Читать книгу The Vegetable Tanning Process - A Collection of Historical Articles on Leather Production - Various - Страница 6

THE tannins may be described as a class of substances found in many plants, which have the common properties of precipitating gelatine from solution and of converting skin into leather. They are all colloid, that is uncrystallisable, and for this reason few of them have yet been obtained in a pure form. They are feebly acid, and are sometimes called “tannic acids,” but it is uncertain whether they are strictly acids, since many phenols, of which “carbolic acid” is a type, have also slightly acid properties. All natural tannins¹ are benzene derivatives, either from the dihydric phenol catechol, or the trihydric phenol pyrogallol, and another trihydric phenol phloroglucol is also often present. The positions of the OH groups are shown by the following diagram, the carbon atoms at the other angles being combined with H.

They are consequently usually divided into “catechol” tannins giving green-blacks with iron salts, and “pyrogallol” tannins giving blue-blacks, and often used as inks; but it is certain that the distinction lies deeper, and is rather one of structure than of the particular phenol. The two classes, however, whatever the cause, possess a marked difference in tanning properties, the iron-blueing tannins causing a white deposit of crystallised ellagic acid in the leather while the iron-greening (with a few giving violet-blacks) deposit dark brown substances called “reds” or “phlobaphenes.” Whether these are in all cases products of the tannins themselves, or rather of other bodies associated with them, is still doubtful.

The tannin-yielding materials are so numerous that but very few, even of those in commercial use, can be mentioned here. The oldest and formerly the most important in this country is the bark of the oak, usually stripped in spring when the sap has begun to rise, because at this time the bark is more readily separated from the trunk. It is a somewhat weak material, only yielding 10—12% of substances absorbable by hide, but it has the peculiarity that good leather of almost all descriptions, both light and heavy, can be made by its use. Very little leather is now tanned exclusively with oak-bark, though other oak products are largely used. Oak wood also contains tannin, though in less quantity than the bark; but by chipping and hot extraction, and subsequent decolorisation and concentration of the infusion by evaporation, an extract of 25—30% is obtained, and is now made from the waste wood and sawdust in very large quantity in Slavonia and Northern Italy, where oak is still abundant. An extract hardly to be distinguished from that of oak wood is made from the wood of the edible chestnut Castanea vesca. The oak principally used in Europe is Quercus robur (with its sub-species Q. scssiliflora and Q. pedunculata) but most oaks contain tannin. Very important is the large bearded cup of the acorn of evergreen oaks from Greece and the Levant known as valonia, which is extremely rich (30—40%) in tannin, and largely imported. Oak-galls (from Q. infectoria) are a source of the druggists’ “taimic acid,” but unimportant as a tanning agent, though the often repeated statement that the so-called “pathological tannins” from insect galls will not make leather is absolutely without foundation. It is somewhat curious that these oak tannins are by no means chemically identical. The bark-tannins distinctly belong to the class we have called catechol tannins, though that of the common oak contains some mixture which gives blue-black with iron and produces “bloom.” The wood tannins belong to the pyrogallol class, and the gall tannin is the typical “gallotannic acid” which is a pure pyrogallol derivative.

Another important material of the pyrogallol class is myrobalans (30—40%), the dried fruit of an Indian tree, and divi-divi, the pod of Cæsalpinia coriaria, a tree allied to logwood (40—50%) and sumach, the leaves of Rhus coriaria (25—30%), while the most important sumach adulterant, Pistacio lentiscus, is a catechol tannin.

To the catcchol class belong also gambier, an extract from the leaves of Uncaria gambia (30—40%); the bark of the Australian mimosa or “wattles” (25—40%); quebracho extract from the very hard wood of a South American tree (dry 60—70%) and many others. It will be noted that all these materials are very much richer in tannin than oak-bark, and it is to the stronger liquors obtained from them, rather than to any “chemical tanning,” that the shortened time of modern sole-leather tannage is due. Whether the shortened process gives so durable a leather as the older method may be questioned, but the leather is honestly and thoroughly tanned with mixtures of natural tannins very closely approximating to that of oak-bark; while in the old process much time was positively wasted by ignorant mismanagement.

The way in which tannins tan and the chemical nature of the leather formed still need elucidation. The view most probable at present is that leather is rather a colloidal than a strictly “chemical” compound. The tannins all yield colloidal solutions, and gelatine and hide-fibre are typical colloids. It has been shown in the author’s laboratory and elsewhere that, in presence of the trace of acid essential to tanning, the particles of tannin have opposite electrical charges to those of gelatine, and it is well known that two colloid solutions in which this is the case, when mixed, are mutually precipitated as a colloidal compound.

