Suppliers of raw crocodiles skins frequently ask whether it would not be more profitable to tan skins locally rather than export them in the wet salted state. This is a difficult question because circumstances vary from one producing country to another. It is appropriate at this point to give some idea of what is involved in tanning and finishing of crocodile skins. Production policy can then be considered further.
As mentioned in part 1 (2.6) the dermis (corium) of a crocodile skin contains interwoven bundles of fibres. The fibres consist of a protein called collagen which has a very high tensile strength. It is collagen, for example, which makes tendons so strong.
The tanning process replaces water in the skin with a tanning agent (tan) that combines with and coats the collagen fibres making them resistant to heat and decay.
Different tanning agents produce different qualities in the leather. The characteristics can be further modified by finishing to the required thickness, moisture content, oil content, suppleness and appearance.
In its simplest form leather may be man's earliest manufactured material but modern leather manufacture, using a wide range of natural and synthetic tans and an ever increasing variety of special treatments, has become a complex technology.
The basic operations are common to all kinds of leather manufacture but the tanning and finishing of reptile skins is so specialized that a conventional tannery is not equipped for it and a reptile skin tannery can not be used for other purposes without modification.
The processes listed below have been described in some detail by Fuchs (1975) though, as that author points out, the exact procedures, equipment and chemicals used are subject to change and variation between tanneries.
The beamhouse is that part of the tannery where skins are prepared for tanning. Crocodile skins are given the following treatments:
The purposes of soaking are to clean the skins and remove salt and preservative, to allow the skin to re-absorb some of its lost water and to dissolve out some soluble proteins. Inadequate soaking can cause problems in liming (5.1.3) but too much soaking can rob the skin of desirable substance.
Skins intended for a classic finish may be soaked in pits but ossified skins are soaked in 'paddles' - i.e. vats with a power-driven rotating paddle. To ensure uniform soaking skins of similar size and type are treated in batches.
This is a continuation of the scraping or fleshing that was done by hand after flaying. In the tannery it is completed on a shaving machine. It has the effect of stretching the skin as well as cleaning the flesh side.
In pits, paddles or sometimes rotating drums the skins are treated with chemicals which dissolve away the scales and epidermis, convert skin fats into soaps, open up the skin structure and make dark pigments in the skin soluble in preparation for bleaching. Slaked lime may be used in temperate climates but at higher temperatures sodium sulphide has a more important role and lime might not be used at all. Other chemicals such as common salt and degreasing agents may be included. The process lasts for about two days.
This is essentially a partial neutralization of the skins which become strongly alkaline during liming. Deliming can also affect the condition of the skin - particularly the state of swelling of the fibres. It is usually done with ammonium salts and lasts about 12-14 hours. Agitation of the skins (as occurs in the revolving drum or, to a lesser extent, in the paddle) speeds up the process but the skins at this stage are very sensitive to abrasion so occasional paddling is the only agitation given.
Skins are left in the bath after deliming and 'bated' which gives them the required degree of softness, pliability and elasticity. To achieve this enzymes are used which act upon the skin proteins. Classic crocodile skins require very little bating (a matter of minutes) as they must remain firm enough for a glazed, classic finish.
Caiman skins, which cannot be given a classic finish because of the osteoderms need much more bating.
Traditionally, bleaching is done in a revolving drum in two stages. A solution of potassium permanganate is used in the first stage and sodium bisulphate in the second. Each process takes no more than an hour. Fuchs (1975) considers that an oxidative (chloride) bleach is preferable for crocodile skins as it does not affect the collagen fibre structure and permits a higher tear-strength in the leather.
This is an acid bath treatment in which skins are prepared for bleaching and tanning with synthetic tans (syntans) or mineral tans. Classic skins can be pickled in a pit for about two days but ossified skins undergo a special pickling to remove the bone deposits as much as possible. This process takes several days and is best done with the skins suspended in the acid bath and given gentle agitation. Hydrochloric acid is generally used for crocodile skins together with salts and sometimes other acids.
Pickled skin can be stored for months - especially if preservatives have been added to the pickling solution.
