Special Qualities of Chrome Leather.—A few words on the special peculiarities of the leather formed by chroming will not be out of place at this stage. One of the greatest disadvantages of the chrome tannage has been the absence of what is known as the "crust" or "rough leather" stage. In chrome tanning, the finishing operations have had to follow on immediately after the tannage. Chrome leather, after tanning, may be dried out like other leathers, but if thoroughly dried, or if kept in a dried condition for any time, it will not "wet back" again with water. Various suggestions have been made to overcome this difficulty but none yet have found much favour in practice. The discovery of the reversibility of the tannage, however, ought to solve this difficulty, and the author would suggest that any of the substances used for "dechroming" might also be suitable for "wetting in" chrome leather which has been well dried out. A piece of chrome leather, dried out well after neutralizing, and kept in a warm place for four years, wetted back easily in ammonium acetate, in the author's laboratory.
Another peculiarity of the chrome tannage is that any defects in the raw material always seem more obvious in chrome than in vegetable leather. This often necessitates the use of a better quality hide or skin. Weak grain or loose grain becomes very obvious. The presence of short hair which both unhairing and scudding have failed to remove also is usually more evident.
A more serious disadvantage of chrome leather is its tendency to stretch. In the case of belting leather this feature is an obvious nuisance, and has inevitably led manufacturers to use powerful stretching machines upon the goods before they are marketed. In chrome sole leather also there is a tendency to spread and throw the boot out of shape.
Further disadvantages arise from the fact that the chrome tannage is an "empty" tannage. Compared with the vegetable tannage, very little of the tanning agent is adsorbed. Hence there is little matter of any kind between the hide fibres isolated during tannage. The inevitable effectof this is that the leather has not the same solidity and firmness, and needs filling out with other materials. A commercial consequence is also that it is impossible to obtain the same yield of leather from any given quantity of raw material. In trade parlance chrome tannage does not give good "weight." Another consequence is that (even when well filled with greases in finishing) chrome leather tends to be "woolly" on the flesh side or at cut edges.
On the other hand, chrome tanning has very many advantages over the older process. The most obvious of these is the great saving in time. Many chrome tannages involve only a day or two, and none more than a week or two. A chrome leather factory therefore needs less capital on account of the quicker turnover. If, moreover, the market be unfavourable, a chrome tanner can stop or reduce his output in a very short time, whereas a vegetable tanner is committed to many weeks' supply of the goods he is manufacturing. Another notable advantage of chrome leather is its durability. In the finishing processes more grease is usually employed than in vegetable tannage, and this has a preservative effect upon leathers which often get wet. Chrome sole leather and hydraulic leathers are cases in point. Chrome leather will also stand changes of temperature and friction much better than vegetable tannages. The light chrome tannage results, further, in yielding a leather which has great tensile strength, and it is not surprising to find that chrome harness and chrome picking bands are highly thought of. The empty nature of the tannage necessitates the use of stuffing greases, but such large proportions of these may be used that chrome tannage becomes obviously suitable if one wishes to produce a waterproof leather. Hence the popularity of chrome tannage for waterproof soling and hydraulic leathers.
The advantages of the chrome process are very real, and very obviously such as will appeal to manufacturers. Chrome leathers have now been for some time in the forefront as far as boot uppers are concerned, especially for the best quality goods, in which the popular "box calf"and "glacé kid" are so largely employed. There seems little doubt that this will continue to be the case. It is an unfortunate fact that in this important branch of tanning, British manufacturers have not quite risen to the occasion. Their products have in the past been faced with very serious competition from Continental and American manufacturers of chrome uppers, and there can be no doubt that these competitors produced a better article, and produced it more economically. The disorganization of the Continental factories owing to the war should give British manufacturers a valuable opportunity of putting such businesses on a better basis. For sole leather also the chrome tannage makes constant headway, and the relative proportion of it becomes gradually greater. A great impetus to chrome sole leather has been given by the war conditions of Britain. Owing to the submarine campaigns of Germany the tonnage question became all-important, and the bulky imports of vegetable tanning materials became a serious item. British tanners were therefore encouraged to make more chrome sole and less vegetable sole. The urgent need of leather for our armies also assisted in the same sense. The production of chrome sole progressed therefore enormously during 1917 and 1918, and although some reaction will doubtless occur, there seems little doubt that chrome sole leather has taken a definite and permanent leap forward. Once the general public fully appreciate its qualities of waterproofness and durability its future will be assured.
