Leather

Leather is a tough, water-resistant material that is produced by tanning the skin of animals such as cows_, snakes_, and alligators_. (Are they always tanned?)

Some leather is prized for its ability to develop a beautiful patina.

Contents

1   Function

Leather is more durable, flexible, and less susceptible to decomposition than raw animal skin.

Leather is used to produce shoes, briefcases, handbags, and watch straps.

2   Properties

Leather is durable, flexible, and water resistant. Leather derives its strength and flexibility from the collagen within the dermis.

2.1   Corrected-grain

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 vegetable-tanned or aniline leather. The imperfections are corrected or sanded off, and an artificial grain impressed 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.

3   Production

Leather production consists of three parts: preparatory stages, tanning, and crusting.

3.1   Preparatory stages

The preparatory stages remove unwanted raw skin components (i.e. meat, fat, and hair) to prepare the skin for tanning.

3.1.1   Curing

Typically the hide is cured with salt to prevent putrefaction of collagen from bacterial growth. Curing removes excess water from the hides and skins using a difference in osmotic pressure.

3.1.2   Beamhouse operations

The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They include, in order, soaking, liming, removal of extraneous tissues (unhairing, scudding, and fleshing), deliming, bating (including puering), drenching, and pickling.

3.1.2.1   Soaking

During soaking, the hides are soaked in clean water to remove salt (left over from curing), blood, dirt and dung, and to increase the moisture so that the hide or skin can be further treated. Skins are soaked in batches, which are put into a drum to which cold water, detergent, salt, and biocide are added. The process is finished when the water remains clean and the skins have been rehydrated. [2] This takes between 18 and 72 hours, depending on the method of curing.

3.1.2.2   Liming

Liming is the process of treating skin with milk of lime (a basic agent) that may involve the addition of "sharpening agents" (disulfide reducing agents) like sodium sulfide, cyanides, amines etc.

The goal of liming is to: remove keratinous matter (i.e. hair, nails), interfibrillary soluble proteins, and natural grease and fats.

Sodium sulphide or sodium hydrosulphide is added to the soaked skins. After an hour or so, these chemicals would have penetrated deeply into the hair and grain of the leather, causing the breakdown of the keratin, the main protein constituent of the hair and epidermis

Lime (caustic soda), a strong alkali is then added, enhancing the breakdown which actually dissolves the hair root and epidermis away. No damage due to liming is done to the collagen (dermis) part of the skin

Another effect of the liming process is to remove the inter fibrillary proteins from the dermis.

3.2   Tanning

Tanning is the process of treating skin.

The tanning process draws its name from tannin.

Tanning permanently converts the protein of the raw hide or skin into a stable material which will not putrefy and it suitable for a wide variety of end applications.

A large number of different tanning methods and materials can be used; the choice is ultimately dependent on the end application of the leather.

3.2.1   Vegetable tanning

Vegetable tanning is a traditional tanning method that employs tannin.

Hides are stretched on frames and immersed for several weeks in wooden drums of increasing concentrations of tannin. Tannins bind to the collagen proteins in the hide and coat them causing them to become less water-soluble, and more resistant to bacterial attack.

Vegetable tanned hide is flexible and is used for luggage and furniture.

Vegetable tanned leathers are more valuable and thus sold at a higher average price compared to chrome tanned leathers.

3.2.2   Chrome tanning

http://media.saddlebackleather.com/Archives/Copious+S3+Server+Files/saddleback_production/497.jpg

Chrome tanning leaves the leather a pale blue color, which is usually called "wet blue".

Chrome tanning is a tanning method that employs chromium.

In the production of chrome tanned skins and hides, the chromium sulphate binds very strongly to the fibres. The resulting complexes are very stable, and this enables the tanners to remove the water both mechanically, and then still further by the use of vacuum drying.

In vacuum drying, the skins are flattened out on a heated bed, a cover is then placed over the skin, and the air pumped out. The subsequent partial vacuum that is formed allows the water to be "boiled off" at a substantially lower temperature in a matter of a few seconds.

The resulting chrome tanned leather has a strong blue colour, is damp to the touch, but certainly dry enough for sorting with a good degree of accuracy. Chrome tanned hides and skins can therefore be dried, sorted and back into production within one day of tanning. This allows a very fast throughput of skins, minimizing the amount of stock necessary to allow the tannery to operate. [3]

3.2.3   Combination tanning

Combination tanning is a tanning method in which leather are tanned using both chrome and blends of vegetable extracts along with emulsified or unrefined oils and waxes.

3.3   Crusting

Crusting is when the skin is thinned, retanned and lubricated.

