How is Leather Made?

Leather-making is an ancient art. It’s likely that ancient people discovered animal hides that were tanned naturally by decaying plant material. They discovered that they too could take an animal’s hide, remove the hair with stone tools, and use animal fat, wood smoke, or the tannin-rich juices of plants to preserve the hide. Cave paintings from the Paleolithic period show people wearing leather clothing. In ancient Egypt and in Rome, leather was used for many of the materials that we use it for today, including containers, clothing, and footwear. Today, leather comes from the skins of many different animals, particularly cow hide. From shoes to couches, so many objects in our lives are made from leather.

The science of leather

 The standard definition of leather describes it as the hide or skin with its original fibrous structure more or less intact, with or without hair or wool, and tanned to safeguard against decomposition or putrefaction (“imputrescible”).

Leather can be made from animal hide or skin. An animal hide is basically the skin or pelt of a larger animal like a cow, buffalo or horse, while animal skin is the external covering of a small animal like a sheep or pig. The skin of several animals can be used to produce leather; from deer and kangaroos to exotic leather from ostriches, snakes and alligators.

 An animal’s skin largely comprises of protein collagen. The chemistry of this fibrous protein, the properties it confers on the skin, and other components of the skin are of concern to the tanner, as they have an effect on the final leather product. Let’s now see what tanning is and then proceed to the main objective of the article, telling you how leather is made.

What is tanning?

TLeather Tanning Processhe word ‘tanning’ is derived from the medieval Latin word tannāre, which in turn is a derivative of tannum, meaning the bark of an oak tree; it refers to the use of oak barks in preserving hides. In the simplest terms, tanning is a chemical process that converts raw animal hides and skins into usable leather. It is the conversion of a putrescible (liable to decay) organic material into a stable material that resists putrefaction brought about by bacteria. The process is conducted in a tannery to achieve the following results:

 Appearance : To change the horny and translucent dried raw pelt into opaque leather with or without a change in color.

Pliability : To add a certain degree of softness to the hard raw pelt.

Smell : Tanning agents may be used to impart a specific smell to the leather, such as plant extracts for vegetable tanned leather and cod oil for chamois leather.

Resistance to wet heat (hydrothermal resistance) : An increase in denaturation or lowering temperature to make the leather more UV and heat resistant.

Resistance to decomposition by microorganisms.

The tanning process also helps ensure that the changes achieved in the process remain permanent.

How is leather made?

Let’s look at a step-by-step process of how leather is made.

The tannery is the place where everything goes like clockwork to convert hides/skins into usable leather. As the hides are preserved in salt to protect against bacteria, the first step at the tannery is to shake out the salt to avoid polluting the waterways.

Liming Process

Liming Process

The next step is liming, where the hides are soaked in lime to chemically dissolve hair, separate unwanted proteins and open up the collagen fiber structure. The process also causes the leather to swell. This is followed by a process known as fleshing which removes the fleshy bits of the dead animal that may be attached to the hide. The hides are put through a fleshing machine with cutting blades to remove the unwanted fleshy matter and achieve a regularity in thickness.                                                                                                                            

The next stage is trimming, where workers cut away the useless material from the edges of the hides for a better trim/shape. Next comes splitting, where the hide/skin is cut horizontally into two layers :  the top grain, comprising of the sturdy upper part of the animal, and the split, which is the lower layer.

 Now, one of the most important processes of leather-making – tanning – starts. The hides are moved to a rotating wooden drum built exclusively for this purpose. Tanning agents work on the raw fibers of the hide to convert it into a durable product, improving its resistance to abrasions, heat and flex. After this, the hides are transferred through a machine that squeezes out excess moisture, in a process known as wringing. The leathers are now called ‘wet blue’ and ready to be selected or graded. Once they have been selected, the next step is to reduce them and achieve a uniform thickness by putting the leather through a shaving machine with revolving bladed cylinders.

 Re-tanning and dyeing follow to alter leather’s properties. Depending on the type of shades and effect the tanner wants, the now uniformly dyed leathers proceed to the final wet operation known as fatliquoring, which imparts a permanent elasticity and softness to the leather.

