nature of the hide-tannin compound and its bearing upon tannin analysis

3
Dec., 1920 THE JOURNAL OF INDCSTRIAL AND ENGINEERING CHEMISTRY 1 I49 I ORIGINAL PAPERS I NATURE OF THE HIDE-TANNIN COMPOUND AND ITS BEARING UPON TANNIN ANALYSIS’ By John Arthur Wilson and Erwin J. Kern LABORATORIES OP A. F. GALLUN & SONS CO., MILWAUKEE, WISCONSIN Received September 2, 1920 In an earlier paper2 the authors described a new method of tannin analysis which they believe gives the true t,anning value of vegetable materials. Compar- ative tests showed the official method of the American Leaeher Chemists’ Association to be greatly in error, exceeding 200 per cent for typical samples of gambier extract. (The earlier paper should be referred to for a working description of both methods.) The much higher percentages of tannin obtained by the A. L. C. A. method for every material examined were at- tributed to the formation of readily hydrolyzable com- pounds between the hide powder and a variable frac- tion of the non-tannins. At the 17th annual meeting3 of t h e A. L. C. A. a formal discussion4 of this paper was staged, and the chief aim of the opposition was apparently to show that the low results obtained by the new method were due to losses of tannin in the manipulation. It was contended that a certain pro- portion of the tannin of a liquor will form a stable compound with hide only after long contact, and, fur- ther, that even tannin which has already combined with the hide will be removed to an appreciable ex- tent during the washing required by the new method; but no really conclusive evidence was offered in sup- port of these contentions. The object of the present work was to investigate these points because they are of great importance, not only to the new method, but to the theory and practice of tanning itself. TANNING -MATERIALS EXAJIIKED Certain differences in behavior of the several differ- ent tanning materials have caused a widespread belief that some tannins form more stable compounds with hide than others; for example, the tannin from gambier is supposed to form a compound with hide less stable than that from hemlock bark. It has also been sup- posed that mixtures of tanning materials behave dif- ferently in this respect from the individual materials. We have therefore chosen for examination typical com- mercial tanning extracts ranging in properties from the extrernes of gambier and sumac to those of hemlock and quebracho, and have also included two mixtures, each consisting of a pair of extracts of very different nature. The analyses of the 8 extracts and z mixtures, both by the new method and by the official method of the American Leather Chemists’ Association, are given in Table I, along with the percentage errors involved in the A. L. C. A. method, assuming that the new method is correct. 1 Pi.esented before the Section of Leather Chemistry at the 60th Meeting of the American Chemical Society, Chicago, I!l,, September 6 to 10, 2 “The ’ h i e Tanning Value of Vegetable Tanning Materials,” THIS 8 Atlantic City, N. J., May 27 to 29, 1920. 4 Printed in full, J. A m . Leather Chem. Assoc., 16 (1920), 451. 1920. JOURNAL, ia (ISJZO), 465. TABLE I-COMPARATIVE ANALYSES OF EXTRACTS STUDIED, BY NEW METHOD AND BY THE A. L. C. A. METHOD ---PERCENTAGE ANALYSIS OF MATERIAL-- Per- ----A L. C. A. METHOD-- centage -Soluble Matter- New Error in Insoluble hTon- Method A.L.C.A. EXTRACT Water Matter tannin Tannin Tannin Method Quebracho ... ... . 