All About Leather
Leather is created through the tanning of hides & skins, converting the putrescible skin into a durable & long lasting material. Leather is not leather until the skin or hide has been tanned.
The Leather Making Process
British Standard BS 2780 Definition of Leather
“A general term for hide or skin with its original fibrous structure more or less intact, tanned to be imputrescible. The hair or wool may, or may not, have been removed. Leather is also made from a hide or skin which has been split into layers or segmented, either before or after tanning, but if the tanned hide or skin is disintegrated mechanically without combination of a binding agent, is made into sheets or other forms, such sheets or forms are not leather. If the leather has a surface coating, this surface layer, however applied, must not be thicker than 0.15mm”
The actual process of making a hide or skin into leather is not just one simple task but a number of processes. These different processes can be broken down into four sections.
* Beamhouse Operations
* Post Tanning
The beamhouse area of a tannery is for the removal of unwanted materials from the hide, such as hair, non-structural proteins and fleshy tissue.
Soaking: This process is done to remove dirt and blood from the surface of leather. The hides are soaked in water kept at 20 degrees Celsius to avoid shrinkage and detergents can be added with an alkaline chemical that aids the uptake of water. This process also moisturises the hide.
Unhairing & Liming: Again the hides are soaked in alkaline conditions, this is to loosen the hair ( unhair ) and to swell the collagen fibre and break down non structural proteins that would harden the final leather if not removed.
Fleshing: The hides are washed for fleshing. They are passed through a machine that removes unwanted flesh, connective tissue and fat.
Splitting: The hides can now be split into two. The top layer is used in most upholstery as it contains the grain pattern or can be corrected and have an embossed pattern applied to it. The bottom layer (split) is then used for suede or can have a pigment applied to it and becomes a finished split (commonly used on the backs & sides of upholstery).
Deliming: The hide is prepared for tanning by reducing the swollen and rigid fibres. This is done by removing the alkali with acidic chemicals such as ammonium chloride or ammonium sulphate.
Bating: This process gives the hide a smoother, flatter grain appearance and it is also the stage that makes the final leather softer and stretchy.
Pickling: This process is used to adjust the hides ready for tanning. This is done by introducing an acid into the hide to lower its pH. The mixture may also contain salt to further reduce the swelling.
This is the process most people are familiar with, and is the most important part of making leather. Remember the definition of leather? If a hide or skin is not tanned it cannot be classified as leather.
The objective of tanning is to convert the collagen protein of the hide into a stable material, which will not putrefy and will be stable under conditions of heat and moisture.
There are a good few different methods of tanning hides but for upholstery there is only one main one you need to know about – Wet Blue Also known as chrome tanning. This is the name given to leather that has been tanned using chromium salts. Why? Because it is both wet and blue after being tanned!
The hides are processed in large drums (usually these drums can take up to 300 hides at a time). The blue colour in the chromium binds to the collagen in the hide making it blue in colour. The hide is fully tanned when it is resistant to heat and will not shrink at 100 degrees Celsius. The benefits of this kind of tannage are good strength, light fastness and heat resistance. It is these properties that make wet blue the most popular method of upholstery tanning.
At this stage the hides must then dried to remove some moisture. This is done by passing the hides through a machine with felt pads that squeezes the moisture out.
Neutralise: Mild alkalis are added to the leather, which prepare it for later chemical processes..
Dyeing: Simple – the leather is dyed in drums to give it colour! Anionic dyes are very common as they are negatively charged and so latch onto the chrome leathers well.
Fatliquoring: This process uses fats and oils to lubricate and soften the fibre structure of leather. The manner in which the oils are introduced into the hide coat every fibre. This is where leather gets its smell!
Drying: The leather is then dried. This can be done by hanging it on pegs or laying it on a board and applying heat. This method is poplar as it can dry a hide in just a few minutes.
Staking: This process softens the leather. The leather is passed over a series of blunt pins that pummel and flex the leather to soften it up.