On the whole the position and prospects of chrome tanning are good. The chrome tannages are making headway in all directions, and undoubtedly threaten the existence of many of the older processes of vegetable tanning.
REFERENCES.Procter, "Principles of Leather Manufacture," pp. 198-220.Bennett, "Manufacture of Leather," pp. 210, 355.Bennett,J.S.L.T.C., 1917, 176.Stiasny,Collegium, 1908, 117.
It has been previously pointed out that the chrome tannage is an "empty" one; the primary principle in the wet work of goods for chrome leather is to avoid anything which will make this feature more obvious. In the vegetable tannages relatively larger amounts of the tanning agents are used, and these fill the interfibrillar spaces; indeed, as we have seen (Part I., Sections III., V. and VI.), effort is made to increase these spaces and to fill them to their maximum capacity, thus yielding a leather of which 50 per cent. is the tanning agent. In chrome tanning, however, the tanning agent may only be approximately 5 per cent. of the finished leather, so that any trouble taken to split the hide fibres or to dissolve hide substance is usually not only superfluous, but also calculated to enhance the "emptiness." The governing principle of all the preparatory processes for chrome tannage is therefore the conservation of hide substance, and this principle determines the modifications of the processes of soaking, liming, and deliming, which are in vogue. Now, in most of these processes there is usually some loss of hide substance, and it is the particular problem of chrome tanning to reduce this loss to a minimum in each stage. Whether the loss of hide substance be due to alkaline or fermentive hydrolysis, or to solation of the hide gel, the effect is increased by swelling, and in the wet-work for chrome, therefore, any variations in the degree of swelling are objectionable. The preparatory processes should be carried out with as little change as possible in the volume and elasticity of the pelt.Whether also the loss of hide be due to hydrolysis or solation, it is increased by time, hence short processes are (other things being equal) much to be preferred. Fermentive hydrolysis is minimized by cleanliness, alkaline hydrolysis by avoiding strongly alkaline liquors, and solation of collagen is reduced by both, and also by avoiding, as far as possible, the presence of calcium and ammonium salts.
Soaking should be quick and clean. The use of the paddle or drum gives the greatest efficiency and also assists in procuring the softness so essential to the bulk of chrome leathers.
Liming chrome leather satisfactorily is almost an impossible ideal. Every conceivable arrangement has some objection to it. The time of the process may be shortened either by the use of sulphide or by the use of mellow or old limes. To shorten time by the use of sodium sulphide unfortunately involves the employment of more alkali than is desirable, with a consequent plumping effect and tendency to harshness. If sufficient sulphide be used to make the liming very short, then the grease is not "killed" (saponified or emulsified). If the harshness and alkalinity be removed by using also an excess of calcium chloride, then the lyotrope influence of this substance enhances the solation of the hide gel. On the other hand the use of old lime liquors avoids the plumping effect, but increases considerably the bacterial activity, and the bacterial enzymes produce both hydrolysis and solation of the pelt. In practice what is generally done is to shorten time by both methods and so to admit both disadvantages to a limited extent. This is theoretically more sound than would appear, for in mellow limes sulphide has less plumping power but is just as strong a depilatant; whilst, on the other hand, a mellow liming shortened by sulphide is less objectionable as there is some evidence that bacterial activity is relatively less in the first few days. Hence a mellow sulphide liming of 7-10 days is very common in practice, but sometimes a 3-4 days' processwith more sulphide is also found satisfactory.
