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Leather Tanning: The Tanning Process Explained

Leather Tanning: The Tanning Process Explained

From animal skin to leather. How do they do it? Leather-making is an ancient practice that has continued to exist for many centuries now. This ancient practice of turning raw animal hide into something strong and durable greatly relies on one certain phase. This is the leather tanning phase. This step is what turns rawhide into the reliable material we all know and love.

Worker at a tannery in Morocco. A visit to a Moroccan tannery is not only a visual impression that stays, also the smell is unforgettable. The procedure of making leather has not changed much over the last decades and even centuries, neither have the working conditions, we were told. In Marrakesh the tannery is an enterprise run by about 50 families. The reputation of Moroccan leather was so great that the French word "maroquinerie" has become the general term for everything made from leather.
Source: Unsplash

Moreover, have you ever wondered what exactly happens during the tanning stage? What happens to the animal skin as it soaks for days in a drum? What likewise changes in the skin as the days go by? Why do we even need to tan animal skin in the first place? Additionally, what are the pros and cons of tanning? Expand your knowledge as we continue to explain the what, the how, and the why of tanning leather.

How is Leather Made?

Making leather is a long process that requires years to master and perfect. It involves quite a handful of steps in transforming raw hide into strong, durable, and beautiful leather products. There are also various ways to make leather; different methods involved in transforming rawhide to leather.

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Of course, if you simplify the leather-making process there are merely three steps to it. The first step is to prepare the animal skin, the second is to tan the skin, and the final step is to finish the leather product. However, in this article, we are all about focusing on the tanning process. Truth be told, making leather mainly focuses on the leather tanning process. This is the step where the actual change happens. It is where animal skin transforms into something great. For many great centuries, the tanning process turned animal skin into useful things like clothes, shoes, and tools. Even if it is an ancient practice, you can make and tan your own leather with a little bit of knowledge and a whole lot of time and practice.

What Exactly is Leather Tanning?

Tanning is simply the process of treating the skin or hide of the animal to make leather. The place where the tanning process usually happens is at the leather tannery. A tannery, like the one in Morocco, is usually large-scale. It has numerous drums that are capable of tanning hundreds of animal skin at the same time. Additionally, the tannery in Morocco is really old. It is proof that tanning and leather production is an ancient art form. Likewise, two ancient civilizations like Sumer and India have also been practicing leather tanning for thousands of years.

tannery
Source: Unsplash

Additionally, tanning is not just an ancient art form. While the animal skin soaks up the tanning agent it goes through chemical and physical change. The end product is leather, however, not all will have the same quality. This is because tanning also involves different techniques and methods. Each technique or method or solution applied to the leather will produce a different change. This, therefore, makes the leather tanning process interesting and important to learn.

Why do we need to tan leather?

Now that we get the gist of what exactly is leather tanning the next question we need to as is “why”. Why do we need to tan leather? The simplest answer would be “to make leather”. But that is not enough. Why do we need to tan animal skin to turn it into leather? What is in the tanning process that changes ordinary animal skin into leather? Why is there a need to tan the animal skin in the first place?

The answer is really simple. It is to keep the animal skin or hide from rotting, decomposing, and putrefying. Once the animal is dead, the skin will take its natural course and begin to break down. There are many elements that usually break down and decompose the animal skin. It may be because of bacteria that eats away the flesh or the elements which will try to harden the skin or both. Tanning, therefore, prevents all of this from happening.

Tanning will permanently alter the protein structure of the animal skin. It will make the skin more durable and less likely to decompose. Different solutions offer different changes to the skin. Changes such as the change in color, flexibility, softness, or strength. However, whatever method or technique you use, the primary purpose of tanning is to preserve the skin. Tanning, therefore, preserves the animal skin and likewise creates leather in the process. Additionally, tanning will not only preserve the skin but also protect it from the elements. Protecting it from decomposing when it gets wet.

Different Methods of Tanning Leather

Being an ancient form of art, many have developed different methods or techniques of tanning. There are many ways to tan animal skin or hide. There are however two methods that are the most common. They are the vegetable tanning method and the chrome tan or chromium tanning method. There are also other alternative tanning methods that we will also be briefly discussing.

What is Vegetable Tanning?

