Volume 3 - Spring 1997


The Manufacture of Leather - part 3

by William McLean

The previous instalment of The Manufacture of Leather covered the various stages of processing which take place in preparation for tannage. These preparatory operations are mainly concerned with the removal of unwanted components to leave the structural material which gives the skin its strength. The next step is to stabilise this fibrous residue so as to give it a long term resistance to further breakdown. This tanning stage is the crux of leather-making and has given its name to the entire process.

As mentioned in Part 2, the fibrous tissue is made up of a protein called "collagen". At a molecular level the protein consists of three amino acid chains wound together in a tri-helical formation. The molecules have a tendency to line up end to end and to aggregate into larger macro-molecules called "fibrils" and these, in turn, join up and twist together with others to form fibres and ultimately the fibre bundles which can be seen by the naked eye. The unique physical properties of leather come from the way in which these fibre bundles are woven together into a three dimensional network which extends throughout the bulk of the skin.

The snag is that untreated collagen is susceptible to putrefaction when wet and although this decay can be halted by drying, the resultant material is hard and inflexible. Furthermore, if the pelt is allowed to become wet again the breakdown will continue. The fundamental job of the tanner, then, is to transform the pelt into a durable leather which can dry out whilst remaining flexible and which can undergo repeated wettings and dryings without further biological decay.

In order to understand what these changes are we must look more closely at the untreated pelt in both the wet and dry states:

A large proportion of the water content of wet skin is present within the spaces around and between the fibres. Removal of this water during initial drying has little effect. The remainder, however, which is approximately one third of the (dry) weight of the collagen, is very closely associated with the collagen molecules which are said to be fully hydrated. The bound water forms a "sheath" around the protein chains and, in effect, keeps them separate from adjacent chains. This water is attracted to the protein by weak inter-molecular forces and as drying continues the protein structure collapses to the point where the same forces begin to act between adjacent collagen molecules, in effect, gluing them together. This force gives rise to a mechanism known as "hydrogen bonding" and is due to the fact that there is a subtle variation of charge along the protein chain with an attractive force between areas of opposing charge. These bonds are fairly easily broken by the re-introduction of water.

There is also the possibility of much stronger chemical bonds forming if appropriate sites on adjacent chains come together. Generally, this sort of cross-linking is much less easily broken which can lead to a degree of irreversibility during drying and re-wetting.

In the wet state protein is susceptible to a process known as hydrolysis - molecules or groups of molecules become detached from the chains and fail to reattach. In other words, the protein gradually dissolves. The process may be accelerated by the presence of certain enzymes or other chemicals but will still occur even in their absence ("auto-hydrolysis"). Incidentally, the effects of hydrolysis are much more serious when dealing with the skins of younger animals (e.g. calfskins) as the bonds holding the individual molecules together are significantly weaker than those in older more mature skins. It should be noted that hydrolysis takes place throughout the pre-tanning stages especially during unhairing, liming and bating. Tanners refer to this as "loss of hide-substance" and strive to minimise it without compromising the benefits of these processes.

In essence, then, the primary purpose of a tanning agent is to:-

  • Inhibit cohesion at a molecular level so as to retain mobility and flexibility after drying.
  • Modify the chemical structure of the protein to make it less soluble and hence lessen or eliminate the effect of hydrolysis of the resultant leather.

There are a great many substances which are able to interact with pelt to bring about these requirements to varying extent. They fall into a number of broad categories, as follows:-

Vegetable Tans

Extracts from many hundreds of plant species have been shown to have a tanning effect. They contain large poly-phenolic molecules which act by displacing the bound water from the protein and taking up many of the exposed hydrogen bonding sites. These molecules have acidic groups on them which are attracted to, and can form stronger bonds with, the basic side-groups on the protein1, further reducing the bound water. The size of these molecules is important in that they are able to fill many of the voids and spaces within the structure. The extracts also contain many other components referred to as "non-tans" which are more easily washed out but which have an important influence on the final properties of the leather.

Synthetic Tans ("Syntans")

Originally these were the results of the chemical industry's attempts to copy naturally occurring vegetable tans. They produced a stable leather but rarely gave the fullness and desirable handle of a genuine vegetable tannage. The category has now grown to include a huge variety of products with widely varying properties. Some of them are used as sole tanning agents, for example in the production of white leather, but mostly they are now used as modifiers in combination with other tanning materials.

Mineral Tans

These include compounds of chromium, zirconium, aluminium and iron. (The use of chromium salts has dominated the tanning industry in recent decades and this remains as the most widespread method of tanning today although there is growing pressure in the developed countries for replacement by materials with a lower environmental impact. The commercial benefit of the use of chromium salts is such that any change will be gradual.) Mineral tans act in a different way to vegetable and traditional syntans in that the fixation takes place with the acidic side-groups and involves strong dative or co-ordinate bonds. Furthermore, the molecular size of these tans is, initially, much smaller giving rise to significantly less filling of the leather. The principle feature of the tanning compounds is that they are able to link together into much larger complexes which can grow sufficiently to bridge between adjacent protein molecules giving a high degree of stability to the structure. The importance and predominance of chromium stems from its ability to form these large complexes.

Aluminium does not form the large complexes nearly as readily as chromium. Furthermore, at the higher pH values which favour good fixation with the protein, aluminium salts have a tendency to be sparingly soluble and hence to precipitate out of solution. The result is that a traditional alum-tawed leather is not particularly stable and the aluminium salt is easily washed out. The fixation does improve with time but this is a slow process.


Formaldehyde and glutaraldehyde are used commercially, and readers will be familiar with the use of formaldehyde as a preservative for tissue samples. These materials react to form very strong bonds with the un-ionised basic side-groups and have a tendency to polymerise under alkaline conditions to create large molecules which are able to cross-link protein chains. The resultant leather is rather thin and empty but has an extremely high resistance to washing out of the tanning material. Chamois leather production which involves the treatment of pelt with raw cod oil is also an instance of this type of tannage, since complex aldehydes are created by oxidation of the oil during the tanning process.

There are many other materials which have some tanning properties but which are not used in commercial leather production, generally because they are too expensive or are hazardous to handle.


Different amino acids share the ability to link up with each other by the formation of a peptide group between molecules. This joining involves a condensation reaction and creates a very strong covalent bond. One might visualise the resultant long chain molecule as having a central poly-peptide core with a variety of side chains (the "bodies" of the individual amino acid molecules). The side chains or groups exhibit quite different properties. For example, some show acidic behaviour and become ionised (negatively charged) under alkaline conditions while others show the opposite effect. In addition to these ionic properties, certain of the side groups and, in particular, the peptide group itself have a marked unevenness in the electron density distribution. This gives rise to the charge variations which favour hydrogen bonding.

The next article will look at vegetable tanning materials and combination tannages in more detail plus methods of application and how these can affect the final properties of the leather.


Skin Deep - Volume 3 - Spring 1997

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