Amongst the finishing processes dyeing holds an important position. The nature of the process has many points of similarity with that of tanning. The great specific surface of pelt is probably more enhanced than otherwise during tannage, at any rate with light leathers, owing to the isolation of fibres, and consequently leather is as liable as pelt to exhibit adsorption. The dyestuffs, on the other hand, are substances very easily adsorbed. Some (like eosin and methylene blue) are crystalloids, some (like fuchsin and methyl violet) are semi-colloids, whilst others (like Congo red and night blue) are undoubted colloids forming sols (usually emulsoid) with water as dispersion medium. The crystalloids and semi-colloids may also be obtained in colloidal solution, sometimes being so changed on the mere addition of salts to the solution. In addition, the pelt has been mordanted with tannin. If, however, leather has been kept long in the rough-tanned or "crust"state, this may not be so effective, owing probably to the secondary changes in tanning (Part I., Section III., p.46), but such leathers are usually "retanned" or prepared for dyeing by sumaching (which process also incidentally bleaches). The tannin mordant assists materially in the fixation of the dyes. In the case of basic dyestuffs, lakes also are formed,i.e.there is a mutual precipitation of oppositely charged colloids (+dye,-tannin). The dyeing of leather is thus a case of colloid reactions even more complicated than that of tanning.
Another finishing operation typical of the light leathers is "graining" or "boarding." In this the skins after dyeing and drying are worked by a board which is covered by cork, rubber, perforated tin or other material, and so grips or "bites" the leather. The object of "graining" is to work up the grain pattern by pushing or pulling a fold on the skin with the board. The nature of the grain varies with the thickness and the hardness of the skin, with the amount of pressure applied, with the nature of the board, with the direction of the boarding and with the total number of directions boarded. There is thus infinite scope for variety of finish, and hence arise bold grain, fine grain, hard grain, straight grain, cross grain, long grain, etc. The operation requires considerable skill and experience. In the case of skins with little natural grain (such as sheepskin) embossing and printing machines impress the desired pattern.
In seasoning, a dressing is applied containing essentially albumins and emulsified fats,e.g.egg albumin and milk. Colouring matters are also often added to intensify or modify the shade. After seasoning the goods are usually "glazed" by a machine which rubs the seasoned grain with considerable pressure, by a glass or hardwood tool, and so produces a high gloss, for which the seasoning is very largely a preparation. Light leathers are very lightly oiled with linseed or mineral oil.
REFERENCES.Procter, "Principles of Leather Manufacture," pp. 220, 394.Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112, 312, 332.Wood, "Puering, Bating and Drenching of Skins."Lamb, "Leather Dyeing and Finishing."
Goatskins are amongst the most valued raw material for the manufacture of light leather. The leather obtained from them is of the very finest quality in respect to durability and adaptability to the principal purposes in view. The texture of the fibres in goatskin is exceedingly compact and very strong, whilst the grain exhibits naturally a characteristic pattern which renders it most suitable for a grained finish. Hence for purposes like upholstery, bookbinding, slippers, it forms almost an ideal material. The tanning and finishing of goatskins into "morocco leather" may indeed be taken as a quite typical example of light leather manufacture.
The skins are obtained from all quarters of the globe where goats exist, and the excellent quality of the leather produced has created a demand which is greater than the supply. This is due not only to the demand for morocco leather, but also to the popularity of the goatskin chrome upper leathers such as "glacé kid" (see Part III., Section IV.). The large American trade in the latter has produced the saying that wherever there is a goat there is an American waiting for it to die! The European supply of skins is somewhat limited. They are obtained from the Balkans and Bavaria, in which case they are small, fine-grained and plump skins. The Swiss goatskins are larger, and have also a fine grain; they are well grown and well flayed. Scandinavian skins have a poor reputation, being very flat. The African supply is important; Abyssinian skins are exceedingly compact and tough, and are very suitable for "bold grain" finishes. The Cape skins are particularly large, strong and thick, but their quality is often impaired by the cure, the skins being flint-dry, and, like hides so cured, prone to unsoundness. Large quantities ofgoatskins also come from the East. Many of these are imported in a tanned state (E.I. Goat). These skins are tanned with turwar bark, which contains a catechol tannin. They are also heavily oiled with sesame oil, and need degreasing. The tannage is also stripped as far as practicable, and the skins retanned with sumach before finishing. They make good morocco leathers for many purposes, but the primary catechol tannage renders them ineligible for finishing under the specifications of the Committee of the Society of Arts. The skins have a Persian or Indian origin. India also supplies a large number of raw dried goatskins which are small and of variable quality. These, however, are more extensively used for chrome uppers.
Goatskins are imported in either a salted or a dried condition. The great aim of soaking is to obtain the skins in a thoroughly soft condition. Hence the soaking is prolonged, and some mechanical treatment is desirable in addition to various steepings in water. To be certain of softness it is desirable to avoid the use of alkalies in the soak waters, for although they cause hydration of the fibres by imbibition, they also have a plumping effect which is not wanted at this stage. Salted goatskins are first immersed in water and left until the following day. This dissolves the salt. They are then stretched and given a fresh soak liquor of water only to soften further, clean, and remove the rest of the salt. This second water lasts only a few hours, and the goods are then drummed well in running water. This not only cleans quickly, but has an excellent softening effect. They are again returned to a soak liquor, then softened mechanically by working them over a beam. This treatment must be repeated, drumming again if necessary, until the skins are perfectly relaxed and thoroughly softened. If the treatment be very prolonged it becomes advisable to use antiseptics in the soak waters after the first drumming. Solubilized (or emulsified) cresols of the "Jeyes fluid" type are the most suitable antiseptics, but too much must not be used or the sterilization affects the liming, in which bacterial action is needed. Flint-dry skins areleft longer in the first soak, which should be of water only. They are then given a fresh soak liquor containing 0.2 per cent. of sodium sulphide. Sometimes a 1.0 per cent. solution of borax is used instead; it softens excellently, is antiseptic, and avoids the plumping effect, but is rather expensive. The goods are next drummed well, and resoaked and worked as for salted skins. In either case the soaking takes about a week.
The liming of goatskins presents some points of contrast with the methods used for other skins. These differences are due to the exceedingly tight and compact nature of the skin fibres. This compactness of texture makes it quite necessary to dissolve the interfibrillar substance to a greater extent than usual, and also to plump the fibres and split them into the constituent fibrils. These effects are essential to obtain a rapid and complete tannage and a soft leather. Too much bacterial action should be avoided, however, or the brightness and soundness of the grain may be impaired, which would be a fatal defect in such a leather. Hence the liming is long rather than mellow, and sharp limes rather similar to those required for sole leather are often used. Another result of the tight texture of goatskin is that depilation is not easily effected. This feature is rather intensified by the deepness of the hair-root. Hence it is usual to employ sulphides to assist the depilation. In one method two rounds of five pits are used. The skins are given about two days in each pit, so that the liming lasts approximately three weeks. In the first round, which consists of rather mellow limes, arsenic sulphide is used to assist depilation. Up to 6 per cent. on the weight of lime is added during slaking. This is a comparatively large amount of arsenic sulphide, and the depilation is considerably hastened; the skins indeed are unhaired after passing through this round,i.e.after about 10 days' liming. In the next round the object is plumping, and caustic soda (or carbonate) is added to the lime liquors in quantities comparable to those suggested for sole leather (Part I., Section V., pp.55, 56). Inthis round the goods stay also for about 10 days. An alternative to the above process is to hasten the earlier part of the liming by employing sodium sulphide instead of realgar. More sulphydrate may be obtained in solution in this way, and the unhairing may be in about half the time. The sulphide of soda also commences the plumping action which follows in the next round, but this alternative has the disadvantage that the skins are unhaired whilst the pelt is swollen with sulphide, which renders the grain both harsh and tender and consequently more liable to damage by the unhairer's knife.
