Bourne Archive: Muspratt: Tannin Sources

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The Bourne Archive

 

Muspratt’s Chemistry, Theoretical, Practical & Analytical (ca. 1859)

Extracts Concerning Leather, 3: Materials Used in Tanning: Sources of Tannin.

The web pages linked from this introduction are from an article on tanning, under the heading ‘Leather’. The original article is presented here in several web pages, respectively dealing: 1, with leather; 2, with tanning materials; 3, Sources of tannin; 4, crushing mills; 5, varieties of skin; 6, hide preparation; 7, the common tanning process; 8, finishing processes; 9, fancy and speciality leathers.

Vol. 2. pp. 500-508

Substances Used in Tanning.—Those which are available for the purpose of tanning, embrace a wide range if the juices, barks, leaves, roots, fruit, and excrescences of trees, and also vegetals 1 which contain tannin of one kind or other, be taken into account. In looking over the labours of chemists, many hundred substances which yield tannin have been discovered; but practice, or the trade, recognizes only a few of these, probably owing to the quantity which can be annually produced, their yield of tannin, or because they offer some advantage either in making a better leather, or in the time required to effect the operation. The tannin and other distinct principles in vegetal growth of every kind, are derived from the sap or soluble matters, imbibed by the rootlets from the soil in which the plant exists, and the gasses absorbed by the foliage. In many plants which elaborate tannin, it is met with in small proportions, owing perhaps to its being only a secondary, and not a primary secretion. It is found in some vegetals equally in all parts, whilst in others certain parts are richer in this substance, the remaining exhibiting only slight indications of its presence. The particular portions of plants that yield the greatest amount of tannin are, therefore, preferred in making leather.

The following is a list of the materials which contain tannin in the largest quantities, and which may be employed in the manufacture of leather 2.

Inspissated3 and Prepared Extracts,

Kino,

Catechu, Terra japonica, or Cutch.

Vegetal Excrescences

Galls.

Tree Leaves,

Of the different kinds of willow.

Of the quercus robur—common oak; quercus cerris—Turkey oak; quercus ilex—evergreen oak—and other varieties.

Of the erica vulgaris, or common heath, and other varieties of the same plant.

Of the cyndra scolymus—garden artichoke.

Of the sloe-tree—prunus spinosa.

Of the conium maculata—spotted hemlock.

Of arbutus uva ursi—bear-berry.

Of the thea Chine sis—the leaves of the several varieties of green and black teas yield tannin—and the rhus glabrum, known as sumac.

Flowers and tree Tops, Fruits,

These are hardly ever employed, although many afford extracts which precipitate gelatine and salts of iron.

Seeds and Bulbs,

Of the grape.

Of the hydro-sapathum and wild cornel.

The hulls of the fruit of the cæsalpina coriaria, known under the title of divi-divi, or leby-dibi.

The bulbs of the scuilla maritima.

The dried acorns of the prickly-cupped oak—quercus œgilops.

Woods,

All the woods or trees, the barks of which yield tannin, also afford more or less of this substance; they are, however, scarcely ever employed.

Roots,

Of the dentillaria, or lead-wort—plumbago Europæa.

Of the male fern—aspidium filex mas.

Ratany—krameria triandria—root.

Of leopard’s bane—arnica montana.

Of statice, or marsh rosemary.

Barks,

[Return to bark properties]

Common bark, of which there are several varieties; birch, chestnut, and horse-chestnut barks; sassafras, or the bark of the root of the root of the laurus sassafras; larch, hazel, beech, Lombardy poplar, blackthorn, pomegranate, ash, elm, cork-tree, cinchona, willow, sycamore, tulip-tree, wattle and oak barks; besides these, however, there are other barks—such as that of the sumac shrub, winter’s bark, et cetera—that afford tannin in available quantities.

It will not be uninteresting to state briefly some of the properties of the substances mentioned in the foregoing table, with respect to their uses for tanning purposes.

Kino.—Of the inspissated 3 extracts which are found in the market, kino or gum-kino, is the richest in tannin, containing, as it does, according to Vauquelin’s analysis, seventy-five per cent. of this compound. Nevertheless, it has not hitherto come into use for tanning purposes, but is solely devoted to medical uses. For its more general properties, refer to page 316, Vol. II 4.