A tanning method that has been in practice since ancient times. The vegetable tanning method uses natural tannins from the bark and leaves of trees and plants. After the animal skin has been skinned, cleaned, and prepared the hides are placed in the tanning pit. These tanning pits are usually drums that contain tannin solutions of various strengths.

vegetable tanned leather

The vegetable tanning method takes time. The animal skin soaks inside the tanning pit for over a period of two to three months. According to ancient practice, the tanners move the animal skin from one drum to another. They typically move the animal skin to the next drum that has a solution progressively stronger than the last. The tanning agents penetrate the skin as the solution gets stronger and stronger. Additionally, since this method uses an organic solution, fermentation occurs. This gives the leather its distinct color and appearance. It also leaves the final product a distinctive sweet and woody fragrance. A smell that typically indicates real leather.

They continue this process until they reach the desired final product.

“Natural” Tanning

Natural tanning happens when specific conditions occur. These conditions are usually present in bogs. The conditions include highly acidic water, low temperature, and a lack of oxygen. These certain conditions preserve the skin. It can, however, leave a severe tan that is similar to or identical to that of mummified dead bodies, also known as bog bodies.

Brain Tanning

Another form of natural tanning. This tanning technique is quite difficult to pull off. It is a labor-intensive way of tanning animal hides. This method usually uses skin from deer, elk, or moose. Tanners use the natural tannic acid from the animal to tan its own hide. First, they clean the skin thoroughly by scraping all the flesh, fat, and membrane off the flesh of the skin. The tanner then washes the skin and stretches it on a frame to dry. What is so unique about its method is that it uses a very unique and fascinating tanning solution. It actually uses a mixture of warm water and the mashed-up animal brain to tan the hide. Once they finish applying the solution they smoke the skin to complete the process.

What is Chrome (Chromium) Tanning?

Unlike the ancient practice of vegetable tanning, chrome tanning or chromium tanning is relatively recent. It is also one of the most practiced methods of tanning leather. This is because it has certain advantages in comparison to other techniques of tanning, especially against vegetable tanning.

This recent method of tanning leather has two stages. The first stage involves soaking the animal skin in baths that containing acidic salts. The animal skin soaks inside the bath until they are ready for the next step. The next step is the chrome tanning proper. Once again the animal skins get to soak inside a bath. This time the bath contains chromium tanning agents such as chromium sulfate and chromium salts. Unlike the first stage, the acidity of the bath is reduced until the hide begins to absorb the chromium tanning agents. This process produces blue hides. They are better known as “wet blue hides.”

chrome tanned leather

The advantage of chrome tanned leather is that it is faster than vegetable tanned leather. Vegetable tanning usually takes a few months to finish. However, chrome tanning can take up to as fast as one day, with aid of modern machinery. Chrome leather also costs less because it is less difficult to manufacture. What usually makes vegetable tanning expensive is because it is labor-intensive.

Alternative Chemicals / Synthetic Tanning

This technique is the frequent alternative to chrome tanning. They do not apply it on their own but apply it together with other tanning solutions. Tanners usually combine it with either vegetable or chrome tanning solutions. This method of tanning uses artificial tanning agents such as formaldehyde, glutaraldehyde, phenols, and acrylates. They call these types of leather wet whites. Wet whites can also be made using aldehydes, aluminum, zirconium, titanium, or iron salts, or a combination of any of them. This method is likely to produce very soft leathers that are usually on car seats.

Mineral Tanning

Another method that uses chromium salts is mineral tanning. This method also speeds up the tanning process to days or even hours. It usually produces soft, pliable leather that is light.

Tawing

This method uses alum and other aluminum salts together with binders such as egg yolk or flour to tan the skin. This process is known to increase animal skin pliability, stretchability, softness, and quality.

How is leather tanning done today?

Today, the leather tanning industry is now a more industrialized process. Back then, the animal skin and the solutions or tanning agents soak inside a drum or container. The hide and the solutions just sit there until they are ready, although tanners casually move them around. Back then it was indeed a tiring process. However, today we have very large rotating drum containers that fill up and drain themselves after every stage. Thereby removing the hassle of having a different set of drums for each tanning step. Today, tanning can be more efficient with the help of machines. Proof of great progress the leather industry is making. However, there are still some who prefer to do it old school.

An Overview of the Leather Tanning Process

There are several steps involved in the ancient art of tanning leather. It also involves different techniques and methods that produce varying colors, textures, and finishes. In order for us to easily understand the tanning process, we divided them into four different phases. The four phases are namely the Beamhouse phase, the Tanyard phase, the Retanning phase, and the Finishing phase. Naturally, the leather tanning process takes months to complete. However, as we learn, later on, there are different methods that speed up the process.

