All tannages have for their object the conversion of the readily putrescible hide tissue of the corium (the pelt) into an imputrescible, insoluble and permanent material called "leather" which, possessing considerable strength and pliability, is capable of application to a variety of useful purposes. The conditions necessary for this transformation have been clearly stated by Procter.[1]For the production of leather from pelt "it is not only necessary to dry the fibres in a separate and non-adherent condition, but so to coat them or alter their chemical character that they are no longer capable of being swelled or rendered sticky by water." Whatever substance will secure this permanent dehydration of the hide fibres in a separate condition is called a "tanning material." The change from pelt to leather is known as "tannage," the process is termed "tanning," and those who undertake it are "tanners."
[1]"Principles of Leather Manufacture," p. 184.
[1]"Principles of Leather Manufacture," p. 184.
In "vegetable tannage" the tanning materials are of vegetable origin, and contain a group of organic compounds called "tannins" which are extracted by the infusion of these materials with water. Pelt, when immersed in these infusions, is converted into leather, rather slowly; but a gelatin solution gives an immediate precipitate of "amorphous leather," even if the tannin infusion be exceedingly dilute. The tannins are aromatic compounds of phenolic character, and contain carbon, hydrogen and oxygen only, but our knowledge of their chemical constitution is exceedingly small owing to their instability and colloid nature, which make impossible their preparation in a pure state. They are all, however, derived from either catechol or pyrogallol, and yield these substances if carefully heated to about 200° C. The tannins aresoluble in water, alcohol, acetone, ethyl acetate and acetic acid, but insoluble in benzene, chloroform, carbon disulphide, petroleum ether, dilute sulphuric acid andpureethyl ether. The aqueous infusions of the tannins are in reality colloidal solutions;i.e.heterogeneous systems of two phases. The systems are lyophile, or, more particularly, hydrophile,i.e.there is an affinity between the two phases. As usual with lyophile systems the two phases may be considered as both liquid, and an aqueous infusion of tannin forms an emulsoid sol, which therefore is subject to the phenomenon of adsorption. The tannins are all precipitated by solutions of basic lead acetate and copper acetate, and many of them with varying completeness by solutions of many other metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They give dark colorations with ferric salts.
The tannins are widely distributed in plant-life, but only in a limited number of cases do the plants contain sufficient tannin to render them of commercial importance. Tannin is found in all parts of plants, but usually in greatest amount in the bark or fruit. The tannins are classified into "pyrogallol tans" and "catechol tans," according to the parent phenol. This classification is confirmed by their chemical, analytical and practical behaviour, and the vegetable tanning materials may be classified into the same two groups, for, although even the same plant contains both pyrogallol and catechol tans, it is usual to find in any one part of the plant that one group is predominant.
Pyrogallol tans, which are oftenest obtained from fruit or leaves, contain usually about 52 per cent. of carbon. Used alone they produce a rather soft and porous leather. Associated with them—in many cases probably as decomposition products—are certain other substances of well-known properties and constitution. These substances are not only typical of the group, but also form the most valuable clue to the chemical constitution of the group and the key to their chemical behaviour. One of these substances is gallic acid (3:4:5trihydroxy-benzoic acid C6H2(OH)3COOH), which possesses properties very similar to the tannins, but does not precipitate gelatin and will not itself make leather. Another of these substances is ellagic acid C14H6O8, a double lactone of a hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an insoluble yellow powder from infusions of many pyrogallol tans, by boiling with dilute acids only, allowing them to stand for a few days. In practice the deposit is found as mud at the bottom of the tan pits, and also upon the leather, to which it strongly adheres. It is technically known as "bloom." It is insoluble in acids and cold alcohol, but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans yield very different amounts of bloom. Other associated substances are the sugars. In practice these sugars ferment to lactic, acetic, and other acids which cause "sour" liquors. Such liquors plump the hides and tend to give firm, thick leather. These acids also probably cause increase of adsorption of tannin by the hide and therefore assist in giving "good weight." Solutions of pyrogallol tans all give a blue-black colour with a dilute solution of ferric alum. If a solution of sodium arsenate be added to an infusion of pyrogallol tan diluted until no longer distinctly coloured, and the mixture allowed to stand for about two hours, a green colour develops at the surface of the liquid. The reaction is due to gallic acid or a similar grouping, and is, in the author's experience, the most satisfactory qualitative test for the group. Another test is to mix equal volumes of a 0.4 per cent. infusion of tan and a 10 per cent. solution of sodium bisulphite; a few drops of 10 per cent. potassium chromate are added, and either a transient blood-red colour or a more permanent deep purple is obtained. The former colour is due to gallic acid. If a tannin infusion be largely diluted with hard water and a little iodine solution added, the pyrogallol tans yield either a purple-red or a dark blue colour, the former being a reaction of gallic acid. Pyrogallol tans yield no precipitate with bromine water. They yield a yellow or brown colour when one drop of infusion is added to concentrated sulphuric acid.
Myrabolans is one of the most important of the pyrogallol tanning materials. It is a name given to the dried fruit ofTerminalia chebulaand other species of Indian trees. The nuts resemble an elongated walnut. They are dried and exported from many parts of India to all parts of the world, but largely to this country. The varieties of commerce are named according to origin and quality: thus we have "J1's,"i.e.Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1), "B1's" (Bhimley, No. 1), etc. The little difference in tannin strength (about 32 per cent.) in these varieties is usually compensated by corresponding differences in price. The quality of myrabolans cannot be safely judged by appearance. Much bloom is deposited by myrabolans liquors, especially by "J's." Myrabolans are amongst the most sugary of tanning materials, containing up to 5½ per cent. It is therefore one of the best materials for giving a plump leather. Broadly speaking, those varieties which yield most sugar yield least bloom, andvice versâ. Myrabolans tannin has a small affinity for hide substance and penetrates the hide very slowly. It gives a "mellow" tannage, but a bright, good colour, which characteristics are imparted to the leather when the material is blended with other materials containing dark or astringent tannins. When used alone it yields a rather spongy leather, and it is not considered a good weight-giving material, though its acid-producing powers are very helpful to other more astringent tannins.
