The mode of finishing which was formerly, at least, in vogue in Lancashire and Cheshire may be taken as a type of the best work. (In the present day, the various methods are so widely known that they have ceased to be local, and are varied according to the quality and tannage of the goods.) The butts, which in earlier times were largely bark-tanned, are taken wet from the pits, and scoured on a rounded beam or “horse” with stone and brush, till the bloom is completely removed, and are then lightly oiled on the grain, half dried (“sammed”), laid in pile to temper, and “struck out” with the “pin,” a two-handled tool of triangular section shown in Fig. 29. The use of this tool has now been largely superseded by Wilson’s striking machine Fig. 39, in which knives or sleekers (or stones and brushes), held in jointed arms, are made to work on the butt, which is extended over a slowly rotating cylinder. The object of the pinning is not so much to remove bloom or dirt, which has been previously effected by the scouring, as to smooth and flatten the grain. After further drying, a second pinning is generally given, and the goods are then twice rolled, first with a light weight, and somewhat moist grain, and then more heavily with the grain nearly dry. This was formerly accomplished by a sort of box or car, heavily loaded with weights, supported on a smooth brass roller of about 5 inches diameter and 9 inches long, and manipulated with a long wooden handle on a floor of hard wood, or zinc plates. One type of the machines which have now almost entirely replaced this primitive contrivance is shown in Fig. 40, but is principally used for offal and common classes of goods. For better work, traversing rollers, such as Wilson’s ingenious double bed roller shown in Fig. 41, are to be preferred. After rolling, the goods are dried pretty rapidly by the aid of moderate heat, and, after polishing with a brush (hand, or machine, Fig. 42), are ready for sale. It may be pointed out that although the tools are different, the process is almost the same as that used for “vache lissée” in France and Belgium, and closely resembles that of currying harness leather except that the “stuffing” with fats and oil is omitted.

In contrast with the rather elaborate method just described, we may place the American finish of red hemlock sides, which are tanned throughout with a material which yields no bloom. On these, the scouring and “striking” is altogether omitted: the goods are completely dried out from the pits, which is found to fix the dark-coloured liquor, and result in better colour; they are then damped back, and tempered, and heavily rolled under a rapidly moving pendulum roller, which polishes at the same time that it smooths the leather. The saving of cost by so simple a process is not inconsiderable.

In the West of England, much heavy leather is still manufactured from South American hides, which are tanned with a large proportion of valonia; and which consequently are heavily bloomed. No attempt is made to remove this bloom, which would too much lessen the weight and firmness, but the goods, after a light oiling to preserve the colour, are hung up and partially dried, and are then laid in pile to temper. The grain side is now wet with soap and water, with which a little oil is often mixed, and the bloom is “struck in” with the pin or machine; a somewhat blunt pin being used, or a blunt tool in the striking machine; which is held at such an angle as to smooth and compress the grain without taking too much hold on it. After a little further drying, the striking is generally repeated, the goods are washed over with water, and rolled “on.” They are now coloured with a mixture of pigment colour, generally containing a large proportion of whitening, or sometimes of French chalk coloured with ochres, chrome-yellow and orange, or whatever may suit the tint preferred by the tanner, or best imitate the colour of a clean-scoured tannage, and usually mixed with size and oil, or sometimes with oil and tan liquor. This mixture is well rubbed in, and smoothed over with a cloth, and then polished by brushing, when the goods are “rolled off,” rapidly dried, and again brushed. If the work has been well done, it is not easy to distinguish from clean scouring, and is much cheaper.

A method intermediate between this and the first described, and which was formerly much used in London, was to proceed as above, but using more water and holding the pin in the first striking so as to scour out as much bloom as possible, and assisting this by the free use of water and the brush. Instead of using an opaque pigment-colour, the goods were generally coloured either between striking and the first rolling, or between the two rollings, with a transparent colour, such as dissolved annatto, or a mixture of aniline dyes, so as to conceal the traces of bloom, and to render slight damages to the grain less conspicuous.

