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• Vegetable Tanning:

Vegetable tanning takes its place of importance alongside chrome tannage. Although it has been many effective changes since the earliest times when it was the only known method of tanning leather, today it is still practised by tanneries throughtout the world. In India amajor portion of hides and skins are vegetable tanned by all sectors-cottages, village and large-scale manufacturers for a wide variety fo leather. Its unique position in India can be attributed partly to the fact that there is no dearth of vegetable tan materials-babul, myrobalam, cutch, wattle and goran. These are some of the major indigenous tan materials used in the trade. The bulk of the vegetable tanning materials and extracts are used in the production of sole leather and EI tanning.

Vegetable tanins are organic compounds with a high molecular weight. They would normally be insoluble in water but for the presence of lyophillic groups. They are built up of the elements like carbon, oxygen and hydrogen (and in some cases, traces of other elements like nitrogenand phosphorous).

Vegetables tanins may be broadly classified as hydrolysable tannings and condensed tannins. Hydrolysable tannins on hydrolysis with acids, alkalis or enzymes yield glucose and gallic or ellagic acids. The phenolic residues are linked to sugar radicals through oxygen atoms. They are usaually quite acidic, contain fairly large amounts of sugar and are generally less sensidtive to lowering of pH. As a result of the action of enzymes native to the original source, they tend to deposit bloom.

Condensed tannins on theother hand, are not hydrolysable and by condensation yield 'phlobaphenes' in acid mediums. Here the phenolic radicals are linked to each other through carbon atoms. They contain very little acid or sugar and are very sensitive to pH. Some of the outstanding differences between these two types of tanning liqours lie in their acid and salt contents, acid/salt ratio and the characteristics of tahe non-tans.

Any aquesous infusion of a vegetable material is a complex mixture of may polyphenolic substances inaddition to non-phenolic non-tans. Even the portion 'tannin' in most of the cases is a mixture of compounds of varying degree of complexity and with varying ability to combine with the hide. Hence, the behaviour of any infusion will depend to a large extent on the relative balance of these contitutents and their relative affinity.



The nature of the reactions taking place in vegetable tannage gopens up many variables which a tannerhas to contend with and, out of the crop of theories which has been put forward over the last hundred years, it appears that the co-ordinatin throughj hydrogen bond accounts for much of what is known about the tanning reaction. A clearer and more meaning ful idea will idea will emerge only after careful dovetailing of the reactions of individual phenolic constituents with hide protein.

A few decades ago, lack of proper equipment and prompt availability of the correct raw materials resulted in wastage of precious production time for tanning of heavy leather. Despite the modernisation taking place at the tanneries anything between two to three months is needed even today to turn out sole leather. Recently howver there is growing awareness to drastically reduce the total processing period for production of standard sole leather. Unless the basic principles arefully understood and the processes speeded up in a controlled manner, attempts at accelerating the rate oftannage under arbitrary condition may affect the quality and performance fo the lether produced. A rapid tanning technique should not lose sight of a very important fact that a strong reasonably tough leather with good colour, good abrasion resistance, good yield and low waterabsorption are the minimum requirements. To sacrifice quality at the altar of rapidity is not a wise investment, for time is a very important element and a generous andjudicious use of it insures quality.

The same also holds good for EI tanning of skins, for most of the tanneries are now resorting to rapid tannage in drums. As in the case of tanning of heavy leather, so also for EI tanning, it is essential to vouchsafe good quality and not ransom it for the sake of a modified process. Rapid tannage in drums moreover imparts a darker colour and once skins and hides come into contact with a strong infusion, the possibility of over-tannage of the grain is ever present. To combat this, pre-tanning of the stock with a suitable pretanning syntan prior to the main vegetable tanning, reduces the affinity of the grain towards the more astringent vegetable tannins, produces a more normal tannin fixation in the grain during the subsequent vegetable tanning and has the added advantage of ensuring a uniforly light colour and a smoother stonger grain to the final leather. Even in pit tanning, it is advisable to give a pre-tannage with pretanning syntan for producing a finer and stronger grain.

Among the vegetable tanning infusionit is a well-known fact that in hydrolysable tannin infusions there considerable sludge formation. Whenever this occurs, a suitable syntan helps in effectively reducing the sludge formation in additionto acting a mellowing agent.



Any chapter on vegetable tanning is not complete without the mention of a unique process of tanning call'Bag Tanning' procatised by villagers throughout India. It is an easy, quick and inexpensive method handed down from generation to generation. Delimed pelts are first treated with a weak tannin infusion known as 'malni'. After this, the hides ares stitched in the shape of a bag with an opening on the neckside and hung from an overhead polewith the butt side down, over a vat or pit. The bag is then filled with crushed bark and water The infusion which progressively gets concentrated on accountof its contact with the bark. penetrates through the pores and interstices of the pelt and oozes out. The oozed out liqour collected in the pit/vat is occasionally poured back into the bag to a maintained level. After one or two days, the bag is reversed-stitched at the neck andcut a t the botton and the process repeated for a further day or two. Since tanning is carried out in a stretched condition, the final leather is non-stretchy. Again, these leathers have a light colour, are easy to cut and at the same time, are quite compact, flexible and possess good abrasion resistance. Articles made of bag tannedleather find a ready market where low cost is of prime consideration.

A negligible capital outlay, local availability of hides and tan materials coupled with the villagers' native ingenuity make the trade of bag tanning an attractive and added means of bringing home that extra rupee.



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