3.3.1   Drying to crust

Whichever method of tannage has been used, it is normal for the leather to be dried out at this stage even if further wet processing is to take place later. This drying or "crusting out" is very important in that it assists in the final stages of tan fixation. This is also a convenient point at which to grade the leather and sort into batches for subsequent treatment. The dry leather may also be readily stored or transported in this state. [4]

The skins are dried out following their initial tannage to form crust leather, as this is a convenient point at which to grade the skins. The method, and degree of drying is dependant on the type of leather being produced, with any future transportation of the skins also an important issue. [5]

In the production of vegetable tanned leather, after the skins have been tanned, they have to be "horsed up" for several days to allow for the fixation of the tannin to proceed. The reason for this is that the bonds formed between the fibres and the vegetable tannins, as mentioned in the previous discussions, are not as strong as those produced with chromium, the hydrogen bonds that do form take several days to stabilises. Following on from this initial horsing up, the skins are then mechanically de-watered using a sammying machine. A sammying machine is basically an industrial version of the mangle that would have been used to wring water out of clothes in most households until recently and consists of two felt rollers through which the skins are squeezed. The resulting leather is still moist and the skins have now to be hung to dry in a cool room for around a week to ten days - if the temperature is too hot, the loose tannins would be drawn to the surfaces and cause cracking of the grain. [5]

3.3.2   The crust warehouse

Once the skins have been dried, they are taken to the Crust Warehouse for sorting. The name crust coming from the fact the skins are very hard at this time having only had a basic tannage and minimal oiling. It is worth noting at this stage that skins are often bought or sold in the crust. The reason for this is that in hot humid climates, tanning the leather is the safest method of preserving the skins from rotting. [5]

3.3.3   Sorting

The sorting of the skins in the crust state is very important for the profitability of any tannery. In all cases, premium prices can charged for the top-quality aniline leathers, whilst the poorest embossing grades are generally sold at or below cost price. The grader looks for many different types of flaws in the skin, as were mentioned in our first article – insect damage, scratches, flay damage, grain rot etc. [5]

The majority of these flaws have been in the skin since it's arrival in the tannery, however it is only when the skin has been unhaired, tanned and dried that the grading can effectively take place with any degree of accuracy. Grading of leather is very subjective since skins are very rarely perfect, and the skill of the grader comes into play in deciding whether particular fault(s) warrant the skin being downgraded or not. On top of sorting for faults, the sorter may also be grading for size, length, and substance (thickness). [5]

Having been sorted, the skins will then be allocated to specific orders for customers, after which they will be shaved to substance, retanned and dyed, and finally finished to the customer's requirements. [5]

3.3.6   Fatliquoring

Fatliquoring is usually the last operation in the aqueous phase before drying. This process is generally carried out using either fish oils or synthetic oils that have been emulsified to allow their use in aqueous solutions. [6]

Like the retannage, it is of decisive importance for the quality and properties of the leather. The fatliquoring process largely determines the mechanical and physical properties of the leather. If the leather is dried without fatliquoring, it becomes hard and tinny, because the fibres are not lubricated. [6]

The function of the fatliquoring is to separate the fibres in the wet state so that they do not stick together too much during drying. [6]

3.3.7   Drying

3.3.7.1   Air drying

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Air drying involves hanging the hides on hooks so that moisture can evaporate naturally, this is the slowest process and results in shrinkage of the hide. Bad for yield, but great for giving leather a "full" or "round" hand. An air dried hide will routinely shrink by 25% (approximately 6 square feet). This also yields an article that is pliable with a bit of stretch. Chromexcel is an example of an air dried leather. [7]

3.3.7.2   Pasting

static/images/leather_pasting.jpg

Pasted leather that has been dried by fixing it to large glass or ceramic plates. Pasted leathers have very little stretch and smooth grain character.

Pasting is the application of leather to large glass, steel, or ceramic "plates." A plant based starch is mixed into a paste, which is used to adhere the hide to the plate. The drying process takes about 5 hours and utilizes steam, heat, and moving air. Think of a pasting unit as a big combination oven with different zones. This yields a leather with "flat," uniform grain. By sticking the leather against a medium that is static, it keeps the leather from shrinking. Good for yields, and good for products that need to not stretch -- like load bearing straps. [7]

3.3.7.3   Vacuum drying

static/images/leather_vacuum_drying.jpg

Vacuum drying, which is probably the most complex of the four options, is basically the same as pasting. The main difference is that we can drop the pressure inside this (massive) machine and draw the water out. This is the most time effective of the processes, and requires skill and diligence to do correctly. [7]

