[L]Hunt (Jour. Soc. Chem. Industry, April 1885) states, that saturation with salt causes partial precipitation of gallic acid when present, and that results agreeing more closely with those obtained by absorption with hide are obtained by employing a mixture of 50c.c.liquor, 25c.c.2 per cent. gelatin solution, and 25c.c.saturated solution of salt containing 50c.c.of concentrated sulphuric acid perlitreand a teaspoonful of kaolin. This approaches very nearly to Löwenthal's original method, but with the addition of the kaolin, and as in it, it is to be feared that a portion of the tannate of gelatin will remain in solution. For accurate work, therefore, absorption by hide-raspings is preferable, though even that has been shown by the writer to remove gallic acid and other matters beside tannin. Hunt states that raw hide also absorbs catechin.
[L]Hunt (Jour. Soc. Chem. Industry, April 1885) states, that saturation with salt causes partial precipitation of gallic acid when present, and that results agreeing more closely with those obtained by absorption with hide are obtained by employing a mixture of 50c.c.liquor, 25c.c.2 per cent. gelatin solution, and 25c.c.saturated solution of salt containing 50c.c.of concentrated sulphuric acid perlitreand a teaspoonful of kaolin. This approaches very nearly to Löwenthal's original method, but with the addition of the kaolin, and as in it, it is to be feared that a portion of the tannate of gelatin will remain in solution. For accurate work, therefore, absorption by hide-raspings is preferable, though even that has been shown by the writer to remove gallic acid and other matters beside tannin. Hunt states that raw hide also absorbs catechin.
A slight error is introduced by the presence of a trace of oxidisable matter in the gelatin, and when very great accuracy is required, it is well to make a blank estimation of "not-tannin" without tannin infusion, and deduct1/2of the permanganate consumed as a correction from the not-tannin; but this may usually be disregarded. Each titration should be made twice, and successive tests should not differ by more than 0·1c.c.of permanganate.
Reagents.—Solutions are required of (1) Pure potash permanganate, 1grm.perlitre. (2) Pure soda or potash sulphindigotate, 5grm., and concentrated sulphuric acid, 50grm.perlitre. (3) Pure oxalic acid, 6·3grm.perlitre(decinormal). The sulphindigotate (indigo carmine), must be filtered, and when oxidised by permanganate, should give a pure clear yellow, free from any trace of brown or orange. Any contamination with indigo-purple, which gives brown oxidation-products, is quite fatal to the accuracy of the analysis. The permanganate solution is standardised by measuring 10c.c.of the (decinormal) oxalic acid solution, adding a little pure sulphuric acid and distilled water, warming to 136° F. (58° C.), and running in the permanganate till a faint permanent pink is produced, for which about 32-33c.c.should be required. The indigo-carmine solution should be of such strength that 14-16c.c.of permanganate are required to bleach the quantity employed, which may be 20-25c.c., as convenient. (4) Gelatin solution: 2grm.of Nelson's or other good gelatin are allowed to swell in distilled water for two hours, melted by setting the glass in a pan of boiling water, and made up to 100c.c.This will not keep. (5) Dilute sulphuric acid: 10c.c.of pure concentrated acidare added to 90c.c.of distilled water. (6) Good table salt. (7) Purified kaolin.