22.28 9.61 7.24 60.87 46.84 30 Gambier . . . . . . . 50.17 7.43 16.79 25.61 7.87 225 Quebracho - gam - bier mixture ... 44.53 7.39 14.86 33.22 20.67 61 Oak bark .... , . . . 52.43 3.10 18.28 26.19 15.52 69 Chestnut wood ,._. 52.47 3.64 18.19 25.70 13.99 84 Hemlock bark.. . 56.10 6.23 10.99 26.68 23.47 14 Chestnut wood- hemlock bark mixture .... .. . 53.89 5.94 14.53 25.64 18.73 37 Larchbark ...... 51.26 7.42 18.36 22.96 11.29 103 Sumac. ... , , , . . . 51.24 1.01 22.24 25.51 16.36 56 Wattle bark ... .. 50.20 1.50 14.75 33.55 24.66 36 It is interesting to compare the errors found for the sumac and hemlock bark extracts with those for the actual leaves and bark noted in the earlier paper. Extract manufacturers often remove only about 80 per cent of the tannin from the raw materials, because in order to get the last 20 per cent they would have to ex- tract a large proportion of undesirable non-tannins that would considerably lower the purity of their products. The raw materials used were extracted completely and therefore contained a higher ratio of non-tannin to tannin than the commercial extract, and this, as the authors have shown, increases the error obtained by the A. L. C. A. method. RESISTANCE OF LEATHER TO WASHING Three methods of measuring possible losses of com- bined tannin during the washing of leather were con- sidered. The first was to wash the leather free from soluble matter and analyze it for tannin, and then to wash it further and analyze it again. The second was to use the gelatin-salt reagent (IO g. gelatin plus IOO g. NaCl per liter) in testing the wash waters for tan- nin; provided all tests were negative, it would show that only a very small amount of tannin, if any, was washed out. The third was to collect and weigh the soluble residues from a number of wash waters. Of these, the first method appeared most important and consequently received greatest attention. TABLE 11-SHOWING EFFECT OF EXCESSIVS WASHING OF THE TANNED HIDE POWDER UPON THE PER CENT OF TANNIN POUND BY THE NEW METHOD Hide Substance in Powder Per Cent Tannin in Extract Used to Extract. Value Obtaiqed Grams Detannize from Analysis of 200 Cc. Soln. 15 25 50 in 200 Cc. -Tanned Powder Washed- EXTRACT Soh. Grams Times Times Times Quebracho ......... . . 3.80 10.44 46.84 47.25 46.90 Gambier.. . . . .. . . . . . 10.00 10.44 7.87 7.89 7.67 Ouebracho-gambier - - mixture’. . . . _. , . . , 6.90 10.44 20.67 20.34 20.43 Oak bark ..... .. . .. . , 13.60 10.40 15.52 15.36 15.35 . . . . .3 13.99 13.93 Chestnut wood. . . . . . . 13.60 10.32 Hemlock bark. . . . . . , 13 .OO 10.32 23.47 23.38 23.50 Chestnut wood-hem- lock bark mixturez. 13.30 10.32 .,... 3 18.73 19.05 ..... 3 11.29 11.28 Larch bark .... . . . , . . 10.32 Sumac . . . . . , . . . . 13.00 10.39 16.36 16.29 16.39 Wattle bark.. . . . . , . . 8.00 10.32 24.66 24.16 24.73 13.60 1 Mixture of 19 parts solid quebracho extract to 50 of gambier extract. 2 Mixture of 68 parts of chestnut wood extract to 65 of hemlock bark 3 Calculation not made because 15th wash water gave test for non- extract. tannin with ferric chloride. PROCEDURE AND RESULTS-Portions of all IO sam- ples were dissolved in water to give the concentrations noted in Table 11. For each sample 12 g. of air-dry