Deliming is by puering and drenching, and is often associated with a further mechanical working of the goods. The skins are inserted into a puer liquor at 85° F. and thoroughly pulled down. The caustic alkalies should be completely neutralized. A slight cut into a thick part at the butt end should develop no pink colour with phenolphthalein. The skins should be thoroughly relaxed, and the swelling so much eliminated that they are quite soft, weak and "fallen." The resilience and elasticity of the plumped skins should have quite disappeared, and the impressions of hand or thumb should be readily retained by the pelt. The grain should appear white and possess a soft and silky feel. In this condition they are again worked over the beam to soften further if possible. They are then rinsed and again worked over the beam. Drenching follows with 10 per cent. of bran on the pelt weight, the operation commencing at 85° to 95° F., and lasting till next morning. The skins are next scudded thoroughly to remove all dirt, but carefully so as not to damage the grain.
In tanning, sumach and oak bark are the staple materials. Sumach gives a much lighter colour, and hence it is used alone for goods that are to be dyed the lighter shades, but oak bark is a "faster" tannage and more preferable for dyeing in those cases where blacks and very dark shades are wanted. For ordinary purposes a blend is usually employed. A feature of oak bark, also, is that it tends to make a firmer leather, so that the proportion used must be adjusted with this fact in mind as well asthe question of colour. For firmer moroccos the skins may pass through a handler round of oak-bark liquors (10°-20°) in which a certain amount of sumach is added to the liquors. The sumach is leached and assists both in tanning and bleaching as the liquor works through the round. The old liquor is run to a paddle, and the tannage is commenced by paddling the drenched skins in this liquor. It is advantageous both for the tannage and for the efficient "spending" of the sumach if this liquor be slightly warmed. In the early pit liquors the goods are very frequently handled. There is, however, the usual tendency of the times to save labour in this direction, and hence it is common to have several paddles with liquors of gradually increasing strength, followed by a shorter round of handlers in which the handling is more infrequent. Instead of paddles latticed drums may be inserted into pits containing liquors. These, however, are not quite so convenient. In some tanneries, especially where sumach only is employed, the tannage is in paddles throughout. A new liquor is made up with fresh sumach and is used repeatedly until exhausted. A three-paddle system sometimes obtains, in which case the operation closely resembles the three-pit system of liming (Part I., Section II., p.19), and the skins pass through an "old" liquor, a "medium" liquor and a "fresh" liquor. The goods need not be paddled the whole day through, and indeed in the later stages this is undesirable. The packs remain several days in each liquor and take up to 14 days to tan. Two to three bags of sumach are needed for about 20 dozen goatskins. This method of tanning is efficient and convenient for bold-grain finishes, on account of the constant tumbling and bending of the skins which tends to work up a grain. For very soft leathers and fine-grain finishes, however, the "bag-tannage" or "bottle tannage" is favoured. In this method the pelt is stitched up by machine to form a bag, grain outwards, leaving a "neck" in the hind shank. The bag is nearly filled with a fairly strong infusion of sumach, inflated with air and tied up at the neck. The bags are then placed into a vat of warmsumach liquor, in which they just float. The bags are pushed down and the liquor stirred up, so that the goods are in constant motion. After a few hours they are piled on a rack, and the tan liquor of the interior is caused to diffuse through the skins by the pressure due to the weight of the pile. The bags are refilled with fresh and stronger sumach liquor and the process is repeated. The skins are thus lightly but effectively tanned in about 24 hours, and the leather has very fine grain and soft feel. However tanned the skins are dried out after tanning, and sorted in the "crust" according to size and colour. The larger skins are preferred for upholstery and the smaller for fancy goods and bookbinding.
To illustrate the course of finishing operations, the case of hard-grain morocco for bookbinding may be given as typical. The goods are wet back with warm water and drummed for 1-2 hours in warm sumac to prepare for dyeing. They are then struck out by machine, sammed and shaved. Dyeing follows, with acid colours, in a drum. The goods are run first in a little water and the dyestuff added very gradually through a hollow axle. The acid required (preferably formic) is added later to develop the full shade. Warm solutions are used, and the dye bath is practically exhausted. The goods are next placed in cold water to wash off superfluous liquor and free the skins from acid. They are then horsed to drain, struck out and hung up to samm. They are seasoned with milk and water and piled to temper. They are "tooth rolled" in the glazing machine two ways: right-hand shank to left fore shank andvice versâ, and piled again. After wetting back again they are "wet grained" by hand with a cork board in four directions: belly to belly, shank to shank, and across as before, and finally from neck to butt. They are immediately hung up in a warm shed to dry, and to fix the grain. They are then softened by "breaking down" with a rubber board, top seasoned, piled to temper and dry, brushed lightly, piled again, brushed more heavily, and dried out. They are finally softened by graining in threedirections: shank to shank and across, and neck to butt. They are then brushed again. If these skins are wanted for upholstery they are shaved after dyeing, and nailed on boards to samm. They are also dried out in a cooler shed or "stove," to ensure softness.
REFERENCE.Bennett, "Manufacture of Leather," pp. 39, 55, 89, 111, 204, 344, 396.
A special class of morocco leather is manufactured from the skins of seals. This should not be confused with the "sealskin" of popular parlance, which is manufactured from the skin of a different animal. All the fin-footed mammals (Pinnipedia), except the walrus, are termed seals, but they are divided into two families. TheOtariidæare known by their possession of small but distinct external ears: into this class fall the fur-seals whose skin is dressed with the fur on, for women's jackets, muffs and caps. ThePhocidæare that family without external ears: the skins of many species (Phoce Greenlandica,Phoco barbata, etc.) of this family are unhaired and given a vegetable tannage, thus forming the raw material of sealskin morocco leather. It is with the latter that this section will deal.
As the seal is a marine animal and is partial to the colder seas, its skin is very oily. The skins are imported in a salted condition from both the Arctic and Antarctic regions. North Europe, North America and Newfoundland supply many skins, and the southern material is supplied chiefly through the Cape. Sealskin shares with goatskin the properties of compact texture, strength of fibre, and great durability, all of which fit it for the manufacture of moroccos for upholstery, bookbinding, etc. It is, however, readily distinguishable from goatskin by its characteristic grain pattern.
In soaking sealskins the object is not only to soften thoroughly, but also to effect the recovery of as much seal oil as possible before the liming commences. This is desired because the oil is in itself a valuable bye-product, and because its removal is essential to a satisfactory liming and tannage. The removal of the oil is materially assisted by raising its temperature, so that the soaking of sealskinsis often done with warm water (85°-88° F.), after which treatment they are laid over the beam and scraped with a blunt knife on both flesh and grain. The oil flows away into a special receptacle. This treatment is repeated until the bulk of loose oil is removed. The process is known as "blubbering" or "brushing over." After some soaking the skins are drummed to ensure softness. The skins are then fleshed. More oil may be obtained from the fleshings.