The principles of the manufacture have been fully explained in previous sections, up to the time when the goods are taken into the tanning liquors. At this stage complicated reactions take place between the lime in the butts, the free vegetable acids in the liquors, and the tannins; and on the right adjustment of these three factors much of the success of the operation, and indeed of the whole manufacture depends. If the lime is in excess of the acids present, it forms insoluble compounds with the tannins in the surface of the hide. If these are protected from the air, they are generally redissolved as they advance into more acid liquors, but they readily become oxidised into dark-coloured matters, which can no longer be removed. Their presence in the finished leather is one of the great causes of darkening in drying. If the hide in the limy condition has been exposed either to the carbonic acid of the air, or to free carbonic acid, or acid calcium carbonate dissolved in water (“temporary” hardness, p. 94), a precipitate of calcium carbonate will be formed in the surface, which is much more difficult to remove than free lime, and which is perhaps the most common cause of the stains and discolorations which are so serious a source of loss to the sole-leather tanner. These stains may, if not too much oxidised, be removed by treatment of the tanned leather with weak warm sulphuric acid, but this remedy brings other evils in its train, and should not be required. The great remedy is to keep the goods from the time of unhairing till they go into the liquors, under water in which there is always a trace of caustic lime, or which at any rate are free from carbonic acid. In deliming sole-leather with acids, it is best to give the full dose of acid required, at once, and not gradually, so that it may act most powerfully on the exterior, and remove any carbonates present, before it penetrates to and becomes neutralised by the excess of lime in the interior. This is exactly the reverse of what is advisable with dressing leather, where the object of the tanner is to remove lime as uniformly and completely as possible, without excessive acidity of any part. Of course hides should not, even in the case of sole-leather, be allowed to go into the liquors while any acid swelling of the surface remains, but this will soon disappear if the goods are suspended for a time in cold water after deliming, unless excess of acid has been used (cp. p. 153 et seq.).

If the proportion of free acid in the suspender liquors is as it ought to be, it is probably rather advantageous than otherwise for a little lime to remain in the interior of the hide, as it keeps the pelt in a plump condition during the first stages of colouring, quickens the penetration of the tannin, and lessens the tendency to “drawn” or wrinkled grain, which arises when the goods go into the liquors in a flat or fallen condition. The causes of drawn grain are often a little obscure. Of course that case needs no elucidation in which the hides are submitted to the tanning liquor in a creased or wrinkled condition, which is simply fixed and made permanent. This may arise, either from carelessness in handling the goods before taking into the suspenders, or from the way in which they are slung to the sticks, which often draws them into long wrinkles, afterwards difficult to remove. Drawn grain in general, however, arises from the grain-surface becoming tanned and fixed in area, while the substance of the hide is in a more extended condition than that which it assumes as tannage proceeds. Hides in a flat and unswollen condition are thinner, the fibres are slenderer and looser than when swollen, and consequently the hide has a larger area. If, after the grain is tanned, the substance of the hide becomes contracted in the liquor, either by swelling with acids, or by the direct action of the tannin on the interior fibres, the grain is certain to be shrivelled, like the skin of a dried apple. A similar effect, produced in a mechanical way, may always be noted where a hide has been coloured hanging grain-side out over a pole, so that the surface is extended at the bend, on which long wrinkles are formed as soon as it is straightened.

A hide in a slightly alkaline condition colours, and even tans more quickly than one which is acid. In presence of a trace of lime, and deficiency of free acid in the suspender liquor, tannages of valonia and bark give the butt a sort of lemon-yellow colour, which is not in itself injurious, and which disappears as the hides advance into more acid liquors, but which is a sign of danger, as showing that no excess of acid exists in the suspender-liquors. Gambier gives pelt perfectly free from lime a pale buff colour, but where lime is present, the colour is always reddish, and much darker, and this coloration does not disappear so readily as that with valonia, so that if gambier is to be used in the first liquors, care should be taken to remove all lime from the surface. The only known tannin which gives no insoluble compound with lime is that of the babool pod (sometimes called “gambia-pod”), which is frequently used in India as a bate, and which would probably prove very useful in colouring liquors (pp. 165, 288).