There are certain steps to take before the actual tanning process begins. Naturally, we start with the preparation stage. This stage mainly involves skinning the animal once it arrives from the farm, slaughterhouse, or wherever the animal came from. In this stage, they will detach the skin from the animal and remove any scraps of flesh left from the process.

The four phases begin after this.

An open windows lights the a dark and dusty room fool of carpets with a couch and a round table.
Photographer: Fabio Santaniello Bruun | Source: Unsplash

First Phase: Beamhouse Operations

The first phase is the beam house operations. According to the Merriam-Webster dictionary, a beam house is a part or section of the tannery where the hides are prepared. Basically, the first phase is also all about preparing the leather. This phase mainly focuses on the animal hide, how to clean it, how to treat it, and why it is necessary to treat the animal hide. While all other steps are important, I believe that the first phase is the most important.

Step 1: Curing

The first step to any leather-making process is to stop the animal skin from decomposing. Otherwise, you would just have wet, rotting animal skin. This process is usually done as soon as possible. Decomposition usually begins within the first 5 hours, depending on the elements and bacterial growth. The way to stop the decomposition is to “cure” or preserve the hide. There are a few ways to cure a hide, here are some reliable methods.

Curing animal hide

Sun Drying

Probably the oldest method when it comes to curing hides. This works by stretching the hide and exposing it out in the sun to dry. The hide will stop decomposing as soon as it dries thereby preserving it. However, this method is not practical for areas that are warm and humid. Moreover, the leather produced from this method is likely lower in quality. This is sometimes due to overexposure to the sun.

Freezing

Of course, another way to stop bacterial growth and decomposition is by freezing. Unlike sun-drying, this method is quick and easy. However, both sun drying and freezing lead to low-quality leather. This is because the water molecules found inside the skin will expand and cause structural damage to the hide.

Dry Salting

This method involves heavily covering the hide with salt. It is then left to dry. As it dries, the salt will draw out any moisture left inside the hide. This will make the hide resistant to bacterial growth. It will likewise make the hide stiff.

Wet Salting

Likewise, salting involves soaking the hide in a high-concentrated salt solution. It works basically like the dry salt method where it draws out water inside the hide. This will likewise prevent any bacterial growth that causes decay. Many prefer wet salting to dry salting because it responds better later in the tanning process.

Controlled Drying

This method works similarly to the sun drying method. However, this method yields more consistent and high-quality leather. This is because the hides are in a more controlled environment usually inside a controlled drying chamber. The downside to controlled drying is the cost. The chamber usually needs something to control the heating, cooling, humidity, and airflow.

Once the hides are done curing they are now ready for storing and/or transporting. Storing and transporting the hides will also differ according to the method of curing you use. Like for example, you must store sun-dried hides in a dry place, preferably low in humidity. Likewise, if you freeze the hide then you must also store and transport them while there are still frozen. The hide is also sorted according to its weight and quality. Tanners also remove unnecessary or extremely damaged portions from the hide. The scraps are either discarded or reused to make lower-quality leather.

Step 2: Soaking/ Washing

After curing, the hides are usually salty and very stiff. To make them pliable and ready for tanning tanners soak them in water. This will reintroduce moisture to the leather fibers making them more usable. The water usually contains some chemicals like disinfectants to prevent the risk of bacteria growing.

Step 3: Liming

In this step, the hide is treated to remove any remaining organic substance on the hide. Examples are the epidermis, the roots of the hair, undesired fats, and soluble proteins. The hide is treated in a mixture of alkali and other additives. This solution will weaken the hair and its roots, any proteins, fats, collagens, and more. It basically makes it easier to remove unnecessary things for making leather.

Step 4: Fleshing and Unhairing

After successfully liming the hide, it now becomes easier to remove any unwanted parts of the skin. Most parts of the hide are now weakened and unhairing can now being. the old school way of unhairing was to use hand tools and scrapping/scudding them off. It was a very labor-intensive process despite the hair already being weakened. These days unhairing is usually done mechanically. There are machines, rollers, and blades that do the job. efficiently.

Leather tanning, scudding

After removing the hair, the hide can now be fleshed. Fleshing removes the epidermal layer of the skin as well as the remaining hair roots. This process is generally known as “scudding”, which mainly refers to doing it by hand. However, like unhairing, it can also be done mechanically.