By fleshing before liming a more regular action of the lime is obtained. This is necessary to "kill" the grease still remaining in the skin. A long and mellow liming is given for the same reason. Fully three weeks are given, and old limes are much preferred, partly to obtain the maximum lipolytic action and partly to avoid the intense ribbing of the pelt which new limes so easily impart to the older animals. These ribs are very difficult to eliminate in the subsequent work. Some factories find it necessary to finish up in new limes, however, in order to plump and split the compact fibre bundles into their component fibrils. The plumped pelt is also easier to split green. No sulphides are usually employed. Sweating (see Section IV., p.113) is sometimes used for depilation, and in this case the ribbing of the pelt does not take place.
The puering is unusually thorough with sealskins. This is to obtain the maximum softness and take full advantage of the lipolytic action. The puer liquor is fully 95° F., and the skins are paddled for about three hours, or until fully pulled down and completely delimed. Scudding follows, now usually by machine. The skins are then well drenched. The action is intensified by the use of peameal in addition to the bran. About 10 per cent. of the mixture on the weight of pelt is used. It is customary, however, to drench at a lower temperature (68°-70°) than in the case of goatskins (Section II., p.102), but the goods are left in the drench overnight only, as is usual in drenching. It is quite possible that drenches worked differently may have also a somewhat different fermentation and be due to other organisms than the symbioticbacteria discovered by Wood. It is equally possible that the acids produced are also different, in relative proportion, if not in nature, and that consequently there is a real difference in the practical effect. In the Author's opinion, the great probability is that in the drench are several fermentations, and that if the action be reduced by lowering the temperature, but intensified by adding peameal to the bran, some of these fermentations are encouraged at the expense of others.
The tannage of sealskins depends upon the size of the skins, the purpose for which they are intended, and whether they have been split or not in the limed state. The largest and coarsest skins intended for boot uppers, and those which have been heavily scratched on the grain and are only suitable for enamels, are given a tannage which may last about 5 weeks. The liquors are made from oak bark and mimosa bark, and are made up to 35° with gambier and possibly myrabolans extract. For fancy work also heavy skins are used, but a softer tannage is needed. If for blacks the tannage is with gambier and chestnut extract. Two sets of handlers are given (10°-15° and 15°-20°), using only gambier in the green sets. They are well sumached after tanning to bleach and to mordant. If for colours, only sumach and oak bark are employed. The skins are first paddled for 3-4 days in sumach liquors, in which they are coloured through. The liquors may be warmed; this quickens the tannage and also leaches the sumach. The skins are then split, and the grains pass through a handler set with liquors made from oak bark (8°-24°). The skins are in this set for 3 weeks, in the first half of which they are very frequently handled. They are finished off by paddling for 1 or 2 days in a fresh liquor containing much sumach, which mordants the skins and bleaches the bark tannage. The flesh splits are given a drum tannage in chestnut and quebracho extracts. If small skins are being tanned for bookbinding purposes, sumach only is employed, and usually the tannage is entirely in paddles.
In finishing many types of grain may be obtained, in blacks and incolours. The finishing of "black levant" may, however, be selected as a typical case. The skins are soaked back, tempered, and either split or shaved, according to their substance and the size of grain wanted. The thin skins of course give the fine grains. Mixed tannages need scouring and possibly sumaching. The skins are then oiled up with linseed oil, sammed, set out and blacked. In this last operation the grain is brushed over with a solution of logwood and ammonia, and afterwards with the iron mordant which often contains glue. They are next hung up for a while and then "wet grained" in four directions—belly to belly, shank to shank, across, and neck to butt. After hanging up in a hot stove to set the grain, they are cooled, fluffed on the flesh, and seasoned on the grain with a solution of milk and blood. A little black dyestuff may be added to the season. The season is well brushed in, the skins dried somewhat, and then glazed. They are then grained four ways again as above, dried out in the stove, and lightly oiled with warm linseed oil on the grain.
REFERENCE.Bennett, "Manufacture of Leather," 40, 56, 90, 112, 206, 251, 312, 346, 383.
The most numerous class of skins for light leathers is from the common sheep. These skins have particular value inasmuch as they include the wool as well as the pelt. This wool, which is actually the most valuable part of the sheep's skin, is the raw material of our woollen industries, and is one of the most important of animal proteids. We have, therefore, in this section to consider this dual value of sheepskins, the proteid of the epidermis (wool), and the proteid of the dermis (pelt); one the raw material of the woollen industry, the other the principal raw material of the light leather trade. The first problem is to separate the two proteids. With other skins and hides the ordinary liming processes were sufficient and appropriate, but in the case of sheepskins the method is unsuitable, because the exposure of the wool to the action of caustic lime and possibly other alkalies would seriously impair its quality and reduce its commercial value. Hence this separation of wool from pelt is usually quite a separate business, viz. that of the "fellmonger," whose occupation it is to collect the sheepskins from butchers and farmers, to separate the two important constituent proteids, and to hand the wool in one direction to the "wool stapler," who sorts it according to quality, and to hand the pelt in another direction to the light leather tanner, who tans and finishes the pelt to fit it for light upper work, fancy goods, etc.
In the first instance, therefore, we have to consider the work of the fellmonger, the separation of wool and pelt. In this work the wool receives first consideration, and the raw material of the fellmonger is usually classified accordingly into "long wools," "short wools," and "mountain breeds." The skins vary very largely in quality of wool and in quality of pelt, being influenced very strongly by the conditions underwhich the sheep lived, and by the precise breed of animal from which the skin has been taken. As in the case of hides (Part I., Section I., p.8), animals exposed to extremes of weather develop the best pelts, whereas those sheep which have been carefully bred and reared for the sake of their wool yield a thin and poor class of pelt. In Britain, and more especially in England, are reared the finest and most valuable sheep. This is evident from the prices paid for them by foreigners and colonial breeders when seeking new blood for their flocks and fresh stock for their lands. As much as 1000 guineas have been paid by an Argentine firm for a single Lincoln ram.
Long wools are obtained from some of the best and most extensively bred animals. The "Cotswolds" are the largest, and probably the original breed of England are still found on the Cotswold Hills. They have long wool, white fleeces, white faces, and white legs, and have no horns. The wool is fine, but the pelts are particularly greasy, especially along the back. A later breed originating in the Midlands was called the "Leicester" long wool. This breed gives a great cut of wool and much coarse mutton. It is very extensively distributed in the North of England and has been much crossed, so that many sub-breeds are now well known,e.g.the "Border Leicester"—the general utility sheep of Scotland—and the "Yorkshire Leicester" or "Mashams," much bred in Wensleydale. "Lincolns" are another long wool found only on the Lincolnshire Wolds. They also have white faces and shanks and yield a large pelt with fine grain. They give a big crop of wool. "Devons" are a smaller breed common in Somerset, Devon and Cornwall. They yield a fairly long wool of great strength, but not quite white. Romney Marsh sheep ("Kents") are also long wools. They have white legs, white faces, a tuft of wool on the head, and no horns. The pelt is large and good. "Roscommons" are an Irish cross-breed with much Leicester blood. They yield a long wool and a spready pelt.