It would seem probable that the real solution of the problem would be found by a different process altogether. In this connection it is interesting to note that a Continental proposal to unhair by enzyme action only has been found most practicable with goods for chrome, and, in the author's opinion, some development on these lines, in which a lipolytic enzyme is used in addition to a proteolytic, might solve the difficulty, and give a rapid depilation which dispenses with liming, plumping and deliming with the consequent loss of valuable hide substance.
In the usual short, mellow, sulphide liming it is clear that there is not much advantage in a "round" or "set" of pits. Hence the one-pit system is popular on account of the less labour involved.
The above remarks are less applicable in the case of chrome sole leather. In this case weight is a great consideration and plumping is necessary. In such leather the chrome tannage is supplemented by the use of waxes, which fill up the spaces between the fibres and give solidity and waterproofness to the finished article. With this leather an ordinary sole leather liming in sharp liquors is not unsuitable, handling the goods from "mellow to fresh," but there is, on the whole, a tendency to shorten the process to about a week by using more sulphide.
Processes for deliming pelt for chrome leather should also be chosen by our guiding principle of hide substance conservation. Here again short processes involving little change in swelling should be preferred. Now, the ordinary bating and puering processes give (1) neutralization of lime by organic acids combined with weak bases; (2) the solation of some hide substance; and (3) a "pulling down" effect on the swollen pelt. Now, neutralization is quite superfluous, as the acids of the chrome liquor (one-bath or two-bath) can quite well accomplish this; the solvent effect is undesirable altogether; and the "pulling down" effect is also unnecessary if the goods are not plumped up. With any method ofliming, however, some plumping is obtained, and this creates a problem of practical importance. In the huge quantities of pelt which go for chrome upper leathers, a primary consideration is the soft, "kind," or mellow feel of the grain in the finished leather. This is obtained only by tanning the pelt when the grain at least is in a thoroughly deplumped and inelastic condition. It is essential to delime not only so that the alkaline plumping effect is completely removed, but also so that no acid plumping effect succeeds it. The practical problem is to decide whether, in any particular instance, dung puers and bates are necessary to obtain this result. Bating is clearly not very desirable, on account of the length of the process, during which hide substance would be lost unnecessarily, and also because there will usually be a slight alkaline swelling. Puering with dog-dung infusions is preferable; it is not such a long process, the liquor is just acid to phenolphthalein, and the action is more intense, and by puering for a short time only the loss of hide may be confined to the grain and flesh only, whilst the desired inelasticity of grain-pelt is soon obtained. Many large firms have admittedly found themselves unable to dispense with puering, but others have succeeded in substituting for it the use of non-swelling deliming and lyotrope agents like ammonium chloride and boric acid. In all cases it is futile to delime or puer the grain and then allow the goods to stand until the centre lime has diffused outwards. The goods must pass into the chrome liquors when in the correct condition. For heavy chrome leather a surface deliming with boric acid is all that is necessary. Even that is superfluous when the goods are to be pickled before tanning.
Types of Two-bath Chrome-Tannage.—Although the original process of the Schultz patent is quite a practicable one, many modifications have been introduced. These modifications have been made to suit the class of goods under treatment, to suit the particular mode of application which is available or suitable, and to effect economies of chrome and othermaterial, and of time, and also to combine with the tannage a pickling effect or a partial alum tannage. Other modifications arise from the precise acid, neutral, or alkaline condition of the pelt, being for example obviously necessary when pickled stock replace neutral pelts. The many two-bath processes which have been found useful have been classified previously by the author[6]into three types: (1) The "Schultz type," in which such quantities of dichromate and acid are used that there is no excess of free acid (other than chromic), but an excess of unaltered dichromate; (2) the "Acid type," in which the chromic acid is completely free and the liquor contains also some excess of mineral acid also; and (3) the "Neutral type," in which neither of these main constituents is in excess, just sufficient mineral acid having been used to liberate all the chromic acid from the dichromate.
[6]"Types of Two-bath Chrome Tannage,"Leather, 1909, 227-259.
[6]"Types of Two-bath Chrome Tannage,"Leather, 1909, 227-259.
