Leather: How’s it made.

Leather is a byproduct of the meat and dairy industry. Yup, the Good Ol’ Slaughterhouse. From the slaughterhouse, flayed hides are taken to tanneries. Here, they are turned, from a potential putrid and decaying heap, into one of the most durable and luxurious materials. Leather comes from beef and dairy cows. Next in line are pigs, sheep, water buffalo, and goats. None of these animals are raised for hide value; they are raised for meat and dairy. The leather industry is a very useful byproduct. There are, of course, some leathers sourced from animals bred 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?

Tanning raw hides is the first step to turning them into leather and preventing them from decaying. (There are always some weirdos out there who want to wear or store in their pockets decomposing animal skin).

From now on out, we will simply refer to raw hides as 'hides' because they go through a series of processes that can be loosely divided into three stages as follows. It is important to prepare for tanning, to tan, and to treat yourself after the tanning procedure

Tanning Preparation

Tanning requires desalting, soaking, liming, fleshing, bating, and depending on the type of tanning, pickling.

After flaying, hides must be preserved to prevent decay while they are transported to a tannery. Usually, the hides are dry salted or wet salted, i.e., soaked in salt water and then sprayed with salt. The hides are sometimes refrigerated, but this requires significant infrastructure and is reserved for a very small percentage of the hides.

The process of desalting and soaking

This process involves removing the salt from the hides, as the name implies.

If the hides have been dry salted, they are tumbling in a desalting drum, which is similar to a cage. The tumbling removes all the salt from the surface. Salt is then removed from the hides by placing them in fresh water. Desalting is not necessary for refrigerator-stored hides, so they move straight to the next stage.

After the hides have been soaked in water again, they are dried. The benefits of soaking are two-fold: one, it cleans the hides of contaminants like dirt, manure, etc., and two, it "plumps" the fibers in the leather, allowing them to slip over one another for the liming process to work effectively. Fresh water is typically treated with biocides to prevent putrefaction and lower quality leather. The later stages of the process also include the use of fungicides to avoid mold growing on wet hides.

Fleshing

A curved knife on a rounded wooden beam or, in a more modern tannery, a fleshing machine is used to remove excess fat from the underside of the hide. Fleshing machines use large cylinders with spiral blades to remove excess fat and adhering flesh

Liming

Liming is crucial for high-quality leather because it achieves a number of objectives. During limeing, hairs are removed, the hides are swollen, natural fat and grease are removed, and some interfibrillary materials are removed. To prepare the hides, they are first immersed in "Old Lime", a lime that has been used twice before. Hair can be removed from hides with lime teeming with bacteria. Hair and inter-fibrillary matter are primarily removed by these bacteria. The hides are then placed in an "Once used lime" and then in a new lime for a few days. Although it takes longer, this process is more efficient and also results in better leather. Softer leather is produced by spending a longer time in "old lime".

Dehairing and plumping can be accomplished much more quickly with sodium sulphide and a New Lime solution. The more common and cheaper approach for Chrome Tanned leather that needs to be firm, such as sole leather and shoe leather

Deliming and Batting

During liming some of the lime (calcium oxide) reacts with water and fats in the leather to form lime soap. Deliming is used to remove free lime and lime soap.

The hides must first be soaked in fresh water before drying. A weak organic acid solution is then applied to the leather or, in case of chemical deliming, ammonium salts are applied. 

It was historically quite unpleasant to bate or puer. A combination of fermented bran and hen, pigeon or dog dung was used to make leather softer and to make hides softer. The smell of that is well known. In fermented bran or dung, specific enzymes change the structure of hides, allowing them to be more pliable. By using dog dung, the most stretchy and pliable leather was achieved. 

These days the baiting process is achieved with synthetic enzymes made in factories.

Pickling

Chrome tanned leather is the only leather that undergoes this step. The hides are treated with sulphuric acid and salt in this process. What is the purpose of this step? To make chrome tanning work, the hides must be acidic. After the previous step, the hides are alkaline/basic. 

Tanning 

Vegetable Tanning

There is no doubt that vegetable tanning is the most complex form of tanning currently in use. In addition, the scientific community do not understand all aspects of tanning in their entirety. Therefore, to produce high-quality vegetable-tanned leather, you need a team of highly skilled tanners. An interesting characteristic of vegetable tanning is its ability to develop a patina, which makes it one of the most beautiful leathers. In the initial stages of vegetable tanning, undyed vegetable leather can either be pinkish or reddish brown, depending on the source of tannins. Eventually, if the leather sees heavy use and is properly nourished, its color will turn rich brown.

Types of Vegetable Tannins

Tanned leather is made from bark, bark extract, and sometimes fruits as well. A variety of plant sources are used in vegetable tanning today, including Black Wattle, Myrobylan, Quebracho, Chestnut, and Tara tree pods. In India, historically, leather has been tanned with extracts from Babul/Babool (Aciacia nilotica), Khair (Acacia catechu) and Dhava (a mangrove species).