When sole-leather first goes into liquors, it is generally swollen with lime to some extent. If the liquors contain, as they usually do, sufficient free acid (acetic, lactic) in addition to the tannins, these combine with and neutralise the lime, and the pelt, without absolutely becoming flat and thin, loses its firmness, and becomes soft and spongy. This is a favourable condition for the absorption of tannin, but care should be taken not to allow the pelt to be squeezed or pressed, or water will be squeezed out, and the pelt will not easily resume its plumpness. As the tannage proceeds, both the tannin and the acid of the liquors penetrate deeper into the pelt, the former tending to contract and the latter to swell the fibres. Thus a given quantity of acid will cause the greater swelling, the less tannin is present; and therefore in strong tanning liquors more acid is required. The presence of certain products of bacterial putrefaction has a great but unexplained effect in preventing hide from swelling with acids; and in hot weather, much better swelling is obtained by sterilising and deliming the hides with one of the coal-tar products mentioned on pp. 30, 162. Boric acid may also be satisfactorily used for this purpose, but should not be allowed to get into sole-leather liquors, as it tends to produce a soft and loose tannage, and from its inorganic and indestructible character, is apt to accumulate in a yard in which it is used. The same reasons render unadvisable its introduction into any liquors which are to be returned to the leaches even in the tannage of dressing leather, though its presence in the colouring liquors is otherwise very useful in lessening the astringency of the tannins (“mellowing the liquors”), and making a fine grain. Its mode of action is by no means clearly explained, but is in some way connected with its tendency to produce “conjugated acids” (L.I.L.B., pp. 37, 46).

The so-called “mellowness” of old liquors requires a word of comment. It is well known to practical tanners, that old liquors are much less liable to produce drawn grain, and a harsh surface, when used to colour green goods, than liquors, even equally weak, which have been made from fresh materials. This is probably due, in part at least, to more than one cause. Most natural tanning materials contain tanning matters of varied degrees of astringency and power of attaching themselves to the leather-fibre. It is obvious that if a tanning liquor is used, the most astringent and energetic tannins will be first removed from it, leaving those of a milder character. It is also known that the presence of neutral alkaline salts of weak acids has considerable influence in producing mellowness; the addition, for instance, of sodium acetate has a marked effect. This effect is probably due in the first place to the action of neutral salts in diminishing the energy of weak acids (see p. 81), and secondly to the fact, that their bases combine to some extent with the tannins; and that, as was perhaps first pointed out by the writer, such tannins are, as it were, partially paralysed in their action on hide (p. 339). Sodium sulphite acts powerfully in this way, and may perhaps prove of technical value in temporarily diminishing the astringency of liquors in quick tannage. Borax has a similar effect, but is too alkaline, and, unless used with extreme caution, spoils the colour of the liquors by causing oxidation. It is probable that similar causes explain the mellowness of palmetto extract, which contains large quantities of alkaline salts, and of some extracts which have been treated with sulphites, when used undiluted in drum tannage. The addition of free acid will generally restore these tannins to an active condition.

As the tannage proceeds and penetrates further into the hide, the liquors are used stronger, as the outside, once tanned, is to a large extent protected from their action, and it is only by continuously increasing the strength of the liquors that more tannin can diffuse into the interior, since diffusion only takes place from a stronger into a weaker liquor. The liquor in the interior of the butts is always exhausted of tannin so long as any part of the hide-fibre remains untanned, but as the layer of tanned fibre between this and the outside gets thicker, a greater difference is required to maintain a reasonable rate of exchange, just as a greater head of liquor is required to maintain a flow of liquor through an increased number of percolation-leaches. If the strength of the liquor outside be allowed to fall off, this graduation of strength from the outside to the inside of the butt is disturbed, and takes some time to re-establish. As the liquors become stronger in tannin, they may also become somewhat stronger in acid, since, as has been stated, the two act to some extent in opposition to each other. The acid-swollen fibre absorbs the tannin more slowly than if it were in more neutral condition, but it absorbs it apparently in larger quantity, and at any rate, makes a firmer, solider, and less flexible leather.