This step is the last of the beam house operations. At this stage, the hide is clean and ready for the next phase of the leather tanning process.

Second Phase: Tanyard

The second phase of the leather tanning process is also the main leather-making stage. This phase basically takes the cleaned and disinfected hide and turns them into usable leather material. This phase will also take time depending on what method or technique of tanning you are using. Like for example, the vegetable will usually take around a few months to complete whereas chrome tanning takes only a few days. Machinery, tools, and the overall manufacturing capability are also a factor. Here are the steps for the second phase.

leather tanyard
Photographer: Adam Jang | Source: Unsplash

Step 5: Bating

This step aims to improve the properties of the leather such as its softness and pliability. To do this they must bring down the PH level of the leather to a lower level using buffering salts. This will reverse the previous liming process done during the beam house operations. This process is usually known as deliming and buffering. However, the whole process is collectively called Bating. This step is very important in terms of manipulating the preferred end state of the leather and its characteristics. To manipulate the leather specific enzyme agents are introduced. These agents work better at a lower PH.

Step 6: Pickling (for chrome tanning only)

This step of the leather tanning process is applicable only for chrome tanning. Chrome tanning agents are not soluble under the current PH level we are now. The process of pickling drops the chrome tanning PH to 2. Pickling will lower the PH of the collagen in the hide. This will allow tanning agents to easily penetrate the hide.

For vegetable leather, pickling is not necessary as the tanning can function at a higher or more basic PH level.

Step 7: Tanning

After going through all the steps of preparation, the hides are ready for tanning. We will briefly discuss the two mains methods of tanning, namely, vegetable and chrome tanning. We will not discuss the other methods of tanning since it has already been mentioned earlier.

Vegetable Tanning

An ancient practice and probably the oldest known common method of tanning. This method of tanning uses a naturally occurring tanning astringent from plants and tree bark. Some examples are chestnut, hemlock, mangrove, and oak bark. This natural solution is better known as tannins. The vegetable tannin will basically bind and cover the collages on the hide. This will make the hide less susceptible to bacterial growth. It will likewise be less water-soluble and more flexible or pliable. It can also be referred to as bark tanning since it mainly uses tree barks to create the tannin.

Contrary to common knowledge, the vegetable tanning process can take as fast as 2 days in the accelerated process. However, it usually takes up to 30 days or more in the standard process. Back then vegetable tanning is usually carried out in large pits. Tanners continue to agitate the leather within the solution, a very laborious process. Now, you will usually find them inside rotating drums which makes things easier. Additionally, tanners continue to expose the hide to stronger and stronger levels of the tannins as the process continues.

Photographer: Ben Ostrower | Source: Unsplash

This method of tanning produces soft yet strong and a bit stiff leather. They are perfect for leather goods such as saddles, upper leather, tooling leather, sole leather, and belts. The color varies and is available in a few shades of yellow, brown, and even in reds.

Chrome Tanning

This method of tanning leather has become very popular due to its efficiency. It typically produces leather that is thin, soft, and stretchable. Chrome tanned leather is also resistant to shrinking in warm/heated water. The secret lies behind the changes within the leather itself. Unlike veg tanned leather, the tanning liquor not only binds to the collagen but also increases the space between the proteins in the hide. Chromium also penetrates better and at a much faster rate than the tannins. The whole process usually takes less than one day and requires less labor. The hides are milled in a single solution until the tanning process is complete. It is usually done in what they call a “one-bath” method where the PH level is around 2-4.

Step 8: Drying

Once the hides absorb all the tanning solutions it needs to dry. There are generally two ways to dry the leather. We call this method wringing. You basically squeeze the excess moisture out of the leather with large, heavy rollers. Another way is to dry using vacuum suction. After the leather is dry you proceed to the next step.

Step 9: Splitting

In order to turn leather into different products, it must be of the right thickness. For example, leather gloves and wallets require thin leather. Bags and saddles on the other hand require something thicker. This is why there is a need to split leather. It also makes leather easier to work with. Splitting is the “longitudinal cutting” of the leather into thinner layers. When leather is split, its different layers also have different characteristics and quality. A diagram of a leather’s cross-section will help you identify the different layers and their names. They are full-grain leather, top-grain, and genuine leather to name a few.

Furthermore, if you want an even more refined or thinner leather you can do so by shaving. Shaving can remove thinner layers of leather than cutting.

If you wish to know more about the different layers of leather click on the links below.

