The second bath of the two-bath chrome tannage consists of a solution of sodium thiosulphate acidified with hydrochloric acid. The reactions in this bath are somewhat complicated, several occurring simultaneously.

Broadly speaking, the final result is due to (1) the reduction of the chromic acid to a chromic salt by the sulphurous acid; (2) the formation of a basic chromic salt owing to the excess of thiosulphate; (3) the reaction of the added acid and thiosulphate to give free sulphur, which is deposited in and on the leather. The relative intensity of these effects is variable, according to the conditions of operation, _e.g._ the amounts of chemicals used, their concentration, the nature and condition of the goods, the time of application, the manner of application, etc. In practice the most favourable conditions are usually discovered empirically, but, broadly speaking, the goods are usually added soon after the thiosulphate and acid are well mixed. There is some evidence that the reduction is in steps, intermediate products such as sodium tetrathionate and chromium dioxide are known to be formed. The goods change from yellow to dark brown, then to green, and finally to the familiar blue. The sulphur makes the final colour a lighter blue than in the case of a one-bath tannage, hence the two-bath process is often preferred for "colours."

On account of the empirical character of this "hypo bath," it is impossible to fix any exact relation between the quant.i.ties of material used in the chroming bath, and the quant.i.ties of "hypo" and acid used in the reducing bath. The following rules, therefore, must be understood as rough approximations for practical use, and though they have been empirically discovered their theoretical significance is often fairly obvious.

1. The amount of hypo necessary is almost directly proportional to the amount of dichromate used. In chroming with baths of the acid or neutral type, the percentage of hypo should be about three times the percentage of dichromate used. Thus 4 per cent. dichromate needs 12 per cent. hypo; and 6 per cent. dichromate needs 18 per cent. hypo on the pelt weight. In baths of the Schultz type a less proportion of hypo may suffice, but the 10 per cent. hypo for 5 per cent.

dichromate, recommended by the Schultz patent, is generally considered rather insufficient.

2. The proportion of hypo is increased somewhat for the heavier cla.s.ses of goods, and may even reach 20 per cent. of the pelt weight.

3. An increase in the proportion of hypo is usual with an increase in the amount of free acid in an acid chroming bath.

4. The percentage of hydrochloric acid in the reducing bath is roughly half that of the hypo, but is the most variable factor. The quant.i.ty varies with the rate and mode of addition, the cla.s.s of goods under treatment, and the composition of the chroming bath.

5. In baths of the Schultz and neutral type it is better to add some acid to the hypo bath before adding the goods, but this is less essential for goods from an acid chroming bath.

6. In the case of goods from acid chroming baths, the amount of acid used in the reducing bath is an inverse function of the excess of acid in the first bath, _e.g._ take the following two processes:--

------------------------------------+------------------------------- Chroming bath. | Hypo bath.

---------------+--------------------+----------+-------------------- Dichromate. | Hydrochloric acid. | Hypo. | Hydrochloric acid.

---------------+--------------------+----------+-------------------- 4 | 4 | 12 | 5 4 | 15 | 15 | 1 ---------------+--------------------+----------+--------------------

7. There should be some excess of hypo at the end of the process. This acts as a feeble alkali, and commences the neutralization.

The process can be carried out in paddles or in drums as preferred, for reasons similar to those applicable in the case of the first bath. On the whole, however, drums are less popular for the second bath, for the dilute solutions of the paddle effect some economy of sulphurous acid, which is apt to escape into the air. A preliminary "hypo dip" is sometimes used to prevent the "bleeding" of the chromic acid. The use of many other reducing agents has been suggested as subst.i.tutes for hypo.

Sulphides, sulphuretted hydrogen, polysulphides, sulphites, bisulphites, hydrogen peroxide, nitrous acid, lactic acid, etc., have been used, but none are so easy to manipulate as thiosulphate.

=Types of One-bath Chrome Tannage.=--The one-bath process is simpler than the two-bath process inasmuch as only one kind of liquor is involved, viz. one in which the chromium is in the chromic state. Hence the variants of the one-bath process consist mainly of variations in the composition of this liquor. The chief point of variation is in the readiness with which chromium hydrate is adsorbed. This is determined by the extent to which the chromic salt is hydrolyzed to form the tanning sol and free acid, and by the concentration and nature of this free acid as well as of other substances. It is difficult unfortunately to express these factors in terms which are comparable under general conditions.

Chromic salts are usually hydrolyzed to some extent, but this extent is very different even in water, according to the nature of the acid radical. The degree of hydrolysis is also largely affected by the extent to which the solution has been "made basic" by the addition of alkalies.