Now:—K2Cr2O7+ 2 HCl = 2 KCl + 2 CrO3+ H2O204 73
Taking the commercial hydrochloric acid as a 30 per cent. solution, 73 parts will be obtained in about 250 parts of commercial acid. Hence 294 parts dichromate need 250 parts commercial hydrochloric acid for the above reaction;[7]in other words, 5 per cent. dichromate needs 4¼ per cent. commercial acid. Similarly 6 per cent. and 4 per cent. of dichromate need 5.1 per cent. and 3.4 per cent. respectively of commercial acid. If therefore such quantities be used we have the so-called "Neutral type" of chroming bath. If less quantities of acid be used we have the "Schultz type," and if greater quantities of acid be used we have the "Acid type." The original Schultz patent used 5 per cent. dichromate and 2½ per cent. hydrochloric acid, and well exemplifies its type, for there is much undecomposed dichromate. The composition of some chroming baths in common use on a practical scaleare given below under the heading of their type:—
[7]Commercial acids of course vary in strength, and the amount needed varies accordingly.
[7]Commercial acids of course vary in strength, and the amount needed varies accordingly.
All the figures are percentages of the weight of pelt. As K2Cr2O7has a molecular weight of 294, and Na2Cr2O7· 2H2O a molecular weight of 298, in practice they may be considered as interchangeable, weight for weight. The sodium salt is cheaper and more often used. The corresponding amount of chromic acid, 2CrO3, has an equivalent weight of 200, hence any weight of dichromate may in practice be substituted by two-thirds the weight of commercial chromic acid. Equivalent weights of commercial sulphuric acid are sometimes used in place of hydrochloric. The quantity depends upon the strength of the sulphuric acid used. Aluminium sulphate, Al2(SO4)3· 18H2O (mol. wt. 666), may be replaced by ordinary potash alum, K2SO4· Al2(SO4)3· 24H2O (mol. wt. 948). In practice 7 parts of the former and 10 parts of the latter may be considered equivalent. It should be remembered that both these salts are hydrolyzed in solution, and therefore increase slightly the amount of free acid present. Their presence decreases the amount of chrome taken up, and as little or no alumina is found in the leather,there is usually small advantage in their employment. The use of salt is common but often unnecessary. It is considered desirable in baths of the acid type to prevent swelling by the excess of acid, and in baths made up from commercial chromic acid to replace correspondingly that normally formed from the reaction of dichromate and acid. It is used also in all baths which are intended to treat pickled goods. Like all electrolytes its presence decreases the adsorption of chromic acid.
All these conceivable modifications will make good leather, and the choice of a process often depends largely upon market prices. On the whole the tendency is to prefer the neutral or acid type, on account of the greater ease and completeness with which the bath may be exhausted. Pickled stock may be depickled before tanning, by a bath of salt, mixed with borax, whitening, or basic alum solutions. It may also be placed direct in the chroming bath, but the amount of acid thus added with the goods must be determined and allowed for when making up the bath. No allowance is usually necessary, however, if the "pickle" consist only of alum and salt.
The chroming operation is carried out usually in drums or paddles. Drums are preferable because more concentrated baths may be used; these solutions penetrate quicker and are easier to exhaust economically. They are also preferable for hides and heavy skins. Paddles are preferable where grain is important, and for light skins in which little time is needed. Small variations in the ratio of chrome to pelt, or in concentration of liquor, have little influence upon the resulting leather.
