In the early years of the 19th century it became apparent in many countries that demand for leather was outstripping local supplies of the necessary natural tanning materials.
Tanners accordingly looked for supplementary tanning materials from outside their own boundaries. As a result, Valonea was important from Greece, Turkey and Asia Minor, Myrabolam nuts from India and various other materials from around the world. This slow replacement of indigenous material by imported vegetable tanning agents gathered momentum throughout the 19th century.
This period also coincided with a growth in industrialization and urbanization in places like the USA and Europe. As a consequence, in some cases tanners found it easier to move the hides to the forest where the tannins were rather than bring the bark into tanneries in the town. This was most noticeable in the famous Swamp area of New York – between Wall Street and what was to become Brooklyn Bridge – where tanning began early in the 17th century. These tanneries moved first to the Catskill Mountains and then to the Adirondacks where hemlock was available. In the USA ownership of forests become important assets for some vegetable tanners, and in particular the United States Leather Company which was founded in 1893 when 80 tanning companies decide to unite into one business to fight the abattoirs for lower hide prices. It was the largest company in the US at the time of its formation and became one of the initial founding companies of the Dow Jones Ave.
At about this time chestnut extract was coming into use from France and in Italy for the European market whilst a similar extract was being made in the United States from native chestnut trees. Shortly afterwards, South American Quebracho wood – a comparatively newly discovered material with a high tannin content -appeared on the market. The leaching of the wood in the tannery presented problems and soon Quebracho extract when exported to North America was found to be lower in cost per unit of tan than the local Hemlock and Oak Bark which were fast diminishing in supply. A little later, Mimosa bark became available to tanners. Originally a tree native to Australia, it was introduced into Africa where growing conditions were more favourable for its success. In view of the fact that the bark was ready for stripping from the trees in a matter of seven to ten years after planting it readily lent itself to schemes of rotational planting and cropping.
The introduction of these and other exotic tanning materials complicated the previous simple, one material tannages, for though it was not fully appreciated at the time these “new” materials had quite a different tanning properties. Oak bark has quite a low tannin content and liquors made from it are very weak. Consequently, tannage with it is very slow but, it has a great inherent advantage in being a mixed tanning material – both catechol and pyrogallol types being present – and is thus very effective as a material used alone. This results in a well-balanced established tannage which is successful and requires very little technical control.
The craft of the tanners of the day, however, duly surmounted these difficulties and as a result leathers were made and described as “mixed tannages”. Blending of various tanning materials enabled leathers of quite different characteristics to be made by taking advantage of the varying properties of different materials.
Of prime importance was the fact that the introduction of tanning extracts made from such materials as Chestnut, Quebracho, Mimosa, Valonea etc. all made from materials having a far higher tan content than Oak bark, enabled tanners to obtain liquors of considerably higher gravity. This fact considerably shortened the time necessary to complete the tannage of hide.
The tenants of the Middle Ages had produced their leather mainly by stacking their hides in pits in the ground with Oak bark chippings packed between and around the goods, filling up with water and allowing tanning to take place extremely slowly as tan was dissolved out of the bark and taken up by the pelt. Over a long period the system gradually changed to one where the Oak bark or other local tanning material was leached with water in a large pit to produce a tan liquor in which the height could be steeped. The strongest liquor obtainable by this method was only of the order of 7°Bé (50°Bk), and the necessary tanning fixation was obtained by laying away for long periods of time the liquor tanned hides in fresh tan liquors with crushed raw tanning material strewn between the goods. How important the time factor was considered is shown by the stringent regulations of the old craft Guilds observed in those days. The sale of heavy leather tanned in less than one year was heavily penalised.
Leaching of the bark, as carried out at the time, was crude in the light of modern knowledge. Extraction of tan was far from complete, the liquors were weak, and due to the length of time spent in leaching, severe tan losses were incurred due to micro-biological effects. The liquors were infested with yeasts and moulds so that further losses were experienced during the long tanyard process. Low extraction efficiency has a further disadvantage from the tannery point of view in that soluble non-tanning substances in the material tend to leach out far more rapidly than the tannins, unless conditions are rigidly controlled. As a result, a more mellow or less astringent liquor is obtained than would be the case in a modern extract factory where leaching is a closely supervised physico-chemical operation. Another drawback was the labour requirement in filling, emptying, and disposing of the exhausted tan spents, even in those days when labour was far less expensive than now.
At about the time that extracts were first introduced into the tanning industry (1860-1870) the typical sole leather tonnage had developed from the old single material system to a “mixed tannage” of several different vegetable tanning materials. In essence, this tannage was usually counter current, consisting of a series of “suspender” pits where the commencement of tanning began for partially delimed pelt. Then would follow a period in a “handler” pit where the goods would be frequently taken out and put back and then stacked away in pits for a further period for “dusting” and ‘layering”. The whole process took a minimum of five months and the strongest liquor would be only 7-0°Bé (50°Bk). Liquor running with this tannage was relatively simple as the liquor from the emptied “layers” was fed back to the “dusters”, the liquor from which passed down to the “handlers” aand from there to the “suspenders”. Here it was virtually depleted and run to thee sewer.