Valonia has been the other staple tanning material of the heavy leather trade. It is the acorn cup of oaks common in Asia Minor and Greece, chiefly the Turkish oak (Quercus ægilops). The fruit is gathered when ripe and dried in layers of about one foot deep until the acorn drops out, Smyrna is the great export centre. Greek valonia is obtained from many parts of the Archipelago and mainland. It is gathered in a more immature condition and includes the acorn. It is considered slightly inferior in strength and colour to the Smyrna valonia. The exterior of the acorn cup is covered with rather scaly protuberances known as"beard," which contains usually about 40 per cent. of tannin. The cup alone contains usually about 25 per cent. tannin, and the whole about 30 per cent. The valonia tannin has been thought to contain two chemical individuals, only one of which produces bloom. Parker and Leach[2]found that the tannin of the cup produces more bloom than that of the beard, and that Smyrna valonia yields more bloom than Greek. The more bloom is deposited, the less acid will be produced. Under all conditions the yield of bloom is large, and its deposition in and on the leather assists materially in giving the weight and water-resisting powers associated with sole leather which has been largely tanned with valonia. The valonia tannins have only a moderate affinity for hide, which, like myrabolans, they penetrate very slowly. When used alone the leather is less yellow than that from myrabolans, and is also duller. After most of its bloom has been deposited valonia makes a very suitable tannage for dressing leather, and in conjunction with gambier has been largely thus used. Since the outbreak of war the Turkish product has, of course, not been available for importation.
[2]J.S.S.I., 1903, 1184.
[2]J.S.S.I., 1903, 1184.
Sumach[3]is the other pyrogallol tan of commercial importance. It consists of the leaves and small twigs of the Sicilian sumach (Rhus coriaria) cultivated in Italy extensively for export. The leaves are hand picked, dried and often ground to powder. It contains 26-28 per cent. of a tannin which yields little or no bloom, but much gallic acid. It is an unstable tannin, and its infusion rapidly ferments. Sumach is a very valuable tanning material, and when used alone gives an exceedingly durable leather of excellent light colour. It gives a soft mellow tannage, and is therefore most suitable for light leather tanning, and is extensively used for this purpose. It is used, nevertheless, in large quantities by the heavy leather tanners for finishing purposes, for it contains some organic reducing agent which exerts a powerful bleachingaction on other tannages, and which assists to brighten as well as lighten the rather dull appearance of leathers largely tanned with valonia. It is rather an expensive tannin, but most manufacturers find that its results are worth its cost.
[3]Also spelt Sumac and Shumac, and always pronounced like the latter.
[3]Also spelt Sumac and Shumac, and always pronounced like the latter.
Other pyrogallol tans are also used to a limited extent. Algarobilla and divi-divi are the fruit pods of several species of Americancæsalpina. They are strong in tan (45 per cent.) and yield a light-coloured and bright leather, but are unstable tans, yielding much bloom. Babla is a small pod yielding a mellow tannage and much gallic acid. Celavinia is another pod containing no colouring matter and giving an almost white leather. The tannin is closely similar to that of oak galls. These last were once extensively used for tanning in Austria. Willow bark is used for tanning in Russia and Denmark. Valuable pyrogallol tannins are obtained from oak wood and chestnut wood, but the woods are not used in tanning as the percentage of tan is so small.
Catechol tans, often obtained from barks, contain usually about 60 per cent. of carbon. They are seldom used alone, for they usually have little or no sugar associated, and hence their liquors do not either "sour" or "plump." They can be used alone if artificially acidified, but without acidifying or blending would give a rather flat leather, though possibly firm. They yield no bloom or gallic acid, but have associated with this other characteristic substances. Of these the catechins are the most typical, and have been considered as the parent substances of the catechol tans.
The catechins are white crystalline substances, apparently isomers with the general formula C15H14O6. They have different melting-points, and varying amounts of water of crystallization, but are otherwise exceedingly similar in properties. They are sparingly soluble in cold water, but freely in hot, and in alcohol and ether. They are precipitated by lead acetate, mercuric chloride and albumin, but not by gelatin, tartar emetic or alkaloids. In gambier liquors they are especially strong, and sometimes crystallize on the side of the pits,being thus known as "whites." The phlobaphenes or "reds" are also typical of catechol tans from which grow catechins; they can be formed by boiling with dilute mineral acids. They are considered to be anhydrides of the catechol tans. They are difficultly soluble in cold water, but freely in hot, and in cold alcohol and dilute alkalies. They are true tannins and alone are capable of making a red leather, but in practice are often found as mud in the tan liquors owing to their limited solubility. They naturally influence the colour of leather made with catechol tans, which is usually distinctly redder than the leather made from pyrogallol tans. Infusions of catechol (cp.catechin) give a green-black colour with iron alum. The sodium arsenate test gives a red colour due to catechin. The chromate and iodine tests mentioned for pyrogallol tans give negative results with the catechol tans, but bromine water gives a precipitate, and sulphuric acid a crimson colour.
Mimosa bark is one of the most important catechol tans. It is usually obtained in this country from Natal ("Natal bark"); but the tree (Sydney green wattle,Acacia mollissima) is a native of Australia. It is being cultivated now extensively in South Africa, and forms a most valuable portion of the Empire's stock of tanning material. Its more extensive use has been long recommended by the author[4], but its gradually increasing employment in British tanneries has been greatly accelerated by the war, which has prevented its delivery in Germany and has cut off Turkish valonia from Britain. It yields about 30 per cent. of a stable and excellent tannin, and will produce a firm, durable leather, with a colour much less red than that obtained from many other catechol tans. It is an astringent tan, and if carelessly used yields a harsh or even "drawn" grain. Most of the tannin is easily extracted, yielding a clear infusion which penetrates fairly quickly and gives good weight. It contains less than 1 per cent. of sugar, which unfortunately rapidly ferments to carbonic acid, so that it is not a good plumping material.It makes in all respects an excellent blend with myrabolans. Like all catechol tans, the resulting leather darkens on exposure to sunlight.