Catechu.—Catechu is another of the same class as the preceding, and forms a most useful article for the tanner. There are several varieties which are distinguished by various names, such as cutch, terra japonica, and gambir. Formerly, this extract, from its brownish-red color, was supposed to be a kind of earth, hence the title terra given to it. Catechu is extracted from the acacia catechu, a tree which grows to the height of twenty of thirty feet, and abounds in the forests from latitude 26° to 30°, known as the Bornese territory on the Malabar coast, and called Cancan. The heart and the bark of the wood are boiled in water, and the solution evaporated, which leaves the astringent extract known as Cancan catechu. According to Nees Von Esenbeck 5, most of the catechu exported from Bombay is prepared from the acacia catechu, whilst that brought from Bengal, is derived from the uncaria gambir, a shrub cultivated in the countries lying on both sides of the shores of Malacca 6. It is obtained by boiling the wood, bark, and leaves of this shrub together with the inspissated juice in water, and evaporating, then adding a little sago to give it consistency; it is finally exsiccated in the sun, and then cut into square or circular cakes to suit the purchaser. Bombay catechu, which is the richest in tannin, is of a dark brownish-red hue, both externally and internally, and possesses a specific gravity of 1.38. Davy examined the Cancan catechu, and found it to be 1.39; he also found in two hundred parts of this, as well as of the Bengal catechu, and known by the title Pegu, from the province of Bengal where it is prepared 7, the following constituents:—

 

Common or    Bombay catechu

Pegu or Bengal catechu

Tannin,

109

97

Extractive,

68

73

Mucilage,

13

16

Earthy residue,

10­­__

14__

 

200

200

The genuine catechu, of whatever variety, contains, on an average, about half its weight in tannin, and its efficacy in making skin into leather, has been estimated as being five times greater than the best oak bark; but this seems exaggerated. Besides the real tannin, the extractive matter contained in it is another definite substance, which is called catechuic acid 8, and which doubtless plays a part in the process of tanning. To it is ascribed the property of coloring the leather so deeply. The latter body differs from the tannin in being insoluble in cold water, although a solution of the tannin of catechu takes up a small quantity of it. The varieties of catechu of commerce are:—

1. Cake catechu, from its being in circular cakes. Their color varies from a light-brown to a black, and their weight from several ounces to two pounds.

2. Pegu catechu,—As already stated, this variety obtains its name from the province where it is prepared. It is generally imported in masses of a hundred pounds weight, but as seen in the shops, it presents the appearance of angular irregular fragments in double layers, with leaves between. It has a compact shining fracture, and a deep brown color.

3. Bengal catechu is manufactured in rectangular cakes, but in the course of transit they get reduced to fragments. Externally, it has a rusty brown color, and internally, the shade varies from a brownish-grey to dark-brown.

4. Bombay catechu,—This variety occurs in globular lumps of the size of an orange flattened, and two pieces generally adhering together. In color it resembles that brought from Bengal.

5. Gambir.—It is stated by M’Culloch 9 that no less than four thousand six hundred tons of catechu, under the name of gambir, is produced annually by the Chinese settlers in Rhio 10. It takes its name from the shrub which is its source, the uncaria or nuclea gambir. It has deep yellow or reddish-brown color on the outside, but within it is paler, and presents a dull earthy fracture. It comes to these countries in solids of about a cubic inch. Esenbeck, who examined this variety, states that it yields from thirty-six to forty per cent. of tannin. Boiling water entirely dissolves it.

6. Arecha catechu.—The nuts of an Indian palm known by this title, afford this kind of astringent substance. These fruits are macerated 11 with water, and the decoction evaporated, when a better sort of catechu results, known by the term kassu, and the semi-exhausted residue upon further treating it with water, affords a solid extract, which is distinguished as coury. The former is a black color, and intermixed with husks, and the latter of a yellowish-brown, with an earthy fracture. The coury is, of course, inferior to extract known as kassu.

With regard to the application of catechu to the manufacture of leather, the natives of India have long practised it; but its introduction into the trade of this country for a similar purpose, is of modern date. Its richness in the tanning agent causes it to be very rapid in producing a gelatino-tannate of the substance of the skin, or, in other words, of making leather. The qualities of the article manufactured from it, are not, however, so satisfactory as the rapidity of its effects, either to the tanner or the public: for the leather is very permeable to water, light and spongy, hard, and of a dark-reddish fawn color. The characteristic deposits from oak bark and a few other tanning agents, known as bloom 12, is not produced by catechu, and this want is a material objection to its use, as the existence of bloom upon leather is considered as a kind of guarantee of its goodness. One pound of catechu, of first rate quality, is capable of producing one pound of leather, and, consequently, in tanning power is equivalent to five of oak bark, or thereabouts, as already stated.