It has been mentioned that in the latter stages of the process, solid tanning materials are generally strewed between the butts in the tanning liquor. It may be pointed out that many materials vary in their tanning effect, according to whether they are used in solid form or merely in liquors. It has been shown by Youl and Griffith[126] that such materials as valonia, oakwood and chestnut extracts, and myrobalans, which contain both gallotannic and ellagitannic acids, lose strength rapidly when kept in the form of liquor, the ellagitannic acid becoming decomposed with separation of insoluble ellagic acid. Now it is just this ellagic acid, which deposited in or on the leather, gives weight, solidity and bloom, and the investigation points out not only an important source of loss in the tanning industry, but also, why valonia, which in sole-leather tannage is known to give hard and heavy leather, can be used in large quantities on dressing leathers in Yorkshire, with gambier, in the form of liquor, giving a soft and mellow leather almost destitute of bloom. If weight and solidity are required from the use of such materials, it is obvious that they must be brought into immediate contact with the leather to be tanned, so that as large a part of the bloom as possible is deposited in, and not outside the leather. With many other materials, such as hemlock, quebracho, and mimosa, which yield no bloom, but “difficultly soluble” tannins (reds or phlobaphenes), the same rule holds, since in contact with the hides, the small proportion of these materials which is soluble in the liquors, is replaced from the materials as rapidly as it is absorbed by the leather, while, when liquors or extracts only are used, the greater part of these solidifying and weight-giving constituents remain unutilised in the spent tanning materials. At the same time, the long “layers” afford an opportunity for the acetic and lactic fermentations to go on which are the principal source of the natural acidity of liquors. It must be understood that what are called layers in England, are not to be identified with the Sätze, but rather with the Versenke of the German tanner, the former being layers given in much the same manner as was current in England 150 years ago; in which the leather, with thick layers of tanning material between it, is laid into the empty pit, which is afterwards filled up with liquor, often of a comparatively weak character. In such layers, the acidification, and the solidification of the leather both go on to a still greater degree; the acid formed, apparently gradually penetrating to the heart of the leather-fibres, and producing a solidity, and cheesy texture which can hardly be obtained by layers of the English kind; which nevertheless have the advantage in rapidity and cheapness.

In drying sole-leather, one of the great objects which must be aimed at is to remove the dark coloured liquor, with which the goods are saturated, from the surface, and to prevent further portions of it from finding their way there from the interior. If a strip of filter-paper be allowed to rest with one end in a basin containing a little liquor, and be placed in a draught of air, the exposed end of the paper will rapidly become dark brown or black, the liquor which evaporates there being constantly replaced by fresh portions sucked up by capillary attraction from the basin. A similar action is constantly seen, when filtering liquors through paper, if the latter be allowed to project above the edge of the funnel. Precisely the same effect occurs, perhaps increased by the oxidation of the tannins, on the edges and other parts of a butt which are most exposed to draughts of air. The use of oiling the grain is not only, to a certain extent to protect it from oxidation, but also to check evaporation, and the consequent accumulation there of the dark-coloured solids contained in the liquor. A very similar result is attained by wetting the grain-side, and allowing as much of the evaporation as possible to take place from the flesh.

The process of sole leather tanning has been discussed in considerable detail on account of its simplicity and importance. It is now time to point out in what respects the tannage of the lighter leathers differs from it in principle. Taking the case of ordinary dressing leathers, such as kips and shaved hides, the first point to remember is that these goods come into the liquors not merely almost entirely deprived of lime by bating, but in a very flat and fallen condition from the action of the bacterial ferments of the bate. As a general rule in this country the colouring is done in paddles, but where a very smooth grain is required, the use of suspenders is to be recommended, and in America is largely adopted. Indeed in the States the entire tannage of much of the cheaper leather is done in suspension, and the sides are only removed from the laths to which they have been nailed, when they are required for splitting. It is obvious, from what has been said of sole leather, that as the hides are brought into liquors in a very fallen and extended condition, the grain will be likely to be wrinkled; and indeed this is sure to be the case unless, by suspension, the hide is more or less kept in tension till its fibres are fixed by tanning. The free motion in the paddle favours the formation of a “pebbled” grain, since the hide is bent now this way, now that, and minute wrinkles and creases are formed in all directions. For many purposes, and especially if a grain is afterwards to be raised by “boarding” the curried leather, this graining in the paddle is not disadvantageous, so long as it is not excessive. In some other cases it causes much trouble and labour to the currier before it is removed, and if the English tanner and currier are ever to compete with the American in smooth grain finishes, it will be necessary for them to obviate this source of wasted labour. The graining is the less considerable, and the easier to remove, the weaker and more mellow are the liquors employed in colouring and the more gradually their strength is increased.