[4]J.S.C.I., 1908, 1193.
[4]J.S.C.I., 1908, 1193.
Oak bark, fromQuercus robur, is the ancient tanning material of Britain, and is still used to a limited extent. It contains about 13 per cent. of tannin and is mainly a catechol tan, but also contains a pyrogallol derivative. It yields catechin, and gives a red colour with the sodium arsenate test, but also will yield some bloom and gallic acid, and gives a blue-black with ferric salts. The tannin itself is exceedingly similar to that of mimosa bark, but the material contains about 2½ per cent. of sugar, which makes it possible to employ oak bark alone for making sole leather. It is noted for yielding a sound, durable leather of good typical tan colour. Its tannin combines well with hide and penetrates quickly. The fatal disadvantage of oak bark is its weakness in tannin strength compared with other materials. This results in heavy freight and heavy cost per unit tannin, bulky storage, expensive handling in the factory, comparatively large bulk of spent tan, after relatively greater trouble in extracting, and the impossibility of making the strong liquors so necessary in these days to produce good weight in a short time. No satisfactory extract has yet been made from it.
Pine bark, fromPinas abies, is one of the staple materials of the Continent. It contains up to 14 per cent. of a catechol tan, and, unlike most of this group, contains a high proportion of sugar and will give good results alone. Hemlock bark has been the staple tanning material of North America. It is obtained from the hemlock, orPinus canadensis. It contains up to 11 per cent. of tan and much phlobaphene, and yields a characteristic red leather of good quality, but which rapidly darkens with sunlight. It contains some sugar, but is usually employed in conjunction with sulphuric acid or with sugary materials. Mallet bark yields another catechol tan similar to that of mimosa, but somewhat less astringent and more yellow in colour. Quebracho wood and mangrove barkhave been used, but are now made into extracts (pp.38and41).
Leaching.—Whatever class of leather is being made, and whatever blend of tanning materials is being employed, the tannins must be efficiently extracted by water in order to make the tanning liquors. This process is called "leaching." The tanning materials, after being ground, crushed or shredded, are placed in large pits arranged in "rounds," "sets," or "batteries" of 6, 8 or 10 units, through which water is percolated systematically, so as to secure a continuous extraction. Water itself is added to only one of the pits of material. The liquor produced is passed on to the next pit, and then to the next, and is continually gathering strength. After passing thus through the series, the liquor becomes the source of the strong extracted tan liquors which are used in the tannery proper. With this system the stronger leach liquors are being acted upon by fresh material, and the nearly "spent" material is being acted on by the weakest liquors, and finally by water, thus ensuring a complete extraction. In the press leach system, which is now practically universal, the bottom of one pit communicates with the top of the next, and the liquor presses round by gravity flow caused by a few inches "fall." Liquor is thus constantly percolating downward through the material in each pit. The "head leach" and "tail leach" are always adjacent in a double row of pits, and when the material in the latter is quite spent, it is "cast," and the pit is filled with fresh material. The liquor is then pressed round into this pit by adding water to the tail leach. Hot water is used to secure better diffusion. At least two such sets of leaches ("taps" and "spenders") are necessary to spend the material of the average tannery and to obtain liquors of the necessary strength.
The Manufacture of Extracts.—In addition to the use of the natural tanning materials described above, modern leather manufacturers employ also a variety of "tanning extracts,"i.e.vegetable tanning materials in which the tannin has been already extracted, and which are suppliedin form of a solid or concentrated liquid. Such extracts only need to be dissolved in warm water in order to make a tan liquor, and the cost and trouble of leaching is avoided. They are a great convenience as making strong liquors of definite strength. Many vegetable tanning materials are too weak in tan for the tanner to leach, and indeed to justify the cost of importation have been made available by manufacturing an extract at the source of the material. With such weak materials the extract manufacturer has had to secure a much more complete extraction than in ordinary leaching, and to concentrate his infusions by means of steam-heated vacuum pans. With such experience he has naturally begun to make extracts also from the stronger materials, such as myrabolans and mimosa bark, and it is now possible to have a tannery without any leaches at all. Tanners also have begun to realize the advantages not only of more rapid and complete extraction, but also of doing the work for themselves, and extract factories are beginning to appear as an adjunct to the larger tanneries. The more complete extraction of tan also involves a greater extraction of unwanted colouring matters, hence decolorization is a feature of extract manufacture.
Chestnut Extractis from the wood of the Spanish chestnut (Castanea vesca), which contains 3-6 per cent. of a valuable pyrogallol tan very similar to that of valonia. Its weight-giving and water-resisting powers are as good as valonia, and its penetrating power is even better, so that it forms an exceedingly suitable material for the modern short tannage, and also for drum tannages. The extract is manufactured extensively in France. The wood is stripped of bark and usually piled for some months to dry and to allow the resins to become insoluble. Some factories, however, use the green wood direct. There are two methods of extraction, viz. in open vats and in closed vats under pressure. The two methods yield extracts which differ in composition and properties. In either case the vats have a capacity of up to 3000 gallons, and hold up to 6½ tons of wood. They are arranged in series, as in leaching, andthe liquor passes in succession through all the vats over wood less and less spent. The temperature is highest in the vat containing the fresh water and nearly spent wood. In open vats of wood or copper the temperature is near boiling-point, whilst in the closed autoclaves (copper or bronze) the pressure reaches about two atmospheres and the temperature about 130° C. (266° F.). The series may contain 5, 7, 9 or even 12 vats, and the liquor obtained has a strength of 3° to 4½° Beaumé (22° to 33° Bkr.).