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Page 1: Nature of the Hide-Tannin Compound and Its Bearing upon Tannin Analysis

Dec., 1 9 2 0 T H E J O U R N A L OF I N D C S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 1 I49

I ORIGINAL PAPERS I NATURE OF THE HIDE-TANNIN COMPOUND AND ITS

BEARING UPON TANNIN ANALYSIS’ By John Arthur Wilson and Erwin J. Kern

LABORATORIES OP A. F. GALLUN & SONS CO., MILWAUKEE, WISCONSIN Received September 2, 1920

In an earlier paper2 the authors described a new method of tannin analysis which they believe gives the true t,anning value of vegetable materials. Compar- ative tests showed the official method of the American Leaeher Chemists’ Association to be greatly in error, exceeding 2 0 0 per cent for typical samples of gambier extract. (The earlier paper should be referred to for a working description of both methods.) The much higher percentages of tannin obtained by the A. L. C. A. method for every material examined were at- tributed to the formation of readily hydrolyzable com- pounds between the hide powder and a variable frac- tion of the non-tannins. A t the 17th annual meeting3 of the A . L. C. A. a formal discussion4 of this paper was staged, and the chief aim of the opposition was apparently to show that the low results obtained by the new method were due to losses of tannin in the manipulation. I t was contended that a certain pro- portion of the tannin of a liquor will form a stable compound with hide only after long contact, and, fur- ther, that even tannin which has already combined with the hide will be removed to an appreciable ex- tent during the washing required by the new method; but no really conclusive evidence was offered in sup- port of these contentions. The object of the present work was to investigate these points because they are of great importance, not only to the new method, but to the theory and practice of tanning itself.

TANNING -MATERIALS EXAJIIKED

Certain differences in behavior of the several differ- ent tanning materials have caused a widespread belief that some tannins form more stable compounds with hide than others; for example, the tannin from gambier is supposed to form a compound with hide less stable than that from hemlock bark. I t has also been sup- posed that mixtures of tanning materials behave dif- ferently in this respect from the individual materials. We have therefore chosen for examination typical com- mercial tanning extracts ranging in properties from the extrernes of gambier and sumac to those of hemlock and quebracho, and have also included two mixtures, each consisting of a pair of extracts of very different nature. The analyses of the 8 extracts and z mixtures, both by the new method and by the official method of the American Leather Chemists’ Association, are given in Table I, along with the percentage errors involved in the A. L. C. A. method, assuming that the new method is correct.

1 Pi.esented before the Section of Leather Chemistry at the 60th Meeting of the American Chemical Society, Chicago, I!l,, September 6 to 10,

2 “The ’ h i e Tanning Value of Vegetable Tanning Materials,” THIS

8 Atlantic City, N. J., May 27 to 29, 1920. 4 Printed in full, J . A m . Leather Chem. Assoc. , 16 (1920), 451.

1920.

JOURNAL, i a (ISJZO), 465.

TABLE I-COMPARATIVE ANALYSES OF EXTRACTS STUDIED, BY N E W METHOD AND BY THE A. L. C. A. METHOD

---PERCENTAGE ANALYSIS OF MATERIAL-- Per- ----A L. C. A. METHOD-- centage

-Soluble Matter- New Error in Insoluble hTon- Method A.L.C.A.

EXTRACT Water Matter tannin Tannin Tannin Method Quebracho . . . . . . . 22.28 9 .61 7 . 2 4 60 .87 4 6 . 8 4 30 Gambier . . . . . . . 50.17 7 .43 16.79 25 .61 7 .87 225 Quebracho - gam -

bier mixture . . . 44.53 7 .39 14 .86 33 .22 20 .67 61 Oak bark . . . . , . . . 52.43 3 .10 18 .28 26 .19 15 .52 69 Chestnut wood ,._. 52.47 3 . 6 4 18 .19 25 .70 13 .99 84 Hemlock bark. . . 56.10 6 . 2 3 10.99 26.68 23.47 14 Chestnut wood-

hemlock bark mixture . . . . . . . 53 .89 5 . 9 4 14 .53 25 .64 18 .73 37

Larchbark ...... 51.26 7 .42 18 .36 22 .96 11 .29 103 Sumac. . . . , , , . . . 51 .24 1.01 22 .24 25 .51 16 .36 56 Wattle bark . . . .. 5 0 . 2 0 1 .50 1 4 . 7 5 33.55 24 .66 36

It is interesting to compare the errors found for the sumac and hemlock bark extracts with those for the actual leaves and bark noted in the earlier paper. Extract manufacturers often remove only about 80 per cent of the tannin from the raw materials, because in order to get the last 2 0 per cent they would have to ex- tract a large proportion of undesirable non-tannins that would considerably lower the purity of their products. The raw materials used were extracted completely and therefore contained a higher ratio of non-tannin t o tannin than the commercial extract, and this, as the authors have shown, increases the error obtained by the A. L. C. A. method.