Short wools are typified by the "Down" sheep. These sheep areextensively bred on the chalk lands which comprise a very large percentage of the southern counties of England. The "South Downs" are the best and most important, the breed being the general utility sheep of England. They are small but well-shaped animals with grey faces, no horns and fine close wool. The pelt is only fair, but the mutton is excellent and provides the meat sold in our best shops. This breed has largely stocked New Zealand. The "South Down" is a somewhat delicate animal, and has therefore been largely crossed with Cotswolds and other breeds. Many well-known cross-breeds are found in the eastern and southern counties. The "Suffolks," for example, are found in the eastern counties. They have black heads, faces and legs. "Oxfords" and "Hampshires" are similar, but larger. "Shropshires" are another hardy cross-breed, which yield a heavier fleece. All the cross-breeds are larger than the South Down and yield bigger pelts.
Mountain breeds yield wool of varying quality but give the best pelts. The "Cheviots"—much favoured by the Scotch farmers—have a wool of medium length but with much hair in it. They have white faces and legs and no horns, and yield excellent pelts. The "Black-faced Mountain Sheep" have longer wool but coarse, and yield good pelts. They are kept in the hilly parts of North England and in the Scottish Highlands. "Lonks" yield a large and good pelt, but very coarse wool. The mutton is good. They are a very large breed with much curved horns and black faces. There are also some small breeds, "soft wools," "Shetlands," and "Welsh Mountain Sheep." The wool of the last two is poor, but the Welsh pelts are valued for their fine grain. There are large numbers of sheepskins also imported, from South and Central America, and from Australia, New Zealand and the Cape. The colonies, however, have often done their own fellmongering, and we have imported pickled pelts. They now tan the skins also, and many tanned sheepskins are now imported. There are also many Indian skins imported after tannage with turwash bark (cp. E.I. Goat, Section II., p.100).
The depilation is brought about by "sweating" (or "staling") and by "painting." The immediate object of both these types of method is to avoid using any thing which will affect the wool. The sweating process is the most ancient method of unhairing and is used in America for hides as well as sheepskins. It consists of a more or less regulated putrefaction. The loosening of hair or wool has long been accepted as evidence that putrefaction had commenced in a hide or skin, and it is the aim of the sweating process to stop the action at that stage, before any damage has been done to the pelt. This aim is achieved rather imperfectly by suspending the goods in closed chambers and regulating the temperature and humidity by means of steam and water. Such chambers are known as "sweat pits" or "tainting stoves". In the case of sheepskins the "warm-sweat" system is generally used, and the operation is carried out at 75°-80° F. A satisfactory yield of wool is obtained in good condition, but the pelt is very liable to suffer bacterial damage and show "weak grain." The skins are first cleaned by a few "soaks" in clean fresh water, with intermediate help from a "burring machine" which presents a rapidly revolving set of spiral blades to the wool, and in the presence of a good stream of water quickly removes all dirt from the wool. The skins then enter the tainting stove, and the operation is commenced by a slight injection of live steam. In summer, about a week is sufficient to loosen the hair, but in winter up to two weeks may be necessary. Little control of the process is possible, and all that can be done is to watch the goods carefully near the end of the operation. In one variety of this method of unwoolling the skins are painted on the flesh side with a creamy mixture of lime and water and piled for a day or two until the pelt is distinctly plumped. They are then washed with fresh water to remove the excess of lime, drained, and then enter the tainting stove. By this method the pelts are obtained in better condition and are less liable to damage by local excess of putrefaction. In unwoolling the skins are placed over a beam and the true wool is pulled out by hand. The wool is graded as it is pulled and differentqualities kept separate: ewe wool, lamb wool, hog wool, etc. The hair is next removed from face and shanks by means of a blunt "rubbing knife," and the pelt then immersed in water.
In the other method of depilation, by painting, advantage is taken of the loose texture of the sheepskin fibre and of the fact that the wool root is nearly halfway through the skin. The flesh side of the clean skin is painted with a creamy mixture of lime in a strong solution of sodium sulphide (14°-24° Beaumé). Care is taken to keep the depilatant off the wool. The skins are folded flesh to flesh and left for a few hours or until next day before unwoolling, according to the strength of the sulphide solution. The depilatory action is entirely chemical, being due to the solvent action of the sulphide on the hair root. The lime is sometimes omitted. After pulling, the skins are opened up and washed in fresh water.
The various classes of wool are sold to the wool-stapler and so to the woollen industry. As this is a mechanical rather than chemical industry, its discussion is beyond the scope of this volume. However unwoolled, the pelt still needs further treatment by the fellmonger. It needs liming and unhairing. This is done in the ordinary way in pits of milk of lime, through which the goods pass from old to new limes in the course of about a week. This plumps the fibres, separates the fibrils and kills the grease. Paddles are used also to save handling. Shearlings are sometimes limed 9-14 days and unwoolled without sweating or painting. After liming the skins are unhaired and fleshed, and placed in clean strong limes until sold to the tanner.
Sheepskin pelts are sometimes preserved by pickling. This consists in placing them first in a solution of sulphuric acid (about ¾ per cent.) together with some common salt. The pelts swell up and imbibe the acid solution. They are then placed in saturated brine, which causes a very complete repression of the swelling, the pelts being apparently leathered. In this condition or partly dried out they may be kept foryears. The forces at work in this phenomenon are somewhat complex (see Part V., Section I., p.200). The skins may be depickled by paddling in a 10 per cent. salt solution to which weak alkalies such as borax, whitening, carbonate and bicarbonate of soda, etc., have been added.
The leather manufacturer classifies sheepskins according to the size of the pelts. The large skins are tanned for light upper leathers and similar work. These are called "basils." Many large skins are also split green into "skivers" which after vegetable tannage are finished for fancy goods, bookbinding, etc. The fleshes are often oil-tanned for chamois leather (Part IV., Section III., p.181). Medium-sized skins such as are obtained from the Down sheep are tanned for "roans," and finished as a kind of morocco leather. Small skins are mostly "tawed" (Part IV., Section I., p.174) for glove leathers, but some are made into roller leather by vegetable tannage.
Basils, which represent the heaviest sheepskin work, are tanned and finished in the following manner. The limed pelts are first bated lightly at about 80° F. for two days, scudded and drenched. They are sometimes puered, but more often merely delimed with organic acids. In this last case they are first paddled in warm water to remove excess of lime, and a mixture of organic acids is very slowly added at definite intervals. When nearly free from caustic alkali the skins are removed and drenched overnight. There are two types of tannage. The West of England tannage is similar to those noted for sealskins when oak bark and sumach are employed (Section III., p.108). There is also the tendency to paddle more and handle less, and to use the stronger tanning materials such as myrabs, gambier and other extracts. After about 12 hours' tannage in paddles they are coloured through, and are then degreased by hydraulic pressure. The skins are piled in the press with layers of sawdust or bran between them, and the pressure applied very slowly. Much grease runs out, for the natural sheepskin contains up to 15 per cent. of oil and fat. Degreasing may be postponed till tannage iscomplete, and the grease can then be extracted by solvents (benzene, acetone, etc.). Degreasing after part tannage is usually considered preferable, and the skins may be tanned out in pit or paddle in about a week. The Scotch tannage is with larch bark fromPinus larix, which contains up to 13 per cent. of a rather mellow catechol tan. This material has also some sugars and yields sour and plumping liquors. The basils are paddled in weak liquors (8°-11°) for about 2 days, and when struck through are degreased by hydraulic pressure. They are then soaked back and tanned out in stronger liquors (11°-20°), which takes up to one week. They are then dried out and sorted in the crust. The finishing depends of course upon the purpose in view. If for linings they are soaked, shaved, sumached, struck out well, nailed on boards and dried right out. They are next stained with a solution of starch, milk and red dyestuff. After drying they are glazed by machine and softened with a hand board. For fancy slippers the crust skins are starched and stained directly, then "staked" (see Part III., Section II., p.155), fluffed, seasoned and glazed. If intended for leggings and gaiters a flesh finish is given. The skins are soaked, stretched, shaved and sumached. They are then rinsed, drained, sammed and stained. A brown stain mixed with linseed jelly is usual. This is spread evenly over the flesh and glassed in. The skins are dried out, restained if necessary, and staked to raise a nap. Basils for gaiters are dyed in paddle and fluffed over the emery wheel.