The analytical investigation and control of chroming baths is usually simple. A suitable volume of liquor is titrated with N/10 thiosulphate after acidifying with hydrochloric acid and adding potassium iodide. The operation should be conducted in a stoppered bottle, and the liquor allowed to stand for 10-15 minutes after adding the iodide and before titrating. A little fresh starch infusion should be added towards the end of the reaction. Each c.c. N/10 thiosulphate corresponds to 0.0033 gram CrO3or 0.0049 gram K2Cr2O7. The same volume of liquor should also be titrated with N/10 caustic soda and phenolphthalein. Potassium chromate is neutral to this indicator,i.e.chromic acid acts as a dibasic acid. Any excess of hydrochloric acid is also titrated. More indicator should be added towards the end of the titration, as it is often oxidized. Each c.c. N/10 soda corresponds to0.005 gram CrO3, 0.01 gram "half-bound" CrO3(i.e.present as dichromate), 0.0147 gram K2Cr2O7, or 0.00365 gram HCl. Ifac.c. N/10 thiosulphate andbc.c. N/10 soda be needed the type of chroming bath may be seen at a glance—
If 10 c.c. chrome liquor requireaandbc.c. of thiosulphate and soda respectively—
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) theformation 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 quantities of material used in the chroming bath, and the quantities 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 classes 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 quantity varies with the rate and mode of addition, the class 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:—
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 substitutes 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 radicle. 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 radicle 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 analytical 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 formulæ by assuming that the acid neutralized by this alkali is replaced in the chrome salt by hydroxy groups. Chromic chloride, Cr2Cl6, 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, Cr2(OH)3Cl3. 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 SO4still combined with 52 grams Cr. Thus the salt corresponding to the composition Cr(OH)SO4is 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 samepracticalvalue 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 radicles 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 titrated direct with caustic soda. The titration is at boiling-point, and is continued until a permanent pink is obtained with phenolphthalein. The amount ofSO4corresponding 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 muffle furnace with magnesia and sodium carbonate in equal parts, or fusing in a blowpipe with sodium and potassium carbonates in equal parts. The oxidized residue is dissolved in hydrochloric acid and titrated 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 titrated 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 SO4per 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 classifying 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)SO4is obtained.
Now:—
= 2 Cr(OH)SO4+ K2SO4+ CO2+ 33 H2O
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 Cr2(OH)3Cl3. 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 potassium, 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 quantities 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 oxidationproducts will be irregular and have a varying effect upon the tanning. Molasses can be substituted 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 passing 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. Cr2O3); 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 morepermanent 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 ¼ 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., p.53) in method, but the "fat liquor" is such that it mixes easily with water, and usually contains soap in order to assist 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 assisted by the use of casein, albumen, gelatine, starch, egg yolk in addition to soap andoil. Soda and borax also assist, 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 assisted 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, titanium 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.
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 popularbox calfis 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 re-inserted 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 pueringis 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 toother 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.
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 assists 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 beforedyeing 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 ¾ 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 1½ 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 butt and belly to belly. They are usually re-seasoned, 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 splitand 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 calftypifies 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, 3½ 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 passed 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 4½ 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, 7½ per cent. salt, and 3 per cent. sulphuricacid, 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 4½ 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 substitutes 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 butt—and the season should consist of milk, water and albumin 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 titanium 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.Glacé calfis 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 calfis also a plain finish. The leather contains more grease, and the fat liquor ismade 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.
Immense quantities of goat and sheep skins are chrome tanned for upper leathers. Most of them are manufactured into the well-known and popularGlacé kid, to the manufacture of which this section is chiefly devoted. To be quite strict, glacé 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 glacé is probably enhanced by the brighter and more glassy finish than is usual with box.
As the supply of goatskins is unfortunately too limited, an even more widely used glacé upper leather is made from sheepskins, and often sold as glacé 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 glacé sheep is by no means so good a leather as glacé 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 glacé 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 glacé goat will first be considered. The soakingprocess 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 glacé 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 substituted 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 glacé the two-bath process is mostly preferred. This is partly because the sulphur deposited in the reducing bath assists 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 illustrative 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 pass into the chroming paddle, which contains at first only 2 per cent. dichromate. Subsequently 4 per cent. dichromate, 3½ 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, passed 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 5½ per cent. chromic acid and 6½ 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 passed 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 passed 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 finishingdull kida 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 forcoloured glacé, the skins are mordanted before dyeing by the use of dyewood extracts, antimony and titanium 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 glacé sheep follows the same general lines as glacé 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 5per 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 ½ 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 glacé sheep much the same methods are used as in the case of glacé 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 aschrome 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