Besides the inspissated juices just mentioned, there is another which, though not yet introduced into the European trade, has, nevertheless, been employed by the North-western Indians for tanning purposes. This is gum-butea, the account of which will be found at page 315, Vol. II.

Galls.—There is no other natural product that affords so much tannin as those round, hard, woody excrescences known as galls. Formerly it was supposed that these were a kind of fruit; but naturalists and physiologists have ascertained that this is not the case, but that they are excrescences or tumors which form on the branches of different trees and vegetals, owing to the puncture of certain insects, for the purpose of depositing their eggs. The tree which affords those galls that are commonly known as nut-galls, is a stunted species of oak—quercus infectoria—which is very general throughout Asia Minor. It grows to the height of four or six feet, has a crooked stem, and yields an acorn two or three times larger than its cup. The gall-flies, which occasion the gall, belong to the genus cynips, and, from different accounts, there are several species; that which is concerned with producing the officinal galls, is the cynips gallæ tinctoriæ. The female insect of this variety, by means of an appropriate apparatus, perforates the cortical part of the plat, and in the wound deposits her eggs, together with an acrid liquor. In two or three days the part is, as it were, inflamed, and a swelling appears, and continues to increase till it results in a gall. The eggs which are inclosed in this excrescence are hatched, and in due time the young larvæ appear, and develop themselves, being supported by the juices of the plant till they become a perfect insect, when they perforate the gall and escape. When this happens, the excrescence loses much of its astringent principle, and becomes lighter; but if gathered or harvested before the entombed insect is completely developed, the nuts are not only heavier, but are richer in tannin, and command a better price in the market. Galls gathered before the escape of the insect, have a black or bluish shade, but when the insect has left, their color is paler, and they generally attain to a larger size. To prevent this occurrence, great care is taken to harvest the galls before the insect attains full growth, and eats its way through, leaving them, however, on the trees till they have acquired their greatest weight. In some parts the governor, or aga of the district, levies a tax on the produce; and being thus interested in the success of the crop, he causes the cultivators to traverse frequently the hills and mountains to report on the advanced state of the galls, and wherever the proper growth has been attained, they are immediately collected. The selections thus made are known in the market as green-galls, and come to these countries from Aleppo, Smyrna 13, and the interior of Asia Minor. Those which escape harvesting before the entombed insect has attained its full growth or emerged, are known as white-galls, and are imported from the same place. Another kind of gall, produced upon the oaks growing in many departments in France, are nearly equal in size to the Asiatic green-galls, but they are rounder, and possess a smoother and in some instances a polished surface. They have a brownish color, and rank in their content of tannin intermediate between the green and white Aleppo galls. A variety originating from the puncture of an insect allied to the aphis on the branches and shoots of the distylium racemosum comes from Japan. They are of an irregular shape, having in some instances both ends small, whilst the middle is much thicker, but more generally the stem end is the least and the more swollen part is the other. Hence they are called commercially apple-galls.

Annexed are the analyses of samples of Aleppo and Chinese galls by two distinguished authorities:—

 

Aleppo galls

Aleppo galls

Chinese galls

 

By Guibourt

By Davy

By Bley

Tannin,

65.0

26.0

69.00

Gallic acid,

2.0

6.2

4.00

Ellagic acid,

2.0

—

—

Brown extractive,

2.5

—

—

Starch,

2.0

}

—

Gum,

2.5

} 2.4

—

Sugar,

1.2

}

—

Chlorophyl and volatile oil,

0.7

—

—

Woody fibre,

10.5

}

—

Water,

11.5

} 65.4

8.00

Fat albumen and resin

—

—

3.00

Cellular matter

____

____

16.00­­_

 

100.0

100.0

100.00

Leaves.—Of the foliage of trees containing tannin, very few, if any, are now employed in the manufacture of leather. The leaves of the heath were once extensively used in this country; but this material has long been abandoned, preference being given to oak barks and other substances of native and foreign growth.

The leaves of the sloe-tree—prunus spinosa—were likewise used to some extent in London for tanning calf skins, by employing a decoction prepared by boiling the leaves in barley water. So also it is stated, in the Bibliothèque Physico Economique for 1789, that garden artichoke was resorted to for preparing calf and goatskins for bookbinders’ use, and that the operation was successful as if nut-galls or willow bark had been the tanning agent. The various species of tea yield variable but large quantities of tannin, amounting from thirteen per cent., according to Mulder—to thirty-five and even forty per cent., according to Franck 14. The value of this article, however, as a luxury and dietary, precludes its employment in the production of leather.

