Animal Proteins Part 7

For this cla.s.s of work, flat, spready and evenly grown cowhides are obviously the most suitable material, and are invariably used. It is important, however, that the grain be good, and free from scratches and similar defects. The tannage must be sweet and mellow, _i.e._ contain no acid and little astringent tan. Hence myrabolans and gambier have always been the favourite tanning materials. A soft and mellow tannage is the more important, inasmuch as the leather is not heavily stuffed with grease in finishing. These types of method for tanning split hides will now be outlined, and the nature of the currying then indicated.

_Type 1_.--In this a long mellow liming of 15-16 days is given, much like that described for harness leather in Section III., Type 3.

Only lime is used, but the liquors are not allowed to get dirty. The three-pit system is much the best. The hides are trimmed at the rounding tables, and then bated in hen or pigeon dung for three days at 75-85 F. The deliming is commenced by washing in tepid water before bating, and is completed by a bath of boric acid, using up to 30 lbs. acid per 100 hides as necessary. In this and other processes for split hides it is essential to obtain all the lime out, but to do no plumping with acid. Lactic acid may also be used, but it is not so convenient to hit the neutral point with it.

The tannage consists of oak bark and myrabs together with gambier. These may be partly replaced by Natal bark, valonia, and quebracho respectively. It is sometimes desired to have a smooth finish, but sometimes to work up a "grain." In the latter case the hides are first put through colouring pits containing fresh leach liquor. In these they are constantly handled for a few hours. A little experience indicates which leach liquor will serve the purpose. The hides then go through the "green handlers" (8-20) in two weeks. The liquor is the old forward handler liquor made up with gambier. The hides may be sammed and split up at this stage, but the heavier goods may be tanned further. These heavies and the grains of the split hides now go through the "forward handlers" (20-40) for four weeks, and the heaviest goods given two layers (40) of two weeks each, and making ten in all.

_Type 2_.--In this a shorter liming of 8-9 days is given with the help of sulphide. No dung bate is used, but the goods are washed with water and bated with ammonium chloride and boric acid. The tannage is chiefly of myrabs, but some valonia or Natal bark may be used together with chestnut extract and some quebracho. Gambier is used in the early liquors. The goods are coloured off in drum or paddle and tanned in several sets of handlers, viz. green handlers (15-35) three or four days; second handlers (35-60) two weeks; forward handlers (60-80) 1-1/2 weeks; and floaters (80-90) for three weeks. The tannage is thus 6-1/2 weeks in all. The arrangement of pits is a matter of local convenience, and the number of sets of equal strength is determined by the number of hides being tanned. The hides are split green or after pa.s.sing through the green set. After tanning they are oiled with cod oil and dried out.

_Type 3_ is ill.u.s.trated by American methods. The goods are tacked on laths or racks with copper nails in order to ensure smooth grain. They are then suspended in tan liquors. The tannage is largely with gambier and in weak liquors, which also help to give smooth grain. The tendency is to employ handler rounds involving a rather large number of pits, and to work these on the press system. Handling is also saved by plumping the liquors instead of shifting the goods forward, and by rocking the suspenders instead of handling up and down. The hides are split after about a month, and the heavier grains laid away in hemlock liquors.

_Type 4_.--This is a rapid process throughout. The hides are limed in 6-7 days with the help of sulphide, and "bated" by washing in warm water and then in cold to which hydrochloric acid is gradually added, finishing off again in tepid water. The hides are now coloured off in paddles, put through a small handler round (11-20) for half a week, and then split. The grains are drum tanned in a mixture of chestnut and quebracho extract, over a period of about three days in which the liquor is strengthened gradually from 30 to 50. The fleshes are drum tanned with the old grain liquors after strengthening with quebracho.

The split hide grains for bag work, after tanning, are drummed in sumach, rinsed, drained, and oiled up to dry out, with some setting out.

After wetting back they are shaved if necessary, hand scoured, and heavily sumached again to get a light even colour. The goods are slicked out, oiled up to samm, reset and dried out. They are next stained, sammed, printed by machine, dubbined or tallowed, "grained" (see Part II., Section I.), brushed and rubbed with flannel.

REFERENCE.

Bennett, "Manufacture of Leather," pp. 202, 308.

SECTION X.--PICKING BAND b.u.t.tS

It is the paradox of vegetable tannage that the less the pelt is tanned the stronger is the leather produced. The manufacture of b.u.t.ts for picking bands affords a good ill.u.s.tration. What is required is a leather of maximum toughness, pliability and durability. Any factor reducing the tensile strength of the leather is fatal. Hence, compared with most other tannages, picking band b.u.t.ts are under-tanned. To ensure the desired softness and pliability, moreover, it is necessary to have a mellow liming, rather heavy bating, and a soft mellow tannage in sweet and weak liquors. The required durability and the necessity for weak liquors both point to oak bark as the most suitable tanning material, a.s.sisted by some gambier in the early stages.