After extraction the liquor is allowed to stand, and much insoluble matter settles out—resins, wood, fibre, etc. The clarified and settled liquor is then passed through a cooler up to about 55° C., and then run into the decolorizing plant, a deep vat fitted with a copper steam coil and mechanical stirrer attached to power. The best decolorizer is bullock's blood, which is run into the vat and well mixed. The temperature is next raised to about 70° C., causing the blood albumin to coagulate. It carries down with it a little tannin, but much colouring matter. After standing a few hours the settled liquor is run off direct to the evaporator. A multiple-effect evaporator is usually employed, and the concentrated liquor, which has a strength of about 25° Beaumé, is run into suitable oak casks. The extracts contain 27-32 per cent. tannin. An extract made with open vats has about 7 per cent. soluble non-tanning matters, whilst a "pressure extract" may contain up to 12 per cent. of these "non-tans." Pressure extracts obtain also a better yield of tannin, which more than compensates for the slightly lower price. Open extraction yields, however, the purer product and an extract with better penetrating powers, and is consequently the more suitable for drum tannages. Chestnut extract is extensively used by the heavy leather tanners.
Oakwood Extractis manufactured from the wood of the common oak (Quercus robur). The centre of the industry has been the oak forest of Slavonia. The wood contains 2-4 per cent. of a tannin very similar to that of chestnut wood, but somewhat more astringent.
The manufacture is also similar to that of chestnut extract, but decolorization is often omitted, and greater care has to be taken and in other ways to keep the colour within limits. One of these is to strip the wood more completely of bark. Another is to operate at as low a temperature as possible, about 110° C. The extraction is made in large circular vats about 14 feet high and holding about two tons material. A battery is composed of about eight vats or extractors. Open extraction is used, and the liquor is passed forward after 2-3 hours' boiling, so that the material is spent in about 24 hours. A liquor of about 5° Be (36 Bkr.) is obtained, and the strength of the material reduced from 4 to ¼ per cent. of tannin. Getting rid of insoluble matter is a difficulty, and is attained by settling, by rapidly cooling, and then passing through a filter press of wood. For evaporation a double-effect vacuum pan is preferred, which operates first at about 113° F., and afterwards at 140° F. with a higher vacuum. The liquor is concentrated from 5° to 25° Beaumé (s.g. 1.036 and 1.210 respectively).
The extract has a much higher colour than chestnut, and is not used now as much as some years ago. As the principal supply was German, it has been unavailable.
Quebracho Extractis made from the wood of the South American treeLoxopteryngium Lorenzii, which contains about 20 per cent. of a typical catechol tan. It is associated with a little catechin, much phlobaphene, but practically no sugar. The tannin is very astringent, penetrates quickly and gives a firm red leather which darkens on exposure to light. It is not noted for weight-giving powers. The wood itself, as chips or shavings, has been used in British tanneries, to a limited extent, but the great bulk of the material is made into extract chiefly in South America. The crude "extract," made by evaporating aqueous infusions of the wood, is largely exported for refinement in Europe. It is also refined on the spot to a large extent and converted into solid extract containing 60 per cent. of tannin.
The great difficulty with quebracho has been the disposal of thephlobaphenes, and a great variety of quebracho extracts are now available which deal with this problem in different ways. In some the more soluble reds are simply left in the extract under the idea that they are really tannins and may be of some use in some part of the tanning process; in others they have been removed by settling and filtration at appropriate temperatures and concentrations; in most, however, they have been solubilized by treatment with alkalies, in the presence of reducing agents, notably by heating with sodium bisulphite in closed vats. The base combines with the phlobaphenes, which are made completely soluble and available for tanning. Sulphurous acid is evolved, and its reducing powers assist materially in retaining and promoting a good colour in the product. Such "sulphited extracts" are now extensively manufactured in this country from the imported "crude" extract, and sold as liquid extracts containing 30, 35 or 40 per cent. of tan according to the requirements of the buyer; "mixed extracts" which are solubilized quebracho blended with about 15 per cent. of myrabolans, are also used.
By solubilizing quebracho with excess of bisulphite an extract is obtained which possesses considerable bleaching powers, and such extracts are also extensively manufactured for the "vatting" or bleaching of heavy leather after tannage. The excess of sulphurous acid not only bleaches the leather, but also swells it up and thus permits a further absorption of strong tan liquor, which is conducive to good weight. These bleaching extracts are usually of 36-38 per cent. strength in tan.
Gambieris an extract of the leaves and twigs of the eastern shrubNauclea gambier. It is a catechol tan of peculiarly mellow quality and great practical value. It contains much catechin, but little phlobaphene, and yields a beautifully soft leather, but without weight. It is an exceedingly suitable material for the early stages of tanning, and is much liked for tanning leathers that have to be curried, and is widely used in the manufacture of upper leather. It is, however, an exceedingly expensive tannin, and the extract is made in a very crude way by Chinese and Malays without much supervision. Hence its strengthin tan and general quality is extremely variable. The plant is cultivated for the purpose of extract manufacture, and prunings are taken in the plant's third year. They are bruised and boiled with water in the open. The infusion is strained, concentrated, and poured into cooling vessels in which it sets to a paste. Two varieties of gambier are well known, "cube gambier" and "block gambier." In the latter the extract remains as a paste containing 25 to 40 per cent. of tannin. It is sold in oblong blocks of 1 or 2 cwt., either wrapped in cocoanut matting or in wooden boxes. Cube gambier is made by running the concentrated syrup into trays 2 inches deep and drying in the sun. When partly dry, it is cut up into 1½-in. cubes and dried further on cocoanut matting. The rough "cubes" as imported contain 40-50 per cent. of tannin.
Myrabolans Extractis now largely manufactured in this country. A liquid extract of 25, 30 or even 35 per cent. strength is made for home consumption, and a solid extract for export. The light colour, high strength and easy extraction of the natural material have all facilitated the task of the manufacturer.
The material is extracted in open vats or stills of copper, which take one ton or more of nuts. A battery of 4, 6 or 8 of such stills is usually employed, and the temperature is kept well below boiling-point except in the vats containing the nearly spent material. The liquors move forward quickly, and the material is quite spent in 24 hours. The material when cast contains less than ½ per cent. of tannin. The liquor obtained is 40°-50° Bkr. (6-7½ per cent. tan), and after settling is concentrated at 40°-50° F. in a single effect vacuum pan, which though more costly in steam is quicker than the multiple effects, and gives the low temperature required. For solid extract the more concentrated liquor is run direct into tarred bags, in which it soon solidifies.