3.3.7.4   Toggling

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Toggled hides are allowed to air dry, but they a stretched gently through the use of small clamps called toggles. It allows the leather to shrink and plump some, but holds it firm to reduce stretchability. I have to admit, my motivation for this post was the fact that the toggling room is my favorite room in the building. It’s unnaturally quiet and well lit by natural light – and it’s clean as far as rooms in a tannery go. Toggling is the most time consuming in terms of labor, but it’s not as time consuming as “tacking,” the predecessor to toggling. Tacking is the same as toggling, except instead of reusable clamps, we used hammers, nails, and wood frames. [7]

4   Classification

4.1   Full-grain

http://media.saddlebackleather.com/Archives/Copious+S3+Server+Files/saddleback_production/489.jpg

Full-grain leather is leather that has been made from the unsanded grain of the dermis, showing all natural characteristics of the hide (e.g. scrapes, bites, brands, veins). The natural surface burnishes and beautifies with use and develops a patina over time.

Full-grain leather is the strongest and most durable leather. It is also the most expensive and difficult to work with. Roughly, 2% of all bags are made of full-grain.

Full-grain leathers are typically available in two finish types: aniline and semi-aniline.

Full grain leather is infused with oils, which help to make it supple and durable, but it is not sealed, which leaves it able to breathe.

While lower grade leathers are "painted", full grain leather is either left natural in color or stained, allowing the natural grain of the leather to show through. This results in a product that maintains the ability to strewth and flex and allow it age gracefully.

4.2   Top-grain

Top-grain (= corrected grain) leather is leather that has been made from the grain and junction of the dermis but that has had its top layer split and then undergone correction (sanded and refinished) to remove blemishes, and make it more pliable and thin.

The finishing process leaves the leather sealed and unable to breathe and creates a cold, plastic feel. Compared to full-grain leather, top-grain leather is cheaper cheaper and, weaker, and more resistant to stains (so long as the finish remains unbroken). Top-grain leather will not develop a natural patina.

4.3   Genuine

Genuine (= split) leather is the third grade of leather and is produced from the layers of hide that remain after the top is split off for the better grades.

Genuine can be rough (e.g. suede) or smooth and refinished.

The surface of genuine leather is usually refinished (spray painted and embossed) to resemble a higher grade.

"Genuine" refers to the fact that the leather is real and not a composite material.

Genuine leather is missing the top grain, which gives leather its strength, breathability, and the unique ability to look better with wear. Genuine leather is structurally weak.

Genuine leather will look worse with wear and will need constant upkeep.


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.

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 and finished or sueded.

4.3.1   Suede

http://upload.wikimedia.org/wikipedia/commons/4/40/Suede_Jacket_Adler.jpg

Suede is genuine leather that has been sanded to produce a napped finish.

The term comes from the French "gants de Suède", which literally means "Swedish gloves". [1]

Suede leather is made from the underside of the skin, primarily lamb, although goat, calf and deer are commonly used. Splits from thick hides of cow and deer are also sueded, but, due to the fibre content, have a shaggy nap.

Suede, like genuine leather, is weak and should not be be used in areas where it gets stress. Suede is tougher than cloth, soft, thin, and pliable making it suitable for clothing and delicate uses; suede was originally used for women's gloves. Suede is also popular in jackets, shoes, shirts, purses, furniture, upholstery, bags, and other accessories, and as a lining for other leather products.

Due to its textured nature and open pores, suede may become dirty and quickly absorb liquids.

4.4   Bonded

static/images/bonded_leather.jpg

Bonded leather is leather that is made from a blend of leftover leather and a polymer glue in a process similar to vinyl manufacture. The degree of organic leather in the mix (between 60% to 100%) affects the smell and the texture of the final product.

Bonded leather is the most economical and lowest quality leather available. It is weak and degrades quickly with use. It is commonly used for Bibles covers and commercial upholstery.

4.5   Shell cordovan

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Shells.

Shell cordovan is an equine leather made from the fibrous flat muscle (or shell) beneath the hide on the rump of the horse. It is commonly used for making shoes.

The leather derives its name from the city of Cordoba, Spain, where it was originally prepared by the Moors.

5   Grading

Better leathers have more consistent grain, less variation in finish and thickness, and are not corrected or treated.