The production of a soft leather depends on the fibre being tanned in a fallen and unswelled condition. It is for this reason that bating is in many cases essential, though where somewhat firmer leathers are required, mere reduction of the swelling by removal of the lime is sufficient. For the same reason, no acid-swelling is permissible either before tanning, or in the liquors, and though liquors for soft leathers must be rather acid than alkaline, they are incapable of removing any large quantity of lime, and for the best results, the deliming must be complete before tanning. As mere bating or puering is mainly designed to reduce swelling by the action of bacterial products (p. 172), and is not a very efficient means of removing lime, it is desirable where it is employed, to supplement it by some more active deliming process. In the lighter leathers, drenching (p. 166) generally fulfils this purpose and many of the more intelligent tanners now give bated hides a bath in boric acid before tanning, which not only removes the last traces of lime without acid-swelling, but checks the bacterial fermentation, and prevents its introduction into the liquors. In gambier tannages, a decidedly better colour is obtained by this treatment (p. 228).

In most cases the production of bloom is not desired in dressing leather tannage, and is prevented by relying chiefly on liquors, and avoiding the use of bloom-giving solid materials, which include most pyrogallol tannins. Dressing leather tannages can frequently be advantageously hastened by drumming: which by continuously bending the leather in all directions, constantly widens and contracts the spaces between the different fibres, and, as it were, pumps the liquor through the skin. The softness of dressing leathers is increased, and the hardening action of acids present in the liquors is prevented by the addition of salt, or of some sulphates (sodium, magnesium, ammonium) which exercise a sort of pickling action on the fibre, and prevent its swelling, but at the same time tend to light weight and a somewhat empty tannage. It by no means follows that a hide or skin which is thoroughly coloured through, is really fully tanned; as, though the fibres may be actually tanned or coated on the surface, time is required for the tannins to penetrate them to the centre. This incompleteness of saturation is often found in drum tannages. Such leathers are generally tough, and gain weight and softness in currying. In order to “carry grease” well, that is, to absorb a large quantity without appearing greasy, it is essential that the fibre-bundles should be thoroughly split up or differentiated; and the degree to which this is attained largely depends on the extent of liming. There is also considerable difference in different tannages, as to the amount of grease which they will carry.

It is now not uncommon to combine a degree of alum or chrome tannage with vegetable tannage in the finer dressing leathers. For further information on this the reader must be referred to the next chapter.

The finest sorts of leather such as goat, calf, sheep and seal for bookbinding, upholstery and the like, are mostly tanned with sumach; paddles and drums being largely used to quicken the operation. Leather tanned with sumach has been proved by the researches of the committee of the Society of Arts on the decay of bookbinding leathers[127] to be the most durable leather for this purpose, some other tanning materials of the pyrogallol class coming near it in this respect, while all catechol tannages are found peculiarly liable to destruction by the action of sunlight, dry heat, gas fumes, and traces of sulphuric acid from other sources, although in many cases more durable than the pyrogallol tannages when exposed to mechanical wear and moisture, as is the case with shoe-leather (p. 298). East India sheep- and goat-skins (so-called “Persians”) are tanned with the catechol tannin of turwar or cassia bark.

The finer leathers of which we are now speaking are almost invariably prepared for tanning by puering with dog-dung, and drenching with bran, as colour and softness are the special characteristics aimed at. A somewhat interesting style of tannage is occasionally used for sheep-skins (roans), and calf-skins, in which the skin is sewn into a bag, flesh side out, with only a small aperture left for filling at one of the shanks. It is then turned grain-side out, and filled with strong sumach liquor, and a little leaf sumach, and floated in a bath of warm sumach liquor. After a short immersion, the skins are piled on a stage, so that the liquor is pressed through them by their weight; and when partially empty, they are refilled and the process repeated. The time of tannage is very short, not exceeding about twenty-four hours, and the leather produced is very soft.

CHAPTER XVII.
COMBINATION OF VEGETABLE AND MINERAL TANNAGE.