Skivers are split in the limed state and sometimes immediately degreased. They are next puered at 85° F. for about 3 hours in a paddle, and scudded. They are drenched at a low temperature (68°-70° F.), but often 2 or 3 days. They are again scudded and then rinsed and sent to tan. The skivers are tanned in a few days by sumach liquors working the goods up from mellow to fresh as usual. The liquors are warmed. Care must be taken that the goods do not tear. A great variety of finish is possible, but the "paste grain skiver" for fancy goods and the plain finish for hat leathers are sufficiently typical. For paste grains theyare soaked and "cleared" for dyeing by immersion in very weak sulphuric acid, excess of which is carefully washed out with water. Paddle-dyeing follows, and is preferred to drum dyeing as the skins are so liable to tear. After being struck out they are "pasted," by spreading on to the flesh a glue jelly, using first the hand, then a stiff brush and finally a cloth. The goods are then dried out. They are then seasoned, partly dried and printed cross grain. They are next grained two ways lightly; shank to shank, and across, lightly tooth-rolled and glazed. They are regrained two ways as before, dried out, and finally softened with a graining board. They are sometimes sized on the grain to fix the pattern and give a gloss. For hat leathers the skins are first soaked, sumached and struck out. If for white or cream finishes they are now lead-bleached. This consists of pigment dyeing with lead sulphate. They are immersed alternately in lead acetate and in sulphuric acid solutions until precipitation is sufficient. They are then dyed to shade. If for browns it is common to mordant with titanium and use basic dyestuffs, paddling afterwards in sumach to fix the dye. After dyeing the goods are struck out again, starched, and dried out on boards. They are again starched and rolled to give the plain finish.
Roans are not split. They are degreased, puered, scudded and drenched overnight at 95° F. They are tanned with sumach usually in pits, and take rather longer than usual to tan. They are finished in much the same style as goatskins for morocco leather, but as the sheepskin has little natural grain it needs embossing or printing according to the type required. If for "hard grains," the skins are soaked, sumached, seasoned, dried, glazed and damped back for printing. This is done by the "hard grain" roller, and the goods are dried out to fix the pattern. They are damped back, sammed, and grained in four directions (cp. Section II., p.104), dried out and boarded to soften. If for straight grains they are printed with a straight-grain roller, or grained neck to butt. After tooth rolling they are boarded, dried and glazed. They aresoftened down and "aired off" in a cool store.
Roller leather is a special class of sheepskin leather which is used to cover the rollers used in cotton spinning. The essential requirements are that a smooth plain finish should be given, and the leather must not stretch or be greasy. For this purpose small sheepskins with a fine small grain are chosen, such as those obtained from the Welsh mountain sheep. The pelts are machine fleshed, short haired and often puered, but the deliming is also brought about by organic acids also. The pelts are drenched in pits fitted with paddles, which are used to stir up the infusion occasionally. A thorough scudding is given. For the smooth-grain finish it is necessary to tan in weak liquors, and to give plenty of time so as to ensure complete penetration. An oak-bark tannage is preferred, but a little extract is usual to assist. The goods are coloured through in paddle, like basils, and are then degreased by hydraulic pressure. This should be as complete as possible, and a little heat is used to assist the escape of grease. The pressed skins, moreover, must be quite freed from creases, and this is attained first by paddling in warm water to remove sawdust, and then by drumming in fairly hot water, in which they are left overnight. The skins are tanned out in suspenders, taking about 3 weeks. The crust skins need careful sorting, and are soaked and hand shaved. They are sumached in drum, rinsed, struck out, sammed and set. The striking and setting should be thorough, in order to get rid of stretch. They are next "filled" by coating with linseed jelly or similar material, and dried out on boards in a thoroughly stretched condition. They are then trimmed, seasoned and rolled with a steel roller. They are then staked or perched, fluffed, re-seasoned, dried and glazed. They are carefully short-haired, glazed again and finally ironed.
E.I. sheepskins are imported in a tanned condition. These are soaked back and the turwar bark tannage "stripped" as far as possible by drumming with soda for 20-30 minutes at 95° F.; after washing they are"soured" in weak (½ per cent.) sulphuric acid solution, and retanned with sumach paste for an hour, drumming at 100° F. They may then be finished for basils, moroccos or roller leather as described above, but are often finished as imitation glacé kid. In this case they are drum dyed, lightly fat liquored (see Part III., Section IV., p.163), struck out and dried. They are staked by machine, fluffed, seasoned and glazed. They may be re-staked and reglazed if desired.
REFERENCES.A. Seymour Jones, "The Sheep and its Skin."Bennett, "Manufacture of Leather," pp. 30, 85, 107, 208, 349-354, 385.
Calfskins are the raw material for many classes of leather. The term itself is rather broad. A calfskin may be obtained from a very young animal and weigh only a very few pounds, or it may be anything just short of a kip. Goat, seal, and sheep skins are obtained from adult animals, but calfskins from the young of a large animal. Thus there are many grades of quality, according to age, and the material must be chosen with regard to the purpose in view. Some of these purposes have already been discussed. Heavy calf is treated much like kip as a curried leather for upper work. Even lighter skins are given the "waxed calf" and "satin calf" finishes, and make upper leather of excellent quality. To produce such leathers the treatment is much the same as described in Part I., Section VIII., p.76. Calfskins were also used for very light upper work, in which they were not so heavily greased in finishing, but rather dyed and finished as a light leather. In this direction, however, the vegetable tannage has been almost completely superseded by the mineral tannages, first by "calf kid," an alumed leather (Part IV., Section I., pp.174-177), and afterwards by the now popular chrome tannage of "box calf," "willow calf," "glacé calf," "dull calf," etc. (Part III., Section III., p.156). In this section, therefore, we have only to consider calfskins as used to make a vegetable-tanned light leather, such as may be employed in bookbinding and in the manufacture of fancy goods. For these purposes the skins receive a mellow liming of 2½-3 weeks. No sulphide need be employed, as the goods are soon fit to unhair. In such a mellow liming it is important that the bacterial activity is not too prominent, and hence it becomes advantageous to work the liming systematically in the form of a round of pits. To avoidover-plumping in the newest limes some old liquor is used in making up a new pit, and its bacterial activity is reduced by adding it to the new caustic lime whilst slaking. Thus for a pack of 200-250 skins, 14-16 stone of lime may be slaked with about 30 gallons of old lime, and the pit filled up with water. If it be necessary to shorten the process and to use sulphide, this should be added only to the tail liquors of the round, and with it should be added, if possible, some calcium chloride to reduce the harshness of the soda. The skins should be puered thoroughly to obtain the necessary softness, bate-shaved if desirable, and drenched with 8 per cent. of bran overnight.