Although not strictly vegetable materials, the synthetic tannins recently discovered by Dr Stiasny must be mentioned here, as they will probably have considerable commercial importance. They are coaltar products, produced by the condensation of sulphonated phenols with formaldehyde, and produce almost perfectly white and very soft leathers. Though not identical with any natural tannins, they possess most of the characteristic properties of the class, precipitating gelatine and basic dyes and giving blue-black inks with iron.

THE VEGETABLE TANNING PROCESS

THE tannage of sole-leather, though full of problems and difficulties in commercial practice, may in theory be regarded as one of the simplest; and a clear understanding of its methods will render easy the comprehension of the rest.

The preparation and character of hides for sole-leather have been described in Chaps. VI and VII. To recapitulate, they are somewhat rapidly limed in fresh limes generally somewhat “sharpened” with sulphide of sodium; the whole process being so conducted as to produce good swelling and easy unhairing, with the least possible solution of valuable hide-substance which should go to make a heavy and solid leather. After unhairing and fleshing, the hides are “rounded” or trimmed, as only the thicker and central part called the “butt” is suitable for soles. After washing in water the butts were (and in some tanneries still are) considered ready for tanning, but it is becoming more and more usual first to remove, the surface-lime by a bath of boracic or some other weak acid, not only to secure a brighter and more uniform colour, but to economise the natural acids of the tan-liquors, which in the modern rapid process are much less freely formed than in the old method, in which there was abundant time and material for bacterial and other fermentations.

The butts first go into “suspenders,” a set of 8 or 10 deep pits in which they are hung by string to sticks laid across the top of the pit. Sometimes these sticks are supported on a frame to which a gentle reciprocating motion is given by suitable machinery, which causes a constant flow of liquor between the butts and prevents their remaining in contact, which would cause stains; but otherwise they must be moved and shaken by hand, especially in the earlier stage of the process. The pits are generally arranged so that liquor can flow from the top of one pit to the bottom of the next, so that the fresh liquor is all pumped into the strongest pit, and flows away exhausted from the weakest into which the “green” butts from the limes are brought and moved forward daily in the opposite sense to the liquors, which are appropriately the oldest and most exhausted in the yard; though if fresh liquors must be used, gambier or myrobalans are among the most appropriate materials. The object of using the oldest liquors is not merely one of economy, but because such liquors are what the tanner calls “mellow”; that is, their action on the hide is gentle, and only mildly astringent. This arises from several causes. Even if only one material is used, the tannins contained in it are a natural mixture; and the ordinary liquors of “mixed tannage” contain a still larger variety, varying much in their affinity for the hide-fibre. It is obvious that when a liquor is brought in contact with partially tanned hide, those tannins which are most astringent, and have the greatest affinity for the fibre must be removed first, so that what remains at last is only the mildest and least active part. Most tanning materials also contain a considerable portion of what the leather-chemist styles “non-tannins.” These are partly sugars and other carbohydrates, which during the process are gradually fermented by bacteria and yeasts to organic acids; and partly derivatives of the tannins themselves which, though they do not actually tan, are yet to some extent absorbed by raw hide, and promote its conversion into leather.

We may add to this the gradual accumulation in the used liquors of organic salts of lime and potash which weaken the acidity of their corresponding acids, and so keep the tanning action in check, since a certain degree of acidity is necessary for the process, and sufficient addition of alkaline salts may even bring it to a standstill.

The first process which takes place in the suspender is the neutralisation by its acids of any lime which still remains in the hide, and the consequent reduction of alkaline swelling. The hides, which, if not previously delimed, were at the outset plump and elastic, become very soft, and easily impressed by the finger; but in a properly conducted process should not lose much actual thickness, since the liquid which escapes from the fibres remains between them, and, as the lime is gradually replaced by the weak acids of the liquors, the fibres again swell and the hide again increases in firmness. That this should take place is essential to the production of a firm and solid sole-leather, for pelt tanned in the flaccid and “fallen” condition remains soft and porous, as well as thin. Fibres which have once been swollen by lime are much more sensitive to acid swelling than where this has not been the case, and those unhaired by “sweating” or any other process in which swelling has not occurred, require much stronger acids to produce adequate swelling and differentiation of the fibre bundles. For this reason American “sweated” sole-leather is usually swollen by dilute sulphuric acid in an early stage of the process, when the surface only has been tanned and thus rendered insensitive to the action of acids. If applied without this precaution, mineral acids produce a dark and brittle grain, and though this is prevented by the slight tannage, a dark layer may usually be detected beneath the thin tanned surface. For this reason, if “acid” leathers are “buffed” or glass-papered in shoe-manufacture, the surface is usually blacked and burnished or covered with a coloured “fake” or paint to conceal the actual leather and imitate a better material.