Hemlock Extractis manufactured from the North American pines and imported into this country to some extent. It gives a very red colour.
Mangrove Extractis made from the bark ofRhizophona Mangleand other species of mangrove which grow freely in the tropical swamps of West Africa, Borneo, etc. Much solid and liquid extract has been made from this material, but is not very popular on account of its harsh tannage and dark red colour.
Pine Bark Extract(Larch extract) is made in Sweden from the Norway spruce (Pinus abies). It is slightly sulphited and gives a good colour. It is a liquid extract of about 30 per cent. strength, and is sometimes used as a chestnut substitute. It should not be confused with the so-called "spruce" or "pine wood" extract, which is a paper trade bye-product and contains ligneous matters rather than tannin.
American Chestnut Extract, made from the chestnut oak, is either a liquid or a solid extract in powder form. It gives a wretched brown-black colour, which is quite unsuited to the usual British needs.
Vegetable tannage is a phenomenon of colloid chemistry. The old arguments as to whether tanning was a chemical or a physical process have been rendered obsolete by the advent of a new set of explanations, which, though shedding light on many obscure points, have enormously increased the complexity of the problem. In vegetable tannage an emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor), which immersion results, not in simple reaction or change, but in a series of changes.
One of these changes isadsorption. Pelt is a gel which possesses a great development of surface. It not only exhibits like gelatine the phenomenon of imbibition and dehydration to a very marked extent, but also possesses a very fine fibrous structure due to its organic origin; thus pelt possesses an enormous specific surface, further intensified by the preparation processes previously discussed, which split up the hide fibres into smaller bundles and into much finer constituent fibrils.Tannins, on the other hand, are hydrophile colloids which in water form emulsoid sols, and which may thus be expected to exhibit the phenomenon of adsorption. A tan liquor usually contains several tannins in addition to other closely similar substances, also in colloidal solution, and is therefore a sol of considerable complexity. The immersion of pelt into a tan liquor results in an adsorption, which consists essentially in an inequality of concentration in the sol, the greater concentration being at the interface. This inequality between the surface concentration and the volume concentration of the sol, is due primarily to considerations of surface tension and surface energy, and exists before the immersion of the pelt. The surface layer having excess over the volume concentration, any considerable extension of surface in a fixed volume of sol must produce a very considerable decrease in the volume concentration. This is what occurs when pelt is immersed in a tan liquor, the immersion being the considerable extension of surface. It should be especially remembered that the inequality of concentration is in the sol, on the liquid side of the interface. In adsorption, the substance adsorbed,i.e.the excess at the surface, is too frequently regarded as bound to the solid immersed. This is because the excess is in the layer which wets the solid and remains wetting it when the solid is removed. Thus the immersion of pelt produces primarily only a change in the distribution of the tannins in the liquor. It follows from this that the adsorption is an equilibrium, and that if the sol be diluted, the equilibrium will become the same as it would have been by immersing the pelt directly into the dilute solution. Thus, if pelt be first immersed in one tan liquor and then into a weaker one it will yield tan to the latter solution.
The chief object in heavy leather tanning is to obtain the maximum possible adsorption in the minimum possible time, or in other words, to obtain good weight quickly. The amount adsorbed is proportional to the actual extension of surface,i.e.the adsorption is a function of the specific surface of the adsorbent. Hence, to obtain good weight it isnecessary to develop in the pelt its maximum possible specific surface. This is one of the objects of "plumping," which splits up the fibres. It is attained also by the solution of interfibrillar substance in limes and bates.
The amount adsorbed is also a function of the volume concentration in the sol after equilibrium is reached. Hence the better weights are obtained with stronger liquors.
The adsorption law is
y/m = ac1/n
where y is weight adsorbed by the weight m of adsorbent, and c the volume concentration after adsorption; a and n are numeral constants. Hence weight is determined by the strength of the liquor which the goods finally leave. The commencement of tannage is necessarily in weak infusions, in order to secure the maximum diffusion into interior of the fibres before they become heavily coated on the exterior. As the equilibrium is being established in such liquors the volume concentration diminishes, and thus makes it less likely that good weight will be attained; hence it is necessary in practice to move the goods constantly into fresh liquors of gradually increasing strength, and so maintain the rate of adsorption and save time. A further consequence of the adsorption isotherm is that as y varies as c1/nand n is > 1, y is increased appreciably only by a relatively large increase in c. Hence, though stronger liquors give better weight, there is a limit beyond which any further gain in weight is not justified by the enormous increase in the concentration necessary to attain it. Such great increase in c is impracticable not only on the ground of expense, but also on account of the great viscosity of the sol.
The amount of adsorption depends also upon the exact nature of the sol. It has been previously pointed out that the tannins differ largely in their penetrating and weight-giving powers. Some are readily adsorbable and are deposited in great concentration at the surface of the fibre,but for good weight it is necessary to use also the less adsorbable and more diffusible tans, which penetrate the fibre itself. Hence it is necessary for good weight to use a blend of materials, and so supply many grades of liability to adsorption. It is particularly advantageous to blend judiciously the two main types of material, the pyrogallol and catechol tans. It is also necessary for good weight to present to the pelt the more diffusible and less adsorbable tannins first, in order to secure the maximum diffusion into the interior of the fibre before the exterior of the fibre is heavily coated with the heavily adsorbable and astringent tans. The least adsorbable materials are therefore used in the early stages of tanning, and the most adsorbable materials at the end of the tanning process. Thus gambier is added to the early liquors (suspenders), solubilized quebracho to the later liquors (handlers), and mimosa bark extract to the final liquors (layers). There is also another excellent way of ensuring this progressive astringency of the liquors; this consists in leaching the required blend of materials together (or mixing them in the case of extracts) and presenting the mixed infusion to the nearly tanned goods, which adsorb chiefly the more astringent tannins. The liquor is then used for goods at a less advanced stage of tanning, which again take the most adsorbable constituents. This is repeated until the stage is reached when the fresh pelt is inserted into the nearly exhausted liquor, which naturally contains only the least adsorbable substances. This system is almost universal, and in practice is known as "working the liquors down the yard." It has the additional advantage of being a systematic method of economically exhausting ("spending") the tan liquors. When free acid is present in the tan liquors, it tends to distend the fibres composing the pelt by a strong and rapid adsorption. Thus distended or plumped the fibres present a still greater surface for adsorptive operation, but the distension naturally leaves less space between the fibres for the diffusion of the sol. Hence acid or "sour" tan liquors give in the long run more weight,but tan more slowly. Pelt tanned whilst thus plumped forms naturally a thicker and less pliable leather. This occurs in tanning sole leather, to a less extent with heavy dressing leather, and to a very small extent in the case of softer dressing leathers.