A good quality hide is chosen, and given a long and mellow liming of about 15-16 days. The one-pit system may be used, and the hides are put into an old lime for about five days with frequent handling and then placed in a new lime which is made up in a pit containing about a foot depth of the old liquor. After about twelve days another 1/3 cwt. of lime may be added.

After unhairing and fleshing the goods are bated in pigeon dung for four days at a temperature of about 78 F., handling twice on the first and last days. The bating is stopped and the deliming completed by paddling with boric acid (15 lbs. per 100 b.u.t.ts).

The tannage is commenced by paddling in a spent handler liquor (4) to which a little gambier has been added. The b.u.t.ts then go through the first handlers (5-15), which are rounds of ten pits in which the goods are handled every day in the first week, and alternate days in the second week, and are shifted forward twice a week in the next pit. The goods are therefore in this set for five weeks. Gambier is added to these liquors as needed. The b.u.t.ts next pa.s.s to duster rounds of four pits, in which they are dusted down in a liquor of 20 for four weeks with 1-2 cwt. of oak bark. The liquor is obtained from the leaches, and afterwards run alternately to the leaches and to the first handlers. As many as six layers are now given of 20-25 strength, in which the b.u.t.ts are dusted down with 2-3 cwt. oak bark for three weeks. The layer liquors are received from and returned to the leaches, which are made from the "fishings" from the layers. The tannage lasts, therefore, 27 weeks, of which 18 weeks (two-thirds) are in layers.

Shorter tannages are now often given, using stronger liquors, much as in ordinary dressing leather.

The tanned b.u.t.ts are rough dried, and then wet in for shaving. They are thoroughly scoured, flesh and grain. They are next drummed for three-quarters of an hour in sumach, struck out and hung up to samm.

Hand stuffing is best, to avoid any tendering owing to high temperature, but drum stuffing is also used. After setting out and stoning on the grain they are stuffed with warm cod oil and laid away in grease for several weeks, re-oiling occasionally. They may be stained before stuffing.

REFERENCE.

Bennett, "Manufacture of Leather," pp. 203, 310.

PART II.--SKINS FOR LIGHT LEATHERS

SECTION I.--PRINCIPLES AND GENERAL METHODS OF LIGHT LEATHER MANUFACTURE

The term "skin," like the term "hide," in its widest sense applies to the natural covering for the body of any animal, but is generally used with a narrower meaning in which it applies only to the covering of the smaller animals. Thus we speak of sheep skins, goat skins, seal skins, pig skins, deer skins, and porpoise skins. It is in this sense that it will be used in this volume. The treatment of such skins to fit them for useful purposes comprises the light leather trade. Whilst this branch of the leather industry is certainly utilitarian, the artistic element is a great deal more prominent in it than in the heavy leather branch. Thus the light leathers are often dyed and artistically finished, and their final purposes (such as fancy goods, upholstery, bookbinding, slippers, etc.) have rather more of the element of luxury than of essential utility. The total weight and value of the skins prepared, and of the materials used in their preparation, are naturally considerably smaller than those of the heavy leather trade. In the latter, moreover, one has to consider the purpose in view from the very commencement of manufacture and vary the process accordingly, but in light leather manufacture one aims rather, in the factory, at a type of leather such as morocco leather, and only after manufacture is it fitted to such purposes as may be particularly suited to the actual result. These results depend very largely upon the "grain pattern" which is natural to the skin of any one species of animals. Hence in Part II. of this volume it has been found most convenient to deal with the different cla.s.ses of skins in different sections. Just as the hides of ox and heifer were much the most numerous and important of hides, so also naturally are sheepskins the most prominent section of the raw material of the light leather trade. This is the more true because the skin is valued for its wool as well as for its pelt; indeed, the wool is often considered of primary importance, and receives first consideration in fellmongering.

Unfortunately for the light leather trade, sheepskins, though most numerous, do not give the best cla.s.s of light leather, the quality being easily surpa.s.sed in strength, beauty and durability by the leather from goat or seal skins.

In the wet work for the preparation of skins for tannage much the same general principles and methods are embodied as in the case of hides, but with appropriate modifications. As soft leathers are chiefly wanted, a mellow liming is quite the usual requirement for all skins. It is also usual to have a long liming, for some skins (like those of sheep and seal) have much natural fat which needs the saponifying influence of lime and lipolytic action of the enzymes of the lime liquors; whilst other skins (like those of goat and calf) are very close textured and need the plumping action of the lime and a certain solution of interfibrillar substance. In consequence of the long mellow liming, sulphides are not usually necessary, and indeed sodium sulphide is not usually desirable, on account of its tendency to make the grain harsh.

