Leather History



Ancient Egypt, one of the most developed civilizations in this early period, valued leather as an important item of trade. The Egyptians made leather sandals, belts, bags, shields, harness, cushions and chair seats from tanned skins. Many of these items are in fact still made from leather today.



Similarly, the Greeks and Romans used leather to make many different styles of sandals, boots and shoes. When the Roman legions marched in conquest across Europe, they were well attired in leather armour and leather capes. In fact, right up until the early 18th century, the shield carried by the ordinary soldier was more likely to be made of leather than metal.



The Moors developed remarkable skill primarily in the preparation of beautiful goatskin still known as morocco leather after the country of its origin. In fact the description `genuine morocco` is still very highly regarded today, particularly in the manufacture of small leather goods.



North American Indians took the ashes from their campfires, put water on them and soaked the skins in this solution. In a few weeks the hair and bits of flesh came off, leaving only the raw hide. This tanning method, which used a solution of hemlock and oak bark, took about three months to complete after which the leather was worked by hand to make the hide soft and pliable.

s.



About Leather



The properties of leather vary considerably depending upon the type and quality of both the skins and the tanning process. Every piece of leather has individual markings which relate to its origins and add character to each skin.

Like a fine wine, a good quality leather garment should improve with age. The natural elasticity of each hide means it is flexible and will stretch and return to its original shape. Leather also has a natural tendency to repel liquids and resist staining. It`s also fire resistant, and emits no toxic fumes, even when exposed to intense heat.



Relative to virtually all man-made textiles, leather is very strong and has a high resistance level to tears and punctures. The comfort provided by most leather goods is due in part to leather`s ability to combine breathing and insulating properties. You may have heard... "Leather is hot in summer and cold in winter." In reality, leather adjusts constantly to its environment. Because it is a natural product, leather "breathes" freely, maintaining a comfort level in all seasons.



Types of Hide



Different Hides used for leather garments and Upholstery



Cowhide is the most common leather used in the making of garments, furniture and leather goods. Cowhide as a category covers a wide spectrum of textures and quality, but generally, it is quite durable, easy to care for and resistant to water and dirt. Cowhide leather will maintain its integrity, taking on the shape of the wearer, making it more comfortable with everyday use. This affordable, functional leather offers fashion, value and endless colours and style. It is also highly used in the leather furniture and upholstery industry.

Lambskin is very soft, luxurious leather. Its natural lightweight layers give it a distinctive, velvety touch, which suites form fitting jackets, pants, skirts as well as coats. But don`t let its delicate texture discourage you. With a little extra care, lambskin is very wearable and the ultimate luxury.



Pigskin is by far the most popular and versatile, easily transformed into fashion`s most current looks. When tanned on the outside, it produces smooth Napa finish, often used for jackets and accessories. Tanning on the inside results in a silky suede finish. The natural, lightweight structure of pigskin produces delicate patterns, textures and silky soft naps, perfect for sportswear, shirts and blazers.



Sheepskin refers to the hide of a sheep used with the wool still attached. Usually, the wool side faces into the garment or accessory, but it can also be made reversible. The wool can be ironed, which means straightened to yield a smooth, fur-like appearance, or it can be left naturally curly. Whichever way the wool is styled, this is the warmest leather available.



Shearling is quite similar in appearance to sheepskin; the term shearling refers to hides from lambs which are generally much lighter in weight then sheepskin hides and much softer. Although they may be lighter, shearling coats are just as warm as the heavier sheepskin. They are an elegant alternative to a fur coat.



Finishing Leather Hides



You do need to understand some of the basic chemistry behind the tanning and finishes applied to automotive leather to understand how to renovate or care for it.



Dyeing process



Depending on the desired product, the hides then go through a dyeing process, which also involves adding moisture back into the skin. Hides which have been vegetable tanned are bleached and them soaked with oils, soaps, greases and waxes to make them more pliable.



Rolling



Rolling leather running the skins through a machine, which works to firm the leather and make it stronger. After the rolling process has finished, leathers are stretched, where they dry out in a heat controlled room.



Surface coating



Once trimmed, dependent on customer requirements, the initial application of surface water-based pigment is applied by machine roller coating. The final protective coats are applied by machine spraying.