By the neutralization of the free acid in this way there is further hydrolysis, the extent of which is again influenced by the nature of the acid radical involved and other dissolved substances, especially of organic matters. Again, the hydrolysis is largely affected by the concentration of the solution even when the proportions of the ingredients are constant, and this is practically important on account of the necessity for exhausting the chrome liquors economically. Nor is the matter entirely one of degree of hydrolysis, for (as we have noted in the preceding section) the electrical condition of the chroming sol is of great importance owing to the operation of the valency rule and the possibility of zones of non-precipitation. The alkaline, neutral or acid condition of the goods when first introduced has also its influence on all these points.

It will be readily understood, therefore, that there is some difficulty in expressing the tanning power of a chrome liquor. As near as can be yet said this is determined by (1) the concentration of the actual tanning sol, and (2) its nearness to the isoelectric point. Now, these points are not readily determined by a.n.a.lytical methods, and the best that can yet be done is to determine the conditions which have large influence upon these points. Thus the degree to which the liquor is "made basic" by adding alkali is known, and can be expressed in formulae by a.s.suming that the acid neutralized by this alkali is replaced in the chrome salt by hydroxy groups. Chromic chloride, Cr{2}Cl{6}, with the addition of soda to correspond to half the acid formed upon complete hydrolysis, would be considered then to be a solution of the salt, Cr{2}(OH){3}Cl{3}. This has given rise to the conception of the "basicity" of a chrome liquor, which may be expressed in many ways, the most common of which in practice is the number of grams SO{4} still combined with 52 grams Cr. Thus the salt corresponding to the composition Cr(OH)SO{4} is said to have a basicity of 96. The practical importance of such determinations of basicity has been much exaggerated, for they are but a rough guide to the degree of hydrolysis of the chrome and to the extent to which the sol is positive. Thus if the chrome salt be actually a sulphate, a liquor of basicity 96 has about the same _practical_ value as a chloride liquor of basicity 72, and in each case the figures are of little significance if many organic substances be present. If, however, as is usual in practice, there be approximately the same acid radicals throughout the tannage and about the same relative proportion of organic matters or of inorganic salts, then these determinations have some practical value for comparative purposes. The determination is itself simple: a portion of liquor is t.i.trated direct with caustic soda. The t.i.tration is at boiling-point, and is continued until a permanent pink is obtained with phenolphthalein. The amount of SO{4} corresponding to the soda required is then relative to the amount of Cr in the same volume of liquor. A chromium estimation is therefore also necessary and is most readily done by evaporating a portion of liquor to dryness, igniting the residue and oxidizing the chrome to chromate by heating in a m.u.f.fle furnace with magnesia and sodium carbonate in equal parts, or fusing in a blowpipe with sodium and pota.s.sium carbonates in equal parts. The oxidized residue is dissolved in hydrochloric acid and t.i.trated with thiosulphate as described for the two-bath process.

Another attempt to determine the practical value of a chrome liquor is the empirical test suggested by McCandlish, in which 10 c.c. of the liquor is t.i.trated with standard alkali until the precipitation point is reached and a turbidity appears. The figure thus indicates approximately the degree of nearness to the precipitation point and the amount of free acid in the liquor. The author has found this a useful test taken in conjunction with the basicity determination. It is best expressed in the same units, _e.g._ grams SO{4} per 52 grams Cr.

Another method is the determination of the hydrion concentration of the liquor. This has useful possibilities for research work, but is usually too laborious for rapid commercial control. The results, moreover, are not less empirical, for the hydrion concentration of the liquor indicates but imperfectly the electrical condition of the particles of the tanning sol.

In cla.s.sifying one-bath liquors into types, it is best to take together those in which the usual "basicity" and "acidity" determinations have at any rate approximate comparative value, and this is determined in the main by the method by which the liquor is manufactured. Broadly speaking, there are three types of chrome liquor: (1) those made from chromic salts by adding suitable amounts of alkali; (2) those made from sodium dichromate by reduction with organic matter; and (3) those made from sodium dichromate by reduction with sulphurous acid or its salts.

Of the first type the most common is that in which chrome alum (a bye-product of the dyeing industry) is the starting-point. To a solution of this a solution of washing soda is gradually added, with constant stirring, until the salt corresponding with the formula Cr(OH)SO{4} is obtained.

Now:--

K{2}SO{4}Cr{2}(SO{4}){3}24H{2}O + Na{2}SO{4} ___________________________________________/ 998 + Na{2}CO{3}10H{2}O ________________/ 286 = 2Cr(OH)SO{4} + K{2}SO{4} + CO{2} + 33H{2}O

Hence, in practice, for every ten parts of chrome alum 2.86 parts of soda crystals (or 1.06 parts anhydrous soda) are used. A convenient "stock solution" is of 10 per cent. strength. Thus 10 lbs. of chrome alum is dissolved, made basic, and made up to 10 gallons. To dissolve the alum a mechanical stirrer is necessary, for the water must not be more than warm. The disadvantage of this liquor is the limited solubility of chrome alum and the need for its solution in the cold.