Finishing leather is the process of applying coatings to the surface of leather, the benefits of finished leather are;
* Protective surface aids durability
* Easier to care and maintain
* Improves water resistance
* Masks defects
* Enhances colour and appearance
There are three processes involved in finishing leather: Buffing, applying a finish and embossing a pattern. Not all these processes occur, which varies the different types of leather.
If the grain surface of the leather is in a good condition, it may be left unfinished and is known as full grain leather. However there are so few hides good enough quality to be classed as full grain that the price is very expensive for such upholstery.
The hide (on a cow) can get scratched, bitten and cut. All these imperfections show on the grain and need to be repaired. The damages are filled using the 'leather repair compound' (a flexible filler), the leather is then left to dry and then the grain side of the hide is passed through a machine with abrasive material to buff away the imperfections. The type of leather is known as correct grain.
Buffing the hide also creates an excellent key for pigments to adhere to and so serves more than one purpose.
On corrected grain leather a pigmented finish is applied to give a sound covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution, either acrylic or solvent based, which contain binders to adhere to the leather.
After applying the pigment a lacquer is then applied to seal the colour in, giving the leather more protection and durability – not only that but the lacquers are used to adjust the gloss level of the leather.
The pigment can be applied to leather in three ways.
1. Sprayed on by hand using an airgun.
2. Sprayed on by a machine on a conveyor belt.
3. Pressed on with rollers. Often a combination of part 2 (base coat) and part 3 are used.
The lacquers are sprayed on by machine or by hand with an air gun.
After the application of finishes on leather an artificial grain pattern can be applied to the leather. This is done by placing a metal plate containing a grain pattern on top of the leather and pressing it down onto the surface under pressure and heat. Or, by running the leather through rollers with an etched ghrain pattern on them.
The most expensive type of leather due to the exclusive selection of hides. The leather is dyed in a dyebath and no pigmented coating is applied, allowing the leather to breathe naturally showing all its own unique markings and shade variations. Because of this, no two aniline leathers are the same and so when you purchase one you are buying an exclusive item with its own story of nature to tell.
Pros: The most natural type of leather available, it is very soft and delicate to the touch and bares all of natures own markings on the hide. It stays warm in the winter and cool and ventilated in the summer.
Cons: Because the leather contains no protective coating it is very absorbant and so is prone to staining and fading.
Semi Aniline Leather
This type of leather is similar to aniline but with the added advantage of a thin protective coating that helps resist stains. Some natural markings may still be visible but not to the extent of an aniline leather.
Pros: Easy to clean & maintain. It has a nice soft feel and still has some natural markings visible, often with a two tone appearance.
Cons: There isn't much room for complaint with semi aniline leather except that it is slightly less natural than aniline leather.
This type of leather is coated with a fine pigmented spray, which gives the leather a sound covering of colour with no shade variations. Sometimes known as corrected grain leather, this is when an artificial grain pattern is embossed into the hide.
Pros: The process in which the leather is made makes it very durable and hard wearing. It has high resistance to light and is very easy to clean and maintain. The colour is uniform and all defects are masked.
Cons: Reduced breath ability and the grain pattern is masked making this type of leather less natural.
This type of leather has a polyurethane film bonded to the surface creating a very hard wearing and easy clean leather.
Pros: Extremely easy to clean and maintain, uniform colour and good light fastness.
Cons: It has no natural grain surface and so is not very natural.
A very natural type of leather that is designed to distress as it ages because the colour lightens when stretched or scratched.
Pros: Has a very natural and lived in look. All natural markings are visible and has a soft & delicate touch.
Cons: It can be very difficult to clean and it stains quite easily.
This is a pigmented leather with an additional top coat of a darker colour sprayed on top. It is designed to give an antiqued look and is very common with chesterfield style sofas.
Pros: Unique antique style differs from sofa to sofa as the leather is enhanced through use. Protective coating helps resist stains.
Cons: The top coat of colour may wear off after continued use causing the leatehr to look severely distressed.
This is leather buffed grain side to create a velvety surface.
Pros: Nubuck is very natural with a lovely soft and delicate feel.