RESISTANCE O F LEATHER TO WASHING

Three methods of measuring possible losses of com- bined tannin during the washing of leather were con- sidered. The first was to wash the leather free from soluble matter and analyze i t for tannin, and then to wash i t further and analyze it again. The second was to use the gelatin-salt reagent ( I O g. gelatin plus IOO g. NaCl per liter) in testing the wash waters for tan- nin; provided all tests were negative, i t would show that only a very small amount of tannin, if any, was washed out. The third was to collect and weigh the soluble residues from a number of wash waters. Of these, the first method appeared most important and consequently received greatest attention. TABLE 11-SHOWING EFFECT OF EXCESSIVS WASHING OF THE TANNED

HIDE POWDER UPON THE PER CENT OF TANNIN POUND B Y THE NEW METHOD

Hide Substance in Powder Per Cent Tannin in

Extract Used to Extract. Value Obtaiqed Grams Detannize from Analysis of

200 Cc. Soln. 15 25 50 in 200 Cc. -Tanned Powder Washed-

EXTRACT Soh. Grams Times Times Times Quebracho . . . . . . . . . . . 3 . 8 0 10.44 46 .84 47 .25 46 .90 Gambier.. . . . .. . . . . . 10 .00 10.44 7 .87 7 . 8 9 7 .67 Ouebracho-gambier - -

mixture’. . . . _ . , . . , 6 . 9 0 10 .44 20.67 20 .34 20 .43 Oak bark . . . . . . . . . . . , 13 .60 10 .40 15.52 15 .36 15.35

. . . . . 3 13 .99 13.93 Chestnut wood. . . . . . . 13.60 10 .32 Hemlock bark. . . . . . , 13 .OO 10 .32 23.47 23 .38 23 .50 Chestnut wood-hem-

lock bark mixturez. 13 .30 10 .32 . , . . . 3 18.73 19.05 . . . . . 3 11.29 11.28 Larch bark . . . . . . . , . . 10.32

Sumac . . . . . , . . . . 13.00 10 .39 16 .36 16.29 16.39 Wattle bark. . . . . . , . . 8 . 0 0 10 .32 24 .66 24 .16 24 .73

13.60

1 Mixture of 19 parts solid quebracho extract to 50 of gambier extract. 2 Mixture of 68 parts of chestnut wood extract to 65 of hemlock bark

3 Calculation not made because 15th wash water gave test for non- extract.

tannin with ferric chloride.

P R O C E D U R E AND RESULTS-Portions of all I O sam- ples were dissolved in water to give the concentrations noted in Table 11. For each sample 1 2 g. of air-dry

Page 2: Nature of the Hide-Tannin Compound and Its Bearing upon Tannin Analysis

I 1 5 0 T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 12, N o . 1 2

hide powder, of known hide-substance content, were put into each of five wide-mouth, half-pint bottles, 2 0 0

cc. of tan liquor were added to each, and the bottles stoppered and shaken in a rotating box for 6 hrs. A t the end of this time all solutions gave a negative test for tannin with the gelatin-salt reagent. The bulk of detannized liquor was removed from each powder by squeezing through a suitable cloth. Two powders from each set were allowed to dry slowly, without previous washing, and bottled for use in connection with an investigation of the “aging” of leather t o be mentioned later. The remaining three powders of each set were washed as follows: Each powder was re- turned to its bottle along with 200 cc. water and shaken for an hour. The aqueous extract was separated from the powder by squeezing through a cloth, and the wash- ing operation was repeated. Of the three powders of each set, one was washed fifteen times, the second twenty-five times, and the third fifty times. After washing, the tanned powders were allowed to dry slowly, and were then analyzed for water, ash, fa t (chloroform extract), and hide substance (nitrogen X 5 . 6 2 ) . The per cent of tannin was calculated by difference. The results are given in Table 11.