In addition to adsorption, there is another phenomenon of colloid chemistry in operation, viz. themutual precipitationof the sols in the liquid by the gels in the hide. In most sols the disperse phase is electrically charged. The sol therefore possesses electric conductivity, and migration occurs in the electric field to the cathode or anode according to the nature of the charge. Oppositely charged sols precipitate one another, the precipitate containing both colloids. The maximum precipitation occurs when the + charge of one sol exactly equals and neutralizes the-charge of the other. There is thus an electrical equivalence; an amount of sol which is equivalent to a given amount of the other. This is not a chemical equivalence, however, and the precipitate is not a chemical compound in spite of its fairly constant composition. The composition of the precipitate, indeed, is not quite constant, for the optimum precipitation may not correspond exactly with the electrical equivalence, being influenced by the number of particles required, their size (dispersity), the rate of mixture, and the relative concentrations of the sols. This mutual precipitation is exhibited by emulsoids as well as suspensoids, but the charge (+ or-) on an emulsoid is in many instances largely an accidental matter, being determined by the medium in which it happens to be, its normal condition being electrical neutrality. Gelatin and pelt are such emulsoids, and a positively charged gelatin sol has been observed to precipitate a negatively charged gelatin sol. It is thought, however, that gelatin is primarily a positive sol. Pelt (whether delimed or not) is rapidly acidified by the quickly penetrating and strongly adsorbed organic acids of the old tan liquors and becomes positively charged before the tannins are adsorbed. The positive charge increases with the acidity of theliquor. Other emulsoids are not electrically neutral, but are electrically charged and exhibit considerable conductivity. Into this class fall the tannins, and in tanning it is thought that there is a mutual precipitation of the negative tannin sol with the positive hide gel, the precipitation of the negative sol being favoured by the acid condition of the liquor. The effect of increasing acidity soon falls off, however, as a saturation limit is soon reached. This mutual precipitation of colloids in tanning is in reality but an extension of the adsorption theory, which explains the predominant effect of H+, and OH-on the electric charge by stating that these ions are more readily adsorbed than other ions, and that as OH-is more readily adsorbed than H+ most sols are negative to water.
In addition to the adsorption phenomena described, there are in vegetable tannagesecondary changeswhich are slow and "irreversible." These changes are obscure and are difficult to investigate. Oxidation, dehydration and polymerization have all been suggested, but there is little direct evidence. Certain it is, however, that time renders the tannage more permanent. It perhaps should be pointed out that in the very strongest tan liquors the viscosity of the tannin sol is so great that adhesion would be a better term than adsorption. There is no abrupt division between the two phenomena.
In the theory of vegetable tannage there is another factor the importance of which has been strongly emphasized by the author, viz.,lyotrope influence. This has been most conveniently discussed in connection with gelatin gels (pp.200-219), but its effect on hide gels is analogous. It has also an effect upon the diffusion and gelation of the tannin and non-tannin sols (cp. pp.129and174).
Mechanical Operations.—In the tanyard the liquors are almost invariably divided up into sections, called "rounds" or "sets," in which the mechanical operations are different in aim and method. In the first pits entered by the goods there is rapid adsorption in spite of the low concentration and small astringency, and the great aim is to obtain evenness of action and a good level colour. It is also necessary tomaintain the rate of adsorption. All the aims are attained by frequently moving the goods. Heavy leather is suspended vertically in the pits of tan liquor and handled up and down as well as forward from pit to pit. Such pits are termed "suspenders." In the earliest suspenders it is indeed advantageous to have the goods in constant motion. This is done by suspending on wooden frames which are rocked gently by mechanical power; such pits are termed "rockers." For dressing leather in which firmness and smooth grain are not so essential, the goods may be paddled in the first liquors. This is occasionally done with stronger liquors for the express purpose of working up the "grain" pattern. The goods after passing through the suspenders are usually passed to "handler" rounds, in which they are moved less frequently. In these pits the goods are laid horizontally one above the other. One advantage of handlers is that the goods flatten thoroughly and straighten one another by their own weight; another is that more goods can be placed in one pit than in suspenders. They are not so convenient to work, however, as suspenders, and the goods do not feed so rapidly. Hence the tendency is now to tan more in suspension, and to economize labour by an extension of the rockers. The handling of the goods is also saved by pumping the liquors and by working rounds of suspenders or rockers like the press leach system, with the difference that the stronger liquor is pumped in to the head pit, and the liquor passes upwards through the goods.
Finally the goods are placed in "layers" or "layaways," in which they remain undisturbed for a decidedly longer time. These pits contain the strongest liquors of the yard, and their principal function is to complete the tannage and give weight and firmness by the adsorption of bloom, reds, etc., in the interior of the hide. The goods are placed in horizontally, and are dusted in between with fresh tanning material which maintains the local strength of the liquor and keeps the goods somewhat apart. Drum tanning attains a more rapid penetration of the pelt by giving constant motion in stronger infusions. It is of courseliable to result in an under-tannage of the interior of the fibre. After the goods have been "struck through" in the ordinary way, however, drumming in extract is increasingly used as a substitute for much labour in handling, and also to save the time spent in the early layers.