The analysis is performed in the following manner:—20c.c.of indigo solution, and 5c.c.of the infusion of tanning material is added, in a white basin as recommended by Kathreiner, to about3/4litregood water, which it is best to measure approximately, so that if it contains any impurity which affects the permanganate it should be constant, and thus be eliminated with the indigo. Permanganate solution is then allowed to drop in, with constant stirring till the pure yellow liquid shows a faint pinkish rim, most clearly seen on the shaded side. This end-reaction, which is of extraordinary delicacy, is due to Kathreiner, and is quite different to the pink caused by excess of permanganate, being an effect common to all pure yellow liquids. It is not needful to make the titration so slowly as has been advised—the permanganate may be dropped in steadily with vigorous stirring, so long as there is large excess of indigo, but as soon as the bottom of the basin can be seen through the solution, it must be added very cautiously, one or two drops at a time, and with occasional pauses, to allow time for its complete mixture through so large a mass of fluid. The titration is repeated twice, and the results added together and denoted bya. Then take 50c.c.of the infusion, and add 28·6c.c.of the gelatin solution of Nelson's gelatin of 2grm.to 100c.c.After shaking, the mixture is saturated with salt, which brings the volume up to 90c.c., and 10c.c.of the dilute sulphuric acid (containing 1 vol. of concentrated acid in 10) and a teaspoonful of pure kaolin are added. It is best to do this in a flask in which it can be well shaken, after which, filtration may be at once proceeded with, although it is safer to let it stand an hour or two: (the flask may be cleansed with caustic soda solution). 10c.c.of this filtrate (= 5c.c.of the original infusion) are employed for a second pair of titrations, which are added as before, and the result denotedb. If, further,cbe the quantity of permanganate required to oxidise 10c.c.of decinormal oxalicacid, and 10grm.of the tanning material have been employed to make 1litreof infusion,c: (a-b) :: 6·3 :x, wherexis the percentage of tannin expressed in terms of crystallised oxalic acid. If it be desired to calculate the gallic acid and non-tannin substances contained in the infusion, the value in permanganate of the indigo alone must be determined. Calling thisd, ascis to (b-d), so is 6·3 to the percentage of non-tannins in terms of oxalic acid, and for the present it is best invariably to calculate results in this way, since we do not actually know the relation of any single tannin to permanganate, even Neubauer's number for gallotannic acid being probably too high, according to the recent investigations of Councler and Schroeder,[M]and Oser's for quercitannic being at most only approximate. It happens, moreover, that this last equivalent (62·36grm.of quercitannic acid = 63grm.of crystallised oxalic acid) does not differ from that of oxalic acid more than the ordinary limits of error of such estimation, and the substitution is therefore of no commercial importance, while it is surely better to employ a standard which is easily and exactly verified than one which is certain to be modified by further research, and so to run the risk either of having our results made useless for future comparison, or of establishing a false or arbitrary equivalent. What is wanted for practical purposes is not the absolute weight of tannins in the various materials, but only a means for the relative comparison of two samples of the same material; cross comparisons of different tannins being simply delusive. If, however, it is necessary at any time to give actual percentages of gallotannic acid, it is probably best to stick to Neubauer's number for the present, as it is in general use. Neubauer states that 63grm.of oxalic acid consume as much permanganate as 41·37grm.of gallotannic acid. Tshekawa found 41·688 as the equivalent for tannin from Japanese gall nuts (Chem. News, xlii. 274). Councler andSchroeder on the other hand give only 34·3grm.Simand gives 61·1grm.as the equivalent of quercitannic acid. Commercial "pure tannin" always gives results higher than the truth, as the gallic acid which it contains consumes more permanganate than an equal weight of tannin, or even than the tannin which would yield it if boiled with acid. When this is done the equivalent used should be definitely stated, or it will certainly lead to confusion. Neubauer's equivalent is only properly applicable to gall nuts, and possibly to sumach and myrabolans. For oak bark Oser's number or that of oxalic acid is most likely nearly correct; and this may also be approximately true of oak wood and valonia, but as respects all other materials we have no information whatever, and the oxalic equivalent is as likely to be right as any other. (Compare note,p. 128.)
[M]From researches by von Schroeder, published since the above was penned, it seems that the permanganate consumed by tannin is largely influenced by the way in which the titration is conducted, seep. 128.
[M]From researches by von Schroeder, published since the above was penned, it seems that the permanganate consumed by tannin is largely influenced by the way in which the titration is conducted, seep. 128.
A few results are given below, not as showing the relative values of the materials, which, of course, cannot be directly compared by any analytical process, but for comparison with those obtained by other methods and modes of calculation:—
It is proved by experiment that kaolin removes nothing which is oxidised by permanganate, but simply facilitates the precipitation and filtration; and it is often found useful to clarify the original infusions and liquors before the first titration. On the other hand, there is no doubt that the salt and acid of Löwenthal's method precipitate of themselvesa large proportion of certain tannins. In the case of cutch this amounted, in the analysis given, to 67 per cent. of the whole. There is, however, good reason to believe that this would also have been absorbed, or at least removed from solution by hide in the process of tanning. This is shown by the analysis of the spent liquor above given, which originally contained the tannins of oak bark, valonia, myrabolans, gambier, hemlock, and oak wood extracts, &c., to the extent of 10 to 15 per cent., but which was reduced by contact with hide to 0·12 per cent. That a portion had not been absorbed but decomposed is proved by the large accumulation of oxidisable impurities (equal to 11 per cent. of oxalic acid); at the same time this example shows that the method is capable of estimating a very small portion of tannin in presence of much gallic acid and other analogous substances. It is worth remark that such spent liquors become very pale in colour, and also that the filtrates, freed from tannin by precipitation, are nearly colourless, thus proving that the colouring matters present in tanning materials are of the nature of tannins, at least as regards their precipitability by hide and gelatin.
