Animal Proteins Part 12

In any of these methods the chrome alum liquor is suitable, using 10 per cent. alum and 3 per cent. soda on the pelt weight. The glucose liquor has also proved very suitable for chrome calf, and the liquors made with sulphurous acid or its salts have increasing popularity on account of lower costs. Many tanners use bought liquors--"chrome extracts" which are supposed to be specially devised to suit the tannage of chrome calf.

When thoroughly tanned through, as can be readily judged from a sectional cut of the leather, and also by the strength of the liquor remaining, the goods are horsed in pelt overnight, and are then ready for finishing.

In finishing box calf the neutralization should be thorough, or the acid may cause trouble in dyeing and fat liquoring. Imperfect removal of excess chrome salts may cause the formation of "chrome soaps" which are very difficult to remove; the goods should therefore be well washed.

There are two general types of treatment before blacking. In one, the skins are first well washed with water at 110 F., neutralized with about 3 per cent. borax, and well washed again. Striking follows and is usually very thorough, partly because it a.s.sists in producing evenly the characteristic box grain, and partly because the finished leather is sold by the square foot. Machine striking is now almost universal, and may be done several times at different stages in the drying. When half dry ("sammed") the skins are shaved by machine and, at this stage usually, weighed. Dyeing and fat liquoring then follows. In the other type, the goods are merely washed, and then struck out, sammed, shaved and weighed. The skins are then neutralized, washed and immediately dyed and fat liquored. The advantages of this latter course are that the goods remain in the drum for the last four processes, which is economical of labour, and also that by neutralizing immediately before dyeing and fat liquoring there is less danger of a further diffusion of acid.

In dyeing logwood extract is largely used, occasionally a little fustic is used also, and by using a "striker" of iron and copper sulphates a good black is obtained. Logwood is often used also in conjunction with coal-tar dyestuffs. The goods are first warmed in the drum up to 140 F., and the dyestuff solution gradually run into the drum whilst it is revolving. Up to 3/4 hour may be necessary to exhaust the bath, the goods being constantly drummed. The fat liquor is then run in similarly, and the drumming continued until the grease is all absorbed by the leather, which may take another hour. The skins are horsed till next day, during which time the grease penetrates more completely.

The skins are now dried out, sometimes by suspending from the hind shanks and sometimes by nailing on boards or wooden frames. They are damped back for staking by leaving for 1-1/2 to 2 days in moist sawdust.

After staking they are dried strained in a "stove" at about 105 F.

In finishing off, the grain is "cleared" by sponging with 10 per cent.

lactic acid, and seasoned with a mixture of milk, blood and black dyestuff. When dry on the surface the skins are glazed by machine, and grained two ways--neck to b.u.t.t and belly to belly. They are usually reseasoned, dried out, reglazed, regrained, lightly oiled with mineral oil, and finally trimmed. These various operations are fairly typical, but there is obviously ample scope for divergence. Thus one may fat liquor before dyeing, and the skins may be staked before drying out, and may be re-staked after glazing.

Much so-called "box calf" is not made from calf skins. A very close approximation, however, is obtained from rather older animals, and "box-kip" is largely manufactured by similar methods. Light hides are also widely used, being similarly treated except that they are split and also cut into two along the spine. The finished article is sold as "box-sides." To yield the characteristic grain pattern, the goods are frequently printed and embossed. Even the flesh splits are sometimes made into box calf imitations, some filling material being used and an artificial grain pattern embossed.

=Willow calf= typifies the chrome calf which is finished in colours. The soaking, liming and deliming processes are the same as for box calf. The tannage, however, is generally by the two-bath process on account of the lighter colour thereby obtained. This colour is largely due to the deposition of sulphur in and on the leather in the second bath.

In one tanning process the skins are first pickled in 2 per cent.

hydrochloric acid and 10 per cent. salt. They are then drummed in solution containing 2 per cent. dichromate (strength 1 in 60) for about half an hour. A solution containing 4 per cent. dichromate, 3-1/2 per cent. hydrochloric acid, and 5 per cent. salt is gradually added, and the skins drummed until well struck through. They are then horsed overnight and struck out and pa.s.sed through a "hypo dip,"--a 2 per cent.

solution of thiosulphate,--and then into the reducing bath, which contains 10 per cent. of thiosulphate, to which 5 per cent. hydrochloric acid is added.

Another process employs paddles instead of drums. The chroming liquor is made up with 4-1/2 per cent. chromic acid and 10 per cent. salt. The bath is exhausted by commencing the tannage of a succeeding pack. The skins are reduced as in the last process.