The detannized liquor and fifteen wash waters from one of the gambier samples were evaporated separately to show the rate a t which the soluble matter is removed by washing. Table I11 shows that only 1 2 mg. of soluble matter were removed by the fifteenth wash water, which appeared colorless and gave no color upon addition of a drop of ferric chloride solution. The

TABLE 111-SHOWING SPEED OF WASHINO OF POWDERS TANNED WITH GAMBIER

SOLUBLE RESIDUE FROM M g . Original detannized solution. . . . . . . . . . . . 1408 1st wash water. ....................... 776 2nd wash water. . . .................... 396 3rd wash water. . ..................... 250 4th wash water.. ..................... 160 5th wash water. . ..................... 101 6th wash water. .... 8th wash water.. ..................... 33 9th wash water.. ..................... 31

10th wash water.. ..................... 22 11th wash water, . ..................... 1 7 12th wash water.. ..................... 16 13th wash water.. ..................... 13 14th wash water.. .......... 15th wash water.. ..................... 12

Weight

smallness of the weights of the residues made the evaporation of individual wash waters beyond the fif- teenth of doubtful value. However, all wash waters from the twenty-sixth to the fiftieth (5000 cc. in all) were collected from both the sumac-tanned and larch- tanned powders, and evaporated to dryness. The total matter extracted froni the former was only 67 mg., and from the latter only 51 mg. Pure hide pow- der washed in the same way was found to yield even greater residues because of slow hydrolysis, and i t should be remembered that hide powder was used in great excess in these tests. If the 67-mg. residues from the sumac-tanned powder consisted solely of hide substance, it would contain 12 mg. of nitrogen; a Kjeldahl determination gave 14 mg. Furthermore, the residues from the tanned powders resembled those from pure hide substance in odor and general appear- ance.

All wash waters without exception gave negative tests with the gelatin-salt reagent. Baldracco and Camilla1 recently proposed a reagent for which they claimed greater sensitivity than the gelatin-salt re- agent. Their reagent was made by dissolving 0.9 g. gelatin in 50 cc. water and adding IO cc. of glacial acetic acid. While this solution appears to be more sensitive than a gelatin-salt solution which has been kept for some weeks, we found i t much less sensitive than the gelatin-salt reagent immediately after prep- aration. When the gambier liquor was diluted IOO

times and a drop of freshly prepared gelatin-salt re- agent was added, a distinct precipitate was formed, whereas a freshly prepared gelatin-acetic acid reagent gave a negative test. We found that the gelatin-salt reagent must be prepared from the highest quality gelatin and used only on the day it was made for the most satisfactory results. The gambier liquor had to be diluted 2 0 0 times before i t failed to give a test, and all the other liquors even more. Since no tests were obtained for any wash waters, a t the end oE fifteen wash- ings not more than 8 per cent of the tannin originally present in the tan liquor could have been removed by washing. In other words, even if, as is most unlikely, each wash water removed an amount of tannin that would just fail to give a test with the gelatin-salt re- agent, we should have to increase the per cent of tannin found for gambier extract, the material least sensitive to the test, only from 7.87 to 8.55, whereas a figure of 25.61 is obtained by the A. L. C. A. method.

coNcLusIoNs-The results show conclusively that prolonged washing is quite without influence upon the values obtained for the per cent of tannin in var- ious materials by the new method, and, further, that tannin once combined with hide cannot be washed out by any amount of washing that would be practical. Thus the major contention of opponents of the new method is disproved.