Simand (Dingl. Polyt. Jour., ccxlvi. 133) has recommended instead of precipitation with gelatin, the use of the gelatinous tissue of bones to remove the tannin. For this purpose porous bones, such as horn piths, are coarsely powdered, and after treatment with dilute soda solution to remove the fat, are steeped in weak hydrochloric acid till all the calcareous matter is dissolved. They are then thoroughly washed, ground wet through a steel mill, washed again and dried at a low temperature; the tannin is removed more quickly than by raw hide, and the amount of gelatinous matter dissolved by cold water is a very trifling one. This method, or that with purified hide-powder, is to be recommended for scientific research, since no element capable of precipitating substances other than those absorbed by the hide is introduced, while it is not certain in all cases that saturation with salt and acidification may not remove other constituents of the liquorbesides tannins. It has, however, for technical purposes the great disadvantage of requiring a much longer time for absorption of the tannin than is the case with gelatin solution, and of the process being much more difficult of execution. If hide-powder be employed, it must be moistened with a small quantity of water before adding to the infusion, and this water must be taken into account in the quantity of the filtrate employed for the titration of the "non-tannin." The digestion with the hide- or bone-powder must be continued till the filtered liquid does not give the faintest clouding with a drop of clear gelatin solution, and it is always very difficult to be sure that the tannin is so completely removed as with gelatin and salt. Hide- or bone-powder may be employed to determine the actual weight of any unknown tannin absorbable by hide, by evaporating equal quantities of the original infusion and of that freed from tannin by digestion with the powder; the difference giving the tannin absorbed. The evaporation must be conducted as far as possible in absence of air, for instance invacuo, or in a current of carbonic dioxide, and the residues both dried at 212° F. (100° C.) so long as they lose weight. The amount of matter dissolved from an equal quantity of the hide- or bone-powder by water must also be ascertained and taken into the calculation.
Ammoniacal solution of cupric acetate or sulphate has been employed by several chemists to remove tannin from solutions. N. H. Darton of New York, who has a large practice in tannin analysis, employs cuprammonic sulphate in the following manner.
The infusion, for which 20grm.of hemlock bark or a corresponding quantity of other material must be used, is made by exhausting with 2 or 3 quantities of water successively, first cold, and then with heat (by placing the flask in a pan of boiling water), each portion of water being poured off into alitreflask. The last should be almost colourless. The liquor is thus made up to nearly 1litre, 25c.c.of dilute sulphuric acid (about 1 vol. concentrated in10) is added, and the liquor is filtered through a small filter, which is finally rinsed with a small quantity of water. Liquid ammonia is now added till the liquor slightly smells of it, and, if any precipitate is formed, it is filtered off as before; 25c.c.of dilute sulphuric acid is again added (which should give the liquid an acid reaction), and it is made up to 1litre. The titration is done as described under Löwenthal's method, but instead of precipitating with gelatin, 100c.c.is mixed with 100c.c.of a solution of copper sulphate to which sufficient ammonia has been added to redissolve the precipitate first formed, and containing 11/4per cent. of copper sulphate. This is well shaken and filtered, and the "not-tannin" is determined in the filtrate just as with gelatin; a little dilute sulphuric acid being added in the basin to neutralise the ammonia. The writer has examined this method with regard to a few tanning materials. With valonia (and therefore probably with oak bark) the preliminary treatment is unnecessary, and copper precipitation gives results practically identical with the improved gelatin, while it is less troublesome. On the other hand, a sample of Miller's Hungarian Larch Extract which gave tannin equal to 18·08 per cent. (by the gelatin method) gave no precipitate with cuprammonic sulphate, and hence a result in tannin ofnilby Darton's method. It is worth remark that by the copper method it is therefore possible to estimate the valonia tannin alone in a mixture of larch and valonia tannin. Probably this mode of analysis may also be utilised to separate other tannins. With chestnut extract the results seem satisfactory, as regards the precipitation of the tannin by copper, the figures agreeing very closely with those by gelatin, but the preliminary treatment with sulphuric acid and ammonia precipitates about 75 per cent. of what is usually reckoned as tannin, leaving 7·53 per cent. of tannin only instead of 25·53 per cent. as reckoned by the gelatin method; which, judging by practical results in tanning, can hardly be accepted as correct. The results of the gelatin method are found to agree fairly with those of direct absorption by hide-powder, which isstrong confirmation that what is estimated as tannin is what is absorbed by the hide. It is well known that sulphuric acid precipitates many tannins, and in an experiment with cutch it was found by the writer that saturation with salt and the addition of dilute sulphuric acid as for Löwenthal's process, but without the gelatin, precipitated 67 per cent. of the total tannin as usually reckoned.