In another process the "acid" type of chroming bath is used. The skins are paddled with a solution containing 5 per cent. dichromate, 5 per cent. hydrochloric acid, 2 per cent. aluminium sulphate, and 10 per cent. salt. In the reducing bath 14 per cent. hypo and 4 per cent.

hydrochloric acid are used.

In yet another process the skins are pickled first in 5 per cent.

aluminium sulphate, 7-1/2 per cent. salt, and 3 per cent. sulphuric acid, and are then dried out and sorted. The tannage proper is in the drum, using 6 per cent. dichromate, 5 per cent. hydrochloric acid, and 5 per cent. salt. In the reducing drum 15 per cent. hypo is used and 4-1/2 per cent. hydrochloric acid.

Whichever process of tanning has been used, the skins are neutralized and washed thoroughly, as for box calf, sammed and shaved. In dyeing, the skins are first mordanted with a filtered infusion of leaf sumach, used at 110 F. for half an hour. As fixing agent, 4 oz. tartar emetic per dozen skins is then added and the drumming continued for half an hour. The goods are washed, struck out and drum dyed at 140 F. with basic colours, and immediately fat liquored. In the fat liquors olive oil and castor oil, with the corresponding soaps, have been popular, but subst.i.tutes are now used on economical grounds. The skins are next horsed a while, well struck out again and dried strained. They are now finished off as for box calf, except that it is usual to grain only one way--neck to b.u.t.t--and the season should consist of milk, water and alb.u.min only, though sometimes other mucilagenous matters are added. As with box calf, the finishing may be varied in many ways. The skins may be dyed with acid colours after fat liquoring. For pale shades direct dyes are used without a mordant. For darker shades of brown and red, the dyewoods are used both as mordants and ground colours, and t.i.tanium salts are useful as fixing agents.

Both the "box" and "willow" finish are largely a matter of public taste, and the fashion varies from time to time on such points as to whether the grain should be one way or two ways, and whether it should be faint or bold. There are also other common finishes besides the typical box grain. =Glace calf= is made much in the same way as box calf, but there is no graining at all. The goods are usually seasoned and glazed three times. Small skins are preferred for this finish. =Dull calf= is also a plain finish. The leather contains more grease, and the fat liquor is made up with greater proportions of degras. The goods are not seasoned or glazed, but ironed, "sized" with gum, oil, soap and logwood, and after brushing are dried and rolled. In both these plain finishes a one-bath paddle or pit tannage is common, in order to ensure the smooth finish.

REFERENCES.

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

Bennett, "Manufacture of Leather," pp. 55, 84, 105, 227, 360-363, 375.

Bennett, "Theory and Practice in Wetwork of Chrome Calf," _Shoe and Leather Reporter_, Sept., 1909.

SECTION IV.--CHROME GOAT AND SHEEP

Immense quant.i.ties of goat and sheep skins are chrome tanned for upper leathers. Most of them are manufactured into the well-known and popular =Glace kid=, to the manufacture of which this section is chiefly devoted. To be quite strict, glace kid should be made from kid skins, but actually comparatively few of such skins are used, they being reserved rather for glove leathers. The popular upper leather is made from goatskins.

Chrome goat is deservedly popular; it is an ideal upper leather for shoes and light boots. As compared with chrome calf (thickness and other factors being equal), it is not only softer and more pliant, but also more durable. It is usually, however, not quite so thick, and perhaps therefore not quite so warm and waterproof. The popularity of glace is probably enhanced by the brighter and more gla.s.sy finish than is usual with box.

As the supply of goatskins is unfortunately too limited, an even more widely used glace upper leather is made from sheepskins, and often sold as glace kid. From what has been previously said as to the quality of goat and sheepskin leathers (Part II., Sections II. and IV.), it will be readily understood that glace sheep is by no means so good a leather as glace goat. It is perhaps as soft, but is more spongy and loose textured, and is neither so waterproof nor so durable as chrome goat.

The ubiquitous sheep, however, provides an immense supply of raw material, and the resulting leather, which should strictly be regarded as a glace kid imitation, finds a ready sale. When well finished it is indeed a good imitation in respect of appearance, and this fact, together with its comparatively low cost, causes it to meet an undoubted public need.