Finishing (application) a water-based pigmented finish is applied to give a uniform covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution of acrylic polymer, which contains binders to adhere to the leather.



The pigment can be applied to leather in three ways;



•Sprayed on by hand using an airgun.



•Sprayed on by a machine on a conveyor belt.



•Often a combination of a base coat and a top coat are used.



After applying the pigment polyurethane is then applied to seal the colour, to provide protection and durability, the polyurethane is also used to adjust the gloss level of the leather.

The above process is what leads me to the following statement on leather conditioners- I have discussed this issue with many people in the leather tanning / preservation / care products industry and haven`t received a definitive answer.



Some Questions



How much conditioner will permeate the thick polyurethane top coat on the leather?



Chrome tanned leather hide is sealed at the tannery and then pigmented; what could a conditioner do for the hide?



Approximately 90% of vehicle manufacturers are using a split-leather hide and (thermoplastic) polyurethane covering for their interior upholstery.



Vehicle upholstery is chrome tanned and uniquely treated with a light pigmented urethane coating and / or a vinyl covering to make it more viable for automotive seating. It retains the softness of natural top-grain leather but resists fading in direct sunlight, which besides body oil is leathers worst enemy.



The complex tanning process of chromed tanned hides results in the fat liquoring and oils necessary to keep the hide soft and pliable being locked in, the hide is then has a pigmentation coating (a water-based paint) to ensure a uniform colour, this is further sealed by a durable polyethylene covering to protect the hide from abrasion from clothing as well as the dust / dirt introduced by the vehicle’s AC system.

“And now for the rest of the story”



Leather - The Manufacturing Process



Leather is made from animal skins or hides which have been chemically treated to preserve quality and natural beauty. The chemical procedure used to ready raw animal hides for use is called "tanning." A piece of hide or skin which has been tanned produces strong, flexible leather which is able to resist decay and spoilage.



The majority of leather made today is produced from tanned cattle hides, though many types of hides can be used, including those from horses, pigs, goats, calves, labs, deer, kangaroos, reptiles, seals, and walrus.

Stages of tanning (preparation) - raw animal skins go through several steps during the tanning process. Depending on the type of hide used and the desired end-product, the steps taken during tanning can vary greatly.



1. Curing

a) Animal skins or hides - are first "cured," a process which involves salting and/or drying the hide once it`s been stripped from the animal.



Because this step needs to be performed almost immediately upon removal from the animal, it often takes place inside the meat-packing industry or at a nearby factory. Hides can be cured in one of two ways:



b) Wet-Salting- is done by salting the hide and then piling many skins together until they form a moist bunch. They are then left to cure for one month, so that the salt can completely be absorbed into the skin.



c) Brine-curing - is more common than wet-salting, as it`s considered a faster, easier method. During brine curing, hides are positioned carefully in vats and smothered with a mixture of salt and disinfectant. After 10-16 hours, the skins are completely cured and ready to move on to the next stage.



d) Soaking - once the hides have been cured, they are then soaked in water for several hours to several days. The water helps to rid the skin of salt, dirt, debris, blood and excess animal fats.



e) Liming - after being soaked, the hair and epidermis is removed by treating hides with a mixture of sodium sulphide and lime solutions.

f) Flesh removal - after soaking, animal hides are moved through a machine which strips the flesh from the surface of the hide.



g) Hair removal - the hides are then transported to a large vat, where they are immersed in a mixture of lime and water, which loosens the hair from the skin. After a 1-10 day soak, the hair is mechanically removed from the hide.



h) Scudding - stray hairs and fat which were missed by machine are removed from the hide with a plastic tool or dull knife in a process known as "scudding." Scudding is done by hand.



i) De-liming - after the hair and debris has been cleaned from the skin, hides are delimed in a vat of acid. After the lime has been pulled from the skin, hides are treated with enzymes, which smooth the grain of the leather and help to make the resulting product soft and flexible



j) Splitting - the hide is split into layers; the grain layer (top) is used in the manufacture of ‘Top Grain Leather’ (premium upholstery, clothing, etc) the lower split is used for polyurethane protected automotive upholstery and the flesh split (bottom) is sold to other manufacturers for the production of suede, or to the food industry as a source of collagen.



2. Tanning - is the process which 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, while tanned material dries out to a flexible form that does not become putrid when wetted back.