At the same time that the alkaline is being replaced by the acid swelling, the surface of the leather is taking colour, most rapidly indeed if the hides are somewhat limey; and actual tannage gradually penetrates, so that by the end of the suspender period, the hide, if not tanned, should be nearly or quite “coloured through.”

At the end of the suspenders the butts, still soft, are advantageously laid flat to straighten out any lumps or creases which may have formed, before they go to the “handlers.” Though the time of suspension is short and the liquors weak, the latter must be abundantly supplied, for the green hides have great avidity both for tannin and acid, and if allowed to remain in exhausted liquors, putrefaction sets in, which brings down the hides as in a bate, and renders it impossible later to produce a firm and plump leather. Too great astringency, on the other hand, is equally dangerous, since it also checks swelling, and hardens the surface, producing perhaps a “semi-permeable membrane,” which it is very difficult for the tannin to penetrate later. Such liquors also cause “drawn grain,” for if the surface of the hide is fixed by too early tanning, and the interior afterwards swells in thickness and contracts in area, the grain becomes puckered in wrinkles which are the larger and coarser, if through ill-managed liming the fibrous texture immediately below the surface is too much loosened.

The “handlers” are a series, conveniently say of 12 pits, in which the butts are laid flat instead of suspended, since in this way a much larger number can be got into each pit; and the hides and not the liquors are changed from pit to pit. The “pack” from the suspenders is, of course, brought first into the weakest and oldest pit; the “forward” or most tanned pack is removed to the “layers,” or in some cases to a second and stronger “shift” of handlers; and the intermediate packs are each moved forward one pit, all getting thus a change into a stronger liquor. As, however, the strength of the liquors must be maintained by fresh liquors, if possible from the “layers,” the new “forward” pack is again raised (conveniently next day), the whole series is again shifted forward, and the liquor vacated by the last pack is run to the suspenders, and replaced by a new liquor to receive the forward pack. If this and the bringing in a new pack takes place on alternate days, the passage of a pack through the entire “shift” or “round” will occupy four weeks, including Sundays. It is most usual to add to the liquor of the forward pack a few pailfuls of finely ground material, bark, myrobalans, or valonia. The object of this is less to increase the strength of the liquor than to prevent the actual contact of the butts. Before bringing the later packs into a pit, this “dust” is well plunged up to distribute it as evenly as possible between the butts. It is not uncommon to have two or even three head packs or “dusters” which are moved alternately so as to give them a longer time in the new liquors, but this introduces complications which cannot be discussed here.

In the handlers and layers the goods were formerly handled with sharp steel hooks on long poles, but as these are apt to produce serious scratches they have been to a large extent abandoned for the method of handling with strings, described on p. 27, which is also much quicker, and avoids the need of partially skilled labour.

If the liquors are well maintained, and the goods are light, they will probably now be ready for the “layers” that is to say, thoroughly tanned through, but still wanting in solidity and weight. They are no longer capable of rapidly absorbing tannin, but can still fix considerable quantities of bloom and reds (p. 101) on and between the fibres. These difficultly soluble matters are supplied by strong and fresh liquors which are supersaturated, and deposit them not merely on and in the hides, but in the liquors and on the sides of the pits. It is hence desirable to bring the actual tanning materials as closely as possible in contact with the leather, and to maintain the strength of the liquors during the comparatively long periods for which the butts must remain undisturbed. To accomplish this, much larger quantities of strong solid materials, such as valonia, mimosa bark, and (in moderation) myrobalans are employed than in the handlers, and these serve also the purpose of separating the butts and maintaining a larger volume of liquor between them. Instead of throwing this material in three or four portions into the liquor, and trusting to its distributing itself between the goods, each butt as it is drawn on to the surface of the liquor, receives a regular sprinkling; a second is drawn on and similarly sprinkled, and so on, and the whole are allowed to sink as evenly as possible into the liquor. This method of making a layer is now universal in England, but an older method is still largely in use on the Continent, in which the goods (generally whole hides) are spread in the vats with thick layers of dry material between them, and the liquor is only run on when the pit is filled, which is then often left undisturbed for three months or more. The method dates from a period when the solid materials rather than the “weak” liquors were relied on to supply the tannin, and has the disadvantage that the hides are often deeply pitted by fragments of tanning material pressed into them, but it must be admitted that the leather produced in this way, though sometimes unsightly, is very solid and durable.