Tanning with natural tans obtained from plants is still important in the leather industry. Vegetable tans are simple to use and give soft, full leathers. With crocodile skins, however, better results are obtained by using chrome salts. Chrome tannage can be more accurately controlled so as to give more uniform results from one batch to the next and the leather has better physical properties.
Chrome tanning is carried out in drums and takes two or three days. By the end of the process the skins have been converted into a pale blue wet chrome leather. At this stage they are known in the trade as 'wet blue stock' and are chemically more stable than pickled skins. Wet blue stock can be drained of surplus moisture and stored in plastic sheeting for many months.
A special shaving machine is used to shave the tanned leather to a uniform thickness. Small skins of less than 40 cm belly width are eventually shaved to a thickness of less than 1 mm.
After shaving a neutralizing or 'sweetening' process completes chrome tannage. A solution of sodium bicarbonate is used and the skins are treated in a drum for about an hour. Following tanning, however, they may be left on a rack ('horse') for a few days in the damp state. This is said to increase the strength of the bond between the chrome tanning agent and the corium fibres.
Retannage is carried out with a combination of vegetable and synthetic tanning materials. Retannage with vegetable tans alone is said to be unsuitable for crocodile skins as it impairs the tensile strength and makes them more difficult to dye. At the end of retannage the leather is light in colour. It is a relatively short process involving about one and a half hours in the drum.
Traditionally a variety of oils and fats are used in the lubrication of leather. For crocodile skins synthetic fatliquors are used which penetrate the leather during an hour or so in the drum.
Drying is an important process in which the moisture must be removed evenly and not in patches. When dry the leathers are known as 'crust stock' and are sorted in preparation for dyeing and finishing.
Dyeing involves a number of processes. First the dry crust leathers are 'wetted back' by soaking with a suitable wetting agent. Then they are given an appropriate treatment to prepare them chemically for the type of dye to be used. Next they are fatliquored again. All this is done in a drum and takes a few hours. Dyeing, with proper attention to evenness, saturation, light-fastness, colour matching etc. is a very skilled operation.
Again, this involves several processes and there are almost as many types of finish for reptile leathers as there are types of reptile leather. Indeed, there are firms who specialize in contract finishing for tanneries.
After dyeing the leathers may be shaved on a special dry-shaving machine. On classic skins this is part of the process of giving a raised or domed scale pattern (bombé effect). Ossified skins have to be dry-shaved with special care to avoid tearing out what remains of the osteoderms. Heavily ossified skins of caimans cannot be dry-shaved at all.
The high gloss classic finish is obtained by using coatings of natural protein products (e.g. from blood or milk) together with synthetic seasonings which are then fixed so that they become permanently hard and waterproof. Finally they are glazed on a special glazing machine.
Caiman and other heavily ossified skins are best processed into a soft finish (softy croco) by polishing rather than coating the drum-dyed leather.
Reasonably good leather can be made by amateurs but it will be apparent that making the best leather from crocodile skin is a skilled and lengthy operation. The basic processes listed above are often separated by routine treatments such as rinsing, washing, draining or partial drying. Processes may be repeated and exceptional treatments may be used. Needless to say the raw material must be worthy of such an investment in skill and labour. Inferior materials will be tanned and finished as appropriate.
In principle there is no reason why producing countries should not process their own skins, indeed some of them do. Obviously, a great deal of expertise is required and this may have to be imported as part of a training programme. A tropical climate is a disadvantage but it can be overcome.
The question is not whether it can be done but whether it is financially worth doing. The answer depends largely upon the intended market and the supply of raw skins.
An expensive handbag in a Paris boutique is a superbly crafted article in top quality leather. It is also a highly fashionable item. The bag manufacturer is concerned that it shall be so. He is also concerned, when he buys his leather, that he can place repeat orders for an identical product or order different colours and finishes at short notice - perhaps over the telephone. It would probably be impossible for a developing country to break into this small market for high class, finished crocodile leather as long as the manufacturer has a choice of supplier.