In tanning for fancy work and for dark colours, the goods are coloured off and evenly struck through with sumach liquors, and then tanned further with liquors made from oak bark, myrabolans or chestnut extract. The methods are very closely similar to those used for goatskins and sealskins (Part II., Sections II. and III.), and need not be described in further detail. The tannage is finished off in sumach. For bookbinding work, however, a pure sumach tannage is given, using liquor slightly warm (70° F.). Paddle tannages are common, but for bookbinding the bag or bottle tannage is often preferred. The skins are sewn together in pairs, grain outwards, and nearly filled with warm sumach infusion, just as described for goatskins. They are then handled in old sumach liquors for about 3 days, and piled to drain and press. At this stage the bag is cut open, the goods worked on the flesh, and the tannage is completed with separated skins in newer sumach liquors, handling at least once a day for 4-5 days, as necessary.
In finishing there is the usual variety, but a plain ungrained finish is most typical, as the smooth and fine grain of the young animal lends itself to this type of finish better than the skins of goat and seal, and gives a better quality leather than those from the sheep. The crust skins are wet back with water at about 110° F., and, if necessary, sammed and shaved. Sumaching follows, the operation being carried out in a drum for 1-2 hours. The skins are then well struck out. Striking andsetting should always be thorough for a plain finish, and this case forms no exception. Dyeing follows next, the paddle being often preferred to the drum, which is liable to work up a grain. The dyed skins are placed in cold water for a while and again well struck out. They are often nailed on boards to samm, and are then set out, lightly oiled with linseed oil and dried out in a cool shed. Seasoning follows, with milk and water only. The operation may be done with either brush or sponge, after which the goods are piled grain to grain and flesh to flesh to regulate. They may be next perched to soften and fluffed if desired. After top seasoning with milk, water and albumin the skins are hung up for a while, piled to regulate and brushed, first lightly and then more vigorously. They may be then oiled very lightly and dried out in a cool stove to ensure a soft leather.
REFERENCE.Bennett, "Manufacture of Leather," pp. 55, 84, 105, 201, 207, 303.
The leathers which receive a japanned or enamelled finish are usually vegetable tannages, and so may be discussed at this stage. They are popularly known as "patent" leather, but for no obvious reason. The chief object is to obtain a leather with an exceedingly bright and permanent gloss or polish, and this is attained by coating the leather several times with suitable varnishes. The great difficulties are to prevent the varnish cracking when the leather is bent or in use, and to prevent it peeling off from the leather. Almost all classes of vegetable tannage are japanned and enamelled. Hides are split and enamelled for carriage, motor car and upholstery leathers, and enamelled calf, seal and sheep skins are used for boot uppers, toe caps, dress shoes, slippers, ladies' and children's belts, hat leathers, and so on. Broadly speaking, a japanned leather is a smooth finish and is usually black, whilst an enamelled leather is a grain finish with a grain pattern worked up, and more often in colours. Hence japanned leathers are often made from flesh splits or leathers with a damaged grain. It is in any case advantageous to buff the grain lightly, for this permits the varnishes to sink rather deeper and get a firmer grip, and avoids the too sudden transition from phase to phase which is one cause of stripping or peeling. Many flesh splits, however, are printed or embossed to give an artificial grain and are then enamelled, which tends to fix the embossed pattern.
Almost any method of preparing dressing hides for upper or bag work will yield a suitable leather for enamelling and japanning (see Part I.,Section VIII., p.76; and Section IX., p.86). If anything the liming should be somewhat longer and mellower in order to eliminate grease, as the natural grease of the hide causes the stripping of some varnishes. In finishing it is important to obtain even substance, or the varnish is liable to crack. Hides are soaked and sammed in, and often split. Sometimes they are split twice, giving grain, middle and flesh, the two former being enamelled and the last japanned. Other goods are shaved very smooth. The goods should be next thoroughly scoured and stoned to get as much "stretch" as possible removed. They are often sumached, washed in warm water, slicked out again and sammed. They are then lightly buffed on the grain, and after oiling lightly are thoroughly set out and dried. Embossing or printing for enamels is done before the goods are quite dry. Considerable difference of opinion obtains as to the best oil to use in the above oiling. Linseed oil is widely preferred as being most likely to agree with varnishes made from linseed oil. Some manufacturers of japans do not dislike the use of mineral oil, but strongly object to cod oil, tallow or other stuffing greases as tending to cause the varnish to strip or peel. Other manufacturers, on the other hand, will not have leather with mineral oil in it, and indicate that nothing but cod oil should be used. In all probability these various preferences are determined by the nature of the varnish, which differs widely in various parts of the globe.
In this country the varnishes are made largely from linseed oil by boiling it with "driers." This oil contains much triglyceride of an unsaturated relative of stearic acid. The double bonds are very susceptible to oxidation with the production of resinous bodies of unknown constitution. This phenomenon is known as "drying the oil," and has been extensively used in the manufacture of linoleums. The driers are either oxidizing agents or oxygen carriers, such as litharge, Prussian blue, raw umber, manganese dioxide, manganese borate, and "resinate." Prussian blue is most preferred for British japans, as it always materially assists the attainment of the desired black colour.The exact details of the boiling, and the manufacture of the varnishes is still largely the trade secret of the master japanners, and differs indeed for the various stages of japanning. The varnish for the earlier coats is boiled longer, and the drying carried further, than in the case of the later coats. This is partly to obtain a product of such stiffness that it will not penetrate the leather. The driers and the pigments should be finely powdered and thoroughly mixed in. The boiling takes several days when at a low temperature, but if done in 24 hours the temperature may be up to 570° F. In the later coats driers are often not used, and the product is often mixed with copal varnish, pyroxylin varnish, etc., which greatly help in obtaining smoothness and gloss. Turpentine, petroleum spirit and other solvents are also used to thin the varnishes. Before boiling, the oil is often purified by a preliminary heating with nitric acid, rose spirit and other oxidizing agents, which precipitate impurities and thereby assist in obtaining a bright gloss.
Before the application of the varnishes, the leather is first dried thoroughly in a stretched condition. This is accomplished by nailing down on boards which fit like movable shelves into a "stove," a closed chamber heated by steam pipes. The temperature of the stove varies widely in different factories, from 140°-200° F., according to the nature of the varnishes. The first coat of warm and rather stiff japan is laid over the hot leather in a warm room, being spread over first by hand, then by a serrated slicker, and then again smoothed by hand. The goods are then put into the stove for several hours to dry. When dry the surface is pumiced and brushed and a second coat applied in a similar manner, but with increased care. This is repeated with finer japans until the desired result is obtained. Brushes are used to apply the later coats. Up to seven coats may be applied for the production of a smooth japan—three coats of ground japan, two coats of thinner japan, and two coats of finishing varnish.
After the stoving is complete, the product is given a few days under ordinary atmospheric conditions to permit the reabsorption of moisture to the usual extent. Enamelled leathers are then grained to develop the pattern.
REFERENCE.Bennett, "Manufacture of Leather," p. 380.
In these days the manufacture of chrome leather has attained a position hardly less in importance than that occupied by the ancient method of tanning by means of the vegetable tanning materials, and large quantities of hides and skins are now "chrome-tanned" after preparatory processes analogous to those described in connection with vegetable tannages (Part II., Section II.; and Part II., Section I.).
