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process of making leather

The Process Of Leather Making

The extensive procedure of Leather tanning is undoubtedly complicated and time-taking. It requires skilled labor and excellent mechanization. Even though with the advent of top-notch machineries, leather manufacturing has become less labor intensive. The complex series of treatments requires considerable time and energy. The purpose of the tanning process is to bring alterations in the protein structure of the animal skin in order to make it more sustainable and durable as well as improve its texture and overall appeal. There are variegated varieties of existing leather in the market which are broadly categorized into Full grain leather, Top-grain leather, Split leather and Bonded Leather. All types of leathers have to go through four major stages. This incorporates preparatory steps, tanning, re-tanning, and finishing. At times, an extra process i.e. sub-process of surface coating is added to the leather manufacturing procedure. Before transferring it to the tannery, the animal skin needs to be treated properly.

process of producing leather

Although curing is not considered a part of the tanning process, it is an integral step in obtaining leather. The animal is killed and skinned before the natural warmth of the body leaves the tissues. The freshly removed skin or hide is immediately cured with salt to get rid of the 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 the next steps in processing.

Here’s the list of four major steps involved in tanning of leather:

leather tannery

Preparatory Steps

There are numerous preparatory steps after curing of leather and all of them may or may not be followed by all the tanneries. It all depends on the quality and nature of the animal skin.

1) The very first process is soaking, in which the hide is soaked in water for an elongated period of time. This may stretch from hours to days. This step restores the moisture lost during salting and removes dirt, debris, blood and fats.

2) In the second step of Fleshing subcutaneous stuff from the flesh side is extracted. It is passed through a machine to remove the fat, muscle, and flesh mechanically. Hides are split into different layers after this stage.

3) Rollers and blades are used to remove hair at the third stage of dehairing.

4) The fourth process involves cleaning and soaking the animal hide in acids or salts to prevent decomposition. It helps the penetration of tanning agents such as chromium and aldehydes. This is called Pickling.

5) Next, the hide is dipped in sulfuric acid for long hours to lower the pH after pickling.

6) Liming loosens the fibers and helps the tanning chemicals to get absorbed.

7) De-Liming is performed after Liming which includes washing of hide with a mixture of water and ammonium chloride or ammonium sulfate.

 8) Bating signifies the end of the liming process. The skin is treated with enzymes to remove non-fibrous proteins, thus making the skin smooth and silky.

9) Water-based solutions and solvents are used to remove excess grease or natural fatty acids from the skin in the second-last step.

 10) Finally chemical agents are used to making the hide colorless which would help in adding the desired shade at a later stage.  This process is known as Bleaching.

Tanning

The purpose of Tanning is to produce a non-decomposable and durable material from the raw animal hide called leather. Tanning alters the protein of the rawhide into a long-lasting material. The most common tanning processes include Mineral tanning, Vegetable tanning, Aldehyde tanning and Oil tanning.

leather tannery

Tannins extracted from bark, leaves, and branches of trees such as oak, chestnut or mimosa are used for vegetable tanning, unlike mineral or chrome tanning where chemicals like chromium sulphate is used for tanning. On one hand where the natural tannins make the tanning process slow, laborious but environment friendly, on the other hand chrome tanning harms the environment with its chemicals but is less time consuming. Thus, the latter is a cheaper procedure. The Aldehyde tanned leather, which uses glutaraldehyde or oxazolidine compounds, is water absorbent, soft, and can be machine washed. Thus, ideal for use in chamois. At times, emulsified oils are blended with aldehyde chemicals to produce exceptionally soft and flexible leather, called oil tanning.

Re-Tanning

This process converts the tanned leather into finished leather which can be used for manufacturing leather products. The choice of chemicals used in this process depends on the final color and texture required in the final product.

Re-tanning includes Drying of tanned leather pressed between two rolling cylinders to remove the absorbed water. The next process removes left over flesh and creates uniformly thick leather. Then the splitting machine chips off the thick leather into one or more layers. Sometimes, this process is also carried out after liming. Except vegetable tanned leather, all types of leather are dyed. Mostly water-soluble dyes are used, allowing the dye molecules to penetrate inside the fibers. Therefore, it differs significantly from surface coating where dye is applied only on the top layer. Fat liquoring is the last stage in Re-Tanning where fats, oils or waxes are stuffed between fibers to keep the leather soft and malleable. The leather will dry and become stiff if this is not carried out.

Finishing

This is the last major stage in which finishing touches are added to the tanned leather according to the required end product. This includes color, texture, thickness and surface patterns. At first the leather is polished by rubbing it with a velvet wheel and embossed using heated hydraulic or roller presses. As per the requirement, resins, pigments, and dyes are added in layers to the surface using spraying, roller-coating, curtain-coating, or hand coating to add the desired color. Finally, the leather is graded before dispatching to the clients.

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