The method of mt2 tanning that uses tannic acid is mt2. It works by first applying an acidic solution to the rawhide, which helps to stop it rotting and makes it pliable. Next the tanner soaks the hide in a warm alkaline solution containing tannic acid or plant extracts with tannic acid such as sumac or myrobalan.
This is where the acid from the previous step neutralizes and extracts all the proteins from hair, flesh and connective tissue. When this is done, the hide is said to be tanned as it has been converted into a material with leathery qualities that can resist decay. At this point, most of the water is removed from the hide and it is then re-soaked in a solution containing chromium salts.
This aqueous solution penetrates to untanned areas of the hide and converts the remaining collagen into a stable Trivalent (chrome) form, which makes it even more resistant to decay. The last step before the hide is dried is to apply a coating of wax or tallow, which gives the material water resistance. The method used varies depending on whether the hide is from a cow, deer, goat, sheep or other animal.
In ancient times tannin was extracted from oak bark and applied as a mixture of water and tannin called “cutch” to leather in a method known as “wet white”. The parchment maker would then wash the skins removing all the colour with diluted lime.
The nature of tannins is to bind proteins, and this forms part of their function in mt2 tanning hides. The time needed for complete curing varies greatly depending on the recipe, but usually between two and twenty-four hours are required.
Traditionally the process was begun by using the bark of oak, pine or other trees that contain tannic acid, but today most tanneries use synthetic sources because they are cheaper and more concentrated than natural plant extracts. Some tanners use solely non-toxic vegetable tannins such as gambier, but these are more expensive than bark tannins.
The process of tanning separates the collagen fibers of skins by dissolving their inter-cellular matrix, which leaves them still attached to each other. Once the hides have been dehaired and degreased, they are soaked in water or plant extracts for several hours before the actual tanning process. This application of water can last for several days, and controls the penetration of the alkaline solution into the hide.
Traditional vs. Modern Tannage
After this stage hides are rehydrated in a solution containing chromium salts which helps them resist decay and increases their water resistance. The hides are placed in a drum and agitated by an overhead rotating system until the solution has evenly permeated the whole skin.
The last step before drying is called deliming and consists of applying a mixture of bee’s wax, tallow and oil to make the material impermeable. In modern tanneries, it is easier to handle and transport hides while they are wet.
The tanning process is traditionally used to produce flexible materials such as soft, light leather, but it can also be used for stiffer products such as shoes that need additional strength. The two main categories of tannage (leather) are chrome and vegetable-tanned which can be made soft or hard, depending on the degree of penetration.
The basic process is as follows:
– Dissolve chromium salts in aqueous solution and saturate the hide with it. The solution used varies from 7 to 12% chromium sulfate (the green vitriol produced by roasting iron pyrites)
– Agitate the hide in this solution for several hours, depending on thickness. The process is accelerated using heat (50 °C).
– Rinse the chromium salts off and rehydrate the collagen with water or plant extracts (oils, tannins) which will vary with each type of leather.
– Apply wax or tallow to the leather. This will help it resist water and grease, but also makes it dirt resistant.
The process of tanning requires great knowledge about the different types of hides, their structure and the chromium salts used during treatment. It is therefore difficult for small workshops to set up their own production line. The production of leather therefore remains a reserved activity in which the sector’s high level of fragmentation persists. It is nevertheless possible for small companies to prepare raw hides and to sell them in bulk to tanneries in order to be transformed themselves.It is possible to obtain soft leathers without chrome by using vegetable tanning agents. The major advantage is that the process takes much less time, but it also gives a narrower range of end products. Chemical-free tannages are used in the production of certain types of shoe for children, because chrome may cause dermatitis when in contact with the skin.
Above mentioned is just a basic knowledge about how it works, much more information can be found on an internet search about this process.