Chrome leathers are made by tanning pelts with the salts of chromium, and are typical of what are known as "mineral tannages," in which inorganic salts are the tanning agents. Tannage with alum and salt (see Part IV., Section I.) is one of the earliest mineral tannages, but is now of relatively minor importance. Chrome tanning was first investigated by Knapp (1858), who experimented with chromic chloride made "basic" by adding alkali, but his conclusions were unfavourable to the process. A patent was taken out later by Cavallin in which skins were to be tanned by treating with potassium dichromate and then with ferrous sulphate which reduced the former to chromic salts, being itself converted into ferric salt. The product, which was a combination of iron-chrome tannage, did not yield a satisfactory commercial leather. Another patent, taken out in 1879 by Heinzerling, specified the use of potassium dichromate and alum. This in effect was a combination chrome-alumina tannage. The alum had its own tanning action and the dichromate was reduced to chromic salts by the organic matter of theskin itself and by the greases employed in dressing. The process, however, was not a commercial success. In 1881 patents were obtained by Eitner, an Austrian, whose process was a combination chrome and fat tannage. The chrome was employed as "basic chromium sulphate" made by adding common soda to a solution of chrome alum until a salt corresponding to the formula Cr(OH)SO4was obtained. Such a solution is now known to be perfectly satisfactory, but at first it proved difficult to devise satisfactory finishing processes, and to supplement the chrome tannage with the fat tannage.
The first undoubted commercial success in chrome tanning was obtained by the process of Augustas Schultz, whose patent was the now widely known "two-bath process," in which the skins are treated successively with a chromic acid solution and with an acidified solution of "hypo" (sodium thiosulphate). The first bath was made up commercially of potassium dichromate and hydrochloric acid, so that, strictly speaking, it contained potassium chloride also. The second bath contained, in effect, sulphurous acid, which reduced the chromic acid in the skin fibres to the tanning chrome salts. Free sulphur is also formed in this bath and in the skin, and contributes to the characteristic product obtained by this process of tanning. Many minor deviations from the original process of Schultz have been introduced, but the main features have been unchanged, and this method of tanning is widely employed at the present time for both light and heavy chrome leather. In 1893 tanning by basic chromic salts was revived and the use of the basic chloride was patented by Martin Dennis, who offered such a tanning solution for sale. The validity of the patent has always been doubtful on account of the previous work of Knapp and others, but the process itself was commercially satisfactory, and the many variants of this and of the basic sulphate tannages are now generally known as the "one-bath process" in contradistinction to the variants of the Schultz process,and are widely used for all classes of chrome leather. A one-bath process which deserves special mention was published in 1897 by Prof. H. R. Procter. In this the tanning liquor was made by reducing potassium dichromate in the presence of a limited amount of hydrochloric or sulphuric acid by adding glucose. Although a basic chrome salt is the chief tanning agent thus produced, there is little doubt that the organic oxidation products play an essential part in producing the fullness and mellowness of the leather thus tanned, but their nature and mode of action has not yet been fully made clear though lyotrope influence is probable.
More recently Balderston has suggested the suitability of sulphurous acid as reducing agent. A stream of sulphur dioxide gas is passed through a solution of sodium dichromate until reduction is complete. The resulting chrome liquor has been favourably reported upon by some chrome tanners. Bisulphite of soda has also often been used as the reducing agent. Other organic substances are also often used, instead of glucose, to reduce the dichromate.
Theory of Chrome Tannage.—As to the theory of chrome tanning there is still considerable difference of opinion and much room for experiment. Some leather chemists regard the tannage as differing essentially from the vegetable tannages. Mr. J. A. Wilson has even suggested that the proteid molecule is in time partly hydrolyzed with the formation of a chromic salt with the acid groups. The author, however, strongly favours the view that in chrome tanning changes take place which are closely analogous to those which occur in vegetable tannage, the differences being mainly of degree. Thus the hide gel is immersed into a lyophile sol—the chrome liquor—and there follows lyotrope influence, adsorption, gelation of the tanning sol, as well as diffusion into the gel, and finally also, probably, precipitation of the tanning sol at this interface (see pp.41-47and200-219).
In chrome tannage the lyotrope influence is much more prominent than in vegetable tannage, but the effect is in the same sense, viz., to reducethe imbibition of the hide gel. Thus the potassium sulphate in a chrome alum liquor has its own specific action of this kind and contributes to the leather formation. Unhydrolyzed chromium sulphate and the sodium sulphate formed in "making basic" act also in the same sense.
The tanning sol is probably chromium hydrate, formed by the hydrolysis of chromium sulphate: it is a lyophile or emulsoid sol and is in consequence very strongly adsorbed by the hide gel. This adsorption, involving a concentration of lyophile sol, is the first stage in gelation, which occupies a relatively more prominent place in chrome than in vegetable tannage. Some diffusion into the gel also occurs, and both the gelation and diffusion of the sol are affected by lyotrope influence, but to a greater extent than in the vegetable tannage. Thus far the analogy is almost complete.
There remains the question of the precipitation of the tanning colloid at the interface. This is a point which has not yet been thoroughly investigated, and which offers considerable difficulty to a clear understanding, but the matter may be probably summarized thus: the adsorbed chromium hydrate is precipitated at the interface of gel and sol to some extent, chiefly through the neutralization of its charge by the oppositely charged ions of the electrolytes present, but possibly also—in the last stages of manufacture by the mutual precipitation of oppositely charged gel and sol.
To illustrate the matter, the case of a basic chrome alum liquor will be considered. The chromium hydrate sol is primarily a positive sol, just like ferric and aluminium hydrate sols:i.e.in water they are somewhat exceptional in that they adsorb H+ rather than OH-. To cause precipitation therefore it is necessary to make the sol less positive and more negative. The positive charge of the sol, however, is greater than in water, because of the free acid formed in the hydrolysis, which results in the adsorption of more hydrions by the sol. Hence to ensure precipitation steps must be taken to reduce the adsorption of hydrions by the chromium hydrate sol. In practice such steps are taken, and tosuch an extent that there can be little doubt that the chrome sol is not far from its isoelectric point. Amongst these "steps" are (1) making the liquor "basic,"i.e.adding alkali to neutralize much of the free acid, which involves a considerable reduction in the stabilizing effect of the hydrions; (2) the adsorption of hydrions by the hide gel when first immersed in approximately neutral condition; (3) the operation of the "valency rule" that the predominant ionic effect in discharging is due to the multivalent anions. In this case the divalent SO4--ions assist materially in discharging the positive charge on the chrome sol; (4) the final process of neutralization in which still more alkali is added. The operation of the valency rule is the most complex of these factors, for there is also to be considered the stabilizing effect of the kations, especially of the trivalent kation Cr+++ from the unhydrolyzed chromium sulphate. It is quite possible also that in the last stages of chrome tanning there are "zones of non-precipitation" due to the total effect of multivalent ions, and it is quite conceivable that the chrome sol may change its sign,i.e.become a negative sol and thus give also a mutual precipitation with the hide-gel. This is particularly probable where a local excess of alkali occurs in neutralization. However that may be, it is probable that most of the tannage is accomplished by chromium hydrate in acid solution, and it is therefore legitimate to conclude that adsorption and gelation have a relatively greater part in chrome tannage. The operation of the valency rule makes it easy to understand why basic chlorides do not tan so well as sulphates; the precipitating anion is only monovalent (Cl-) and chromic chloride contains no substance analogous to the potassium sulphate of chrome alum and hence contains a less concentration of the precipitating anion. Hence also the stabilizing influence of common salt added to a basic alum liquor, the effect being to replace partially the divalent SO4--by the monovalent Cl-. Lyotrope influence, however, may be here at work.