Liquors may be also made by dissolving chromium hydrate in hydrochloric acid, and making basic to correspond to the formula Cr{2}(OH){3}Cl{3}. Many preparations are on the market containing both chlorides and sulphates with appropriate basicity. Chrome alum liquors have been less often used in Britain of recent years owing to the high price of chrome alum, caused in part by the presence in the salt of pota.s.sium, all the salts of which have been scarce and dear under war conditions.

Of the second type Procter's "glucose liquor" is a good example. Use 5 lbs. sulphuric acid, 6 lbs. sodium dichromate, and 7 lbs. of glucose, or quant.i.ties in similar proportion. The dichromate is first dissolved, and the acid added gradually. The glucose is then added cautiously on account of the brisk effervescence of carbon dioxide. A glucose of good quality is necessary, and the proportion to be used is not quite definite, for sufficient only is needed to effect the reduction, and this amount is influenced by the rate of addition and temperature of the mixture. The reduction should be careful and regular, or the oxidation products will be irregular and have a varying effect upon the tanning.

Mola.s.ses can be subst.i.tuted for glucose, in amounts varying with its strength.

Of the third type the most common is that in which the dichromate is reduced by sulphuric acid and sodium bisulphite. Solid bisulphite may be used, but it is usually dear, and solutions are more commonly employed.

Into this type fall also the liquors formed by pa.s.sing sulphur dioxide gas into dichromate solution. Stock liquors of this type have the advantage that strong solutions may be made (up to 18 per cent.

Cr{2}O{3}); they have the disadvantage that they are liable to contain excess of free sulphurous acid.

The method of application of chrome liquors is usually by paddling or drumming the goods in solutions of appropriate strength--broadly speaking, paddles used for lighter goods and plain finishes, and slowly revolving drums for heavier hides and grained finishes. Heavy chrome leather is often tanned in pits by suspension just as in vegetable tanning. In such instances rockers may be usefully employed.

In any case, the goods are successively brought into contact with liquors of increasing strength, as in vegetable tannage, and the liquors are thus most conveniently exhausted economically. The green goods thus receive first nearly spent liquor and finish out of fresh strong liquor.

The goods may be, of course, handled from drum to drum, or from pit to pit, but the modern tendency is to save labour by moving the liquors instead. Thus in drum tanning the liquor is run out and pumped into the next drum. In pits air ejectors have proved suitable, not only as lift pumps, but also as agitators of the liquor in which goods are suspended.

The press system is also used.

=Finishing Operations.=--In nearly all cases the chrome leather has to be "neutralized" after tanning. This consists in removing the acid "reversibly adsorbed". This removal is necessary to the finishing processes, as well as to bring the tanning sol into condition for more permanent tannage. Neutralization gets rid of soluble chrome salts as well as free mineral acid, and is the final stage in rendering the tanning sol less positive, and perhaps even negative. It is brought about by the use of weak alkalies, of which borax is the easiest and safest, but not the cheapest. Sodium silicate, phosphate, carbonate, and bicarbonate have been also used, and a mixture of soda and an ammonium salt has been suggested by Stiasny. Whitening has also been tried, but is very slow-acting. Considerable economy in alkali may be effected by a thorough washing of the leather before using the alkali. If the water be hard, so much the better, and if warm water be available the process is hastened. For most leathers it is necessary to remove excess of alkali just as much as excess of acid, so that a thorough washing in water generally follows the treatment with alkali. Anything from 1/4 to 3 per cent. borax (or its equivalent) on the pelt weight may be used, and, generally speaking, it is better to use solutions as dilute as practicable in order to avoid local over-neutralization and tender leather.

Fat liquoring is a process which is very largely typical of chrome leather manufacture; it consists in drumming the goods with an oil emulsion, the grease of which is entirely taken up by the leather. It thus strongly resembles drum stuffing (Part I., Section IV.) in method, but the "fat liquor" is such that it mixes easily with water, and usually contains soap in order to a.s.sist in this sense, and may sometimes indeed consist of soap only. Mineral oil is also used frequently in fat liquors. The object of fat liquoring is to give softness, pliability, or waterproofness, and to feed the "empty" chrome tannage. It is also used as a preparation for more complete impregnation of grease, _e.g._ as in "stuffing" chrome harness, and in "dipping"

chrome sole leather. Fat liquors are usually made by dissolving the soap in boiling water and gradually adding the oil with constant agitation.

