Animal Proteins Part 13

SECTION V.--HEAVY CHROME LEATHERS

The term "heavy chrome leather" is taken to include chrome sole leather, chrome strap and harness b.u.t.ts, waterproof chrome upper leathers, motor b.u.t.ts and picking band b.u.t.ts. These will be discussed in turn.

=Chrome sole leather=, as stated in Section I., has made headway in Britain during the European War, the Army authorities having recognized its great advantages in durability and waterproofness. At the time of writing, however, its manufacture has received a set back, and many factories are reducing their output. The primary cause of this is that the Army purchases have largely ceased, whilst the general public have not yet been educated to its value. Men who take chrome uppers for granted talk of chrome sole as a "leather subst.i.tute" with an implication that it is of inferior value. It must be recognized, too, that there is some interested opposition to its development. Cobblers and bootmakers complain that it ruins their tools, being so hard to cut.

Now, it is manifestly impossible for it to be soft to cut and hard to wear out; the complaint is therefore an excellent testimonial. There is also a stupid fear that an article which lasts twice as long will reduce repairs and retail sales by 50 per cent. Even the manufacturer has sometimes a suspicion that a demand reduced in proportion to durability will not be balanced by an extended export trade. These points of view will become minor considerations when the public realize its relative economy, and when the community as a whole grasp that a durable article is a natural a.s.set. Meanwhile credit is due to those firms who persevere in their pioneering work of educating the public.

The manufacture of chrome sole leather presents many a.n.a.logies with the vegetable tannages. The soaking and liming should be about identical, but the hides for chrome are generally given more sulphide and the depilation is reduced to about a week. The methods used for deliming differ widely in different factories. Some delime completely with mineral acids, some even pickle in acid and salt, whilst others merely delime the grain with boric acid. The last is really quite sufficient.

Again, in tanning one finds similar divergences of method. Drum tanning is practised, but tannage in pits by suspension is more usual, though, as this last involves more dilute liquors, it involves also greater time to tan. In drum tannages a few days only are sufficient. In pit tanning at least a week is given, but sometimes up to a month, according to the strength of the final liquor and the rate of progress of the goods into stronger liquors. Liquors containing over 1 per cent. of chromium may easily be spent out so as to contain only 0.01 per cent. Labour and time are saved in pit tanning by the use of rockers. The press system of avoiding handling, however, so complicates the a.n.a.lytical control that its advantage is doubtful, a better way being to shift the liquors by an air ejector, which may also be used as an agitator of the liquor and thus abolish the need for rockers ("Forsare" patent). Chrome b.u.t.ts are tanned out in suspension. No floats or layers are used. The neutralization need not be so thorough as for light chrome uppers, as dyeing is not practised and trouble does not arise with emulsions made from sulphonated oils. Thorough washing is advisable, and the b.u.t.ts are usually then cut into bends and may be oiled before drying if desired.

The bends are dried strained, to obtain flatness and smooth grain, for no machines, such as strikers and rollers, are usually employed. It is necessary to dry very thoroughly, for the bends are waterproofed by dipping the dry leather into molten waxes. The most commonly used wax and the cheapest is paraffin wax with a m.p. of about 127 F. It is rather a brittle wax, however, and as the finished leather consists of up to one-third of the wax, it is better to use at least some proportion of hard fat, j.a.pan wax or ceresin wax, to obtain a stuffing material with less crystalline texture. The use of 10-30 per cent. rosin in the stuffing grease is also usual. This prevents the leather from being so slippery when in wear. The stuffing should take place at temperatures from 150-195 F., according to the melting-point of the grease employed. The bends are taken out and laid in pile to cool and set in a flat condition, and are then finished.

The chrome tannage of b.u.t.ts for strapping and harness backs, and for motor b.u.t.ts and picking bands may be similar to that for chrome sole, but drum tannages are more common and the two-bath process is often used. In the latter case the acid chroming bath is preferred, using 6 per cent. of dichromate and of acid, with up to 15 per cent. of salt, and reducing with 15 per cent. thiosulphate and acid as needed. This process a.s.sists in the production of the light colour which is preferred in the case of some of these leathers.

Strap b.u.t.ts after tanning are very thoroughly washed with cold water in pits, and repeatedly struck out by machine between the washings. They are then oiled with heavy mineral oil, and stretched by powerful machines. They are dried and curried during the stretching. Degras, wool fat and vaseline are greases used, and the drying and stretching finished off at 120 F. They are then fluffed on the flesh, French-chalked and heavily rolled.