It is obvious that it is impossible by analysis to compare the relative value of different tannins, such as those of myrobalans and gambier, or hemlock and valonia. All that analysis can reasonably be expected to do is to give the relative values of different samples of the same substance, or at the most, of materials of the same class. All other comparisons are misleading; and would be so, even if the exact percentage of each tannin could be calculated; since the commercial and practical value of different materials does not depend on the quantity of tannin only, but on the character of the leather it produces, hard or soft, dark- or light-coloured and heavy- or light-weighing.
A Commission of German technical chemists, under the presidency of Dr. Councler of Eberswalde, and including Messrs. Eberz, Kathreiner, Schaun, von Schroeder, and Simand, have recently reported on methods of tannin estimation ('Bericht über die Verhandlungen der Commission zur Feststellung einer einheitlichen Methode der Gerbstoffbestimmung,' Cassel, 1885). After reviewing earlier methods, they recommend the following modifications of the Löwenthal method, for general adoption.
Chemicals employed.
(1)Permanganate solution.10grm.of the purest potash permanganate are dissolved in 6litresof distilled water.
(2)Indigo solution.30grm.dry sulphindigotate of soda (Carminum cærul. opt., "pure Indigotin I" of Gehe & Co., Dresden), air-dry, are dissolved in 3litresof dilute sulphuricacid (1 vol. H2SO4to 3 vols, water), 3litresof distilled water are added, the whole is shaken till dissolved, and filtered. In each titration, 20c.c.are used in3/4litreof water, and reduce about 10·7c.c.of permanganate.
(3)Hide-powdermust be white and in a fine woolly state of division, and should yield to cold water no substance capable of reducing permanganate. Such a powder is prepared by Dr. Both of Berlin,[N]and by the Vienna Research Station.
[N]Messrs. Mawson and Swan, of Newcastle, have kindly undertaken to keep these, and the other reagents mentioned in this book, in stock for the convenience of English tanners and chemists.
[N]Messrs. Mawson and Swan, of Newcastle, have kindly undertaken to keep these, and the other reagents mentioned in this book, in stock for the convenience of English tanners and chemists.
Mode of Titration.
Instead of adding the permanganate solution drop by drop, to the mixture of indigo, water, and liquor (as described,p. 121), it is recommended to add it 1c.c.at a time,[O]vigorously stirring 5-10 seconds after each addition. When the liquid has become bright green, 2-3 drops at a time are cautiously added with stirring, till the liquid is pure yellow. Either a beaker on a white tile or a white basin may be used (comparep. 121). It is advantageous in strong sunlight to shade the window with white tissue-paper.
[O]It has been noted by several chemists, and especially by Kathreiner, and later by Prof. von Schroeder, that the quantity of permanganate reduced by a given amount of tannin varies within rather wide limits, according to the rate at which the permanganate is added; and the "1c.c.method" was suggested by Prof. von Schroeder, to secure uniformity in this particular. It has, however, been found by the writer, in the course of experiments not yet completed, that the quantity of permanganate required, was a function not simply of time, but of the rapidity of diffusion through so large a bulk of liquid; and by the alternate use of a simple glass rod, and of a specially constructed perforated stirrer, he was able, while adhering strictly to Prof. von Schroeder's directions, to obtain results even more divergent by the "1c.c.method" than could be obtained by the drop method previously recommended, when properly carried out. Employed in conjunction with the use of tannin for standardising, as recommended by the Commission, either method gives perfectly dependable results.The explanation of the variation is a simple one. The oxidation in the Löwenthal process should be limited to indigo, and bodies more oxidisable than indigo, but there exist both ready formed in liquor, and among these oxidation products many substances which in the absence of indigo will readily reduce permanganate. When the latter is added rapidly, and with insufficient stirring, it destroys the indigo and tannin in contact with it, and proceeds also to oxidise the other matters present, although in other parts of the beaker indigo and tannin still exist. Thus more permanganate is reduced than corresponds to the indigo and tannin, and this is especially so towards the end of the process, when very little of either remains. The more slowly the permanganate is added, and the more vigorously it is stirred, the more closely it will approximate to the theoretical quantity required merely to oxidise the indigo and tannin. It seems to the writer more scientific to approach this as nearly as possible, than to attempt to establish a purely arbitrary standard such as the "1c.c.method;" but he would rather refrain from committing himself to a definite opinion till his experiments are complete.
