Leather Tanning: Definition, Process & Chemicals

This is the first step in the production of leather. When hides are come and cleaned from blood, dirt, hair, and salt. It needs to be processed to use hide for making products. The process is known as the tanning process. Tanning alters the chemistry inside the fibers of the leather permanently to prepare it tougher for the enzymes from fungi and bacteria to break them down, this also makes it durable. So in short, it is a process that prepares skin and hides for leather formation. Moreover, the process is done in a tannery.

The process is a chemical treatment of the rawhide of different animals to convert them into leather. There are different types of tanning agents. The most widely used are vegetable tannin, mineral salts like fish or animal oil, and chromium sulfate.

Why does leather need a tanning process?

Leather needs a tanning process because, without it, it will rot and putrefy. This process will stabilize the protein of the hide to prevent putrefaction and make it available for different uses.

When untanned dry hides become wet, they will be putrefy. While tanned leather will not putrefy. The process changes the look of the skin and this change will depend on how long it takes and what agents are used for tanning.

This process for leather has been performed for thousands of years in various countries.

History of tanning

The tanning process is used to convert animal hides into a substance suitable for furniture clothing, and other uses.

The word tanning comes from the Latin term Tannere, which is a derivative of tannum and means oak bark. In the old time, people use leather for making bags, clothing, and even homes. Some treatments make it useful for various sorts of footwear. To get products leather tanning process was required.

Tanning was practiced in Mehrgarh as far back as 7000 BCE and continued until 3300 BCE. By 2500 BCE, Sumerians were utilizing tanned leather on their chariot wheels, which were attached with copper studs.

Tanning was often designated an “odiferous trade” as tannin was known for its intense odor. And allowed only specific regions where such trades were located and on the outskirts of towns and cities.

When the skin comes, it is dirty, and stiff and has hair on them. In the old time, it was first soaked in water to make it soft. After it, tanners pound them and scrape off any remaining fat and tissue that may make them stiff again after drying.

They were soaked in a solution of urine to remove hairs, applying paint with an alkaline lime combination, or dipping in salt solution after some months of letting it putrefy. This process loses hair and allows it to be scraped off. In this part, a great deal of the odor was produced.

After hair removing, bating” took place to soften the hide better. This needs hammering dog or pigeon dung into the hide or absorbing it in an animal brain solution. Bacteria produce enzymes and fermentation occurs and produce a smell.

After it, vegetable tanning began. Tannin, cedar oil, or alum would be rubbed into the skin as a tanning agent. Stretching skin help to draw in tanning agent and also moisture to get away.

After 1840, using chromium III was to soak gut sutures for wound repair.

Pros and Cons of Leather Tanning


Leather tanning creates leather that is strong and durable. It is also a material that can be used to make various things from accessories to furniture.


  • Leather tanning required killed animals to make leather.
  • It is a smelly process.
  • The process may be unhealthy for people.

Phases of Leather tanning process

Phases of tanning are given

  1. Soaking
  2. Liming
  3. De liming
  4. Bating
  5. Pickling


Soaking is for removing excess dirt or salt. All excess material remove in this phase to prepare flat and clean pelts.


Liming is for splitting the fiber bundles and eliminating unwanted hairs by alkali. Hair-saving systems yield good results. Lime is usually added before alkaline swelling occurs from subsequent additions of sodium hydro Sulfide and sodium sulfide. This make it able to absorb and diffuses smoothly through the pelts, for optimal fibers separation and splitting. In this stage, the hide is referred to as a pelt.


Pelt needs the removal of lime and neutralization of the remaining alkalinity after fleshing and splitting. This is done better by the use of proprietary organic acids and acid salts that form good soluble lime complexes. More care is needed to sufficiently buffer the de-liming bath and prevent the precipitation of soluble proteins by a fast drop in float pH.


Insoluble elastin fibers network in the high alkaline condition of liming need to be evenly flattened and relaxed by the extended action of adequate enzyme preparations.


Acids are put in to slacken the pelt and increase penetration of the tanning agent.

The Actual Tanning process

By utilizing a tanning agent the collagen pattern of a pelt can change and turn into leather. That is durable and protected against moisture, chemicals, and microorganisms.

Tanning with vegetable tanning

Leather tanning submerged in a bath that has a mixture of vegetable extracts + 0.5 0. For tough and nice leather mostly after cooking hide with extract mixture, tanning proceeds with the method hide is planted and provided a thick extract solution for 2-5 weeks.

The procedure is quick

Early tanning with 200% water and 3% extract of mimosa (Sintan) rotary drum 4-hour dive preceded. Cook for 1 night in the drum after rotating and adding the peyamak material.

With mineral tannery

With chrome tanner

Chrome tanner is the main chromium sulfate form. The most commonly used tanners have a base of 33.33%. 

If this tanner is to be fixed in the skin material, then the base of the chromium liquid should be enhanced to improve the size of the chrome tanner particles.

Tanning with white alum

Rotate acidified skin with.

  • 10% of white alum.
  • 40-50% water.
  • 2% salt, rotate for almost 2-3 hours, stacked for 1 night.