In very early times leathers were produced, which were partly tanned with alum, and partly with vegetable materials. One of the earliest of these was probably the Swedish or Danish glove-leather. The principle has long been applied to the production of certain very tough and flexible leathers known as “green leather,” and used for “picker-bands” for looms, laces for belting, “combing-leathers” and some other purposes where softness and toughness are of principal importance. About twenty-five years since, it was applied in America by Mr. Kent to the manufacture of an imitation of glazed kid, which he named Dongola leather; and since that time, the method in various modifications, has taken a considerable place in the manufacture of the finer leathers for shoe purposes, especially in the United States.

Alum-tanned leathers, as has been already stated, are remarkable for softness and toughness, and the mineral (crystalloid) tannages have the power of penetrating and isolating the individual fibrils of the skin in a much greater degree than the vegetable tannins, and hence are less dependent than the latter on a previous isolation produced by liming. On the other hand, they give much less plumpness and solidity, and more liability to stretch, and are less resistant to the action of water; and are, as a general rule (to which some chrome-tannages are an exception), incapable of producing a soft leather without mechanical softening (staking) after the tannage is completed. Purely mineral tannages have always a woolly fibrous structure, and never the firm and compact flesh which is required in leathers which are to be “waxed,” or finished on the flesh side to a smooth surface, and as they communicate more or less of these peculiarities to combination-tannages, the latter are mostly used, either for grain-finish, or for uses where a soft and velvety flesh-side is required, as in the case of “ooze-” or “velvet-” calf. On the other hand, the partial use of vegetable tannage communicates to them a degree of plumpness, fulness and resistance to water which is not possible to alum-tannages pure and simple, and a softness which is not easily obtained in vegetable tannage without the use of large quantities of fats or oils. A preliminary mineral tannage also greatly increases the rapidity of the penetration of the vegetable tans, by isolating the fibres, and rendering them less gelatinous. Once a leather is thoroughly tanned by vegetable materials, it is little affected by subsequent treatment with alumina, or even with chrome; and on the other hand, though chrome and alumina leathers are still capable of absorbing considerable quantities of vegetable tannins, they always retain, in a degree, the qualities which the mineral tannage has communicated to them. The resulting leathers are thus not only modified by the different proportion of vegetable and mineral tannages which have been given, and by the properties of the particular vegetable tannage used; but by the order in which the several treatments have been given, and always retain, to a considerable extent, the characteristics of that which has been first applied. We have thus in our hands a powerful means of modifying the character of our leather to suit the special requirements which it is to fulfil.

So long as tanners were restricted on the one hand, to the ordinary methods of stuffing tanned leathers with oils and fats, and on the other to the use of egg-yolk, which had long been common in alum-tannages, combination-tannage remained of but secondary importance, and it was the application of the method of “fat-liquoring” by James Kent to his Dongola leather, which gave them the place they now possess, by providing a cheap substitute for egg-yolk, and enabling the tanner to obtain softness and resistance to water, without producing the greasy feel which is common to curried leathers. The process of fat-liquoring has already been mentioned in connection with chrome leathers, to which it was subsequently applied, and we shall return to it, after having given some further details of the methods of tannage.

In the first place we must consider briefly the mutual action of the mineral and vegetable tannages on each other. It has been pointed out by Eitner, and also mentioned (p. 339) in connection with the decolorisation of extracts, that the addition of say ¹⁄₂ per cent. of alum, or aluminium sulphate to tanning liquors, lightened their colour, not only by giving a degree of acidity to the solution, but by precipitating a portion of the less soluble and more darkly coloured tannins. Chrome-alum, and basic chrome salts produce a similar effect, though from their marked colour, the lightening of the solution is not so easily observed. It is therefore advisable if these salts are to be used in actual mixture with the vegetable tans, to allow the solution time to subside, or to filter off the dark-coloured precipitated matters. Larger quantities than ¹⁄₂ per cent. of the alum do not appear materially to increase the effect just described.