Steel Horse Leather Co. | KNOW YOUR LEATHER: FULL GRAIN VS TOP GRAIN

Steel Horse Leather Co. | GENUINE LEATHER: WHAT IS GENUINE LEATHER?

Third Phase: Retanning

Congratulations, the hide has been tanned successfully. At this phase, you may consider the tanned hide as usable material. You might be wondering why we have a retaining phase when we just finished tanning. The purpose of retanning is to refine the leather. After the first tanning session, there are still more tangible qualities on the leather that we can influence. Qualities such as color, softness, flexibility, pliability, and density can still be manipulated.

For efficiency's sake, retanning is done in sequence inside a rotating drum that washes and dries the leather in-between steps. Think of this phase as doing your laundry on an automatic laundry machine.

Step 11: Altering the appearance

Altering the appearance of leather is nothing new. Leather is often colored, or, bleached into a wide array of looks. The most common practice is to dye leather. There are two general classifications of dye: water-soluble and oil-based. Dyeing will also affect the properties of the leather. Depending on what kind of dye you use its flexibility, softness, and even storage life will be affected. There are four main types of dye, which are Acid, Basic, Direct, and Sulfur. Each type of dye offers different benefits from one another.

The Vernon Backpack

On another note, veg tan leather is generally not dyed. This is because they already have such a pleasing and natural appearance that dyeing it would just ruin the leather.

Step 12: Fatliquoring

After going through so much, the leather tends to become dry and stiff. Fatliquoring applies lubricants to the leather to make it stronger and more flexible.

Step 13: Setting and Drying

To ensure that the previous treatments result in an even overall finish the leather is to be set out to dry. The usual practice is to air dry the leather. To ensure that drying is uniform the leather is stretched and/or mounted to expose to as much airflow. However, mechanical drying is also available. This is generally done similar to wringing where they press the leather with large, heavy rollers to push the moisture out. Vacuum suction is likewise another viable method.

Final Phase: Finishing

The final phase to the leather tanning process and the overall leather making process. This phase focuses mainly on refining the leather to a specific quality intended for a specific use. Here are some examples of different ways to apply finishing to the leather.

Conditioning

For leather to last as long as it should conditioners are applied. Conditioners help the leather retains some helpful moisture and oils that keep its fibers in great shape over time. Different types of conditioners will vary upon different types of leather.

leather duffel bag
The Dagny Weekender

Staking

If what is intended is to have a smoother, more supple material leather usually goes through the process of staking. Staking is like tenderizing your meat with a mallet. Machines beat the leather to soften its texture. Of course, you can also do this manually although it will cost more time and manpower.

Buffing

Buffing is usually done when you want the leather to have a smoother touch. This also gives the leather a more grainy look. The process of buffing usually involves sanding down the leather in a sanding drum.

Surface Pressing

Another technique that greatly changes the appearance of leather. Surface pressing uses rollers to imprint a pattern into the leather. This also creates texture on the surface of the leather. A common example of surface pressing is embossing.

embossed leather
Example of Surface Pressing

Pros and Cons of Leather Tanning

Leather tanning is great because it creates leather. A material that is not only strong and durable but beautiful as well. It is also a material that can be turned into many things. From accessories to clothing to furniture. While there are many great things that come out of leather tanning there are also bad things. For example, leather tanning promotes the killing of animals to harvest their leather. That is why many turn to the alternative such as vegan leather. It is also a smelly process. The reason why tanneries used to be far away from towns or a community.

What are the Environmental Impacts of Leather Tanning?

Leather tanning has been called out for its environmental impacts, mainly due to water pollution. It lies between the two common methods of tanning. Vegetable tanning has a smaller impact on the environment since it mainly uses biodegradable and organic materials. After tanning, the solutions are usually dumped into bodies of water which eventually dissolve in the water.

Chrome tanning on the other hand is what causes major problems. It uses harmful chemicals that will affect health and the environment. It is especially harmful to the ecosystem where they dispose of chemical wastes. Moreover, studies found that frequent exposure to these chemicals has frightening health risks. Workers at tanneries are therefore very vulnerable to these risks. Frequent and long exposure may lead to cancer and other diseases.

Conclusion

Leather tanning is a marvelous process that involves science and human innovation. It is a process that requires knowledge, skill, and experience. Leather tanning also requires a great deal of time to finish. Likewise, it requires time to perfect. If you own any leather, it is about time you take a pause and appreciate the whole process.