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As mentioned earlier, there are many varieties of leather. However, all types of leathers have to go through four fundamental stages. This includes preparatory steps, tanning, re-tanning, and finishing. Sometimes, a further sub-process of the surface coating may be added to the process. Before sending it to the tannery, the animal skin needs to be cured. Although curing is not considered a part of the tanning process, it is the first crucial step in obtaining leather.

Usually, the animal is killed and skinned before the body heat leaves the tissues. The freshly removed skin or hide is immediately cured with salt to remove water. The skin is allowed to remain in the shade until it is completely dry. The cured skin or animal hide is then transported to the tanneries for further processing.

A. Preparatory Steps

The cured skin or animal hide needs to be prepared for the tanning process. Although there are various preparatory processes, the purpose of each process is to remove unwanted raw skin components. The tannery may not perform all of them, depending on the quality and type of the desired product.

1) Soaking 

At this stage, the cured hide is soaked in water for several hours to several days. This process not only restores the moisture lost during salting, but helps to remove dirt, debris, blood, and excess animal fats.

2) Fleshing 

This process removes subcutaneous material from the flesh side. The pelt is passed through a machine to remove the fat, muscle, and flesh mechanically. Usually, this process takes place after slaughter, soaking, or liming. At this stage, or after tanning, hides may be split into different layers.

3) Un-hairing 

Hair is removed at this stage using mechanical instruments such as rollers and blades. 

4) Pickling 

This process involves cleaning and soaking the rawhide in acids or salts to prevent decomposition. It helps the penetration of tanning agents such as chromium and aldehydes. Stronger pickling agents are used to preserve hides for several months.

5) De-Pickling 

The hide is soaked in sulfuric acid to lower the pH after pickling.

6) Liming 

This process loosens the fibers and allows the skin to absorb various tanning chemicals. Usually, sodium sulfide and hydrated lime are used to treat the hide, as they remove keratinous material such as hair and wool. Fats get hydrolyzed as the pH increases. Water is absorbed into the skin fibers, resulting in a swollen skin structure.

7) De-Liming 

In this process, the hide is washed with a mixture of water and ammonium chloride or ammonium sulfate. This removes water (along with any impurities) to reduce swelling.

8) Bating 

Bating marks the end of the liming process. The flaccid skin is treated with proteolytic bating enzymes to remove non-fibrous proteins. It cleans the grain and makes the pelt smooth and silky.

9) De-Greasing 

Sometimes, water-based solutions and solvents are used to remove excess grease or natural fatty acids from the skin.

10) Bleaching 

Chemical agents are used to making the hide colorless to add the desired color at a later stage.  

B. Tanning 

The primary purpose of this process is to produce a non-decomposable and sturdy material from the raw animal hide, called leather. Essentially, tanning converts the protein of the rawhide into a durable material. The most common tanning processes include mineral tanning, vegetable tanning, and glutaraldehyde tanning.

1) Vegetable Tanning

Vegetable tanning has been around for thousands of years. Unlike mineral tanning, it uses a naturally occurring polyphenol astringent chemical called tannin. This is usually found in bark, leaves, and branches of trees such as oak, chestnut, or mimosa. As it produces shades of deep brown, beige, yellow, and red, tannin lends a unique color and texture to the leather.

However, the process is time-consuming, laborious, and expensive. There are two types of vegetable tanning processes. The slow process takes about 30 days, while the rapid tanning process only lasts about 36 to 48 hours. Sometimes, however, the slow process can take several months, as it may require multiple treatments.

This process produces highly durable leather. So, the vegetable tanned leather is often used to make products such as saddles or holsters. The unmatched durability and distinct appearance makes this leather suitable for imprinting and intricate leatherwork such as tooling.

2) Mineral or Chrome Tanning 

Mineral or chrome tanning is the most popular tanning process because it’s much quicker, affordable, and less labor intensive than the others. In 1858, it was introduced as an alternative to the expensive and time-consuming vegetable tanning process. The process can be automated and lasts a day at most. Usually, the time for chromium tanning is around 2 or 3 hours for small and thin skins. However, it can go up to 24 hours for thicker ones obtained from cattle.

The size of chrome molecules is small compared to vegetable tannin ions. As a result, chrome ions can penetrate the collagen and remove water molecules effectively. That’s why chrome tanned leather is thinner and softer than vegetable tanned leather. Chromium (III) sulfate is the most efficient and effective tanning agent. Chrome tanned leather is also called wet blue leather due to its bluish color.

However, the chrome tanning process creates a negative environmental impact as it comprises heavy usage of acids and other chemicals. The toxic waste can seep into groundwater and contaminate drinking water supplies. The resulting environmental implications are a major concern, especially in developing countries.

3) Aldehyde Tanning 

This tanning process uses glutaraldehyde or oxazolidine compounds. It is also called wet white leather due to its pale cream color. Aldehyde tanned leather is water absorbent, soft, and can be machine washed. It is, therefore, perfect for use in chamois.

4) Oil Tanning 

Sometimes, emulsified oils are blended with aldehyde chemicals to produce exceptionally soft and flexible leather. This process is called oil tanning.

 C. Re-Tanning 

Re-tanning converts the tanned leather into a marketable product. The choice of chemicals used in this process depends on the desired color and texture in the final product.

1) Drying 

At this stage, the tanned leather is pressed between two rolling cylinders to remove the water absorbed during the tanning process.  

2) Shaving 

This process removes flesh residues and creates uniformly thick leather. The leather passes through two rolling cylinders where the upper one is provided with helical blades.  

3) Splitting 

A splitting machine slices the thick leather into one or more horizontal layers. Sometimes, this process is also carried out after liming. The top grain layer is the most expensive leather. It is used to make high-end leather merchandise. The layer without grain is used to make suede leather. Sometimes, an artificial grain surface can also be applied to it.

4) Dyeing

With the exception of vegetable tanned leather, all types of leather are dyed. More often than not, water-soluble dyes are used, allowing the dye molecules to penetrate inside the fibers. Thus, it differs significantly from surface coating where dye is applied only on the top layer.

5) Fat Liquoring 

Fat liquoring, or stuffing, consists of adding fats, oils, or waxes between fibers to keep the leather soft and flexible. Without this process, the leather will dry and become stiff.

D. Finishing 

This is the final stage where finishing touches are added to the tanned leather – as per the desired end product. This includes color, texture, thickness, and surface patterns.

1) Polishing 

A velvet wheel rubs the leather to create a shiny surface.

2) Embossing 

The process of embossing obtains a three-dimensional print using heated hydraulic or roller presses.

3) Surface Coating 

The surface coating process adds color and different designs to leather. As per the customer requirement, resins, pigments, and dyes are added in layers to the surface using a variety of techniques such as spraying, roller-coating, curtain-coating, or hand coating.

4) Final Grading

Finally, the leather is graded before it is dispatched to the customers. Grading is often based on a variety of factors such as the feel of the leather, color, pattern, thickness, softness, and flexibility.

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