How Is Leather Manufactured?

14/12/2019

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Making leather is a long and complex process to be followed with precision and care in order to produce high-quality leather each time with the same finish. The cycle will start with an animal hide and a piece of leather will be ready for transformation into a wallet, clothing or any other leather item by the end of the journey.

Almost any animal, including pigs, sheep, goats, and crocodiles, can make leather from the hide. The most common hide used, however, is from a cow. The art of making leather uses hides that would otherwise be lost, a by-product of the meat and dairy industries. Rather, when producing leather, the hide of the animal becomes a beautiful and valuable commodity that lasts for decades.

Of course, the cow’s upbringing would affect the quality of the hide that is produced with a good hide that yields 80-90%. For example, cows that were marked, exposed to a lot of bites of insects, kept close to barbed wire fences, or used electric cattle prods on them can damage their hide. Here you will find that the yield of the hide is more likely to be around 60% to avoid blemishes.

The skin is stripped from the animal when a hide is prepared and then the meat has to be extracted, which can be done by hand or using a fleshing machine. If you do it by hand, the process must be done as quickly as possible as the material can easily dry out. You should be left with a dry, white surface at the end of the process.

A hide should be salted or placed in a salt brine at this stage. This must be a very generous salt layer as the salt prevents it from decomposing. When within a few hours of being removed a hide is not salted, brined or frozen, the process of decomposition can begin and the hide can become worthless and be wasted. The hides have to be handled, salted and folded with touching flesh sides and will stay salted until they are ready to be processed if they are to be washed in water to remove any soil or other materials.

If a hide has hair on it, it will need to remove this, which is carried out using a chemical solution containing calcium oxide. This can also be called a lime bath and it can take one to two days for the soaking. The hide will also be softened by this process. If you don’t want to use chemicals, the hair can be removed in a manner similar to the skin being extracted by hand.

The hide will be full of water after all this swimming and washing, so it will swell to be about 4 mm thick and can be broken into two sheets. This is done in order to be able to use separate parts of the hide for different leather product types. For the highest quality leather products such as full-grain leather, the upper part of the leather split is saved. This is because the upper layer has a much tighter fiber structure that makes it more durable. This surface renders a beautiful and supple leather when treated correctly.

To cheaper leathers with less overall quality than the top layer, the hide’s bottom layer will be kept. These tend to be used for top grain and split leathers and are used most frequently for shoes and bags.

There are many varieties of leather, as mentioned earlier. All kinds of leathers, however, must go through four basic levels. This includes preparation, tanning, re-tanning, and finishing steps. Occasionally, an additional surface coating subprocess may be added to the process. Until sending it to the tannery, it is important to cure the animal skin. While curing is not considered part of the tanning process, it is the first important step in obtaining leather.

Tanning Process

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 point, for several hours to several days, the cured hide is soaked in water. In addition to preserving the moisture lost during salting, this process helps remove dirt, waste, urine, and excess animal fats.

2) Fleshing:

This process removes from the skin side subcutaneous material. The pelt is passed through a machine to mechanically remove the fat, muscle, and flesh. This process usually takes place after being slaughtered, soaked, or limed. Hides can be divided into different layers at this point, or after tanning.

3) Un-hairing:

Using mechanical instruments such as rollers and blades, hair is removed at this stage.

4) Pickling:

In order to prevent decomposition, this process involves washing and soaking the rawhide to acids or salts. This allows tanning agents like chromium and aldehydes to penetrate. For several months, stronger pickling agents will be used to secure hides.

 

5) De-Pickling:

Upon pickling, the hide is immersed in sulfuric acid to reduce the pH.

6) Liming:

This method loses the fibers and allows different tanning chemicals to be absorbed by the body. Because they extract keratinous material such as hair and wool, sodium sulfide and hydrated lime are typically used to treat the skin. As the pH increases, fats are hydrolyzed. Water is absorbed into the fibers of the body resulting in a skin layer that is bloated.

7) De-Liming:

In this process, a mixture of water and ammonium chloride or ammonium sulfate is used to clean the cover. This extracts moisture to reduce swelling (along with any impurities).