The liquors used for the last layers are the strongest which can be made and, since the almost universal use of oakwood and chestnut extracts, frequently reach. 100° or even 120° of the barkometer (S. G. 1·100—1·120) while in pure oak-bark tannage it is difficult to exceed 30°, and then only by strengthening repeatedly used liquors. The goods, when it is desired to change the liquors, are pulled out, the liquor pumped off and the partially exhausted solid material removed for further extraction, and the goods are returned to a new liquor in the same pit. Layers of a month to six weeks were formerly the rule, but it is better practice to begin with one even as short as a week or ten days, gradually extending the time as the goods become more fully saturated, and exhaust the liquors more slowly. It is only by constantly maintaining and increasing the strength that the goods will continue to “feed,” and it is waste of time to allow them to lie in a liquor which has fallen below its original strength. Of course, if layers are to be short, less solid material is given. Light goods are fully tanned in two or three layers; heavy ones may require four or five, but the careful tanner will investigate how far the gain of weight and solidity repays for the cost of additional time and material, which, as the goods become fully tanned, produce but little effect.

After the last layer, goods were formerly merely washed in a weaker liquor before being sent to the drying shed, but some sort of bleaching is now almost universal where extracts are largely used. A strong warm myrobalans liquor produces a good effect, but recently “vatting” in warm strong solutions of “bleaching extracts” after scouring has become customary. These are mostly quebracho extracts containing much bisulphite of soda, which not only bleaches by its sulphurous acid, but has the property of dissolving and removing the “reds” deposited in the leather. The effect is to render the leather brighter in colour without removing much weight, but at the same time to make it more porous and much more permeable to water. It is hard, however, to see how the practice is to be avoided so long as the public and the shoe-manufacturers continue their absurd demand for soles of light and bright colour!

The finishing of sole-leather is simple in theory, but, especially with heavy extract tannages, not at all easy in practice. Before drying, the goods are very lightly oiled on the grain side, usually with crude cod liver oil, with the object both of lessening oxidation by the air, which darkens colour, and of checking evaporation from this side, which would tend to bring the dark liquors, still present in the interior, to the surface by capillarity. The first drying must be slow and even, and at a low temperature, and if possible is accomplished without artificial heat. When the goods are half dry (or “sammed”) they are laid in a pile, damped in dry places, and often allowed to heat a little by the incipient growth of mould, which, though somewhat dangerous, facilitates the next process, that of “striking.” This, if done by hand, is accomplished on a horizontal wooden beam of rounded section on the upper surface, and about 7 ft. long. The tool used is called a “striking pin,” and is a two-handled blade of triangular section with three edges, with which the workman stretches and smooths the grain side of the leather, leaning over the low beam, and putting his full weight on the tool. If it is desired to strike the bloom “out,” so as to show the original colour of the leather as fixed in the suspenders, the surface is kept thoroughly wet, often with slightly soapy water, the tool used is pretty sharp, and is aided by a stiff brush. If the bloom is to be struck “in,” and afterwards concealed by colour, a blunter tool, a dryer leather, and a little oil are employed, and loose bloom is afterwards washed off. The process, however, is now generally performed by a machine carrying four blades or “slickers” on separate spring-arms which work outwards from the centre of the butt, and lift on the return stroke (Fig. 11). In place of removing the bloom by striking it is now generally removed by scouring tools of smooth sandstone, aided by brushes and plenty of water, before vatting and usually by machine. After the first striking, the goods are dried a little further, piled again overnight, and again struck to smooth them and remove tool marks, often after rubbing the surface with an oily rag. After the second striking, they are rolled, now almost universally by machines of the type shown in Fig. 12, coloured if desired by sponging with a pigment or dye solution, often containing some mucilaginous ingredient and a little linseed oil to heighten the gloss, again rolled with a heavier pressure, and hung up in a warmed and well-ventilated room to dry off. The whole process of finishing takes nearly a fortnight, and much judgment is needed as to the exact “temper” or degree of dryness required for each of the operations. Such is a brief description of the usual methods, which, however, vary in detail in different yards.

Fig. 10. Striking pin and section.

Fig. 11. Butt rolling machine.

Fig. 12. Wilson’s striking machine.

The common American method is much simpler and rougher. The goods are dried completely after light oiling, damped back and piled to equalize, and rolled twice with pendulum rollers driven at a high speed, which give a good gloss, but not the even colour and freedom from wrinkles which is demanded in English leather.

¹ Fuller information as to structure is given in the Lea. Ind. Laboratory Book, Chap. XIX.