 Sumac.—In its purity it consists of the powdered leaves of a shrub that grows extensively in the south of Europe, in the United States, and in Asia to which it is indigenous. It seems that there are several species such as the rhus cotinus—wild olive—rhus glabrum or coriaria, which is the best and most esteemed for the preparation of the finer kinds of leather. Italy, Sicily, Portugal, Spain and France produce considerable quantities of sumac, varying in quality, and distinguished from one another by the habits of the tree, the color, and other properties. The sumac obtained from the rhus cotinus is for the most part employed in dyeing, and the product of the rhus coriaria is that which is converted to the uses of the tanner, especially in the preparation of morocco 15 and similar leather. The latter shrub which grows wild in Portugal, Spain, and other countries named above, rises to a height of four to eight, and is some cases twelve feet; its stem sis crooked and covered with a reddish-grey bark; the leaves present a green on the upper and a whitish color on the under surface during spring and summer, but they assume a reddish hue in autumn. It flowers in July, the blossom being greenish-red, and yields a cluster of small crimson berries on ripening. Regarding the effect of sumac as a tanning agent, it is stated that it deprives the skin of much of its softness and elasticity, but it offers one great advantage of not coloring it during the process, and is on this account preferred in France and other places, notwithstanding its cost being much greater than other tanning agents, by the fabricators of morocco and glazed leather. It is utilized in the ordinary way of tanning mixed with bark or other matter, and affords good results.

Of the species of sumac in the market, the Sicilian is accounted the best. There are two kinds, on of which, the Alcamo, is the most esteemed. It is a very fine light-green powder, containing very little woody matter, having an agreeable odor analogous to that of the violet, and a strong astringent taste; it contains very little coloring matter, though it gives a yellowish green solution when macerated with water. The second variety inclines to a reddish yellow, has a feebler odor, with a less astringent taste than the foregoing variety. On this account it is not much employed in tanning, though extensively used in dyeing; see vol. i. pp. 579, 629. Sicilian sumac is generally packed in bales weighing about one hundred and a half.

Spanish sumac is various in quality, being less carefully prepared, and consequently, more or less mixed with woody matter. The best sort comes from Priego 16, and is grown in the neighbourhood of Malaga. It is like the Sicilian, finely ground, and affords a color of equal or greater brightness; its odor reminds one of the tea plant. With water it gives a dark and more reddish solution than the foregoing. It is usually packed in bales of one hundredweight. The other sorts, the Molina and Valladolid sumac, are next in quality to the foregoing; they are very similar.

Portuguese or Porto sumac is almost similar to the Priego, but generally dirtier, and contains more mineral salts.

Italian sumac has a dark-green color, is free from woody matter, but feels granular in the hand, and has an odor like that of the bark, which possesses similar qualities to the leaves.

French sumac is similar to the preceding. Three sorts are collected, the Fauvis is almost equal to the Sicilian when well purified, and comes from Brignolles, near Marseilles. If less care be taken in its manufacture, it approaches more to the quality of Malaga sumac. It frequently goes under both these names. A second sort, Donzere, and a third, Pudis, are commonly used in the tanneries. A fourth variety, called redou or redoul, obtained from the coriaria myrtifolia, cultivated in Languedoc, is of a greyish-green color.

The operations involved in the preparation of sumac, consist in collecting the branches of the shrub whilst in full foliage, drying them in the sun, and then separating the leaves by threshing, or other means. Subsequently the detached leaves are ground under vertical mill-stones, and packed in bales for the market. In some cases, the peduncles and more tender branches are ground with the leaves; but as these contain a good deal of tanning material, they do not much injure the quality.