6   Further reading

7   Questions

8   References

[1]http://www.waskerd.com/leather.html
[2]Lanning, David. 1996. The Manufacture of Leather - part 2. http://www.hewit.com/skin_deep/?volume=2&article=2
[3]Lanning, David. 1996. The Manufacture of Leather - part 3. http://www.hewit.com/skin_deep/?volume=3&article=3
[4]Lanning, David. 1996. The Manufacture of Leather - part 4. http://www.hewit.com/skin_deep/?volume=4&article=2
[5](1, 2, 3, 4, 5, 6) Lanning, David. 1996. The Manufacture of Leather - part 5. http://www.hewit.com/skin_deep/?volume=5&article=3
[6](1, 2, 3) Lanning, David. 1996. The Manufacture of Leather - part 6. http://www.hewit.com/skin_deep/?volume=6&article=3
[7](1, 2, 3, 4) August 2010. http://horween.com/101/technique-toggling-and-drying/
[8]Jedibrad. Materials. https://www.reddit.com/r/malefashionadvice/comments/3aubi0/a_comprehensive_spring_summer_2015_guide_part_1/csg04r8

[8]a_robot_with_dreams & 6tfg. Dec 10 2014. Introductory Leather Care Guide. http://www.reddit.com/r/goodyearwelt/comments/1ubpyl/introductory_leather_care_guide/

Leather Types Calfskin[1] is a dress leather due to its fine wrinkling and suppleness. However, it is relatively delicate as a leather when compared to pull-up leathers, and handles wear and water only moderately well. Cowhide[2] and horsehide[3] are two of the primary sources of leather, and can be tanned in varying ways to produce different products (including pullup leathers). Regular cowhide and horsehide are similar to calfskin in that they have a solid, unchanging finish. However, they have coarser wrinkling and are considered less formal. Pullup leathers[4] aren’t necessarily a type of leather, but rather a tannage. In other words, pullup leathers can be sourced from nearly any animal. They are characterized by their response to interior pressure[5] , specifically that they lighten when pushed on from the inside[6] . Horween Chromexcel[7] is likely the most commonly known pullup leather, although there are many types of pullup. These leathers are stuffed full with oils and fats, lending to their resiliency, stretch, and water resistance. Pullup leathers are generally consider more casual. If left untreated, pullup leathers will lighten at the creases with wear. Shell cordovan[8] is arguably a pullup leather, but is discussed separately due to its unique characteristics. Shell cordovan is one of the least stretchy leathers, and is highly prone to cracking under tensile force. However, it is a hardwearing, dense, highly water-resistant leather cherished today its for depth of color[9] and rolling folds[10] . This explains its heritage as a workwear leather and casual nature. For more information on the heritage and origins of shell cordovan, check out this blog post[11] by /u/lordpoint[12] . Roughout[13] leather is regular leather turned with the rough side out[14] , leading to a soft, highly textured feel. Any leather can be used as roughout, although pullup leathers are most common. Roughout is considered casual. Suede[15] is the result of splitting the hide, then taking the bottom half and sanding/buffing it to raise a nap, the fuzzy quality of suede. The result is a soft and flexible leather, but sacrifices water resistance and durability. Due to this lack of weather resistance, it’s often considered a spring/summer material. It’s also very casual due to the texture. Nubuck[16] is similar to suede. It is made by sanding and buffing the topside of a hide to raise a nap. The result is similar to suede, but more durable. Nubuck is casual. Patent Leather[17] is the most formal of leathers due to its smooth, shiny finish, although it Is also used on some designer sneakers. True patent leather is the result of a long process of buffing with many layers of linseed oil lacquer, and it is now rare and commonly faked using plastic topcoats. Scot’s Grain[18] is also known as Scot’s, and describes Scottish leather that had been tanned in old whiskey aging oak barrels filled with barley mash. This gave them an almost shriveled appearance on the surface. True scotch grain leather is exceedingly rare, and most is faked through stamping rather than through tanning processes. This faked Scotch grain is known as pebbled grain. Zug Grain[19] is highly water resistant, to the point of being waterproof, and is commonly used in veldtschoen construction to create naturally waterproof boots. It has a scaly appearance and dark chocolate colour lent from the tannage process, which includes a step tanned in a milky chocolate syrup. Don’t ask how that happened. Exotics are leathers that are uncommon, such as alligator, kangaroo, or shark. They are all unique in their characteristics, and you can find more information about some of the specific types in this comment[20] by /u/Siegfried_Fuerst[21] and related comments in that thread. It’s difficult to describe them all, so ask and we try to track down more information.


[8]_


Full Grain just means nothing has been done to alter the outer layer of a hide. Sure years ago this was a sign of quality when they would only use the most flawless hides for full grain, but if you don’t toss out the rougher hides the result is still full grain.

Tons of not full grain leathers can be awesome: Suedes, Nubuck, Scotch, pebble and Alpine grain...the list goes on.

I get asked how to tell quality leather online and the answer is the same as with suits: look at other markers of quality in the item (admittedly more obvious in leather goods than suits) and then consider a brand’s reputation and reviews.