How Your Hide is Being Tanned - Leather in Furniture (Part 2)

In the previous article we studied the beginning stages of how raw hides are prepared for tanning into leather and talked a little bit about the different methods used to clean and remove unwanted components. We learned that the hides are cleaned in large wooden drums with a mix of detergents and chemicals. Liming is done to the hides by which the removing of the hair occurs. De-liming, batting and scudding are used to soften the leather and make sure that everything has been removed in preparation for the tanning agent. In this article we’ll examine the tanning process. The different types of chemical compound tanning. As well as why tanning is an important to leather.

The process of introducing a tanning agent into the raw hides to create leather is truly a science unto itself. Mankind has been perfecting this process over the centuries and many of the same techniques used long ago are still used today. The process of tanning the hide into leather involves a clear understanding of the chemical properties of the hide and what needs to be done to give it a long term resistance to breaking down and to stabilize the fibrous tissues.

The fibrous tissues of the hide are made up proteins called collagen. Collagen fibers are a major component to most tissues. It gives them tremendous tensile strength and elasticity. It is this part that makes up leather that provides it with longevity and durability that can’t be found in common fabrics and makes it perfect for use in furniture. The problem with collagen though is that if untreated it can become hard and inflexible making it almost impossible to work with. If it becomes wet again then a breakdown in the collagen occurs. This is just one of the many issues that tanners face when creating leather and that is why they must chemically transform the hides wet protein state and avoid hydrolysis where groups of molecules detach from their chains and fail to re-attach. This process most often occurs during the processing of the hide during the pre-tanning stages and tanneries refer to this as "loss of hide substance" which is something they strive to minimize without interfering with the normal processing of the hides.

Introducing the tanning agent

During this stage an agent is introduced into the hide to primarily do two things. The first is to help the hide retain mobility and flexibility after drying by inhibiting cohesion. The second is to change the chemical structure of the proteins and eliminate hydrolysis as mentioned above in this article. There are a variety of substances and agents that can be introduced into the hide to help with these two things and they are referred to as tans. There are a number of specific tans that are used in influencing the final properties of leather and they are as follow.

Vegetable Tans

Extracts from vegetable matter, tree bark and other plant species have been shown to have a tanning effect on hides. These extracts contain molecules which displace the water from proteins and take up many of the hydrogen bonds. These contain acids which bond with the proteins removing the amount of water. From a cellular standpoint these molecules are able to fill the spaces within the cell structure . Vegetable tans also contain components or extracts that aren’t considered tans. These wash out easily and do provide an important part in the properties of the leather when finished.

Synthetic Tans

Also referred to as syntans. This was an attempt by the chemical field to introduce an agent that copies natural vegetable tans. Synthetic tans were able to produce stable leathers. Unfortunately though there were unable to provide the desired hand that vegetable tanning created. However the category has grown to include a wide range or products and is used in combination with other tanning materials as a modifier. It is also interesting to note that a use for syntans is in the creation of white leather. A color not often possible using vegetable tans.

Mineral Tans

Compounds used heavily in third world countries which includes chromium salts. The use of chromium salts has gone on for a good 30-40 years and still remains a widespread compound in leather tanning today. This compound however has an environmental impact in that it may pose a threat to workers in leather tanneries as it has been linked to carcinogenic issues and it is that reason why there is pressure on developing countries to replace it with other compounds. Mineral tans can also make use of aluminum and iron compounds which are safer to work with. The reason for using these compounds is that they offer quite a bit of stability to protein structures and can form complex chemical links within the hide. Mineral tans offer a gradual change in the chemical bonds and is a very slow process in tanning. Lighter shades are possible using this process but does not offer the same hand as vegetable tanned leather.

Aldehyde Tans

Tanning using glutaraldehyde (a colorless chemical liquid) and oxazolidine (formaldehyde alternative) compounds are used commercially and act as a preservative that is used in many medical fields as well for preserving tissue samples. The chemical bond created forms a very strong bond but results in leather that is rather thin and empty. The advantages to using aldehyde tans is that they are highly resistant to washing out of the hide.

In this article we examined the reasons tanning is used on hides bringing us closer to finished leather. Tanning is used to help create long term resistance and strength in the hide and helps to maintain its natural tensile strength and elasticity. We took a look at several different compounds used in tanning which included vegetable tans, synthetic tans, mineral tans and aldehyde tans. In our next article we’ll move into the later stages of tanning and examine this process as it continues.

About the author

Phil Pendleton is a furniture professional with over 15 years experience. He has been involved in furniture sales, production and distribution. He continues to work with other furniture professionals to help promote new trends in leather furniture.