Harness backs are neutralized, machine sammed, and lightly fat liquored with 4-1/2 per cent. soap. They are then struck and oiled with heavy mineral oil and dried for stuffing. Hand stuffing, drum stuffing, and "burning in" are all used (see Part I., Section IV.). Stearin, paraffin wax, ceresin wax, wool fat, sod oil and mineral oil are the greases employed. The b.u.t.ts are blacked after stuffing with lamp black and oil, gla.s.sed well and buck-tallowed on the grain.

Motor b.u.t.ts are fat liquored lightly, using soap only. They have to be softened, therefore, during the drying by being mechanically worked. A boarding machine is repeatedly used during the drying. They are finished off with French chalk on flesh and grain.

Picking band b.u.t.ts are neutralized by using warm water and then borax solution, and are then sammed by machine and very heavily fat liquored with cod oil and tallow and hard soap, to which degras may also be added. Up to 20 per cent. of greases (on the pelt weight) may be used.

They are well drummed in this, struck out, French chalked, and dried out. They are softened finally by machine.

Waterproof chrome upper leathers are manufactured usually from hides tanned by the two-bath process, which is said to give a mellower leather. The neutral type of chroming bath is common. The b.u.t.ts are neutralized, machine sammed and struck, and then fat liquored with 2 per cent. each of neatsfoot oil and soft soap. They are then sammed, shaved and blacked on the grain with logwood and iron, and dried further. They are stuffed then by brushing with an abundant amount of concentrated fat liquor. This gives the waterproofness. They are staked after drying further, and often grained three ways. A further waterproof finish is given consisting of a fat liquor containing beeswax. They are finally brushed and re-oiled with linseed oil, to which some mineral oil may be added. This leather is much the most durable type for a shooting boot, or where waterproof uppers are desirable.

REFERENCES.

Procter, "Principles of Leather Manufacture," p. 198.

Bennett, "Manufacture of Leather," pp. 234, 368.

PART IV.--MISCELLANEOUS TANNAGES

SECTION I.--ALUM TANNAGES

The use of alum for making pelt into leather is several centuries old.

It was the first case of what are called "mineral tannages." The tannage is closely a.n.a.logous in theory to the chrome tannages discussed in Part III., and if soda be added to ordinary potash alum in solution, a basic alum liquor is obtained which is quite capable of yielding a satisfactory leather, and which is thus a strict a.n.a.logy of the basic chrome alum liquor described in Part III., Section II. The range of basicity which is practicable is very limited, however, and it is much more usual to use common salt with the alum instead of soda. The alum is, of course, hydrolyzed and free sulphuric acid is quickly adsorbed, whilst the colloidal solution of alumina is adsorbed also but more slowly. The adsorbed acid tends to swell the pelt and to cause it to take up the alumina less readily. The function of the salt is to repress the swelling by a pickling action. The actual result is thus partly due to the alum tannage and partly due to the temporary tannage given by the pickle. Hence such tannages are not firmly "fixed," nor is the result water-resisting, for much of the tanning material may be washed out. If, however, such leathers be stored for a time in a dry condition, the alumina becomes much more firmly fixed, owing probably to a further dehydration of the alumina gel deposited upon the fibres. The tannage is thus relatively more "irreversible," and such storage is practised in commerce for this purpose, being known as the "ageing" of the leather.

It will be understood that it is possible to use too great a proportion of salt, the hygroscopic nature of which would keep the leather moist and thus interfere with a glossy finish. About one-third the weight of the alum used is usually sufficient.

All that has been said in Part III. as to the empty nature of the chrome tannage is equally applicable to the alum tannages. It is as necessary therefore to employ filling agents. A fat liquor is quite satisfactory for many purposes, but is too dark coloured and greasy for glove leather. Egg yolk is the favourite emulsion in these cases. It contains about 30 per cent. of an oil very similar to olein and in very perfect emulsion. Olive oil is also largely olein and is also used, being emulsified by the egg yolk and effectively reducing the proportion required of this expensive material. Flour is also used as a filling agent. It acts also as a whitening agent and as an emulsifier. Its use enables the tanner to obtain the required fullness without so much greasiness. Thus softness and fullness may be obtained, and yet a glossy finish be possible. It will be clear that the more flour is used, the more oil may also be used.