[O]It has been noted by several chemists, and especially by Kathreiner, and later by Prof. von Schroeder, that the quantity of permanganate reduced by a given amount of tannin varies within rather wide limits, according to the rate at which the permanganate is added; and the "1c.c.method" was suggested by Prof. von Schroeder, to secure uniformity in this particular. It has, however, been found by the writer, in the course of experiments not yet completed, that the quantity of permanganate required, was a function not simply of time, but of the rapidity of diffusion through so large a bulk of liquid; and by the alternate use of a simple glass rod, and of a specially constructed perforated stirrer, he was able, while adhering strictly to Prof. von Schroeder's directions, to obtain results even more divergent by the "1c.c.method" than could be obtained by the drop method previously recommended, when properly carried out. Employed in conjunction with the use of tannin for standardising, as recommended by the Commission, either method gives perfectly dependable results.
The explanation of the variation is a simple one. The oxidation in the Löwenthal process should be limited to indigo, and bodies more oxidisable than indigo, but there exist both ready formed in liquor, and among these oxidation products many substances which in the absence of indigo will readily reduce permanganate. When the latter is added rapidly, and with insufficient stirring, it destroys the indigo and tannin in contact with it, and proceeds also to oxidise the other matters present, although in other parts of the beaker indigo and tannin still exist. Thus more permanganate is reduced than corresponds to the indigo and tannin, and this is especially so towards the end of the process, when very little of either remains. The more slowly the permanganate is added, and the more vigorously it is stirred, the more closely it will approximate to the theoretical quantity required merely to oxidise the indigo and tannin. It seems to the writer more scientific to approach this as nearly as possible, than to attempt to establish a purely arbitrary standard such as the "1c.c.method;" but he would rather refrain from committing himself to a definite opinion till his experiments are complete.
Pl. V.E. & F. N. Spon, London & New York."INK-PHOTO." SPRAGUE & CO. LONDON.SOFTENING THE SKINS.
Pl. V.
E. & F. N. Spon, London & New York.
"INK-PHOTO." SPRAGUE & CO. LONDON.
SOFTENING THE SKINS.
Standardisation of the permanganate.
To avoid the uncertainty involved in comparing tannin (which reduces different quantities of permanganate according to the method of titration) with so dissimilar a reducing agent as oxalic acid, it is recommended to employ tannin, titrated under precisely the same conditions as the tanning material, so that whatever method be employed, the differences will be common to both, and will so be eliminated. Prof. von Schroeder has shown (Report,p. 74) by careful experiment, that with the purest samples of tannin the permanganate value estimated on the total dry substance of the tannin varied by very little from that of the part of the tannin absorbed by hide as determined by Hammer's process, but on the average bore the proportion of 1 : 1·05. The percentage of water in an air-dried tannin must be estimated by drying a portion at 201°-212° F. (94°-100° C.) and determining the loss, and a quantity equivalent to 2grm.must be dissolved in 1litreof water and 10c.c.titrated with indigo in the usual way. If the permanganate value thus obtained be multiplied by 1·05, it will be equivalent to that of 2grm.of chemically pure tannin. It is only necessary to determine the moisture occasionally, if the tannin be kept in a well-closed box or bottle.
To ascertain if a tannin is pure enough for this, use a solution made as above described (it is not necessary to determine the moisture) and 10c.c.are titrated with permanganatein the usual way. 50c.c.are then digested in the cold with 3grm.hide-powder (previously moistened with distilled water and well squeezed in linen) for 18-20 hours, with frequent shaking, filtered, and 10c.c.again titrated. If the second titration ("not-tannin") does not exceed 5 per cent. of the total, it isgood, but it may beusedso long as the "not-tannin" does not exceed 10 per cent.[P]The purest tannin examined by Prof. von Schroeder was Schering's Phar. Ger., which may be obtained of Messrs. Mawson and Swan.
[P]Gallic acid suggests itself to the writer as being a good standard, since it behaves with permanganate like tannin, and being crystalline is easily purified and of definite composition.
[P]Gallic acid suggests itself to the writer as being a good standard, since it behaves with permanganate like tannin, and being crystalline is easily purified and of definite composition.
Fig. 20.
Fig. 20.
Fig. 21.
Fig. 21.