8) Bating:

Bating marks the end of the process of limiting. Proteolytic bating enzymes are used to treat the flaccid skin to eliminate non-fibrous proteins. The grain is cleaned and the pelt is smooth and silky.

9) De-Greasing:

To remove excess oil or natural fatty acids from the body, water-based solutions and solvents are sometimes used.

10) Bleaching:

To incorporate the desired color at a later stage, chemical agents are used to making the hide colorless.

 

Vegetable Tanned Leather

1) Vegetable Tanning:

For thousands of years, vegetable tanning has been around. Like mineral tanning, it uses a chemical called tannin, a naturally occurring astringent polyphenol. This is usually found in trees such as oak, chestnut, or mimosa bark, leaves, and branches. When dark brown, beige, yellow, and red shades are made, the tannin gives the leather a unique color and texture.

The process, however, is time-consuming, laborious and costly. There are two types of processes for vegetable tanning. The slow process takes about 30 days, while the fast tanning process only takes about 36 to 48 hours to complete. However, the slow process can sometimes take several months, as multiple treatments may be required.

This system creates leather that is extremely durable. The vegetable-tanned leather is therefore often used to make products like saddles or holsters. The unparalleled longevity and distinct appearance render this leather ideal for imprinting and complicated leatherwork such as tooling.

2) Mineral or Chrome Tanning:

 The most common tanning process is mineral or chrome tanning because it is much quicker, more economical and less labor-intensive than the others. It was introduced in 1858 as an alternative to the expensive and time-consuming method of vegetable tanning. The process can be automated to a maximum duration of one day. For small and thin skins, the time for chromium tanning is usually about 2 or 3 hours. For thicker ones obtained from cattle, however, it can go up to 24 hours.

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

Mineral or Chrome Tanning

Mineral or Chrome Tanning

Mineral or Chrome Tanning

The chrome tanning procedure, however, has a negative impact on the environment as it requires heavy use of acids and other chemicals. Toxic waste can flow into groundwater and contaminate supplies of drinking water. The resulting environmental impacts, especially in developing countries, are a major concern.

3) Aldehyde Tanning:

The method of tanning uses compounds of glutaraldehyde or oxazolidine. Because of its light cream texture, it is also called wet white leather. Tanned leather with an aldehyde is water-absorbent, flexible and can be washed by machine. Therefore, it is ideal for chamois use.

4) Oil Tanning:

To produce highly smooth and durable leather, emulsified oils are often mixed with aldehyde chemicals. It’s called the oil tanning process.

C. Re-Tanning:

The tanned leather is turned into a marketable product by re-tanning. The range of chemicals used in this process depends on the final product’s desired color and texture.

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 system extracts traces of flesh and produces leather that is reliably dense. The leather passes through two rolling cylinders where helical blades are given to the upper one.

 

3) Splitting:

A slicing robot cuts one or more horizontal layers of thick leather. This method is also sometimes done after liming. The most costly leather is the top grain surface. It is used for the manufacture of high-end leather goods. The grainless surface is used to render leather suede. It can also be applied to an artificial grain layer often.

4) Dyeing:

 All types of leather are black, with the exception of vegetable tanned leather. Water-soluble dyes are used more often than not, allowing the dye molecules to enter the fibers. It therefore differs significantly from the surface coating where only the top layer is added to the dye.

5) Fat Liquoring:

To keep the leather soft and flexible, fat liquoring or stuffing consists of adding fats, oils or waxes between fibers. The leather will dry and become stiff without this operation.

D. Finishing:

This is the final stage where, as per the desired end product, finishing touches are applied to the tanned leather.

1) Polishing:

The leather is rubbed with a velvet wheel to produce a light color.

2) Embossing:

The embossing process uses heated hydraulic or roller presses to produce a three-dimensional print.

 

3) Surface Coating:

The method of surface coating adds leather color and different designs. Using a variety of techniques such as grinding, roller-coating, curtain-coating, or hand-coating, resins, pigments, and colors are added to the surface in layers according to customer requirements.

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