As would be expected, the advent of extracts received a very mixed reception from tanners – reputedly one of the most conservative fraternities – ranging from amusement to plain scepticism, but the new products quickly proved their worth and the demand for extracts began to increase rapidly. It was at this period that leather chemistry began to emerge in an industry which had up to that time been very much a craft. These early chemists and technicians, pioneers of a new science, in their early investigations – rather crude by modern standards – found among other facts that given weight of hide substance required a given amount of tannin to convert it into leather. This theoretical amount of tan was found to be far less than the actual usage of tan than the material employed by a typical sole yard using the conventional materials of the day, such as Oak bark, Chestnut wood or Hemlock bark leached in the tannery. In general, twice as much tannin was being purchased as was necessary according to the chemists’ calculations.
Naturally, attempts were made to explain the discrepancy and it was soon found that the losses incurred by the long duration of heavy leather tannage were largely to blame. Goods were being left far too long a time in liquor where fermentation and other micro-biological actions were breaking down the tannin into non-tanning substances and insoluble sludge. Another weakness of the conventional “material” tannage was shown to lie in the comparatively low gravity liquors which the tanner produced by leaching ground or crushed material in his own yard.
Experiments showed that the leaching systems used by the new extract factories were far superior to those employed in the tannery. It was found that strict control of temperature, duration and other conditions permitted better extraction yields than could be achieved in the tan yard and moreover yielding much more consistent quality products.
Tanning being mainly a diffusion process, the speed of penetration depends largely on the difference between the external and internal concentration of tan. Therefore, if the strength of the tanyard liquors could be raised by using liquors made from extracts instead of leach liquors, the time necessary to complete tannage could be substantially shortened, with consequent reduction in tannin degradation and sludge formation. As the result of such a change, there would be a better economy in tan consumption.
There were other advantages to be gained from changing from material to extract tannage, of a more commercial nature. Tanneries had also had to bear considerable labour expenses in the handling, grinding or crushing, leaching, etc. of imported material. Frequent pit cleaning was necessary and another problem was the disposal of “spents”. The drying down of spent tan to a moisture level at which it could be disposed of by burning was no easy task and could mean the consumption of quite large amounts of fuel or the installation ofequipment entailing much capital outlay.
Extract manufacturers produce their tanning extracts under reduced pressure or vacuum – at a lower temperature – and thus avoid the colour deterioration which arises from the combination of heat and oxidation if evaporation is carried out in open vessels, so that liquors made up from extracts suitably diluted with water invariably produce liquors of far better colour than the tanner could make by leaching bark or wood himself.
Originally, extract factories were located in the vicinity of the main world tanning centres, which at the time were mainly Europe and the USA. Chestnut being indigenous to these areas was the first to be used. Ouebracho logs were shipped from the River Plate to Germany and the USA to be extracted at destination, the tannery receiving the extract in liquid form in casks or railcar loads. Dry Mimosa bark was mostly extracted by the tanneries themselves.
Logically, the next step was for the liquid extracts to be further concentrated into solid products containing 15-20% moisture. Soon extract factories were erected at origin and solid extract gradually replaced both liquid extract and tanning materials. After afew decades the process went a step further and powdered extract appeared made by spray drying the strong leach liquor under ideal conditions in the extract factory. These powders contained even less moisture, mainly in range of 5/8% and by reason of the method of preparation are much more easily dissolved in the tannery, and give even better coloured liquors.
At first, extracts were used as supplements to the “mixed material” tannages described earlier. They were mainly employed to strengthen the basic leached liquor and the “layers” – or final liquors – became stronger and stronger as confidence grew.
As a result of these changes the duration of tannage was progressively reduced as it became proven that the speed of penetration was being considerably accelerated by the use of stronger liquors. At this time tanners began to appreciate that the mechanical effects of drumming could be advantageously employed to hasten tannage, and that the extracts now available were ideal for this type of production.
Nowadays, a wholly pit tannage of sole leather, where all the tanning material used is in the form of extracts can take no more than 15-20 days. This reduction in tanning time has largely come about by extract usage coupled with control of other factors such as temperature acidity, salt content, etc. of the liquors. The bogey of time has been reduced to a minimum. But wholly pit tannages are in the minority today, and most systems employ either pits and drums or drums only. With these methods, tanning time has been still further reduced, as a satisfactory pit and drum tannage can be completed in a week and a full drum tannage, with either a short float or no float at all, finished in a matter of two or three days.
Extracts themselves have changed considerably since their original introduction as concentrates. Quebracho, for example, was probably the first extract to be chemically treated during its manufacture. It was found that a carefully controlled treatment with sodium bisulphite, could improve the colour of the extract and increase the speed with which the tan could penetrate the pelt. Chestnut extract also has been the subject of much research and employment of better methods of extraction, has greatly improved the colour of early extracts. In addition, its tanning characteristics have been found amenable to adjustment with buffer salts and a “sweetened” Chestnut extract with quite different qualities, is now available, in addition to the “normal” type. Mimosa, the other main extract of the times, has also been improved in colour, not only by changes in extraction methods and the use of fresh green bark as opposed to dry bark, but also by silvicultural selection of the variety of Acacia best suited to produce good coloured extracts and tan liquors.