It is possible to make out a rather weak case that the tanning sol isnot chromium hydrate at all, but a basic salt of chrome also in colloidal solution, and to contend that this salt, like most substances, forms a negative sol, but in practice not negative enough, hence the desirability of alkali, divalent anions, etc. From this point of view the analogy with vegetable tannage becomes more complete and the stabilizing effect of the soda salts of organic acids becomes easy to understand.
It is highly probable that the electrical properties of the chrome sol need closer investigation on account of the complexity due to the prominent effect of multivalent ions. It is desirable to bear in mind the remarkable phenomenon observed by Burton (Phil. Mag., 1905, vi,12, 472), who added various concentrations of aluminium sulphate to a silver sol (negative). He observed (1) a zone of non-precipitation due to protection; (2) a zone of precipitation due to the trivalent kation; (3) a second zone of non-precipitation due to protection after the sol has passed through the isoelectric point and become a positive sol; (4) a second zone of precipitation due to the precipitating effect of the anion on the now positive sol. It seems to the writer that similar phenomena may possibly occur in chrome tanning, for whatever the sol actually is, it is not far from the isoelectric point.
A few observations on the vegetable-chrome combination tannages will not be out of place at this stage. Wilson refers to the well-known practical fact that chrome leather can take up about as much vegetable tan as if it were unchromed pelt, and considers this evidence that the two tannages are of fundamentally different nature. "In mineral-tanned leathers the metal is combined with carboxyl groups, while in vegetable-tanned leather the tannin is combined with the amino groups. This strongly suggests the possibility that the two methods of tanning are to some extent independent of one another, and that a piece of leather tanned by one method may remain as capable of being tanned by the other method as though it were still raw pelt" (Collegium(London), 1917, 110-111). To the writer, however, it seems that thefacts are evidence for the contrary proposition, that the tannages are fundamentally of the same nature. On the adsorption theory, one would expect chrome leather to adsorb as much tan as pelt; the readily adsorbable tan is the same, and the chrome leather is an adsorbent of very much the same order of specific surface as pelt. The adsorption theory would find it difficult to account for chrome leather not adsorbing as much tan as pelt. It is quite conceivable that a chrome leather could adsorb more tan than pelt, owing to the more complete isolation of the fibrils by the chrome tannage and to their being coated over by a more adsorbent gel. Adsorption is often deliberately increased by a preparatory adsorption. Thus sumach-tanned goatskins are wet back from the crust and "retanned" in sumach before dyeing, to coat the fibres with a fresh and more adsorbent gel and so ensure the even and thorough adsorption of the dyestuff. Mordanting fabrics has a similar object,—the adsorption of colloidogenic substances which give rise to an adsorbent gel on the fibre. Unless vegetable-tanned leather is so much loaded with tan that its specific surface is effectively reduced, one would similarly expect that vegetable-tanned leather would adsorb the chrome sol. This, of course, is exactly the case of semi-chrome leather. If, on the "chemical combination" theory, the vegetable tan combines with the amino groups and the chrome with carboxyl groups, it is natural to inquire which groups the dyestuffs combine with. As either tannage does not interfere with the adsorption of dye, are we to conclude similarly that tanning and dyeing are fundamentally different processes?
Those who favour this chemical combination theory, and who offer equations for the formation of vegetable and of chrome leather, should likewise suggest an equation for the formation of leather from pelt by the action of dyestuffs—a practical though hardly an economic process.
The remarks made earlier in this volume (Part I., Section III.) as to the occurrence of what have been called "irreversible changes"subsequent to the mutual precipitation of oppositely charged gel and sol, are equally applicable to the chrome tannages. Chrome tannage was once thought to embrace such irreversible changes, but the process can now be "reversed" with ease. The reversibility of the chrome tannage is an easier proposition than that of vegetable tannage, partly because the leather is comparatively much less tanned, and partly because the acidity or alkalinity of the stripping agent may be adjusted, as desired, without the oxidation trouble. In approaching this question from the theoretical side one must consider mainly whether to solate the tanning agent to a positive or to a negative sol. Our imperfect knowledge of the electrical forces in operation in the chrome tannage is thus a serious drawback, but the evidence on the whole points to the precipitation being effected by a negative sol near its isoelectric point but in faintly acid solution. Hence, we should theoretically expect that reversion should take place into a negative sol in nearly neutral or even faintly alkaline solution. Thus, suitable stripping agents for chrome leather would be the alkali salts of organic acids (especially if multivalent). Now, Procter and Wilson have recently accomplished this stripping of chrome leather by the use of such salts. They approached the question from an empirical and practical point of view and found that Rochelle salt, sodium citrate, and sodium lactate would strip the chrome tannage with ease. This important and very creditable achievement will have great practical and commercial importance. Procter and Wilson have deliberately and carefully refrained from offering an exact explanation of this reversible action, but point out that all their stripping agents are salts ofhydroxy-acids, and strongly insist that these form soluble complexes with the chrome. Whilst not denying this in the least, the present author would point out that according to the views advanced in this book, the salts of organic acids which donotcontain hydroxyl groups should, when combined with a monacid base, also strip the chrome tannage. This he has found to bethe case. Thus the chrome tannage is reversible in solutions of ammonium or potassium oxalate and of ammonium acetate. With these salts the full effect of multivalent anions is not attained, so that somewhat strong solutions are necessary. A 10 per cent. solution of ammonium acetate shows some stripping effect after a few days, but a 40 per cent. solution after a few hours. Saturated ammonium oxalate is only a 4.2 per cent. solution, but shows a stripping effect in 2-3 days. Potassium oxalate (33 per cent.) shows distinct stripping in 24 hours. Potassium acetate and sodium acetate show only slight action, because the solution is too alkaline, but strip if acetic acid be added until litmus is just reddened. It is noteworthy from a theoretical point of view that a 40 per cent. solution of ammonium acetate is distinctly acid, and indeed smells of acetic acid. There can be little doubt that such stripping actions are also connected with the solubility of the stripping agent in the gel, for the liquid must pass through the walls of the gel to dilute the liquid in the interior. This view fits in with the facts that hydroxy acids and ammonium salts are particularly efficient, for the tendency of chrome to form ammonia-complexes as well as hydroxy complexes is well known. From this point of view we should not expect a stripping action from a salt such as disodium phosphate, which would form an insoluble substance. Actually sodium phosphate does not strip, and indeed reduces the stripping power of ammonium acetate. Similarly, we might expect some stripping action by ammonia and ammonium chloride, with the formation of chrome ammonia complexes. This actually occurs, a pink solution being obtained. Sodium sulphite does not strip, possibly partly on account of its too great alkalinity, but is interesting theoretically to observe that sodium sulphite as well as Rochelle salt will strip salt stains (see Yocum's patent,Collegium(London), 1917, 6; also Procter and Wilson,loc. cit.). This points to the formation of a negative sol, and suggests many other substances for removing salt stains.