Flowers and Fruits—Valonia.—Hitherto flowers and flower-tops, though containing tannin, have not been used in the preparation of leather on the large scale. The same might be said of fruits, with the exception of the acorn-cups of the quercus œgilops—prickly-cupped oak,—a tree which grows in abundance in the Morea and the adjacent countries. In commerce, they pass under the title valonia, and are imported from Turkey, Greece, Italy and India. When the fruit is gathered, it is conveyed to the nearest port to be shipped; there it is stored in a warehouse during several months, being laid out in beds of three to five feet in thickness, A slight heating or fermentation sets in during the above period, and as the moisture escapes, the long spreading scales, which hitherto confined the acorn become contracted, and allow the latter to fall out of the cup. After being well dried the whole is picked, and the acorns which contain no tannin and the damaged cups are separated from those of the latter that are dry and good. The cup of the acorn, so long as it is kept dry, retains a bright drab color 17; but when exposed to moisture, it loses this appearance and turns black, losing by the change its tanning properties. Doubtless, to the long exposure of the upper cups to the disengaged vapour from the bed, is owing their being invariably more or less damaged. Ordinary or common valonia, the cups of which average about two inches in diameter, differs from that kind known as camata, or camatena valonia, which are only about the size of a large cherry. The latter is said to be the fruit of a smaller species of quercus than that which affords the common valonia. It is in greater demand for silk-dyeing than for tanning.

Leather prepared with valonia is said to be harder and less permeable to water than that made with oak-bark; and, besides, it presents the advantage of readily depositing a rich bloom upon the leather, a characteristic much sought by the traders in this article. Not less than from ten to twelve thousand tons of this tanning agent are annually imported into Great Britain and Ireland, and it is stated that two pounds of good average quality are sufficient for making one pound of leather. The duty, which up to 1842 was twenty shillings per ton, was then reduced to five shillings, and entirely removed in 1845. Its price varies from ten to twenty pounds per ton, according to the stock and seasons

Myrobalans, the dried fruit of various species of terminalia, is extensively employed in tanning and dyeing factories. There are several kinds, all of which come, however, from the East Indies, by Calcutta and other ports. This sort of fruit when ripe is pear-shaped, deeply wrinkled, of brownish-yellow hue, and weighing from seventy to one hundred grains. The whole of the astringent matter which it yields is contained in the husk, which is easily separated from the inclosed nut by bruising the whole; besides the tannin, a yellow coloring matter with mucilage and other principles is extracted. The tannin from this source differs but slightly, according to Stenhouse, from that found in gall-nuts.

Divi-divi is an article which has acquired, within a comparatively modern date, an interest with tanners and commercial men. It may be classed with the foregoing, since it consists of the dried pods of a leguminous shrub—cæsalpina coriaria—which is indigenous to South America, and grows to the height of twenty and even thirty feet. The pods are about three inches long. Of a dark brown color, and curled up as if they had been submitted to a high temperature during desiccation. The whole of the tannin is concentrated in the rind of the pod, immediately beneath the epidermis, and has consequently a very astringent taste, but the inner portion that incloses the seed is very insipid. Besides tannin, it yields coloring matter, and a mucilaginous substance which interferes with its application in dying and printing. The leather prepared with divi-divi is very porous, and tinged brown of brownish-red, according to the density of the ooze, time allowed, and state of exposure to the air. Its formation is attributed to a fermentative change induced by some of the extracted matters; this change occurs as well in cold as in warm weather, but more frequently in the latter. During the reaction, a reddish matter deposits upon the leather in course of preparation, and on the sides of the pit. By preventing oxidation of some of the principles present, through the exclusion of atmospheric air, none of the forementioned substances appears, and the leather retains its natural color. A solution or extract of divi-divi, readily affords a deposit of bloom to leather. Stenhouse has shown that the tanning matter of divi-divi, though similar to that contained in galls, inasmuch as it exerts a like reaction on solutions of sesquisalts of iron, is, nevertheless, different from the latter, since it produced no pyrogallic acid when submitted to dry distillation.

Wood.—This material has not been converted to any useful ends so far as it immediately concerns tanning, notwithstanding, that tannin, as before stated, exists to some extent in many species.

Roots.—Several substances of this nature may be advantageously applied in tanning. The root of the common avens, geum urbanum, contains, as shown by Tromsdorff’s analysis, forty-one per cent of tannin. This plant is perennial, and is indigenous to Europe; it grows wild generally in damp and shady places. The root is about a quarter of an inch in thickness, has long descending fibres, and appears in alternated layers of red and white, the former predominating. It should be gathered in spring, and dried cautiously at an incipient heat 18. It has a slightly bitter and astringent taste, and, in the dry state, an odor not unlike cloves.