The course of analysis is as follows:—
Preparation of the infusions.—Extracts are dissolved in hot water, and if necessary, filtered. Barks and other solid materials are treated in Prof. von Schroeder's extraction-apparatus (Fig. 20) (which seems very well adapted for its purpose). This consists of a perfectly cylindrical vessel of cast-tin, about 12·5c.m.deep and 7c.m.diameter. A strainer covered with fine muslin fits it like a piston.[Q]The powdered material is placed in the cylinder, and stirred up with 200c.c.of cold water. At the end of an hour, the piston is inserted and pressed down gently, the clear liquor is poured off, and the process is 4 times repeated with hot water, at intervals of1/2hour, placing the cylinder in a water-bath. The liquid is made up to 1litre, and, if necessary, filtered (Report,p. 66). The quantity of material used should be suchas to give an infusion of which 10c.c.do not reduce more than 8c.c.permanganate. If it is desired to determine separately the "easily soluble tannin" (viz. that extracted by cold water), Real's Press (Fig. 21) is employed, which consists of a cylindera, through which water may be forced by the pressure of a column of liquid. The small sieved, covered with a disc of linen, is placed ina, next the tanning material previously thoroughly moistened with water, and the tap is closed. The press is then filled with water and left 15 hours under a pressure of 11/2metres. The tap is then opened and 1litreis allowed to run through in the course of about 2 hours, and mixed by shaking. The material is finally exhausted like a new material in von Schroeder's apparatus to extract the difficultly soluble tannin.
[Q]Both this apparatus, and the Real's press, may be obtained from C. Focke, Zinngiesser, Grosse Kirchgasse 3, Dresden.
[Q]Both this apparatus, and the Real's press, may be obtained from C. Focke, Zinngiesser, Grosse Kirchgasse 3, Dresden.
The titration is carried out as before described; in each infusion separately to determine the "not-tannin" 50c.c.are treated with 3grm.hide-powder, and 10c.c.are titrated.
It may be well in conclusion for the writer to state for the information of the non-chemical reader, that though for purposes of comparison of the results of different chemists, it is most desirable to have a standard method of the highest possible perfection; any of the accepted modifications of the Löwenthal method will give excellent practical results in careful hands.
SOLE-LEATHER:—Preparing the Hides.
Theprincipal sources of hides for sole-leather are:—
(I.) Market hides, from the cattle slaughtered for food in the United Kingdom. These are received by the tanner, fresh, or slightly salted, and are either bought directly from the butcher, or, now more commonly, through the auction markets established in all large towns. The latter system, while it perhaps slightly enhances the price of the hides to the tanner, ensures him a better classification according to weight, and, in some cases, as notably in that of Glasgow, a better flaying, through an organised system of inspection and sorting. The Scotch hides, being mostly from Highland cattle, are many of them small and very plump, for, as a rule, the hides are thickest on those animals which are exposed to cold and the hardships of out-door life. On the other hand, the hides of highly-bred cattle are apt to be thin and spreading; and, if they have been kept much indoors, and negligently managed, the grain of the hide is injured by the dung which adheres to it. The Irish hides are usually somewhat roughly flayed.
(II.) South American hides are from the River Plate, Uruguay, and Rio Grande. Those from the River Plate are considered the best, as being stoutest and finest in texture. They are usually cured by salting, and are known as "saladeros," "estancias," and "mataderos," according to the slaughter and cure. The saladeros are the best, and are from cattle killed at large slaughtering establishments on the coast. The estancias are from cattle killed in the interior, and are worse in flaying than the saladeros, but free from the objectionable dark cure of the mataderos, which arekilled by the city butchers. Many hides are brought from Brazil, and are generally both salted and sun-dried, or simply stretched out and dried. Hides are also imported from Valparaiso, both dry and wet-salted.
Chinese and West Indian hides are mostly dried. Chinese hides are occasionally infected withBacillus anthracis, which produces the dangerous "malignant-pustule," or "wool-sorters' disease." Hence any pimple appearing after working with such hides should have immediate medical attention. French market hides have been of recent years largely imported; they are mostly well flayed, and some of them very heavy, but are sold at original butchers' weight, and, in the experience of some tanners, the result in leather is 5-6 per cent. less than from English market hides. They usually lose about 25 per cent. in skulling and salting. Lisbon hides are often well flayed, but are frequently branded, and the grain is injured by insects. They yield considerably more leather than market hides in proportion to weight. Hambro' hides are salted, but mostly wet and ill-flayed. Very heavy hides are produced in the Rhine district and in Switzerland.
For further information about hides, see the Commercial Section.
