanned leather and then treated with an oil helping to preserve it. Usable for work chaps, saddle bags and linings. Not suitable for gun cases, holsters and such.
Vegetable-tanned leather is tanned using tannins and other ingredients found in different vegetable matter, such as tree bark prepared in bark mills, wood, leaves, fruits and roots and other similar 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, so if left to soak and then dried it will shrink and become less supple and harder. In hot water, it will shrink drastically and partly congeal, 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 armour after hardening, and it has also been used for book binding. Due to its process it takes weeks or up to 40 days to produce a piece of vegetable-tanned leather. This is the perfect tooling leather. If your leather is to be tooled, it will have to be vegetable-tanned leather.
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.
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 such as deers, cows, and buffaloes. They are known for their exceptional softness and their ability to be washed.
Chamois leather also falls into the category of aldehyde tanning and, like brain tanning, produces a porous and highly water-absorbent leather. Chamois leather is made by using marine oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to make the fabric the color it is.
Rose tanned leather is a variation of vegetable oil tanning and brain tanning, where pure rose otto replaces the vegetable oil and emulsified oils. Rose tanned leather tanned using rose otto oil leaves a powerful rose fragrance even years from the day it is manufactured. It has been called the most valuable leather on earth, but this is mostly due to the high cost of rose otto and its labor-intensive tanning process.
Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types (syntans, contraction 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-tanned leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Alum-tanned leather is technically not tanned, as tannic acid is not used, and the resulting material will revert to rawhide if soaked in water long enough to remove the alum salts. Its primary use is for lace leather.
Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tanning, 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; it is primarily found in uses such as drum heads and parchment where it does not need to flex significantly; it is also cut up into cords for use in lacing or stitching and for making many varieties of dog chews.
Leather; usually vegetable-tanned, can be oiled to improve its water resistance. This currying process after tanning supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Russia leather was an important international trade good for centuries. Frequent oiling of leather, with mink oil, neatsfoot oil, or a similar material keeps it supple and improves its lifespan dramatically.
Types of leather
In general, leather is sold in four forms:
Full-grain leather refers to hides that have not been sanded, buffed, or snuffed (as opposed to top-grain or corrected leather) to remove imperfections (or natural marks) on the surface of the hide. The grain remains allowing the fiber strength and durability. The grain also has breathability, resulting in less moisture from prolonged contact. Rather than wearing out, it will develop a patina during its expected useful lifetime. High quality leather furniture and footwear are often made from full-grain leather. Full-grain leathers are typically available in two finish types: aniline, semi-aniline.
Top-grain leather (the most common type used in high-end leather products) is the second-highest quality. It has had the “split” layer separated away, making it thinner and more pliable than full-grain. Its surface has been sanded and a finish coat added to the surface which results in a colder, plastic feel with less breathability, and it will not develop a natural patina. It is typically less expensive and has greater resistance to stains than full-grain leather, so long as the finish remains unbroken.
Corrected-grain leather is any leather that has had an artificial grain applied to its surface. The hides used to create corrected leather do not meet the standards for use in creating vegetal-tanned or aniline leather. The imperfections are corrected or sanded off, and an artificial grain embossed into the surface and dressed with stain or dyes. Most corrected-grain leather is used to make pigmented leather as the solid pigment helps hide the corrections or imperfections. Corrected grain leathers can mainly be bought as two finish types: semi-aniline and pigmented.
Split leather is leather created from the fibrous part of the hide left once the top-grain of the rawhide has been separated from the hide. During the splitting operation, the top-grain and drop split are separated. The drop split can be further split (thickness allowing) into a middle split and a flesh split. In very thick hides, the middle split can be separated into multiple layers until the thickness prevents further splitting. Split leather then has an artificial layer applied to the surface of the split and is embossed with a leather grain (bycast leather). Splits are also used to create suede. 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. Manufacturers use a variety of techniques to make suede from full-grain. 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 considered to be a true form of suede.
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 returning to a rawhide state, if wetted. It will be easier to soften, and will help keep leather eating bugs away.
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.
Fish leather is popular for its motifs and its pigmentation. Mainly used for making shoes and bags, the fish skin is tanned like other animal skins. The species used include salmon, perch, sturgeon, etc.
- Salmon : farmed in Iceland and Norway, salmon skin has fine scales. Its strength and elegant look make it the most popular fish leather.
- Perch : coming from Nile, its skin is recognizable with its large, round and soft scales.
- Wolffish : its skin is smooth because without scales. We recognize it thanks to its dark spots, and the ‘stripes’ which are due to the friction of marine rocks.
- Cod : its skin has finer scales than salmon, but its texture is more varied, sometimes smooth and sometimes rough.
- Sturgeon : fish well known for its eggs (caviar), which make it rare. Its leather is thus quite expensive.
- Eel : fish without scales, its skin has a shiny appearance.
- Tilapia : compared with salmon skins the patterns of tilapia leather is more beautiful, but the skins are smaller. The leather has resistant qualities similar to salmon and perch. Tilapia originate from Africa, and are farmed in many tropical countries. It is a popular food fish and is considered an invasive species in much of the United States.
- 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. It is known as the most difficult leather to work due to dished scales of the animal, and it is one of the most expensive leathers.
- Shark is covered with small, close-set tubercles, making it very tough. The handbags made of shark skin used to be in vogue but this keen interest has since fallen as the costs of production and of the leather itself are very high. Moreover, this skin is more difficult to work. Should not be confused with traditional “sharkskin”, a woven textile product.
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, water-resistant leather, possibly due to the animal’s adaptations to its thorny and thicket-filled habitats. Deerskin has been used by many societies, including indigenous Americans. Most modern deerskin 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 such as handbags and wallets. It is the perfect clothing leather.
Goatskin is soft but tough, and is used for items such as thorn-resistant gardener’s gloves.
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.
Russia leather is a particular form of bark-tanned cow leather. It is distinguished by an oiling step, after tanning, where birch oil is worked into the leather to make it particularly hard-wearing, flexible and resistant to water.
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, vegetal-tanned leather.
Napa leather is chrome-tanned and is soft and supple. It is commonly found in wallets, toiletry kits, and other personal leather goods.
The following are not “true” organic leathers, but are materials that contain leather fiber. Depending on jurisdiction, they may still be labeled as “Genuine Leather”, even though the consumer generally can only see the outer layer of the material and can’t actually see any of the leather content (think of the $30 belt you’ll get at Wal-Mart):
Bonded leather or reconstituted leather is an economical type that uses leftovers of organic leather (often from leather tanneries or leather workshops) which are shredded and then bonded together with polyurethane binders or latex on top of a fiber sheet. The varying degree of organic leather in the mix (between 10% and 90%) affects the smell and the texture of such product. Due to its reduced cost it is becoming a popular choice for furniture upholstery, especially for commercial use, where durability is needed; however durability can vary widely, depending on the formulation.
Bycast leather is a split leather with a layer of polyurethane laminated to the surface and then embossed. Bycast was originally made for the shoe industry and later adopted by the furniture industry. The original formula created by Bayer was strong but expensive. The result is a material that is slightly stiffer but cheaper than top-grain leather but has a much more consistent texture. Because its surface is completely covered in plastic, is easier to clean and maintain, but is not easily repaired.
(Information is based on http://en.wikipedia.org/wiki/Leather)