Perfect emulsification is essential, and this is a.s.sisted by the use of casein, alb.u.men, gelatine, starch, egg yolk in addition to soap and oil. Soda and borax also a.s.sist, and degras and sod oil are also useful and are admissible where the leather is to receive a dull finish. The operation of fat liquoring is greatly a.s.sisted by heat, and temperatures of about 110 to 130 F. are usual. Chrome leather may be dyed before or after fat liquoring: if before, the fat liquor sometimes tends to alter the shade; if after, the dyeing tends to be uneven. Logwood extract and iron salts are largely used for blacks. It is common to mordant chrome leather with vegetable tanning before dyeing. Sumach and gambier are often used for this purpose, and the usual "fixing agents" (tartar emetic, t.i.tanium salts, etc.) may also be used.

Of the mechanical finishing operations staking is the most characteristic. It is now done entirely by machines, and the primary purpose is to soften the leather, which otherwise dries out in a non-pliant and stiff condition. In the staking machine, the "blade" is fixed between two rollers, which are however on the other side of the leather. The leather is held by the operator, and the machine "head"

pulls a fold of the leather over the blade. Seasoning and glazing are also common for many chrome leathers.

REFERENCES.

Procter, "Principles of Leather Manufacture," pp. 198-220.

Bennett, "Manufacture of Leather," pp. 210, 312, 355, 375.

Bennett, "Types of Two-bath Chrome Tannage," _Leather_, 1909, Aug.

and Sept.

SECTION III.--CHROME CALF

The tannage of calfskins by the chrome processes for the manufacture of upper leathers is one of the most extensive branches of leather manufacture. The deservedly popular =box calf= is typical of these leathers, and the observations of this section are primarily applicable to it. A chrome-tanned calf skin, fat liquored and blacked, provides as suitable an upper leather as could be desired for ordinary boots. It is at once supple and durable. It is also sufficiently waterproof, but can be given a bright glazed finish.

In regard to the wet work for chrome calf, the general principles and methods discussed in the previous section are much to the point. It is essential to avoid undue plumping and the loss of hide substance. The skins should be washed clean as soon as possible. Three fresh waters are desirable, the goods remaining only a short time in each. Salted skins need more time, but the liquors must be kept sweet. Drumming the skins in running water is very suitable for the first and last stages of soaking.

The liming should be short but not "sharp," _i.e._ mellow sulphide limes are suitable, depilation being carried out after about 7 days. The one-pit system is usual, but two liquors may be given, the green goods being first inserted into a used liquor, and after handling reinserted into the same pit with a new lime liquor made up with lime, sulphide and a proportion of the old liquor. Scudding should be carefully done, as hair on the finished leather is very objectionable.

In deliming it is essential to have the grain of the skins thoroughly relaxed and pulled down. The finished box calf should have a characteristic soft and silky feel, and this is only attained by procuring the inelastic pelt. It is not surprising that a light puering is a popular method for attaining this, but there is also a tendency to use artificial bates such as are made from ammonium chloride and pancreatin, together with organic acids, or non-swelling acids like boric acid. Drenching is also common after a preliminary deliming with acid. The skins may be half or two-thirds delimed with lactic acid, rinsed and drenched over night at 85 F. with 6 per cent. bran on the pelt weight. Less acid may be also used, in tepid water, and the drench made up with 10 per cent. bran and a little pea meal. It is very common to pickle the skins in 5 per cent. alum and 5 to 10 per cent. salt before tanning. This is often of doubtful advantage, but sometimes prevents drawn grain when the goods are moved rapidly into strong chrome liquors. This pickling is said to give fullness to the leather.

The tannage of box calf is usually by the one-bath process, though the two-bath process gives quite as good a result and is sometimes used.

Again, drum tannages are the most popular on account of their speed and the economy of chrome. The practical problem is to use up all the chrome, and to tan quickly without "drawing" the goods. It is, in any case, usual to commence the tannage in a used and nearly spent liquor and finish in a fresh liquor. The most appropriate way depends largely upon local convenience, the number of drums available, supply of labour, etc. In a one-drum system the goods may be started in an old liquor, which is run off when exhausted by the green goods. Fresh stock solution is then added at intervals of an hour or two and the drumming continued till tannage is complete, which is usually in less than 24 hours. The remaining liquor is used to commence the tannage of the next pack.

In another system the operation is similar except that the liquors are weaker, and the goods are then removed and finished in another drum. A three-liquor system, however, is often combined with a one-drum method; the goods are thus not handled. The liquors are run off and pumped to other drums, the once-used liquor to a drum containing goods already treated with a twice-used liquor; the twice-used liquor to a drum containing green goods, and the thrice-used liquor pumped to the drain.