Ratany root is another substance very rich in tannin. Morfit describes this shrub—krameria triandria—as being indigenous to Peru, growing in mountainous districts, and flowering at all seasons. As found in commerce, it is in pieces of irregular shape and size, some not thicker than a pipe-stem 19, and others an inch in diameter, and two or three feet long. Water at 212° 20 dissolves the valuable principles, and the decoction, which has a deep-brown color, is abundantly precipitated by the mineral acids. The alkalis only change its color to that of urine. Sulphate of iron produced a black, and acids a fawn-colored precipitate. Gelatin proves the presence of tannin. The following analyses by Gmelin and Peschier indicate the composition centesimally:—

 

Ratany root

 

Gmelin

Peschier 21

Tannin,

38.3

42.6

Gallic acid,

—

0.3

Sweet matter,

6.7

—

Nitrogenous matter,

2.5

—

Mucilage,

8.3

—

Lignin,

43.3

—

Krameric acid,

—

.4

Gum-extractive and coloring matter,

—

56.0

Loss,

.9___

—___

 

100.0

100.0

Tournal has drawn attention to a perennial plant growing wild in the south of France, the dentillaria or leadwort—plumbago Europæa—on account of its contents of tannin. The same chemist has made reference to the statice or marsh rosmary, and suggested its being employed as a substitute for oak bark. The Kalmucs, it is stated, use a decoction of this plant and sour milk, for tanning the skins of sheep and goats so as to be available for clothing.

Barks.—This species of tanning material is by far more generally preferred in tanneries than any other, and that which ranks of highest importance for the purpose is oak bark. There are several kinds of this substance, varying more or less in their amount of tannic acid; these, however, come under two classes—the evergreen and the deciduous oak barks. Like most of those substances already described, the virtues of the oak and other barks are not disseminated equally in the several parts; indeed, in the rind of the common oak of these countries, there are portions in which no tanning or other valuable secretion is found. In examining the exterior covering of trees generally, a very great analogy will be observed between it and the skin of animals. For instance, the epidermis or outer thin, dry, and occasionally transparent layer of matter, has it analogue in that of the skin; and, like the latter, it is thrown off as the tree grows old and increases in size, owing to the formation of internal layers of other matter, which cause the epidermis to fracture and ultimately to fall off. The matter beneath the epidermis is a layer of cellular substance, which partakes more or less of the vitality of the tree. It is generally the seat of the green color, and, like the rete mucosum of animals, is being constantly dried up into the epidermis externally, whilst the secretion of herbaceous matter from the interior maintains it in the normal state. Beneath this is found the cortical tissue, of which, as the tree grows older, the woody matter is formed. In it are found all the valuable secretions of the tree, such as gum, sugar, resin, oil, and other matters which are so generally applied in medicine and the arts. It is constituted of a number of elongated cells, divided into layers by a variety of longitudinal woody fibres. This is more particularly the case with the inner portion of this layer, and which is called the liber from its organized appearance. It immediately overlays the real ligneous matter, or wood of the tree. In its general construction it resembles in a measure the last-mentioned, only that there are more longitudinal fibres, forming as it were leaves or laminæ, being reticulated and bound together by cellular substance. These layers may be separated by macerating the liber in water, which removes the cellulose, so that they appear like the pages of a book, and hence the term liber applied to it. It is this portion that is annually converted into wood, and which gives to the transverse section of the heart of the tree the annular appearance it presents, and which is generally termed the grain. Each of these rings represents a year’s growth of woody fibre.

As regards tannin, the parts of the cortex, or true bark, in which it is mostly contained, are the exterior layers of the portion known as the liber, and the interior of the cortical tissue—the inside portions of the former, and the most exterior of the latter, yielding very little of this principle. The same observation is true of other matters, such as quinine and the like. The various dyes are seated frequently in the exterior portion of the cortical tissue. The sap always ascends through the cellulose of the real bark; and as this fluid is the source from which tannin is secreted, it is evident that there will be more of it in the bark, when the flow is greater than at other periods. Experiments have proved this to be the case as regards oak, and the same observation applies to the barks of other trees, such as the willow, elm, pine, birch, beech, et cetera, with equal force.