REFERENCES.Procter, "Principles of Leather Manufacture," pp. 220-350.Bennett, "Manufacture of Leather," pp. 113-179.Bennett, "Celavinia and Babla," L.T.R., 1914, 122.Dumesny and Noyer, "Manufacture of Tanning Extracts."Theory:—Meunier and Seyewetz, Collegium, 1908, 195.Stiasny, Collegium, 1908, 117-159, 289, 294, 337.Procter and Wilson, Collegium (London), 1917, 3.Wilson, Collegium (London), 1917, 97, 100, 105.Moeller, Collegium (London), 1917, 13, 38, 46, 103; andJ.S.L.T.C., 1917, 22, 56, 92.Bennett,J.S.L.T.C., 1917, 130-133, 169-182; 1918, 40; 1920, 75-86;S.L.R., 1916, March.
After the tannage is complete, leather is hung up to dry. In the case of heavy leather this drying must be very carefully carried out in order to obtain a product of satisfactory appearance and saleable qualities. Associated with the drying are many mechanical operations (scouring and rolling) which assist very materially in imparting the desired qualities. After tanning, however, the quality of the final product is most strongly influenced by the amount of grease added in finishing. Some grease is always used in finishing, partly because even sole leather requires some measure of pliability and partly because a coating of oil over the leather during drying prevents the loose tannin from being drawn to the surface of the leather by capillarity, thereby causing dark and uneven patches and a "cracky" grain. The added grease is also a contribution to the "weight" of the finished article—a primary consideration for heavy leather, which is usually sold by weight. The finishing processes, indeed, tend to be dominated by this consideration, and become a series of efforts to retain as much tannin and add as much grease as are consistent with the requirements of the class of leather being manufactured. Sole leather does not contain more than about 2 per cent. grease, or its firmness is impaired. Belting leather, in which considerable pliability is needed, may contain about 9 per cent., whilst harness leather, which must be exceedingly tough and durable, may contain up to 13 per cent. of fatty matters. Upper leathers, which need to be soft and pliable as well as waterproof and durable, are very heavily "stuffed" and often contain up to 30-40 per cent. of grease. Sole leather is thus rather distinct from the rest, which are called "curried," "stuffed," or "dressed" leathers. The actualdrying out before, after and between the various mechanical operations, each have an appropriate degree of wetness. In this country the drying is usually under the prevailing atmospheric conditions and is known as "weather drying." The goods are suspended by hooks or strings or by laying over poles in special sheds fitted with louvre boards by which the rate of drying can be roughly controlled. Weather drying is cheap, but exceedingly slow, and in unfavourable weather is very unreliable. The goods, moreover, need constant attention to obtain an even result. Steam pipes are usually laid along the shed floors, and are used in winter and damp weather to accelerate the drying, and also in the final shed stove to remove the last traces of moisture. Wet weather, however, will not stand a high temperature, and steam drying is better avoided when possible. Air-dried leather still contains about 14 per cent. of moisture. Many systems of shed ventilation have been suggested to hasten the drying and to secure a better control of the process. In one system a screw fan is fitted at one end of a shed (without louvre boards) and sucks air through the goods from an inlet at the other end. The air can be heated by a steam coil near the inlet. In another system a centrifugal fan blows air through an arrangement of pipes which distributes it to the drying sheds, and discharges it close to the floor by various branch pipes. The outlets are near the roof. A system of dampers permits hot air, warm air and the used wet air to be blended in the desired proportions. In America turret drying has been used. The sheds are vertically above one another and have latticed floors. Heated air is admitted at the bottom and rises through the goods up the building just as in a chimney. For many of the finishing operations it is important to obtain the leather in a uniformly half dry or "sammed" condition. This may be done by careful drying, and wetting back the parts that have become too dry with tepid water or weak sumac liquor, and then leaving the goods "in pile" until of uniform humidity. It may also be done by "wetting back" leather which has been completely driedout. There are also "samming machines" which by means of rollers squeeze out the excess liquor. Sole leather is dried out and finished immediately after tanning, but dressing leather is often "rough dried" out of tan liquors and wet back for finishing when required. Dressing leather is often treated in different factories; tanners selling it as rough leather and "curriers" finishing it.
Scouring is one of the first operations in finishing leather. The grain side is wet and worked with brushes and stones until the bloom and loose tannin are removed. This process aims at producing a good even colour and level surface, but is liable to cause a loss of weight. Dressing leather is often scoured on both grain and flesh, and weak soap or borax solutions are used to assist the process. In this operation hand labour has been now quite superseded by machine work. A great variety of machines have been devised. The mechanical working of leather takes place in various parts of finishing. These operations, known as "striking," "setting," "pinning," "jacking," may be carried out often by the same machine as used for scouring, but with a change of tool. The object of these operations is to get rid of wrinkles and creases, to produce softness, pliability and area, and to remove superfluous moisture, grease, dirt. The tools are of steel, brass, slate or vulcanite. Scouring is often effected by putting the goods into rotating drums together with extract and sumach. The bloom is removed by friction, the colour is improved by the sumach, whilst the extract keeps up the weight.
In finishing sole leather firmness is enhanced by "rolling." A brass roller passes to and fro over the goods with the exertion of considerable pressure. The operation is carried out by machinery.
Shaving is an important operation in the case of many dressing leathers. Its object is to produce a uniform thickness of the leather and an even surface on the flesh side. The sammed goods are laid over suitable beams and shaved with special sharp knives which possess a turned edge. Thishand process, which demanded considerable skill, is fast becoming extinct, and machine shaving is already almost universal on account of its greater speed. The machines consist essentially of two rollers, one of which is smooth, whilst the other is a spiral knife-blade cylinder (cp. Section II., p.23). The sammed goods are held in the hands and placed over the smooth roller, which is raised to the cutting roller by a foot treadle. A number of similar operations ("flatting," "whitening," "buffing") are carried out by a suitable change of tool. In all these operations good samming is important.