The production of glace goat will first be considered. The soaking process is quite similar to that before described for the production of goatskin moroccos (_q.v._) and need not be here repeated. The liming is similar in many respects also, but from what was said in Section II.

about the undesirability of excessive plumping of pelt for chrome leather, it will be clear that caustic soda should be omitted from the limes. The liming should also be shorter for glace than for moroccos, and this is attained both by using a greater proportion of sulphide and by using mellower lime liquors, preferably the latter, as soft pelts are better ensured. Calcium chloride has sometimes been added to the limes: this reacts with the soda from the sulphide, yielding salt and probably precipitating lime, and has its own lyotrope influence, thus reducing the plumping effect possibly in two ways. To obtain either effect it is necessary to use considerable amounts of calcium chloride. As goatskins are so tight fibred, a longer liming and a greater loss of collagen is permissible than with most pelts for chrome. The deliming operations should be exceedingly thorough in order to obtain the desired softness and the smooth grain. Puering is largely used to the full extent, _i.e._ the goods are thoroughly pulled down at 85-90 F., and are carefully delimed in the puer liquor. After puering it is common to give a low temperature drench (60-65 F.), which of course acts slowly over a day or two. The skins must be well scudded after puering or after drenching; sometimes after both. The drenching is often subst.i.tuted for purely deliming processes, of which may be mentioned the use of boric acid and also the use of warm solutions of the commercial organic acids (lactic, formic, acetic, butyric, etc.), together with calcium chloride. In place of the chloride, a salt of the acid may be employed, and the deliming bath may be regenerated by oxalic acid and used repeatedly. Sometimes puering is omitted and the desired result obtained by washing in warm water, nearly deliming with warm solutions of organic acid, washing again and drenching. Skins are also washed often after drenching.

In tanning chrome goat for glace the two-bath process is mostly preferred. This is partly because the sulphur deposited in the reducing bath a.s.sists materially in producing the mellowness and fullness which are so essential, and partly because a large proportion of skins are finished in colours. The two-bath process also lends itself to a paddle tannage, which is necessary for the smooth grain finish. One or two ill.u.s.trative processes may be given.

One process presents many points of resemblance to the first process suggested for willow calf in Section III. (_q.v._). The skins are first pickled in a paddle with 2 per cent. hydrochloric acid and 10 per cent.

salt, and then pa.s.s into the chroming paddle, which contains at first only 2 per cent. dichromate. Subsequently 4 per cent. dichromate, 3-1/2 per cent. hydrochloric acid, and 5 per cent. salt are added to the paddle liquor, and the skins paddled until well struck through. After being horsed overnight the skins are struck out by machine, pa.s.sed through a hypo dip if desired, and reduced with 12 per cent. of thiosulphate and about 5 per cent. of acid. The skins may be left overnight in the hypo paddle, and the excess of thiosulphate, which is a feeble alkali, commences the neutralization.

In another process the chroming bath is made up of 5-1/2 per cent.

chromic acid and 6-1/2 per cent. of salt, and to this paddle liquor 2 or 3 per cent. of aluminium sulphate may be added if desired. The reduction is with 14 per cent. hypo and 7 per cent. hydrochloric acid. A little of the acid is added to the reducing bath; when the liquor turns milky, the skins are rapidly inserted, and the rest of the acid gradually added.

In the finishing processes the mechanical operation of "striking" is very prominent, on account of the necessity of obtaining area and smooth grain. The skin of goats has rather a tendency to bold grain, and this enhances the need of striking. Most manufacturers lay great stress upon thorough neutralization and washing. An important point also is that the staking should be carried out at the proper condition of dryness. If either too damp or too dry, the requisite mellow feel is not obtained.

There is, of course, ample scope for variation and ingenuity, and the following processes for blacks and colours must be taken as broadly typical.

The skins from the reducing bath are first machine-struck, and then immediately neutralized with one per cent. borax until this is thoroughly used up, and the skins are then paddled for many hours in running water. They are again struck out and lightly shaved, possibly after a little drying. There is a tendency to save time by using a stronger borax solution, and by using warm or tepid water, and some factories save borax by washing well first in warm water. If for blacks a common plan is to dye grain and flesh a violet-blue and then black the grain only with logwood and iron. The skins are drum dyed blue with a coal-tar dyestuff, drumming half an hour in the solution at 110 F., and again struck out. They are then paired or pleated, and rapidly pa.s.sed successively through three vats containing respectively cold weak ammonia, a logwood and fustic infusion at 120 F., and a solution of ferrous sulphate containing a little copper sulphate. The skins must be immediately washed well to remove excess of iron. Instead of this process the skins may be pa.s.sed through vats containing coal-tar blacks.