It has already been stated that leather character is very much influenced by the material used for tanning it. As leather is required for a very wide range of uses, the correct selection of the tanning materials to be employed to produce a particular type of leather poses a problem to the tanner. Technology, however, has provided the answer and nowadays it is possible to adjust extracts to varying levels of acidity and buffer salt capacity to yield entirely different specific effects in the tanned leather. Extracts have made tannery production much more versatile.
By the middle of the 20th century, a typical process for tanners making sole leather was to use extracts in a counter-current system mainly with Chestnut, Mimosa and Quebracho. The limed pelt was first de-limed and initially tanned in a short suspender set, usually rockers, with as steep a concentrationgradient as possible between the liquor in one pit and the one next strongest. Acidity of the liquors was controlled and stabilised at a pre-set level by the addition of acid. The goods took a week to pass through the suspenders, then the pack was transferred to a set of pits where the liquor was continually circulated and maintained at summer temperature level. Again, there was as much difference as possible between the gravity of the liquor in the strongest suspender and that in the circulator. After three days, the leather was taken to a second circulator set where the liquor would be about 15°Bé (110°Bk), probably 3°Bé (25°Bk) stronger than the previous liquor.
Three or four days later the goods were given their final tanning in either hot liquors or a drum. If hot liquored they remained for 3-7 days depending mainly on the substance of the leather and its intended purpose, the pH and acidity of the liquor being maintained at a level determined mainly by the constitution of the extracts being used in the blend. The combination of heat and acidity without adequate buffering could be dangerous at too high a level. The extract
blend being employed was adjusted with an organic acid to the required acidity and pH, should the natural acidity of the blend be insufficient. If, on the other hand, drum finishing was preferred, the final operation in the tanyard would be a drumming stage with strong liquor or extract. This type of tannage has the advantage of a considerable degree of flexibility in that sole leather of varying character can be produced simultaneously by altering the hot pitting duration. Secondly, being counter current – i.e. hot pit or drum to circulators, to suspenders – the movement of liquor is simple, exhaustion of liquors is reasonably efficient, and tan losses minimised.
The duration of tannage was further shortened during the next two decades by the adoption of processes in which a short drumming stage using extracts was substituted for a much longer pit stage. Such tannages were developed further and proved particularly successful in Europe, notably in Santa Croce, Italy and in Latin America, by giving thepelt a pre-tannage in pits with either natural or synthetic tanning materials under strictly controlled conditions which ensured a very rapid tan penetration followed by a drumming with strong liquors made up from extracts as a main tannage. The process was simple and required only the minimum of tanyard equipment. The value of using appropriate extracts in drums after the correct preparation of the pelt was clearly demonstrated by the prosperity of the industry in the areas mentioned above, due in large part to the high productivity possible with limited plant and labour.
The production of extract in the form of spray powders has enabled the tanner to reduce his tanning time still further. These powdered extracts are generally used following the initial pre-tannage of the delimed pelt with rapid penetrative agents such as mineral salts or synthetic tannins in drums. The actual tannage with the spray dried extracts is also carried out in the drum – maybe the same drum as was used for pre-tannage – virtually without the float. Satisfactory leather can be, and is, made with such methods. An obvious advantage of such a tannage is the near disappearance of tanyard effluent, the disposal of which is usually a difficult problem for any tanner
In recent years the tanning industry, in common with most other manufacturing undertakings has been severely hit by inflation caused by rapidly rising labour and hide costs. In attempting to surmount these difficulties by reducing production costs, tanners have come to look upon tannages such as the Santa Croce system discussed above, as not sufficiently rapid for their purposes and are now looking to the latest all drum tannages, only needing 2-3 days to carry out the actual tanyard purposes. The capital tied up in the value of the hides in process and liquors in pits and drums is obviously far less and as these short all drum systems are in fact “batch” processes, the production flow can be halted, reduced or increased to take full advantage of market fluctuations in the cost of leather or hides.
It will be appreciated that the heavy leather industry, after centuries of near stagnation, has been re-vitalised over the past few decades by the scientific and efficient production of extracts of great versatility which, used in conjunction with auxiliary products and drumming equipment, enables leather of quality to be made in a matter of days not months.
Some tanneries have done well by making a feature of the traditional processes they use to make sole leather, equestrian leathers or others for a wide range of purposes. Others have been able to take advantage of the newly formulated vegetable products offering additional features, and of new approaches to processing to create new leathers with exciting properties.
A huge new use of vegetable tanning extracts is in tanneries using other tannages for their primary processing, but requiring the properties that can only come from vegetable extracts to give their leather the characteristics of touch and feel, appearance and smell that today’s customers require. The wonderful patina of vegetable tanned leather cannot be achieved by any substitute material.