A second effect produced by these mineral salts on vegetable tannins, is in many cases to develop mordant colouring matters which are present; and thus, since most of these colouring matters are yellow, to produce a yellower leather than would be obtained with the vegetable material alone. This effect is very marked in the cases of sumach, gambier and quebracho. The compounds which these colouring matters form with chrome are mostly of a darker shade than those with alumina, tending to olive, and therefore chrome-combination leathers are generally dull in colour. Potassium dichromate, especially if acidified, generally oxidises and precipitates tannins, and darkens their colours, so that it is not practical to follow a vegetable tannage by the two-bath chrome process; and though the reverse order may be pursued, the single-bath chrome process, and that following and not preceding the vegetable tannage, generally gives the best results. If lightly tanned leathers, such for instance as the imported East Indian tannages, with babool or turwar barks, be treated with a basic chrome tanning liquor, such as is described on p. 215, so large a proportion of chrome will be absorbed, that the leather will possess most of the characteristics of a genuine chrome leather.

Combination-tannages for glove-leathers, such as the Danish and Swedish leathers already alluded to, are generally first tawed with alum and salt, with or without addition of flour and egg-yolk, and are then coloured, and more or less tanned with vegetable materials. That employed on the original Danish leather was willow bark (of Salix arenaria). In France, where this willow is not found, the bark of the commoner Salix caprea was substituted; and as it is much weaker in tannin, additions of oak-bark or sumach to supply the deficiency, and of madder to give a redder colour were made to it. The dyeing of these leathers is frequently combined with the tannage, dyewoods or dyewood liquors being mixed with the tanning liquors. In the manufacture of glazed French kid, indeed, the process is so arranged, by brushing on dye-liquors mixed with tannins, as merely to tan the grain-surface, which is necessary to enable it to be glazed by friction, leaving the substance of the leather of purely alum tannage.

On the other hand, in the “green leathers” (so-called from their greenish-yellow colour, and largely made in the West Riding of Yorkshire), the hides usually receive a light gambier tannage, extending over a week or so in weak gambier liquors in handlers, and are then “cured” by handling in hot and strong solution of salt and alum, in which they are finally left all night, and then dried rapidly without washing out the alum, much of which consequently crystallises on the surface. This is slicked off, and the leather damped back, and heavily stuffed with sod-oil. If, however, the combination-tannage is properly carried out, it will stand liberal washing without losing the necessary alum, and of course a tougher and more satisfactory, though somewhat lighter weighing leather results. It is in many cases a better plan to combine the two tannages in one bath, mixing the alum and salt with the gambier, and handling or paddling the goods in the mixture. This is the plan usually adopted for Dongola leather, in the United States. For skins which are to be glazed, it is important that the surface should be tanned with the vegetable material, and the goods are therefore worked into gambier liquors, to which the salt and alum are only added after the tannage has made some little progress; while for dull Dongola, intended rather to imitate calf-kid, it is best for the alum and salt tannage to begin first. For goat-skins for glazed Dongola kid, about 4 lb. of block gambier, ¹⁄₂ lb. of alum, and ¹⁄₄ lb. of salt are used per dozen, and the tannage occupies in all about twenty-four hours.

After the skins are tanned, they are thoroughly washed out with tepid water, to remove loose alum and gambier, and are then ready for fat-liquoring. As in the case of chrome leather, it is of great importance that this washing should be done thoroughly, as any remaining alum which diffuses into the fat-liquor, will cause it to curdle. If the washing is thorough, the more neutral the fat-liquor and the better; but a somewhat alkaline soap-solution is less liable to curdle. The original fat-liquor used by Mr. Kent was the alkaline liquor which had been used in washing the surplus oil from chamois leather (see p. 380), but now soap- and oil-solutions are generally made specially for the purpose. Most of the remarks in the chapter on chrome tannages are applicable in this case, but probably fat-liquoring is somewhat easier than in the case of chrome. Mixtures of either soft soap or curd soap with cod, sod, and olive oil are frequently employed. Sesame oil also seems well adapted for the purpose. The better these are emulsified, and the more satisfactory is the result; a cylinder of zinc or copper fitted with a plunger, something like that of a “Lightning Egg Beater,” but covered with perforated zinc, or wire gauze, does very good service as an emulsifier on a small scale.