Cons: It is very difficult to clean and maintain as it has no protective coating. Light fastness is poor and the leather is prone to staining.
Leather is a natural product and as such, bares all hallmarks of its origin. Unique and indvidual markings may appear on a hide and should not be considered as defects. Instead, they give leather a unique identity, which enhances its natural beauty and characteristics.
Corrected Grain Leather
Correct grain is where the leathers surface has been corrected to remove the above natural markings. This is done when there is an abundance of marks that cause the hide to look damaged or unslightly, and so the marks are removed.
The process in which this is done can slightly differ depending upon the amount and depth of the marks. When the scuffs and scratches aren't to severe the hide can be buffed (with a type of sander) and then have a pigment applied to the surface. The light buffing reduces the marks and then the pigmented coating covers them. When the marks have some depth to them they need filled before they can be concealed. This is done by spreading a flexible paste (repair compound) over the surface of the leather and then sanding it down. A pigmneted coating is then applied to the surface as before.
Corrected grain leather falls under the category of 'pigmented leather' on our leather types web page.
NOTE: Because the surface is buffed the leather loses its natural grain pattern and so, an artficial grain pattern is then embossed into the leather.
Full Grain Leather
Full grain leather is where the natural markings are minimal and so the leather an be dyed or lightly pigmented without the need for buffing. The natural markings on a hide should in no way affect the strength and durability of the leather as they are mainly superficial. The exclusive selection of hides for full grain leather means that they have a premium price over other leather types.
Full Grain Leather Types: Aniline, Semi-aniline, pull up, oily/waxy pull up
Selection of Hides & Price
The majority of leather used for upholstery in the car & home is pigmented leather. This is because, in the field, a cow can get caught on barbed wire, rub against thorn bushes, and get cut by other animals horns. In addition to this, cows are often bitten by insects, parasites, ticks, and lice, all of which result in markings on the hide.
The majority of hides available for tanning are in such a condition that the surface needs to be 'corrected' before it is suitable or visually accebtible for upholstery. Only a small percentage of hides have few marks and are in good enough condition to be left untouched. So full grain leather, is therefore more expensive. Simply, its a numbers game, there are fewer full grain leathers available, and so, supply & demand dictates that they be charged at a premium price
What type of leather do I have?
Inorder to properly determine a type of leather there are a few tests that we can do.
1. Magnifying Glass Test: Looking at leather under a magnifying glass will show us in detail the finish applied to the leather, or if there is a finish on the leather.
2. Water Absorbtion Test: This test helps us identify how easy your leather will be to clean, and how effective the finish on it is.
3. Visual Test: Looking at leather tells a lot, are there any natural markings visible? what colour is it? etc etc
4. Touch Test: Feeling the leather for softness, smoothness, grain pattern also helps identify the leathers type.
To make things a little easier for you, we can break down the different types of leather into categories, each containing a group of leathers with similar characteristics and so similar care procedures.
Unfinished Leather (full grain leather)
In this group are all leathers without a lacquered finish applied, maybe with a few exceptions but the finish on these is so light the same identification procedures apply. Also not included in this group are suede and nubuck, this is because their surfaces have been buffed to create a nap and so make them unsuitable for cleaning.
What’s different about these leathers? Because they have either a very thin or no applied finish it makes them very absorbent. Being absorbent means they stain and mark easily. Having no finish also means they can fade very easily, the majority of aniline is just dyed with no lacquered finish and so the leather doesn’t have the greatest colour fastness. Pull up leathers are a bit better than aniline but still need careful maintenance.
Finished Leather (corrected grain leather)
All the above have a lacquered finish and so are very easy to clean and maintain, bi-cast has a plastic coating bonded to the finish and so falls in this category. Rub off leather has been starred, WHY? Because the top coat lacquer is very weak, this means when you clean it the black top coat may easily wear away exposing the brighter colour beneath. Be careful when dealing with this leather type.
Nubuck & suede
These leathers have a nap texture and no top coat finish applied. This makes them stain very easily and because these stains are absorbed, very difficult to clean.