Many different tanning methods and materials can be used; the choice is ultimately dependent 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 2.5 and 3.5



Hides and skins are often treated several times during the process of tanning. Which type of tanning procedure is used, depends largely on the hide itself and the resulting product intended. After de-liming and pickling to make the hide more receptive, the hides are tanned to stabilise the collagen fibres and prevent its decay.



This can be done either using a solution of recyclable chromium salts producing `wet blues` or various organic solutions producing `wet whites`. Subsequent to the chrome tanning process (using chromium salts to preserve hides and prepare them to absorb dyes) automotive leather is pigment coated and finished (and some leather then has a polyurethane protective coating applied)



Leather is hygroscopic (naturally absorbs and retains water), meaning it’s also susceptible to losing the moisture necessary to keep it pliant and soft. The denaturing process of leather tanning removes moisture from the hide; water-based products restore the lost moisture of leather hides to maintain its natural flexibility.



a)Vegetable Tanning Hides which have been tanned with a vegetable tanning agent solution produce flexible, but stiff leathers, such as those used in luggage, furniture, leashes, belts, hats, and harnesses. Vegetable tanning consists of stringing hides on large frames, located inside large vats, and exposing them to tannin, a natural product found in the bark, wood, leaves and fruits of chestnut, oak and hemlock trees. Hides are transferred to many different bins during this step, each containing a stronger solution of tannin. Vegetable tanning prevents the skin from decay and shrinkage.



b)Mineral Tanning or chrome tanning is performed on skins which will be used for softer, stretched leathers, such as those found in purses, bags, briefcases, shoes, gloves, boots, jackets, pants, and sandals.



c) Chrome-tanned leather Invented in 1858 and perfected by the American inventor August Schultz in 1884.is tanned using chromium sulphate and other salts of chromium. It is more supple and pliable than vegetable-tanned leather, and does not discolour or lose shape as drastically in water as vegetable-tanned. Also known as wet-blue for its colour derived from the chromium. More esoteric colours are possible using chrome tanning.



d) Fat liquoring Leather, at the time of completion of the tannage does not contain sufficient lubricants to prevent it from drying into a hard mass. Almost all light leathers need a greater softness and flexibility than is imparted by tannage. This is attained in the fat liquoring process by introducing oil into the leather, so that the individual fibres are uniformly coated. The percentage of oil on the weight of leather is quite small, from 3-10 %. The precise manner in which this small quantity of oil is distributed throughout the leather materially affects the subsequent finishing operations and the character of the leather.



Proper lubrication or fat liquoring greatly affects the physical properties of break, stretch, stitch tear, tensile strength, and comfort of leather. Over lubrication will result in excessive softness and raggy leather in the bellies and flanks. Under lubrication, or improper penetration, results in hard bony leather that may crack in use.



To allow a small amount of oil to be spread uniformly over a very large surface of the leather fibres it is necessary to dilute the oil. Although this could be done with a true solvent such as benzene, it is cheaper, safer and more convenient to use the method of emulsification. In an emulsion with water, the oil is dispersed in microscopically small droplets, giving it a white, milky appearance.

It is important that the oil drops in water should remain as an emulsion until they penetrate the leather, and should not separate out as large drops or as a layer of oil, which could not penetrate the leather fibre and would only give a greasy surface layer.



All cowhides are naturally oily, unfortunately, these natural oils are stripped away in the tanning process (tanning is a process using a water vat and chromium salts to preserve hides and prepare them to absorb dyes) and some equivalent oils must be re-introduced after tanning. This step, the replacement of oils, is called fat liquoring.



Over the centuries, a number of oils have been found that have a natural affinity for leather fibres. Every leather tanner has his own, unique, blend of tanning oils. These formulas are closely held secrets, passed down through the generations.