The next day the hide is rotated once more for ½ – 1 hour, and after that, dried in the humid air for almost 2 to 3 days. The hide is stretched by the use of a machine or hand until it is weak.

Tanning with Tannery Oil

The hide to be oiled is often pre-tanned by formalin. The hide is cleaned to eliminate extra formaldehyde and transform and lessen the water, playing with 20-30% fish oil, for about 2 to 3 hours, and assembled the next night dive hung. Then aired for almost 7 to 10 days.

When skin is ripe, signs of skin are elastic and white. Skin cooked washed with 1% Na2CO3 solution.


The skin is stacked for about 1 to 2 days after that milked by hand or machine to eliminate water, then plucked by a crude machine on the meat to regulate the width of the skin to flat. The skin is evaluated to determine the amount of the chemical that need for processes, followed by cleaning with ½ hour of running water.


For this process, the skin was rotated with 150-2005 water, warm (36- 40 0 C). 

0.5-1.0% oxalic acid for almost ½- 1 hour.


Just perform for chrome leather. Chrome leather is acidic (pH 3-4) skin needs to be neutralized once more to prevent interference in the following process. Neutralization generally takes alkali salts like Neutrigan, NaHCO3, etc.


Dyeing aims to get the basic color on the hide.


The following are the aim.

  • Prevent skin fibers from adhering to one another.
  • For lubricating skin.
  • Make your skin waterproof.

After painting the base, played for almost half one hour with 150%,  200% water 40 to 60 0 C, and 4-15% oil emulsion. To break the oil emulsion, 0.2- 0.5% formic acid was used. The oil is on the skin as the water is eliminated. For a night, leather was placed on the horses.


It is performed for vegetable leather sole leather. The objective of lubrication is to conserve the tanning material not out from the skin before the skin dries.


The skin is watered with a hand or machine and then it is dried. This is to end chemical reactions in it. Water quantity in the skin to be 3-14%.


After skin drying, allow in regular air for 1-3 days so that the skin adapts to the humidity of the air. Then, Skin moistened sawdust grown in water including 50 to 55% while on the first night, leather will grab water and get moist. 

Stretching and Time

Skin stretched by machine or hand. The goal of this stretch is to pull the skin to attain the limit of elongation so that if create craft things are not elastic, not change shape.

Final Stage of Completion (Finishing)

The final stage is to beautify the look of the finished skin, maintain the base color, polish, smooth the look of the surface, and cover the deformities or unequal paint color.

When a hide has been tanned, it becomes leather.

Leather tanning chemicals

Chemicals are divided into 4 groups.

Vegetable tanning

Vegetable tanning is the oldest method. Extracts from nuts, shrubs, and wood. it takes a longer time to tan the hide.

Chrome tanning

 Almost 75% of tanned leather manufacturing is chrome tanned. This process needs trivalent chromium which is known as a safe substance.

Aldehyde tanning

This tanning is grouped and known as chrome-free or aldehyde tanning.  by this method, leather will be mostly used in automobiles. Mostly uses glutaraldehyde is used in this method. It usually needs more chemicals as well as a treatment after tanning.

Zeolite tanning

This is a recent method of tanning. These microporous, aluminosilicate minerals have a different property in that they can release or hold water depending on the temperature. Zeolite tanning is chrome-free, heavy metal-free, and aldehyde free.

Some common chemicals used in the tanning process


Water serves as an intermediate or medium to permit other chemicals to be absorbed into the hide.


Salt inhibits bacteria in the skin by absorbing liquid. Salt ensures moisture and salinity balance level in the skin.

Sulphate oil

Fish oil with Sulfuric acid makes oil that can be used for liquoring or anointment in the tanning process.

Sodium sulphide

Sodium Sulfide is used to get rid of hairs.

Sulfuric acid

The acidification ends the enzymes acidification used in the erosion process.

Chromosomal B

It is used during chrome tanning.


Formaldehyde reacts with the amino acid in the skin and makes it durable.

Environmental Concerns regarding the leather chemicals

  • The tanning of hides involves organic substances like tree bark and leaves. This had a smaller impact on the atmosphere since it needs only water which is then released into the river or other.  It breaks down in the liquid and does not pollute it.
  • Brain tanning uses fats and acids to tan the leather, with an impact on water use.
  • Chrome tanning needs different chemicals that can be harmful to health and end up polluting the air, water, and soil. Tanneries are associated with a  strong smell because of the processes. Workers are also at risk of different diseases.


Leather tanning is a difficult task and essential to make leather. There are various kinds and chemicals of leather tanning and hopefully, we have mentioned important points in our article. The detail about leather tanning is very long. We can say the articles about the tanning process are not convening complete information about it but we have searched a lot and tried our best to provide important points in this brief article. Hopefully, it will be sufficient and helpful to you. Process and ingredients may vary and if you want to tan your hides you should be so careful, you should learn more about it. You should have good practice before any important project. Practice can make a man perfect, so try the best method and good ingredients to get good results.

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