ANOTHER VALUABLE P R O P E R T Y O F C E R T A I N NON- TANNINS NOT BEFORE F U L L Y RECOGNIZED

In our earlier paper it was shown that aon-tannins like gallic acid are valuable agents in rendering too astringent tan liquors sufficiently mild for practical use. We now find that these same non-tannins are capable of undergoing chemical change with the forma- tion of substances capable of tanning. During his criticism of the new method, G. W. Schultz2 said, “We have taken the non-tannins and washings and recon- centrated them under a high vacuum to the original volume of 2 0 0 cc. and have tanned hide powder with it, and, by the calculations employed, we have found a definite percentage of tannin.” He mentioned also that the concentrated liquor gave a positive test for tannin with the gelatin-salt reagent. I t might look a t first sight as though the detannized liquor and wash waters, before concentrating, really had contained tannin and Schultz evidently so regarded it. This view, however, would be difficult t o reconcile with the experimental data noted in the preceding section.

1 J . SOC. Leather Trades’ Chem., 4 (1920), 101. 3 J . Am. Leather Chem. Assoc., 15 (1920), 455.

Page 3: Nature of the Hide-Tannin Compound and Its Bearing upon Tannin Analysis

Dec., 1920 THE J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

We confirmed Schultz’s findings while analyzing a sample of gambier extract by the new method. The detannized liquor and 1 5 wash waters, all of which gave n o test with the gelatin-salt reagent, were con- centrated to 2 0 0 cc., whereupon they were found to give a bulky precipitate with the reagent. But, when dilated back to 3200 CG., they still gave a bulky precipitate wi th the gelatin-salt reagent, showilzg that a most impor- tant chemical change had taken place during the 60%-

cenfrating. TABLE IV-GAMBIER EXTRACT

200 cc. solution containing 9.00 g. extract were detannized with 12 g. air-dry hide powder containing 10.40 g. hide substance, and then the tanned powder was wished 17 times with 3400 cc. water. The residual liquor and wash waters were evaporated to 250 cc. and used to tan 12 g. fresh hide powder which was afterwards washed as usual.

Total tannin’ either originally present or formed during the concen- tratinn of the wash waters. 13.50 Der cent. -

-HIDE POWDER TANNED IN- Original Concentrated

ANALYSIS OF AIR-DRY LEATHER Solution Wash Waters Water.. ........................... 17.31 16.24 Ash ................................ 0.16 0.14 Fat . ............................... 0.39 0.42 Hide .substance (N X 5.62). .......... 76.86 79.38 Tannin (by difference). .............. 5.28 3.82, Per 100 g. hide substance:

Tannin found, grams.. ............. 6.87 4.81

Per cerit tannin in extract.. . . . . . . . . 7.94 5.56 Material used, grams.. ............. 86.54 86.54

Another sample of gambier was analyzed by the new method and found to contain 7.94 per cent tannin. The detannized liquor and 17 wash waters (3600 cc. in all) were evaporated to 250 cc., analyzed by the new method, and found to contain 5.56 parts of tannin per IOO of original extract, giving the extract a total of 13.50 per cent tannin. The detailed results are given in Table IV. In order to show that this in- creased amount of tannin would have combined with the hide powder had it been present in the original solution, we made up a new solution of this extract, concentrated and diluted back several times, and then analyzed it by the new method, finding 12.69 per cent of tannin. If the concentrating had been continued a little longer the figure 13.50 would probably have been reached or passed. The results are given in Table V.

TABLE V-GAMBIER EXTRACT (S.ame as noted in preceding table.) Dissolved 60.00 g. extract in 1 liter of water. Concentrated to 250

cc., and diluted back to 1 liter. Repeated 3 times, the fourth time dilu- ting to 2 liters. 200 cc. diluted solution containing 6.00 g. original extract were detannized with 12 g. air-dry hide powder, containing 10.37 g. hide substance. which was afterwards washed as usual.