In may be well to state here, as briefly as possible, the nature and peculiarities of the more important barks referred to at page 500, with a view of showing their adaptability for tanning. In this digest, considerable information has been obtained from Professor Morfit’s very copious work on tanning and currying leather. The bark of the cinnamon-tree, though yielding tannin, ha not yet been employed for the manufacture of leather. Larch and birch barks are resorted to in a very limited way, being in Great Britain and Ireland applied only for tanning bazils and other sheep-skin leather. In the larch-tree bark the tannin amounts to two per cent.; but in that of birch, Davy found 6.75 parts centesimally. Birch bark is very much employed in Russia for making the fancy red-colored leather, known in that country as jucten and other kinds, and the peculiar resisting powers, as well as its odor, are attributed to the assimilation of the oil which the bark contains. The American variety of the chestnut-tree bark yields four per cent. of tanning material, and the leather prepared from it is said to be more flexible than that made with oak bark. The bark of the Spanish chestnut contains, according to Davy, four per cent. of tannin. In the United States, the bark of the hemlock spruce, a species of fir—abies Canadensis—is much used for tanning as a substitute of oak bark. Morfit states that this tree, which grows to the height of seventy to eighty feet, constitutes three-fourths of the evergreen woods in Nova Scotia, New Brunswick, Maine, Vermont and part of New Hampshire; it is less common further South, though found in the middle of the Southern states on the Alleghanies. The bark of this tree is grey when young, but grows lighter when old, and is generally covered with moss. In June the barking is effected, and before being ground half of the epidermis is shaved off. Leather made with this bark is reddish, and is judged inferior to that yielded by oak bark; however, the two barks worked together are supposed to afford a better product than oak tan produces by itself. Davy found in the bark of the beech about three per cent. of tannin. Some varieties are employed by the tanner when oak bark happens to be scarce, but the leather from it is white and inferior. The same chemist obtained from the bark of the Lombardy poplar, 3.12 per cent.; from ash bark, 3.32; from elm bark, 2.7; from the white or European willow, 2.295; and from the Leicester willow, 6.86 per cent. of tanning principle. Lombardy poplar bark makes a light-brown colored leather, which acquires during the tanning a fragrant odor, somewhat resembling that which is given off by birch bark. Elm bark is not much used in these countries for making leather, but it is utilized in this way in Norway, and the remarkable beauty of the glove leather of that country is attributed to the mode of tanning, which is done with elm bark. Willow bark also is restricted to the tanners of Northern Europe, although it is worked up occasionally with oak bark in British and other tan-yards. The Danish and Schoonian leather, much in request for the manufacture of gloves, owing to its peculiar and agreeable smell, is prepared with the bark of the white willow; in Russia, the tanner uses this material occasionally, but impregnates the leather subsequently with the empyreumatic oil 22 of birch bark. The barks of all the cinchona species afford considerable quantities of tannin; still these are more valuable in pharmacy, owing to the alkaloids they yield, and, therefore, they are not resorted to for leather-making. Several other trees afford tannin, when their barks are submitted to the usual process, but they are rarely turned to on any account.

Oak Bark.—The bark of the oak, has been for a long period extensively employed in tanning skins, and if other materials have been used, it still happens that, in Great Britain and Ireland at least, these have not been resorted to by preference, but rather from the scarcity of the staple article, oak bark, which seems to part with the tannin under those circumstances which best avail for its combination with the tissue of the skin, and thus for converting the latter into an article of prime quality. There are several varieties of oak, known as well in Europe as in America, all of which secrete tannin in their bark; but those which are valuable to the tanner are the different varieties of  quercus robur, quercus coccifera, and sometimes the quercus suber, indigenous to Europe; and quercus falcata, quercus rubra, quercus tinctoria, and quercus prinus monticola, indigenous to the American soil. The bark of the quercus robur—which term is applied to designate a group of closely allied species or varieties, and of which the quercus pedunculata, and quercus sessiliflora, form the two principal—is generally preferred by the tanner, with the exception of Norway and the North of Russia and some districts of France. In Norway, as already remarked, the birch and willow are resorted to, and in Russia and France the bark of other species of oak, the quercus glomerata, and quercus coccifera, are occasionally substituted. The latter variety, known also as the kermes oak, is a tortuous branching shrub inhabiting the South of France, Portugal and Spain. It grows to the height of three or four feet in close clumps, the roots interlacing one another, so that the soil, which might otherwise be washed away by the heavy rains, is retained. The bark of the root of this shrub, which is sometimes called coppice oak, is of a yellowish brown hue, and very rich in tannin. It is much in quest in France for tanning sole leather of a superior quality.