Preparation for Tanning.—Market hides should be well washed in fresh water, to remove blood and dirt, before unhairing; but prolonged soaking dissolves a portion of hide-substance, and probably reduces weight, though it facilitates the action of the lime. It is very advantageous if grease and flesh, and also dung can be removed before liming, and if hand-labour is too costly machinery might be employed. Salted hides should be soaked somewhat longer, and in clean water, so as to remove the salt before liming. This water should be frequently changed, since 10 per cent. brine dissolves coriin freely (seep. 19). Dried hides require more lengthened treatment. Before they are prepared for tanning, they must be brought back as far as possible to the condition of fresh hides, and, for this purpose, must bethoroughly soaked and softened in water. There are many ways of doing this: sometimes hides are suspended in running water; sometimes laid in soaks, which may be either renewed, or allowed to putrefy; sometimes in water to which salt, borax, or carbolic acid has been added, to prevent putrefaction.
The first of these methods, were it desirable, is rarely possible in these days of River Pollution Acts; of the others, it is difficult to say which is better, since the treatment desirable varies with the hardness of the hide and the temperature at which it has been dried. The great object is to thoroughly soften the hide, without allowing putrefaction to injure it. As dried hides are often damaged already from this cause, either before drying, or from becoming moist and heated on ship-board, it is frequently no easy matter to accomplish this. The fresh hide, as has been seen, contains considerable portions of albumen, and if the hide is dried at a high temperature, this becomes wholly or partially coagulated and insoluble. The gelatinous fibre and the coriin (if indeed the latter exists ready formed in the fresh hide) do not coagulate by heat, but also become less readily soluble. Gelatin dried at 266° F. (130° C.) can only be redissolved by acids, or water at 248° F. (120° C.). Eitner experimented with pieces of green calf-skin of equal thickness, which were dried at different temperatures, with results given in the following table:—
Hence it is evident that, for hides dried at low temperatures, short soaking in fresh and cold water is sufficient, and,except in warm weather, there would be little danger of putrefaction. With harder drying, longer time is required, and it may be necessary to use brine instead of water. A well-known tanner recommends a solution of 30°-35° barkometer (sp. gr. 1·035, or about 5 per cent. of NaCl). This will have a double action, not only preserving from putrefaction, but dissolving a portion of the hide-substance in the form of coriin. Although this is undoubtedly a loss to the tanner, it is questionable if there is any process which will soften overdried hides without loss of weight: since even prolonged soaking in cold water at too low a temperature to allow of putrefaction will dissolve a serious amount of hide-substance. Water containing a small quantity of carbolic acid has been recommended for the purpose, and will prevent putrefaction, while it has no solvent power on the hide, but, on the contrary, will coagulate and render insoluble albuminous matters. Concentrated carbolic acid, however, tans the grain and renders it incapable of colouring in the liquors. Borax has been proposed for the same purpose, and, in strong solution, certainly prevents putrefaction, but is probably too costly. Sodium sulphide and other sulphides seem to have considerable effect in softening dried hides, from their property of attacking hard albuminous matters, without injuring the true hide-fibre.
For some descriptions of hides, however, and notably for India kips, putrid soaks seem actually to be an advantage, the putrefactive action softening and rendering soluble the hardened tissue. In India the native tanners soften their hides in very few hours by plunging them in putrid pools, into which every description of tannery refuse is allowed to run. Putrefactive processes are always dangerous, as the action, through changes of temperature, or variation in the previous state of the liquor, is apt to be irregular, and either to attack one portion of the hide before another, or to proceed faster than was expected. Hence hides in the soaks require constant and careful watching, and the goods must be withdrawn as soon as they are thoroughly softened, for theputrefaction is constantly destroying as well as softening the hides. It is possible that putrefactive softening is less injurious to kips, and such goods as are intended for upper-leather, than to those for sole purposes, as it is generally considered necessary in the former case that the albumen and interfibrillary matter be removed, and that the fibre be well divided into its constituent fibrils for the sake of softness and pliability; so that the putrid soak, if acting rightly, only accomplishes a part of the work which would afterwards have to be done by the lime and the bate. The actual fibre of the hide seems less readily putrescible than the albuminoid parts; hence the putrefaction may soften the latter better, and even at less expense of valuable hide-substance, because more rapidly, than fresh water. On this point, there is room for investigation. Putrefaction is a general name for a class of decompositions which are caused by a great variety of living organisms, each of which has its own special products and modes of action. It is quite possible that, if we knew what precise form of putrefaction was most advantageous, we might by appropriate conditions be able to encourage it to the exclusion of others, and obtain better results than at present. It will be necessary to revert to this subject when speaking of the bates used in preparing dressing-leather, which also owe their activity to putrid fermentation.