In almost all situations fat liquors are designed to be applied to leather in the form of an aqueous emulsion that is ‘oil in water emulsion’. A fat liquor emulsion can be of various particle sizes. If the emulsion is coarse it will have a particle size of around 0.1 (micron) and will have a milky appearance. In this situation, the penetration of these fat liquors is restricted to the surface of the leather. If the particle size is around 0.03 then the emulsion is translucent, and because of the smaller size these can generally penetrate more deeply into the fibre structure. Emulsions of 0.005 are classified as micro-emulsions; they are relatively clear and give good penetration



Note: 0.3 is the largest particle size that can fit through a HEPA filter



In the fat liquoring process, both the penetration and the fixation of fatty matter take place. In order to optimise penetration into the leather structure, fat liquor must be emulsified in water and added to the processing vessel. The mechanical effects caused by the drum rotation, the surface tension and the capillary action of the hides, all promote penetration of the fat liquor into the structure. Fixation of this fat liquor is achieved by adding acid products at the end of the process.



Fat liquoring is usually carried out in a drum at the highest temperature practical for the type of leather, or about 113.oF (45 C) for vegetable tanned leather and 140-150.OF (60 to 65 C) for full chrome tanned leather. Fr the best results ideal rotational speed of the fat liquoring drum should be around 12-16 RPM for 30 to 40 minutes. After drumming, the leathers are usually struck out on the flesh side, carefully set out to smooth the grain, nailed or toggled out flat to dry, or paste dried. Strict control over the initial pH of the fat liquoring bath & the final pH of the exhaust bath should be maintained so as to achieve uniform fat content and fat distribution in leather and to avoid many fat splitting problems. Source leatherbiz.com Technical Articles Library



Modern fat liquors are technically advanced using high quality specially processed natural and synthetic oils that meet very high specifications. These are typically fully reacted to the fibre structure, and as a consequence only minimal amounts can migrate out of the leather. Because of this low migration continually adding creams and lotions to replace lost oils is therefore no longer necessary.



[Fat liquor is not volatile nor migratory, so leather is not going to lose it. However, along with the fat liquor, the other critical factor is moisture. Any leather is going to lose its moisture in a hot car. Although leather seems dry, it is not. Of course too much moisture quickly leads to mould and mildew problems, so soaking it down is not reasonable. Ideally, exposure to humid atmosphere will help balance the effect of over drying on hot days. Leather is very dynamic with respect to moisture content, meaning moisture comes and goes easily under normal circumstances....] American Leather Chemists Association



3. Finishing:



Dyeing process Depending on the desired product, the hides then go through a dyeing process, which also involves adding moisture back into the skin. Hides which have been vegetable tanned are bleached and them soaked with oils, soaps, greases and waxes to make them more pliable.



Rolling leather running the skins through a machine, which works to firm the leather and make it stronger. After the rolling process has finished, leathers are stretched, where they dry out in a heat controlled room.



Surface coating Once trimmed, dependent on customer requirements, the initial application of surface water-based pigment is applied by machine roller coating. The final protective coats are applied by machine spraying.



Finishing (application)



A water-based pigmented finish is applied to give a uniform covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution of acrylic polymer, which contains binders to adhere to the leather. The pigment can be applied to leather in three ways;



•Sprayed on by hand using an air gun.



•Sprayed on by a machine on a conveyor belt.



•Often a combination of a base coat and a top coat are used.



After applying the pigment polyurethane is then applied to seal the colour, to provide protection and durability, the polyurethane is also used to adjust the gloss level of the leather.





Bibliography



1. Leather Technician`s Handbook by J.H. Sharphouse

2. Practical Leather Technology by T.C. Thorstensen.

3. Books on Leather - Leather

4. How to keep Luxurious Connolly Leather Young and Supple. Krysti Pavlisko 1998

5. How to Restore Auto Leather. D Briggs 2001



Information resource



1. Wikipedia Encyclopaedia

2. American Leather Chemists Association, various technical papers / articles

3. Leather Care Facts. Leatherique 2003 -

4. The Leather Institute (Townsend Leather Group)

5. Article in the Leather Training & Technical Dept Ltd website -

6. IICRC Reference Guide for Professional Upholstery Cleaning (IICRC S300)

7. Upholstery cleaning is based upon methods recommended by the Carpet and Upholstery Cleaning Institute (CUCI)

8. The Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)

9. Leather Research Laboratory , University of Cincinnati, Cincinnati OH

10. Society of Leather Technologists and Chemists (SLTC) various technical papers / articles

11. Leather Master™

12. Leather Magic™

13. Global Care Solutions, LLC , 354 Hiatt Dr, Palm Beach Gardens, FL, 33418

14. IICRC Course Descriptions: Leather Cleaning Technician