Splitting is another important operation on tanned leather. In this process the leather is cut parallel to its grain surface, thus yielding two pieces with the same area as the original, the "grain" and the "flesh split." It is essentially a machine operation, and is carried out by presenting the carefully sammed leather to a sharp knife-edge, towards which it must be constantly pressed. The "band-knife" machine is the most popular arrangement. The knife is an endless belt, which continually revolves round two pulley wheels of equal size. In between these the knife is horizontal, and is then used for splitting. The sammed leather is pushed towards the blade by two feed rollers, and the grain passes above the knife on to a small platform, whilst the flesh or "split" passes below and falls to the ground. Emery grinders and thick felt cleaners in the lower part of the machine keep the knife in good condition. The adjustment of the machine is delicate and requires considerable experience. With care splits may be obtained down to 1/16" thick, and sometimes as many as 6 or 7 splits are obtained from one hide.
Oiling is still usually done by hand, and cod oil is still preferred for many classes of goods. Of recent years there has been a great extension of the use of sulphonated oils, which have the valuable property of forming an emulsion with water or tan liquor. With these materials it is easier to ensure the goods being completely covered with oil. The penetration of the oil into the leather is also quicker and morecomplete. These oils have often the disadvantage of leaving solid fats on the exterior of the leather, which gives it an ugly smeared appearance.
Stuffing the dressing leathers is carried out in a variety of ways and with a variety of materials. The old process of hand stuffing employs a mixture of tallow and cod oil called "dubbin." This is made by melting the ingredients together and allowing them to cool with constant stirring to a nearly homogeneous salve. The dubbin is brushed thickly on to the flesh side of the sammed leather, which is then hung up to dry. As the moisture dries out the oils and soft fats penetrate the leather and leave the more solid fats on the outside. The proportions of tallow and oil are varied with the time of year and with the method of drying, for if the dubbin be too soft it will run off the leather, and if too hard will not penetrate it so well.
Drum stuffing is a more modern development in which a higher temperature is employed, about 140° F. The drum is heated up by steam or by hot air, and the sammed goods are then inserted and drummed for a few minutes until they are warmed. The drum is fitted with a heated funnel containing the melted grease, which is run in through the hollow axle. After a half to three-quarters of an hour's drumming the grease is completely absorbed by the leather. The drumming is continued for a while until the goods have cooled. Whilst still warm they are "set out" to remove creases and superfluous grease. Drum stuffing is not only quicker than hand stuffing, but also makes it possible to use the hard fats, and so make a leather which carries more grease without appearing greasy. Thus in drum stuffing, paraffin wax and wool fat are used, and their penetration assisted by small proportions of cod oil or dégras. If the leather be too wet the grease is not absorbed, whilst if it be drier than usual the leather will take more grease, but the resulting colour is not so good. There is also another method of stuffing which originates from the Continent. It is known as "burning in" and involves the use of still higher temperatures (195° to 212° F.). Wet leather will, of course, not stand this temperature, so that it is firstnecessary to make the leather absolutely dry. This is effected by drying in stoves at temperatures up to 110°-115° F. There are two ways in which the grease is applied. In one method the melted grease is poured by a ladle on to the flesh side and brushed over until evenly distributed. A second application of grease is made to the thicker parts. The hides are then put into warm water (120° F.) for about a quarter of an hour, and then drummed for half an hour. In the other method the goods are completely immersed in the melted fats for a few minutes in a steam-jacketer tank at a temperature of 195° F. After softening in water at 120° F. the goods are drummed. "Burning in" is used for the heavier dressing leathers such as belting and harness. It does not give good colour, but permits the employment of still more hard fats.
REFERENCES.Procter, "Principles of Leather Manufacture," pp. 223, 378.Bennett, "Manufacture of Leather," pp. 251-312.Bennett, "Principles of Leather Stuffing,"Leather Trades Review, 1911, 186.
Leather for the soles of boots and shoes is a matter of essential interest to all, and forms one of the best appreciated applications of animal proteids to useful purposes. Methods for its manufacture are as numerous as the factories producing it, hence all that can be done is to describe broadly the general method which is typical of our time, to classify the many varieties into types, and to indicate the recent changes and present tendencies.
Sole leather is mainly manufactured from butt pelt, and the great aim is to produce a firm, thick, waterproof and smooth grained leather which will bend without cracking. It must have a light tan colour to be saleable, and contain as much weight as possible to be profitable.
The modern mixed tannage of "sole butts" or "scoured bends" generally utilizes ox-hides of the Scotch and English markets, though salted Continentals and South Americans are also employed. After the usual soaking a short and sharp liming is given. The special aim in liming sole hides is to obtain the maximum plumping effect with the minimum loss of hide substance. Both these achievements are necessary to obtain good weight. The limes should be kept as clean as possible, which is best obtained by putting clean hides into work. This reduces bacterial activity and loss of hide substance. The "shortness" of the process is attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of sulphide crystals), by which depilation may be accomplished easily in about nine days. The amount of sulphide should be increased somewhat in the short-hair season and in cold weather. Some factories take up to about 12 days using less sulphide, whilst others will lime in about a week by using the larger quantities. The amount of lime used variesenormously, and is invariably in great excess of the actual requirements. "Probably 2-3 per cent. on the green weight of the hides is all that can be really utilized, the remainder being wasted."[5]This amounts to about 2½ lbs. lime per hide, but in practice it is more frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess is innocuous, owing to the limited solubility of lime. Some excess is desirable, to replace in the liquor the lime adsorbed by the goods in plumping, to assist bacterial activity (p.21), and also because in sharp lime liquors the undissolved portions do not remain so long in suspension. The use of sulphide and other alkalies does not "make it possible" to reduce the amount of lime used, it merely renders the excess more superfluous. The use of sulphide not only shortens the process, but also sharpens it, on account of the caustic soda produced by hydrolysis. Usually for sole leather, however, it hardly sharpens it sufficiently, and it is very common to add also caustic soda (or carbonate of soda) to the limes. About 2 ozs. caustic soda (or its equivalent in carbonate) is used per hide. The hides are limed generally by the three-pit system, giving about three days in each pit. They should be handled each day in the first pit (old lime) and once in the other pits.