ANALYSIS O F AIR-DRY LEATHER Water .............................. 18.23 Ash ................................. 0.18 Fat.. ............................... 0.42 Hide substance (N X 5.62). . . . . . . . . . . 75.62 Tannin (by difference). ............... 5.55 Per 100 g. hide substance:

Tannin found, grams.. . . . . . . . . . . . . . . 7.34 Material used, grams.. .............. 57.86 Per cent tannin in extract.. . . . . . . . . 12.69

I n spite of the great change in the tan liquor pro- duced by concentrating, it is not shown to any appre- ciable extent in the analyses by the A. L. C. A. method shown in Table VI. Concentrating the tan liquor and diluting back caused a rise in per cent of tannin by the new method from 7.94 to 12.69, but the rise in the A. L. C. A. method is only from 26.14 t o 26.40, which difference is so small as even to be attributable to experimental error. The reason for this small dif- ference is probably that the non-tannins which are convertible into tannin all combine with the hide

I 1 5 1

initially, even though they are easily removed later by washing.

TABLE VI-GAMBIER EXTRACT Both the original liquor noted in Table I V and the specially treated

liquor noted in Table V were appropriately diluted and analyzed by the A. L. C. A. method.

PER CENT OF ORIGINAL EXTRACT Original Treated Liquor Liquor

Insoluble matter.. .................... 7.66 8.62 Non-tannin 18.33 17.57 Tannin. .............................. 26.14 26.40

...........................

Just what chemical actions are involved in the con- version of non-tannin to tannin must remain a matter of speculation until more data are available; oxidation, condensation, and polymerization may all be involved. It is conceivable that gallic acid might be converted into digallic acid under suitable conditions, and i t seems extremely likely that a polymerized form of digallic acid would have tanning properties. A solu- tion of pure gallic acid gives no test for tannin, but after boiling for some time i t gives a bulky precipitate with the gelatin-salt reagent, and apparently will t an hide. A detannized solution which gives no test for tannin can be made to give a strong test merely by passing oxygen gas through it. Long exposure to air has a similar action. It is evident tha t the new method furnishes a valuable means of studying the conversion of non-tannins into tannin, and might con- ceivably be applied to a study of the formation of tannins in nature and to the aging of barks.

THE TIME FACTOR I N TANNING AND THE “AGING” OF

The results .of the preceding section suggest tha t the conversion of non-tannin into tannin is responsible for two factors of great importance to tanners of heavy leather, namely, the time factor in tanning and the (‘aging’’ of leather. In the discussion referred to, Alsopl remarked that sole leather tanned slowly not only contains more tannin, but actually consumes less tanning material than the rapid tannages. In a private communication, Professor Procter, of Leeds, has called attention to the fact that leather stored for a long time, or “aged,” before washing contains more tannin than if i t had been washed immediately after tanning. The extent of this Aging is well shown in the following experiment:

LEATHER

TABLE VII-EFFECT OF “AGING” UPON PER CENT OF COMBINED TANNIN I N LEATHER

Two 12-g. portions of hide powder were used to detannize 200-cc. portions of the solutions of tanning materials noted in Table 11. One portion in each case was washed 25 times immediately after tanning: the other portion was allowed to dry without washing, kept exactly 30 days, and then washed 25 times.

TANNIN AS PER CENT

I n Leather I n Leather Washed Kept

Immediately 30 Days after before

EXTRACT Tanning Washing Quebracho. ....... 47.25 53.00 Gambier. ........ 7.89 10.49 Quebracho-gambie 20.34 23.92 Oak bark.. ....... 15.36 17.23 Chestnut wood. ....................... 13.99 18.02 Hemlock bark. . ...................... 23.38 24.87 Chestnut wood-hemlock bark mixture. . . 18.73 20.45 Larch bark.. 11.29 13.22 Sumac.. ............................. 16.29 17.94 Wattle bark. . 94.16 25.89

-OF ORIGINAL EXTRACT-

......... ....................

1 LOC. cii., p, 464.