Another method is to melt the soap with just sufficient water to make it pasty, and to incorporate the oil thoroughly with the mass, which is afterwards dissolved in hot water. Oils are the most easily emulsified when they are somewhat acid. For this reason rancid olive oils are often used for the Turkey-red process, but a similar effect can be obtained by adding a small quantity of candlemaker’s oleic acid to the oil before mixing. The addition of sulphated castor oil (Turkey-red oil) also helps emulsification, and is in itself a very good softening agent. One of the commonest mistakes in fat-liquoring is the employment of too strong an emulsion—even so small a quantity as ¹⁄₂ per cent. of soap, and half that quantity of oil, reckoned on the wet weight of the well-drained leather, will produce a very notable softening effect. Of course, for dull finishes, much larger proportions may be used. Not only combination-tannages, but those entirely vegetable, can be fat-liquored with excellent effect, and the process is now largely used for coloured calf, and other leathers, which are required soft and nourished, but without any appearance of greasiness. Leathers absorb the fat-liquor most readily if put into it in a sammed or partially dried condition, but even if quite wet, they soon take up the whole of the oil and soap on drumming, leaving only a little clear water in the drum. Goods may be blacked while still wet with fat-liquor, but should generally (except in the case of chrome leathers) be dried out before dyeing, as this fixes the oil and soap in the fibres.

Many coloured leathers are now made by a process which may be considered a combination of the Dongola process itself with the ordinary process of vegetable tanning, the goods being coloured and partially tanned as if for a vegetable tannage, and then finished in Dongola liquors with alum, salt and gambier. Very good leathers are made in this way in the United States, with a tannage begun in suspension in hemlock bark liquors.

Imitations of Dongola leather are made by treating East India sheep or goat with alum liquors, and afterwards fat-liquoring (if necessary)[128] and finishing like genuine Dongola leather. The treatment is most effective, if a portion of the original tan be removed by washing with warm water, with a little borax, ammonia, or even soda, and the goods then alumed with a “neutralised” or basic alum solution such as that described on p. 187. Goods treated with a basic chrome-liquor, like that used for the one-bath chrome process, p. 212, are almost converted into chrome-tanned goods, and will even stand some degree of boiling. The use of a liquor made like the Martin-Dennis liquor, by dissolving chromic oxide in hydrochloric acid, was the subject of an American patent[129] which in this country is owned by Wichellow and Tebbutt, but which expires in 1903.

Chrome-combinations may also be made by retanning goods tanned by either of the chrome processes with vegetable materials, of which gambier seems the most suitable. The use, even of very weak liquors of sumach and most other tanning materials, deprives chrome leather of its stretch, and if carried to excess, readily makes it hard and tender.

CHAPTER XVIII.
VEGETABLE TANNING MATERIALS.

As has been stated in the previous chapter, our knowledge of the chemistry of tannins is not sufficiently advanced to render possible any strictly chemical classification, while an additional complication arises from the fact that very different tannins may coexist in the wood, bark, fruit, galls, etc. of the same plant. It therefore seems best to follow the example of Prof. Bernardin in his ‘Classification de 350 matières tannantes,’[130] and arrange the plants under the orders of the natural system of botany, as has already been done by von Höhnel[131] and A. de Lof.[132] In the following pages, only those materials which from their high percentage of tannin, or from some other cause, are of commercial interest or value, are included, as the tannins are so widely distributed in the vegetable kingdom, that any exhaustive list would be quite out of the question.

Tannins are not confined to any particular part of the plant, though they are usually most abundant in barks and fruits. Insect-galls are often very rich in tannin, usually gallotannic acid; while in several cases woods are of commercial importance from their cheapness, though the percentage of tannin they contain is not generally high. The function of tannins in the vegetable economy is not well understood. In some cases they are probably a waste product of plant-life, and may help to ward off attacks of insects. They usually exist as cell-contents, and as vegetable cells have frequently thick and impermeable walls, and the diffusive power of tannins is low, much time is required for extraction, unless the cells have been previously crushed or broken.

It would be beyond the scope of this text-book to describe in detail the structures of the tannin-yielding parts of plants; but barks are of such general importance, that some particulars seem desirable.

The detailed structure of bark varies greatly in different trees, though its general principles remain unaltered. One of the best short accounts of these is given by Prof. H. Marshall Ward on page 199 of his little book on ‘Timber and some of its Diseases,’[133] and further information may be found in Van Tieghem’s ‘Traité de Botanique’ and other works on structural botany.