Beside merely soaking the hides, it is necessary to work them mechanically, to promote their softening, which was formerly accomplished by "breaking over" the hides on the beam with a blunt knife. This process is now usually superseded or supplemented by the use of the "stocks"; these consist of a wooden or metallic box, of peculiar shape, wherein work 2 very heavy hammers, raised alternately by pins in a wheel, and let fall upon the hides, which they force up against the side of the box with a sort of kneading action. The ordinary form of this machine is shown inFig. 22. A more modern form, which seems to possess some advantages, is the American double-shover, seen inFig. 23.
Fig. 22.
Fig. 22.
Fig. 23.
Fig. 23.
The number of hides which can be stocked at once naturally varies with the size of both hides and stocks, but should be such that the hides work regularly and steadily over and over. The whole number should not be put in atonce, but should be added one after another, as they get into regular work. The duration of stocking is 10-30 min., according to the condition and character of the hides. Hides should not be stocked till they are so far softened that they can be doubled sharply, without breaking or straining the fibre. After stocking, they must be soaked again for a short time, and then be brought into an old lime. A small quantity of sodium sulphide added to the soaks or in the stocks has been recommended as of great value in softening obstinate hides, and probably with justice, from its well-known softening action upon cellular and horny tissues.
In Continental yards, another machine is in use for softening hides, and which seems to present some advantages over stocks, as being less severe on the thinner portions of the hide. It consists of a pair of rollers, arranged like those of a wringing machine, and pressed together by springs, but not allowed to come into actual contact. One of them is studded with rounded pegs, which correspond in position to grooves round the other, and the hide when passed between them is thus subjected to a very thorough kneading and stretching. Tumbler drums of various forms may also be used with good effect for softening purposes, especially for skins.
SOLE-LEATHER:—Unhairing Hides.
InEngland, lime is the agent almost universally employed for loosening the hair, though every tanner admits its deficiencies and disadvantages. It is hard, however, to recommend a substitute which is free from the same or greater evils, and lime has one or two valuable qualities which will make it very difficult to supersede. One of these is that, though it inevitably causes loss of substance and weight, it is also impossible, with any reasonable care, totally to destroy a pack of hides by its use; which is by no means the case with some of its rivals. Another advantage is that, owing to its very limited solubility in water, it is a matter of comparatively small consequence whether much or little is used; and even if the hides are left in a few days longer than necessary, the mischief, though certain, is only to be detected by careful and accurate observation. With all other methods, exact time and quantity are of primary importance, and it is not easy to get ordinary workmen to pay the necessary attention to such details. Again, the qualities of lime, its virtues and failings, have been matter of experience for hundreds of years, and so far as such experience can teach, we know exactly how to deal with it. A new method, on the other hand, brings new and unlooked-for difficulties, and often requires changes in other parts of the process, as well as in the mere unhairing, to make it successful. As our knowledge of the chemical and physical changes involved becomes greater, we may look to overcoming these obstacles more readily; for the power of dealing successfully with new difficulties constitutes one of themain advantages of a really scientific knowledge over an empirical one.
Slaked lime is soluble in water at 60° F. (15° C.), to the extent of 1 part in 778. Unlike most substances, it decreases in solubility at higher temperatures, requiring 972 parts of water at 130° F. (54° C.), and 1270 parts at 212° F. (100° C.). Its action upon animal tissues increases rapidly, however, with temperature, though no doubt it is moderated to some extent by the lessened solubility. Calculating from Dalton's numbers, pure lime-water at 60° F. (15° C.) contains 1·285grm.[R]of CaO perlitre, and should require 459c.c.of decinormal acid to neutralise it. This estimate in some cases appears to be slightly too high; e. g. a saturated lime-water from Carboniferous limestone at 56.5° F. (13° C.) required only 433c.c.of decinormal acid, which equals 1·211grm.of CaO perlitre, and this lime-water, kept with excess of lime, gave nearly constant results for many months together. A magnesian limestone lime-water tested at the same time required 472c.c.of decinormal acid, confirming the old observation of tanners, that such lime is stronger than that made either from chalk or carboniferous limestone. This increased strength must arise from the presence of some soluble base other than lime, and may be due to the magnesia, which, however, is very slightly soluble. Magnesian limestone contains a very large amount of magnesia, and hence would not go so far as a purer limestone; but as a very large proportion of the lime ordinarily used is thrown away undissolved, this is perhaps of little practical moment. (For the chemical examination of limes, seep. 102).