Natural sources contain two types of tannins. Tannins like pyrogallol (hydrolysable) and Catechol (concentrated). It is common for natural sources to contain both types of tannins, though in varying proportions. Condensed tannins produce a very firm leather by not only tanning it but also depositing themselves throughout the interfibrillar spaces. Tannins derived from pyrogallol produce leather with greater flexibility.  As an example, leather used for soles, saddlery, and belts is typically tanned from a source rich in condensed tannins. With condensed tannins, leather becomes darker and more reddish. Often, this is balanced by tanning with myrobalan on the final step.

Vegetable-tanned leather from different countries differs for many reasons. Due to market forces and availability, tanners in different countries favour different tannin sources. There will be a difference in the proportion of Pyrogallol and Catechol tannins depending on what source is used.

The vegetable tanning process

There are several ways to tan leather, including cross current liquor pits, layer pits, and tanning drums.

Usually sole leather is not made this way anymore since the layer pit method is only used for sole leather. A whole year may be required to tan oak bark leather using this method. In this type of process, delimed hides are layered with powdered or shredded bark in a pit, followed by another layer of hide. By alternating the layers, the pit will be filled with water and then the layers will be replaced. Leather produced by using the Layer pit method and a tannin that is rich in condensed tannins has a high density, a degree of water resistance, and is very firm and strong.

High quality vegetable tanned leather is most often produced using the crosscurrent liquor pit method. Using this method, hides are hung on rods and immersed in pits with varying tannin levels. Tanners begin by immersing themselves in the mellowest tanning liquor. At each stage of the tanning process, the tanning liquor is strengthened by adding more hides (depending on the thickness and the characteristics of the leather). Liquor is moved to the previous pit after each stage (lower tannin levels) has been completed. The leather moves from a low concentration of liquor to a high concentration, and the used liquor moves the other way. Using this method allows for very efficient water and tannin use.

Chrome Tanning

There is no doubt that chrome tanning is the most efficient and effective method of tanning leather. A chrome tanned leather can be tanned in one day rather than 2-4 weeks for a vegetable tanned leather. Tanning occurs in a revolving drum where chrome tan liquor is applied to hides. The drum rotates, allowing the hides to tumble. A combination of this mechanical action and the fast-acting properties of Chrome liquor results in a quick tanning process.

Leather tanned in chrome is far more resistant to the action of water and heat than leather tanned in other methods. Vegetable-tanned leather becomes quite stiff after getting wet and drying. Vegetable-tanned leather can lose its shape if it is dried too quickly. Vegetable-tanned leather is also significantly more susceptible to heat than chrome-toned leather. When exposed to high heat, vegetable-tanned leather dries and crosslinks, similar to thermosetting plastic, and becomes extremely stiff. In fact, soldiers' Armor was made with this very property of vegetable tanned leather in the past.

Combination/Vegan & Retanned

After chrome-tanning, the leather can be re-tanned with vegetable tannins for up to a day if necessary. It may or may not have many of the disadvantages associated with vegetable tanned leather. 

Alum Tanning

This type of leather produces a very soft and stretchy leather that is white in color. The leather used for these gloves has been used historically for fine gloves. In this tanning process, potash alum plays a major role. A transparent piece of this stuff was probably rubbed on your face by the barber when you had your hair cut. After soaking in a warm solution containing potash alum, egg yolk, and flour, the goat or pig skin is dried by air. It is technically not tanned since leather will decay when exposed to water for an extended period of time.

Oil Tanning

It's exactly what you think and not what you see when someone claims their leather is oil-tanned. There is only one type of oil-tanned leather: chamois. 

Be aware of the fact that oil-tanned bags, wallets and jackets are fake. A "pull-up" effect is just the result of adding additional oil to leather after tanning. A leather that has been bent will have its oils move away from the bend, making it lighter. Upon removing the bend, the oils move back in and make it dark again.

Typically, oil-tanned leather or chamois leather is produced by tanning the leather with cod oil. Goat skins are first split. In addition to grain split leather, other types of leather are also made from grain split leather. Cod oil is then soaked on the flesh side of the split and forced into the leather by a machine called "Faller Stock" exposed to atmospheric oxygen. Repeat these steps until the oil is fully absorbed by the leather. A permanent chemical bond is formed between the oil and the skin during this process, which turns the oil into leather.

Post Tannage Treatment

This stage involves dyeing and curring "crust leather", that is, the tanned but not further processed leather. A process of currying is the introduction of oil, fat, and waxes into 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. 

Wrap Up

There is no doubt that making high quality leather involves a lot of work, no matter which tanning method you use. It is very common to hear about different types of leather on the internet, such as full grain, top grain, etc. In spite of these differences, leather can be compared only if it has been tanned and finished by tanners and curriers equally skilled. 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 improvements in making the process more efficient and repeatable, master tanners and curriers still have their “secret sauces” and a body of traditional knowledge which makeleather from some tanneries vastly better and different than others.