 Next come the leather drying operations. The first step, known as setting-out, removes excess water and wrinkles. The leather can then be dried using any of the following methods: (a) vacuum drying, where leathers are placed between hot plates to suck out water through vacuum creation (b) suspension drying, where the leathers are suspended from the ground on a chain to allow for normal, effective drying at room temperature, and (c) toggling, where the leathers are fixed on frames and put through a dryer. The leathers are then pounded by pins as they pass through a machine, to open them up and soften them. This process is known as staking.

Vacuum drying

Vacuum drying

The scratches or parasitic damage in the grain of some leathers is filled with a paste called stucco. This is followed up by sanding, which creates a smooth surface for the finishing operation.

The finishing stage is where the leather’s aesthetics are enhanced to impart a natural look. The finish  can be applied using different methods. In the rotary spray method, revolving spray guns achieve consistent coating over the leather for the desired tones and effects. Roller coating is another method where the finish is directly deposited on the leather. Grain patterns can be then created through embossing. Bycast is the third application technique of finishing leather for uniform softness and thickness. Here, the finishing film with adhesive lining is pressed on the leather.

After finishing and enhancing, each piece of leather is measured by machine. As leather is dispatched/sold by area, accurate measurement becomes critical.

A look at various tanning methods

 It is clear that treating the skins and hides of animals is essential to how is leather made. There are different methods of tanning as outlined in the table below.

Method

Description

Ingredients

Time

Applications

Vegetable tanned

Most traditional tanning method that produces rich, warm and natural looking tones, though the colors obtained can be limited.

Natural tannins (powder or liquid form) derived from different parts of a plant, including bark, wood, leaves, fruits and fruit pods. The most frequently used tannins are obtained from oakwood, Chinese gallnut, chestnut wood and mimosa bark.

Can take up to 60 days to produce certain types of leathers.

Book binding, apparel, leather stamping and carving

Chrome tanned

The most common tanning method, responsible for almost 80% of leathers in the world.

A solution of chemicals, salts and acids

Only takes up to a day; the ease and agility of this method makes it a popular choice.

Saddles, harnesses, footwear, personal leather goods

Aldehyde and formaldehyde tanned

Aldehyde-tanned leather has a white or pale cream color. Formaldehyde tanning, another tanning method, is being discontinued due to its harmful effects on workers, many of whom tend to exhibit sensitivity towards formaldehyde

Aldehyde tanning uses glutaraldehyde or oxazolidine compounds; formaldehyde tanning uses formaldehyde, a chemical used for disinfecting medical equipment

Up to a week

Automobile upholstery, kids’ shoes, specialty leathers

Brain tanned

Brain tanning produces soft, breathable and durable leather. Its earliest origins date back to the northern Tungus who used reindeer’s spinal fluid to tan reindeer skin. The Native Americans also used the method to tan bison hides

Emulsified oils from the animal’s brain; deers, cows and buffaloes have enough lecithin, a brain oil, to tan their own hides

Up to a week

Clothing, bags, moccasins and lodge covers

Chamois tanned

Chamois tanned leather is porous, gentle and water-absorbent. It originated before the 18th century and was used by France’s glove making industry

Cod oil or any other easily oxidizing marine oil is used to produce the aldehydes to tan and color the leather

Up to two weeks

Jackets, small bags, automobile drying and buffing, shoe and jewel polishing cloths, grips on sporting gear

Rose tanned

Rose tanned leather is considered an extremely valuable leather, offering a powerful rose fragrance even after years of usage.

Rose Otto oil, which is distilled from rose petals without any chemicals or fillers

4-5 days

Wallets, watch bands, key fobs, handbags, leather belts and other accessories

Synthetic tanned

This white colored leather was invented during the World War II when the the supply of vegetable tannins was severely affected

Aromatic polymers like Novolac or Noradol

Up to one week

Sporting goods

Alum tanned

Alum tanning produces only very light shades of leather. This tanning method traces its origins to the Roman and Hellinistic periods

Aluminum salts, proteins (egg yolk and flour) and binders

Up to a week

Saddlery such as reins, straps and whips

 

Now that you know how leather is made, you should also know that the primary leather manufacturing processes have not changed much over the years. Which is fine, because the present day products made from top grain and full grain leather boast a luxurious feel and character that no other material  can achieve.