Instead of blue backing the skins may be drum-dyed black on flesh and grain with either coal-tar blacks or with logwood and iron. In the latter case the skins must be drummed in water for an hour to remove excess of iron. However dyed, the skins are often struck out again after dyeing, and sammed slightly for fat liquoring. Neatsfoot oil is a popular ingredient of the fat liquor. The skins are drummed dry for a few minutes in a hot drum, and the fat liquor added at 130 F., and the drumming continued after the grease has been taken up in order that it may be thoroughly distributed. The skins are struck out again, rapidly dried out, and wet back for staking in damp sawdust. The staking should be thorough, and, if necessary, repeated when the goods are rather drier.

In finishing off the skins may be fluffed if desired, and are then "cleared" by sponging with 10 per cent. lactic or acetic acid. They are then seasoned and glazed after some drying. This is repeated until the required gloss has been obtained. They are finally oiled lightly with a mixture of linseed and mineral oils. On finishing =dull kid= a heavier fat liquor is given, in which degras is used, and the skins are not seasoned and glazed, but are ironed and oiled. In finishing for =coloured glace=, the skins are mordanted before dyeing by the use of dyewood extracts, antimony and t.i.tanium salts being used as fixing agents. The fat liquor should contain less soap and more egg yolk, and for fancy shades even egg yolk only is sometimes used.

The production of chrome glace sheep follows the same general lines as glace goat. There is less difficulty in obtaining smooth grain, so that "striking" is perhaps less prominent, and drum tannages are preferred, whether one bath or two bath. The skins are received after fellmongering (see Part II., Section IV.) and need thorough puering to remove scud, and may be then rinsed through boric acid. Pickling is very common with these goods. In the pickled state they are often sorted out before tanning. The pickling is usually a one-bath process in which vitriol and salt or else alum and salt are used, but sometimes all three substances. The skins may indeed be received in a pickled state. They may be depickled by paddling with salt and borax, bicarbonate, or basic alum solution. They may also be tanned without depickling if the composition of the pickle be allowed for in the first chroming liquor. A commonly used pickle consists of 3 per cent. aluminium sulphate and 9 per cent. salt. If these goods are to be dried out, flour also may be used with the pickle, which thus becomes practically a light preliminary alum tannage (see Part IV., Section I.). A commonly used acid pickle is of 5 per cent. commercial sulphuric acid and 25 per cent. salt.

The delimed or depickled stock may be tanned as now described. The two-bath process may be used with drums. The chroming bath contains 5 per cent. dichromate, 5 per cent. hydrochloric acid, and 10 per cent.

salt. After the skins are thoroughly penetrated they are horsed overnight and reduced with 20 per cent. thiosulphate, up to 7 per cent.

of hydrochloric acid being added after half an hour in thiosulphate only.

Alum pickled or tawed skins are wet back by drumming for about an hour in water, and are then tanned by the one-bath process in drums. Only a few hours are needed. Towards the end of the operation about 1/2 per cent. of bicarbonate of soda may be added to the chrome liquor. Acid pickled skins may be wet back with 10 per cent. salt, and depickled by adding a basic alum solution and the chrome tannage superimposed after about half an hour without handling the goods. The basic chrome alum liquor is suitable for this purpose.

In finishing glace sheep much the same methods are used as in the case of glace goat. Sheepskins are perhaps more lightly fat liquored, being naturally soft and porous. Degreasing is often necessary to obtain an even finish. As sheep gives an empty pelt and chrome an empty tannage, a slight retannage is often given in gambier, especially for blacks, in which case the skins are well mordanted. This retannage makes the leather less stretchy. Logwood and iron blacks are usual. For colours, fustic or sumach are the usual mordants, with tartar emetic to fix. If for glove leathers, skins pickled in alum and salt or tawed should be preferred, and flour may be used in the fat liquor.

Sheepskin splits are sometimes given a chrome tannage and finished as =chrome chamois=. This leather may be used for linings, but not for polishing silver on account of the sulphur originating from the reduction bath. The splits are puered heavily, and pickled in 6 per cent. vitriol and 24 per cent. salt. They are paddled in this pickle liquor, and 4 per cent. dichromate added in successive portions. The fleshes are horsed overnight and reduced in 15 per cent. thiosulphate, to which a little hydrochloric acid is added if needed.

In finishing the splits are washed in warm water, neutralized in weak soda, and washed again. They are sammed by machine striking, and fat liquored, using much soap. They are then horsed, struck and dried out.

They are staked several times after damping back, drying out again between stakings. They are finally fluffed.

REFERENCES.

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

Bennett, "Manufacture of Leather," pp. 55, 84, 105, 230, 364.

Bennett, "Theory and Practice in Wetwork of Chrome Goat," _Shoe and Leather Reporter_, Sept., 1910