Maybe Too Much Information about Leather

There are a number of processes whereby the skin of an animal can be formed into a supple, strong material commonly called Leather.

• Vegetable-tanned leather is tanned using tannin (hence the name "tanning") 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. 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. This is the only form of leather suitable for use in leather carving or stamping.
• 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 leather used in chrome-free leather often seen in infant's shoes and in automobiles made with chrome-free leather. 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. 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 oxidizes easily to produce the aldehydes that tan the leather.
• Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types. This leather is white in color and was invented when vegetable tannins were in short supply, i.e. 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 tanned using aluminum salts mixed with a variety of binders and protein sources, such as flour, egg yolk, etc. Purists argue that alum-tanned leather is technically "tawed" and 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-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, 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.

Leather, usually vegetable-tanned leather, can be oiled to improve its water resistance. This supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Frequent oiling of leather, with mink oil, Neatsfoot oil or a similar material, keeps it supple and improves its lifespan dramatically. Leather with the hair still attached is called hair-on leather.

In general, leather is sold in three forms:

• Full-Grain leather or Top-Grain is referring to the upper section of a hide that contains the epidermis or skin layer. It refers to hides that have not been sanded, buffed or snuffed (otherwise known as Corrected) in order to remove imperfections on the surface of the hide. Only the hair has been removed from the epidermis. The grain remains in its natural state which will allow the best fiber 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 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. For these reasons only the best raw hides are used in order to create Full-Grain or Top-Grain leather. Full grain leathers can mainly be bought as two finish types: aniline and semi-aniline.

• Corrected-Grain leather is any Top-Grain leather that has had its surfaces sanded, buffed or snuffed in order to remove any imperfection on the surface due to insect bites, healed scars or brands. Top-Grain leather is often wrongly referred to as Corrected-Grain. Although Corrected-Grain leather is made from Top-Grain as soon as the surface is corrected in any way the leather is no longer referred to as Top-Grain leather. The hides used to create corrected leather are hides of inferior quality that do not meet the high standards for use in creating aniline or semi-aniline leather. The imperfections are corrected and an artificial grain applied. Most Correct 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 that is created from the fibrous part of the hide left once the Top-Grain of the raw hide has been separated from the hide. During the splitting operation the 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. Splits are also used to create Suede. The strongest suede 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. 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. Large quantities are still tanned from wild deer hides in historic tanning towns such as Gloversville and Johnstown in upstate New York. Deer skin is used in jackets and overcoats, professional sporting equipment 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 fibers, 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 chrome-tanned and 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 fibers (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 Bible 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. Bicast was originally made for the shoe industry and recently was adopted by the furniture industry. The original formula created by Bayer was fairly strong, but creating Bicast from the original recipe is expense. Most of the Bicast used today is created using inferior generic chemicals resulting in an inferior material. The result is a much stiffer product that tends to delaminate resulting in bubbles and cracking.

The vast majority of leather is sold according to its area. The unit of measurement is square meter, square decimeter or square foot. The thickness is also important, and this is measured using a thickness gauge (the unit of measurement is millimeters, 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.

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