The primary raw material for leather, animal hide, is a byproduct of the meat and dairy industry. Yup the Good Ol’ Slaughterhouse. From the slaughterhouse, the flayed hides are taken to tanneries where they are turned, from a potential putrid and decaying heap, into one of the most durable and beautiful materials. A vast majority of all leather come from beef cattle and dairy cows. Next in line are pig, sheep, water buffalo, and goat. None of these animals are raised for the value of the hide, they are raised for meat and dairy. Leather just happens to be a very useful byproduct. There are of course some leathers which are sourced from animals raised or hunted primarily for their leather, however I'd like to keep this post civil and rather not get into emotional, political or otherwise fomenting views
How is leather made?
First things first, raw hides need to be tanned to prevent them from decaying and turn them into leather. Nobody, wait let me rephrase, almost nobody wants to wear or store in their pockets decomposing animal skin (There are always some weirdos out there).
The raw hides, which we will simply call 'hides' from here on out, go through a series of processes loosely grouped into three stages. Preparation for Tanning, Tanning and Post Tannage Treatment
Preparation for Tanning
Preparation for tanning consists of a desalting, soaking, liming, fleshing, bating, and depending on the type of tannage, pickling.
Hides need to be preserved after flaying, to prevent them from decaying, while they wait to be transported to a tannery. This is typically achieved by either dry salting (a bunch of salt is applied to the hides) or wet salting where the hides are saturated with salt water. In some cases the hides are refrigerated, however this needs significant infrastructure and account for a small minority of hides.
Desalting & Soaking
The name of the process says it all, in this process the salt is removed from the hides.
If the hides have been dry salted, they are put into a desalting drum, which is sort of like a cage, and tumbled. The tumbling removes all the surface salt. The hides are then “soaked” in fresh water to remove the salt from the hides. Refrigerated hides obviously do not need desalting, instead they move directly to the next part of the stage.
The hides are then soaked in water again. Soaking achieves a couple of things, one is that it help clean the hides of contaminants like dirt, manure, etc. and two it “plumps” the fibers in the leather and allow the fibers to “slip” over each other to allow the next process (liming) to work well. Some type of biocides are typically added to the fresh water to prevent putrefaction and subsequently low quality leather. Some fungicides are also added in the later stages to prevent wet hides from growing mold.
In this process the excess fatty tissue on the underside of the hide is removed either with a curved knife on a rounded wooden beam or in a more modern tannery on a fleshing machine. A fleshing machine has a large cylinder with spiral blades which remove the excess fatty tissue and adhering flesh
Liming is done to achieve a number of objectives and is crucial in obtaining a high quality leather. Liming allows removal of hair, swelling the hide, removing natural fat and grease in the skin and removing some inter fibrillar materials. Hides first spend a few days in “Old Lime”, i.e., lime that has been used twice. This type of lime is teeming with bacteria needed for removing hair from the hide. The action of these bacteria is what primarily allow the removal of hair as well as removing some inter-fibrillary matter. The hides then spend a few days in a “Once used lime” and then a few days in New lime. This process takes longer but makes better use of water and also results in much better leather. A longer time spent in the “Old Lime” makes for a softer leather.
A much quicker approach is using Sodium Sulfide in combination with a New lime for dehairing and plumping. This is the more typical and cheaper approach for Chrome Tanned leather which needs to be firm, for example chrome tanned shoe leather and sole leather
Deliming and Bating
During liming some of the lime (Calcium Oxide) reacts with water and fats in the leather to form Lime Soap. The major reasons for deliming are removing free lime and lime soap.
The hides are first run through fresh water for a while. Then the leather is washed in a weak organic acid solution or in case of chemical deliming in a solution of ammonium salts.
The bating or puering process was historically quite nasty. Fermented bran and hen, pigeon or dog dung was used to make hides and the resulting leather softer. You can well imagine what that smelled like. The dung or fermented bran contained specific enzymes which affect the structure of hides making the hides more pliable. The most pliable and stretchy leather was attained using dog dung.
These days the bating process is achieved with synthetic enzymes made in factories.
This step is only performed for chrome tanned leather. In this process the hides are treated in a bath of sulphuric acid and salt. The reason for this step? In order for Chrome Tanning to work the hides have to be acidic. At the end of the previous step the hides are alkaline/basic.
Vegetable tanning is quite possibly the most nuanced type of tanning currently in practice. In addition not all aspects of the tanning process are understood in their entirety by the scientific community. This means, in order for vegetable tanned leather to be of high quality you need a team of highly experienced tanners. Vegetable tanning produces some of the most beautiful leather and has a unique quality, the development of a patina. If you start with an undyed vegetable tanned leather, depending on the source of tannins, the color of the undyed hide changes to either a pinkish or a light reddish brown in the initial stages. If the leather sees heavy usage and is nourished appropriately the color will eventual turn into a beautiful rich brown.
Types of Vegetable Tannins
Leather is tanned using bark, bark extract and in certain cases fruits. The most widely used sources for vegetable tanning today are Black Wattle, Myrobylan, Quebracho, Chestnut and Pods of the Tara tree. In India, historically, leather was tanned using extracts from Babul/Babool (Aciacia nilotica) as well as its fruit, Khair (Acacia Catechu) and Dhava (a mangrove species).
There are two types of tannins present in natural sources. Pyrogallol (hydrolysable) and Catechol (condensed) tannins. Most natural sources contain both types of tannins but in different proportions. Condensed tannins produce a very firm leather by not only tanning the leather but also because they deposit themselves in the interfibrillar spaces in the leather. Pyrogallol tannins produce a much more flexible leather. For example, leather intended for making soles, saddlery and belts is typically tanned with a source high in Condensed tannins. Condensed tannins produce a darker leather with reddish notes. This is sometimes balanced of by doing a final finishing tannage with Myrobalan to make the final color lighter.