The materials mentioned, viz. alum, salt, flour, egg yolk and olive oil, are all mixed together into a paste with some amount of water. The goods are drummed in this paste and then dried out. This operation is known as "tawing." The goods are then "aged" for several weeks and finished as required.

The manufacture of "glove kid" from lambskins and kid skins is the most typical example of alum tannage. Lambskins are unwoolled very usually by painting the flesh with a mixture of lime and sodium sulphide. There must not be too much of the latter on account of its tendency to give harshness, a fatal defect in glove kid. The addition of calcium chloride is desirable, and the skins, which should be pulled as soon as possible, should be quickly placed in soft water or weak lime. For kidskins a set of lime liquors may be used, and in preference to sodium sulphide red a.r.s.enic is employed. About one per cent. realgar on the weight of the lime is used, but more often larger quant.i.ties are preferred, even up to 6 per cent. The liming is thus shortened to 4 or 5 days. Fresh lime liquors are sometimes used without any sulphides. Another method is to place the skins in a paste of lime to which realgar has been added in slaking. In any method it is necessary to saponify or emulsify the grease on the grain, or difficulties occur in dyeing and finishing.

Skins which are to be tawed for glove kid are both puered and drenched.

They are heavily puered at 70 F. for 3 hours, or even longer for the heavier skins. After scudding they are drenched with 10 per cent. bran and some pea meal at 95 F. for a few hours only.

In preparing the tawing paste, the flour should be mixed with tepid water; the egg yolk should also be diluted with tepid water slightly, and strained if necessary, and then added to the flour. The oil is then carefully mixed in. The alum and salt are dissolved separately at 110 F. and added to the flour and oil. The tawing paste should be used at about 105 F. For every hundred medium-sized lambskins there will be required: 10 lbs. flour in 2-1/2 gallons water, 1 quart preserved egg yolk, 3-3/4 lbs. alum and 1-1/4 lbs. salt. The skins are drummed in this for an hour or so and dried out on poles rapidly, but not with great heat. This is essential to get "stretch." They are next wet back, staked, dried and staked again. They are then "aged."

To wet back for dyeing and finishing the skins are drawn through warm water and then drummed in water at 95 F. for 15 minutes to wet evenly and thoroughly. This liquor, which contains much of the tawing material, is run off and replaced by the dye solution, _e.g._ fustic or turmeric, with which the goods are drummed for half an hour. Iron, chrome or copper salts may be used for saddening. After this "bottom" colour is obtained, a coal tar colour is added for "topping" and the drumming continued until the required shade is obtained. The excess liquor is now run off, and the materials lost in soaking are replaced by drumming further with egg yolk and salt for 15 minutes. This is known as "re-egging." Blacks are obtained with logwood and iron. After re-egging, the skins are dried out and staked. They are "seasoned" with a weak emulsion of soap and oil, dried, oiled lightly with linseed oil, ironed, re-oiled and finally brushed. Whites are undyed, and 10 lbs.

French chalk per 100 skins is used in re-egging.

"Calf kid" is a once popular but now obsolete upper leather made by tawing calfskins. The skins were well plumped in limes, delimed by washing and drenching, tawed much as for glove kid, split, dried out rapidly, staked and aged. They were finished dull and black with soap and wax.

The various white leathers used for belts, laces, whip lashes, ap.r.o.ns, covers for stoppered bottles, etc., are very usually made with an alum tannage. Alum, salt and flour only are used. Whitening is also mixed in and acts as neutralizing agent as well as pigment dye.

Wool rugs are manufactured from suitable sheepskins by an alum tannage.

They are first well cleaned, using soap on wool and flesh. They are next degreased by painting with fuller's earth paste and drying. They are tawed by painting the flesh with a strong solution of alum and salt, or even by rubbing on the solid salts. They are dried out, aged and sorted for suitable colours. The dyeing is rather difficult, as many artificial dyestuffs are of no use. It is usual to bleach the skins first in a weak solution of bleaching powder, and afterwards to dye with infusions of the dyewoods, _e.g._ logwood, fustic, sandalwood, terra j.a.ponica, quercitron bark, turmeric, indigo, etc. Vat dyeing is usual. After dyeing, retanning with alum and salt is necessary, on account of the loss of these in bleaching and dyeing. Rugs are usually finished black, white, grey, brown, walnut, crimson, blue or green.