At the other extreme, poorly made goods from inferior leather are to be seen in many parts of the world. Tourists buy them together with such curios as ash-trays made from crodocile feet, stuffed baby crocodiles or mounted crocodile heads. Between these extremes there is a range of quality and prices both in leather and craftsmanship. It may be that a skin-producing country could find a niche somewhere in the market for tanned and finished skins which would give a better return than selling wet-salted skins to the main tanneries. But the market would need to be thoroughly researched.
The species of crocodiles involved largely determines the value of the raw material for the world of fashion but it is far less important for 'down market' products.
Should it be decided that skins are to be exported for tanning the question of a compromise arises. Would it not be more profitable to the producer to do part of the processing but stop short of dyeing and finishing so that the mainstream tanner still has his options open?
This is a distinct possibility. The pre-tanning operations pose expensive problems of effluent disposal in densely populated cities. There is consequently an advantage to the tanner if the beamhouse operations have been done elsewhere.
However, largely because of the diminishing supply of skins, the number of reptile tanneries has contracted to a few well-established firms. These, in one way or another, seem to have come to terms with the pollution problem and strongly prefer to buy their raw material in the wet salted state. It is a situation that could change if as a result of wise management of the resource an increased supply of skins can be sustained and tanning activities expand.
The preference for wet-salted skins is not difficult to understand. In grading a salted skin the tanner/buyer is assessing its potential as leather. The state of preservation can be inspected but this is not the case with partly processed skins. In the words of one buyer 'I like to see what I am buying'.
In addition it is a fact.that certain pre-tanning processes have an influence upon subsequent treatments. For this reason a tanner prefers to be in control of every operation from start to finish. If he must start with partially processed skins then the less that has been done to them the better he likes it.
There are three stages at which skins can conveniently be packed and exported.
i. Pickled (5.1.7). Wet and packed in plastic-lined containers.
ii. Chrome tanned (wet blue stock 5.2.1). Still damp and packed in plastic-lined
containers.
iii. Crust tanned (5.2.6). Dry, light and packed in protective containers.
Most tanners would probably be prepared to try pickled skins and provided that the liming (5.1.3) and other processes had been carried out to their satisfaction would be prepared to pay more, commensurate with the reduction in their own tanning costs.
The next question is whether pickling would be profitable to the skin producer.
A small pickling plant for up to 10.000 skins a year requires only a modest investment in plant and machinery. A building of no more than 150 square metres would house the entire facility.
A supply of clean water is essential and is needed at a rate of about 30-40 litres per kilogramme of wet salted skins (A skin of 40 cm belly width weighs about 2 kg).
The electricity used would amount to something in the order of 240 KWh/day depending on the capacity of the drums and paddles used.
Drums and paddles could probably be made locally as could soaking vats. The drums are
constructed in wood with cast iron frames. An electric motor supplies the rotating power.
A flesing machine would probably have to be imported and would be the most expensive item
of equipment.
In total not more than 12 people would be exployed at such a plant.
Effluent disposal could be managed with a series of small pits in which brine could be evaporated and sludge could be settled and treated.
Financial projections for such an enterprise would have to include not only local costs but the availability and cost of expertise at least for the establishment period and first year of operation.
Predicted market prices for pickled skins could be difficult to determine in view of the stated preference of leading tanners for wet salted material.
The extended storage life attained by pickling could be a significant advantage to the producer, offering him more flexibility in negotiating his sales.
A fundamental concern would be the supply of raw skins. Seasonal peaks and lows are not important provided storage facilities are adequate but a short fall over a longer period could result in a small plant lying totally idle for part of the year. This could more than negate any gains from the pickling.
It is evident that a crocodile pickling plant could easily be incorporated into a conventional tannery and this would probably present the safest position from which to test the economic viability of pickling classic skins for export.
Similarly, if less valuable skins are to be tanned and finished for direct sale to other than the high fashion manufacturers, a multi-purpose tannery could be specifically designed. This would provide a broader base for the investment and more flexibility for finding the optimum scale of operations - something which might not-be possible with reptile skins alone.