This is one of many reasons, for differences in vegetable tanned leather structure and feel from different countries. Tanners is different countries favor different tannin sources due to availability and market forces. A different source equals different proportions of Pyrogallol to Catechol tannins.
The Vegetable Tanning Process
Leather may either be tanned in cross current liquor pits, layer pits or in tanning drums.
The layer pit method is only used for Sole leather and really not much sole leather is made this way these-days. Oak bark leather produced by this method takes about 1 whole year to tan, maybe even more. This type of a process involves taking delimed hides and putting them in a pit one layer of hide followed by one layer of powdered or shredded bark. Alternating the layers till the pit is full and then filled with water. The Layer pit method used in conjunction with a tannin high in Condensed tannins produces a leather with high density, some degree of water resistance, very firm and high in flexural strength.
The cross current liquor pit method is the method used by most manufacturers of high quality vegetable tanned leather. In this method hides are hung on rods and immersed in a series of pits containing different levels of tannins. Tanning is begun by immersing in the mellowest of tanning liquor. Over a period of 2-4 weeks (depending on the thickness of the leather and the characteristics required) the hides are moved from one pit to the next, increasing the strength of the tanning liquor at each stage. After each stage is over the liquor is moved into the previous pit (lower tannin levels). The leather moves from low to high concentration of liquor and the used liquor moves in the opposite direction. This allows a very optimal use of water as well as the vegetable tannins.
Chrome tanning is by far the most efficient and effective method of tanning leather. Instead of the 2-4 weeks it takes to tan a vegetable tanned leather, chrome tannage take only a day. Tanning is done in a revolving drum where the chrome tan liquor is added to hides. The drum is rotated which tumbles the hides in the drum. This mechanical action couple with the fast acting nature of the Chrome Liquor makes short work of tanning.
Chrome tanned leather is far more resistant to the action of water and heat. Vegetable tanned leather become quite stiff if it gets wet and is then dried. Sometimes, if the vegetable tanned leather is too dried quickly it will lose it shape. Heat also affects vegetable tanned leather significantly more than chrome tanned leather. If exposed to high heat vegetable tanned leather dries and crosslinks, much like a thermosetting plastic, and becomes very stiff. In fact, this very property of vegetable tanned leather was used in the past for making armor for soldiers
If necessary the chrome tanned leather is then tanned again with vegetable tannins for a few hours or a day. The resulting leather has some or a lot of characteristics of vegetable tanned leather without the negative effects.
This type of leather yields a very soft and stretchy leather which is white in color. This type of leather was historically used for fine gloves. The major ingredient in this tannage is potash alum. If you have ever been to barber for a shave, the guy probably rubbed a translucent piece of this stuff on your face to stop the micro bleeds. The goat or pig skin is soaked in a warm solution of potash alum, egg yolk and flour and then air dried for several weeks. This is technically not tanned since the leather on prolonged exposure to water will decay.
This is exactly what you think also exactly not what you see when you see some seller claiming their leather is oil tanned. The only oil tanned leather is Chamois leather.
If you ever see a bag, wallet or jacket claiming it is oil tanned, know that it is not oil tanned. It just has additional oil added to the leather after tanning to produce a "pull-up" effect. When such a leather is bent, the oils move away from the bend making it lighter. When the bend is removed, the oils move back in making it dark again.
True Oil-Tanned leather or Chamois leather is typically produced by tanning the leather with cod oil. Skins of goat are first split. The grain split is used to make other types of leather. The flesh side split is then soaked with cod oil, the oil is forced into the leather with a machine called “Faller Stock” exposed to atmospheric oxygen. These steps are repeated many times until the oil fully penetrates the leather. During this process the oil oxidizes and forms permanent chemical bonds with the skin turning it into leather.
Post Tannage Treatment
In this stage “crust leather” , i.e. tanned but not further processed leather, is dyed and curried. Currying is the process of introducing oil, fats and waxes into the leather.
This is done in a few different ways:
1. The most common method is to make and emulsion of oil in water and run the leather in a drum with the emulsion. The oil is taken up by the leather and the water remains behind.
2. Melted waxes and tallow are mixed with oil. This mixture is then added to a rotating drum which has been preheated with the leather. Running the drum this higher temperature allows the hard waxes and tallows to penetrate the leather fully
3. When a high quantity of hard waxes and tallow are to be added to the leather, very dry leather is immersed in a vat of molten waxes and tallows, quickly removed and then put into tepid water. Any extra waxes are removed after removal from the water
4. The least common method is “Hand Stuffing”. In this method, a stiff mixture of tallow and oil are applied on the grain side of damp leather manually with brush on a hard flat stone table. The leather is allowed to absorb the oil and the tallow is removed with a metal squeegee. This process is repeated until the leather has absorbed enough oil.
As you can see, making high quality leather is a very involved process, no matter which method of tanning you use. There is a lot of noise on the internet about different types of leather, such as full grain, top grain, etc. While these differences do have an impact on the qualities exhibited by leather, the comparison is only useful if they have been tanned and finished by equally skilled Tanners & Curriers. There is a lot of difference between leather that is made well and leather that is not skillfully tanned. The importance of Master Tanners and Curriers cannot be overstated. While scientific understanding of the process has allowed significant improvement in making the process more efficient and repeatable, Master Tanners and Curriers still have their “secret sauces” and a body of traditional knowledge which make leather from some tanneries vastly better and different than others.