It is used, however, for unwoolling sheepskins, in such a manner that the grain is not touched. Similarly caustic soda is seldom required, and the yield of pelt by weight is usually a small consideration. Systems of liming show some variety. The one-pit system is very common, and is less objectionable for a long mellow liming, but rounds of several pits are also used, and in some cases even more than one round. This is obviously conducive to regularity of treatment, and as the work involved in shifting the goods is much less laborious than in the case of heavy ox hides, it would seem a preferable alternative. The depilation of sheepskins involves very special methods of treatment (sweating and painting) on account of the importance and value of the wool, the quality and value of which would be impaired by putting the skins through ordinary lime liquors. The pelts, however, are limed after unwoolling.

In deliming light leathers the process of puering is widely used. This consists in immersing the skins after depilation in a warm fermenting infusion of dog-dung. In principle this disgusting process presents a close a.n.a.logy with bating, and indeed the two terms are both used somewhat loosely, but there are nevertheless several points in which the two processes are radically different. The dog-dung puer is a process carried out at a higher temperature than the fowl-dung bate; it is also a much quicker process, and the infusion employed is generally more concentrated. Whilst the fowl-dung bate is always slightly alkaline to phenolphthalein the dog-dung puer is always acid to this indicator, and the course of the puering may be conveniently followed by testing the pelts with it. The mechanism of the two processes is also probably somewhat different. The mechanism of the dog-dung puer has been largely made clear by the researches of Wood and others, and been found due partly to a deliming action by the amine salts of weak organic acids and partly to the action of enzymes from a bacillus of the coli cla.s.s, which received the name of _B. erodiens_, and which effects a solvent action on the interfibrillar substance. As we have noted (Part I., Section II.), the fowl-dung bate involves two fermentations, in each of which (aerobic and anaerobic) several species of bacteria are probably active.

Wood found the bacteria of the bate to be chiefly cocci, and ascribed part of the difference in mechanism by the nature of the media, which in the bate includes also the urinary products. In the dog-dung puer, also, a lipolytic action is probably an essential part of the total effect.

The puer gives a much more complete deliming and a much softer and more relaxed pelt than the bate, it is therefore particularly suited to the needs of light leather manufacture. The puering action has been imitated fairly successfully by artificial methods. "Erodin" (Wood, Popp and Becker) involves the use of _B. erodiens_ and a suitable culture medium including organic deliming salts: "Oropon," "Pancreol" and others involve the use of ammonium chloride and trypsin, together with some inert matter.

Light-leather goods are usually drenched after puering. They are also often split green after the wet work. Sheepskins thus yield "skivers"

(the grain split), whilst the flesh split is often given an oil tannage (see Part IV., Section III.). The greasy nature of sheep and seal skins necessitates the processes of "degreasing." In the case of sealskins this is done largely before liming, but with sheepskins either after being struck through with tan, or after tannage is complete. Sheepskins are often preserved in the pelt by pickling with sulphuric acid and salt, which process forms a temporary leather. The fibres of the pelt are dried in a separate condition, but the adsorption is easily reversible and the pelts may be "depickled" by weak alkalies and afterwards given an ordinary vegetable tannage.

In the vegetable tannage of skins for light leathers, the same theoretical considerations have force as in the heavy-leather section, but the former has its own rather special requirements and aims.

Generally speaking, a softer and more flexible leather is required, but these qualities must not be imparted by stuffing with grease as in the currying of dressing leather, because a bright and grease-free result is usually required. Hence it is important that a sweet mellow tannage be given. The durability of the leather is also a primary consideration for goods intended for bookbinding, upholstery, etc., and the tannage must be arranged to impart this quality and avoid anything tending to cause the perishing of the fibre. Thus oak bark is a popular tanning material, and sulphuric acid very definitely avoided. The tannage must be fast, and take the dyestuffs well, and for the production of light shades of colour in dyeing must be a light-coloured tannage. All these qualities are imparted by sumach, which also fits in excellently with the other general requirements, such as softness, brightness and durability.

Hence sumach is the princ.i.p.al light-leather tanning material, but the tendency is to employ other materials--oak bark, myrabs, and chestnut extract--to do much of the intermediate tanning, so that the expensive and useful sumach may be used for setting the colour and grain at the commencement, and for brightening, bleaching and mordanting the leather at the end of the tanning process. Weight is generally no consideration, but area is often a definite aim, partly because some goods are sold by area and partly because the striking out, setting out and similar operations improve the quality of the leather by giving evenness of finish. Leather well struck out, moreover, is less liable to go out of shape. As the grain pattern is so important in the finished leather, appropriate care must be taken during tannage. If a smooth or a fine grain finish is wanted, for example, the goods must not be allowed to get wrinkled, creased, doubled or unduly bent to and fro during the tanning. For such goods, suspension, careful handling and even the "bag tannage" may be desirable, whilst for coa.r.s.er and larger grains paddles or drums may be more extensively used.

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.), but such leathers are usually "retanned" or prepared for dyeing by sumaching (which process also incidentally bleaches). The tannin mordant a.s.sists 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 alb.u.mins and emulsified fats, _e.g._ egg alb.u.min 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 gla.s.s 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."

SECTION II.--GOATSKINS

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 princ.i.p.al 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.