REFERENCES.

Procter, "Principles of Leather Manufacture," pp. 184, 236.

Bennett, "Manufacture of Leather," pp. 239, 371.

SECTION II.--FAT TANNAGES

For the manufacture of a permanent leather the essential requirements are that the fibres of the hide or skins gel should be dried in a separate condition, and that they should be coated by some waterproof or insoluble material. Many substances fulfil the first but not the second of these conditions. For example, the dehydration only may be accomplished more or less by salt (as in curing hides), still better by salt if a little mineral acid be used (as in pickling), and by other salts such as pota.s.sium carbonate and ammonium sulphate, and dehydrating agents such as alcohol. Such "temporary leathers," however, are not water-resisting, as the second requirement has not been fulfilled, viz.

the coating of the fibres with some more or less waterproof material.

Thus if pelts dehydrated with alcohol be treated with an alcoholic solution of stearic acid, the second condition is fulfilled and a permanent leather is obtained.

Now, many tanning agents accomplish these two requirements only imperfectly. As we have noted in the preceding section, the alum-tanned leathers are not very water resisting, and much of the tannage will wash out. Leathers made by the vegetable tannages usually contain some excess of vegetable tanning matters which are soluble in and removed by water, though much tannin can no longer be thus removed, owing to the mutual precipitation of the oppositely charged tannin sol and hide gel. The necessity for fulfilling the second requirement mentioned is one reason for the practice of following these tannages by applications of oil, fat or of both. In this way the isolated fibres are not only dried separately, but are coated with a typical water-resisting material.

In the fat tannages an attempt is made to fulfil this second requirement without the use of any specific "tanning agent" for producing the first requirements; _i.e._ an attempt is made to dry the fibres separately in an "untanned" condition, and to coat them simultaneously with fat so that a permanent leather is obtained. It is only possible to do this, if the pelt is constantly during drying subjected to mechanical working, _e.g._ by twisting, folding, bending, drumming, staking, etc. The resulting leather is often called "rawhide leather," and presents a real advantage over other leathers in its great tensile strength. Where toughness is an essential quality, there is much to be said for the fat tannages. It is also possible, of course, to effect compromises between ordinary tannages and the straight fat tannages; thus picking band b.u.t.ts, which must be tough, are often very lightly tanned with oak bark or chrome, and then given what is practically a heavy fat tannage. In the most typical of fat tannages, moreover, it is often common to "colour" the goods by a brief immersion in a weak vegetable tan liquor.

Further, the employment of fats in the currying of dressing leather is in effect a fat tannage superimposed upon the vegetable tannage. (See Combination Tannages, Section VI.)

The fat tannage is undoubtedly one of the earliest methods for making leather. Prehistoric man discovered that the skins of animals killed in hunting could, by alternately rubbing with fats and then drying slightly, be eventually converted into a useful leather, whereas without the fat it was stiff and h.o.r.n.y. Even yet similar methods are in use, thongs of raw hide being continually twisted during drying, with intermittent application of fats.

In the modern fat tannages drums are used to give the necessary mechanical working to the goods. The raw hide leather produced in the U.S.A. is made by drumming the nearly delimed goods with tallow and neatsfoot oil. In this country the fat tannages have been typified by the "Crown" and "Helvetia" leathers. The hides are thoroughly limed in mellow limes, and after the beam work are delimed by drenching, scudded, and sometimes fleshed again, and then coloured off in tan liquor. After partial drying, they are drummed warm for some hours to ensure isolation of the fibres. After further drying they are coated with the tanning paste, which consists essentially of soft fats and flour to produce partial emulsification. Equal parts of soft fats and of flour may be used, to which may be added smaller proportions of degras, cod oil, mutton tallow, salt, together with about 25 per cent. water. The goods are coated with this mixture, drummed, and dried further, and this routine repeated as often as necessary to fill the interstices thoroughly with fat. The temperature in the drum may reach 95 F. In finishing an attempt is made to stuff further with grease. The goods are thoroughly set out, dried a little, and coated again, flesh and grain, with a mixture of tallow, cod oil, glycerine and degras, and dried further. The excess grease is slicked off and the goods again set out and grained. They are then dried out.

REFERENCES.

Bennett, "Manufacture of Leather," pp. 245, 246 and 376.