Barking of Trees.—In the foregoing pages are given the varied natural products which have been more or less employed, or offer advantages for tanning. The bark of the oak is, however, the most extensively consumed, and, therefore, much care is paid to its collection and harvesting. For a long time it was supposed that the rind of old oaks was more valuable for tanning than the product from younger woo; but experience is rectifying, or has done so, this mistake, for tanners have by a long course of working ascertained with certainty, what, through the labours of Sir Humphrey Davy and others, has long been on record, that the bark of young trees is richer in tanning principle, than that of old ones; and, not only does the bark of the former offer this advantage, but the leather prepared with it is softer and whiter than what old bark produces, owing to the amount of coloring and extractive matter which they yield. Doubtless, the best age at which the trees should be barked, is from eighteen to twenty-five, or thirty years; but owning to the importance of the timber for building and other purposes, rarely are trees of this age felled in England or the European countries. In France, however, they harvest the bark of oak of this age, but the wood is not turned to further use, excepting for the manufacture of charcoal 23, owing to the variety being unadapted for the builder. In France, too, the advantage may be gained of collecting the bark in spring, at a time when the sap is in full flow, and when there is most tannin contained in it; but in other countries, where the bark of the tree constitutes only an inferior secondary product compared with the wood, this season is not chosen, in consequence of the timber being cut when the sap is in an active circulation and thus liable to decay. In England, Holland, and America, the period for stripping the bark is about June; but it varies according to the mildness of the winter and the spring. The operation of barking, is performed by cutting two circular bands round the trunk, at a distance of two, three or more feet, then a longitudinal strip from one band to the other, loosening the bark at the upper , and stripping it off in bands towards the lower end. When stripped, the pieces are spread out to dry in beds, a shady site being preferred, care being taken to turn them occasionally, so as to prevent the beds heating. If the harvesting of the bark is not strictly attended to, considerable injury is sustained, not only in the loss of tannin, but more so in the kind of leather it affords in the pits. For the most part, or indeed in all cases, the bark should rest on hurdles elevated more or less from the ground in an inclined state, and the fragments ought never to be heaped together more than from twelve to eighteen inches in thickness, Provision ought to be made to protect the bark from the rain, and the whole, should be turned at least once a week till quite dry. After this it is usually stacked in larger rectangular heaps, and protected by thatch, if not by a roof, from rain or wet.

Quality of Barks.—It is important, that in regard to this article of so much value to the tanner, he should have some means of ascertaining its quality. By physical characters, the man of practice and experience will often form a good opinion in this respect, so as to decide between a good and bad quality of the same kind of bark; but this will not enable him to tell what amount of tannin it contains. Thus, if the bark has a whitish color exteriorly, and a reddish interiorly, and combines with these a dry, fragile, and clean fracture, and an astringent taste, the bark has been well harvested, and other properties being taken for granted, it is of good quality. So also when the epidermis and liber are thick, dry, and ligneous, with large crevices and a blackish aspect, it is deemed of an inferior quality, for the former characteristics indicate age, and the latter, that it has undergone a heating or fermenting change. There are other indications also from which persons acquainted with tanning materials form a judgement as to its goodness; it happens, however, that there are exceptions, so that an article presenting one or more of these appearances, may be of first-rate quality, and make good leather; but, under any circumstance, they will not lead the purchaser to the knowledge of the per centage of real tannin they yield. To attain to this end, a determination of the tannin must be made by chemical means, and even then the results are often only approximative.

Estimation of Tannin in Barks, et cetera.—Several methods have been devised for estimating the amount of tannin in barks and other products which are used in tanning, most of which are founded on the behaviour of tannic acid with gelatine, and similar bodies; but mention will be made here only of those of Davy, Stephens, and latterly, Warington. The process of Davy consists in treating three hundred and sixty grains of the powdered bark with a pint of boiling water, the vessel being frequently agitated during the digestion, which ought to last twenty-four hours. After filtering off the extract, it should be mixed with an equal volume of solution of jelly or isinglass, prepared by dissolving sixty grains of either of those substances in a pint of hot water. The precipitate of tanno-gelatin is the collected on a tared filter, exsiccated 24 and weighed, and four-tenths of the entire taken as the proportion of tannin contained in it. Modern research has shown, however, that it is almost impossible to separate the precipitate by filtration from the liquid, so that at best this method is tedious; but it is inaccurate, inasmuch as the first portions which fall contain about fifty per cent. of tannin, which diminishes towards the end of the saturation. Sulphate of quinine, acidulated with a few drops of sulphuric acid, may be employed for precipitating the tannin from its solutions completely, and the deposit, unlike the gelatin compound, is constant in its composition. The tanner, however, in his yard, bases his operations on the indications of what is called the barkometer—an instrument like a hydrometer—and according to the extent to which it sinks in the extract, so is the strength of the ooze, or tan-liquor estimated.

Stephens, to avoid the error to which Davy’s method is liable, proposed to estimate the tan by combining it directly with the cortical tissue, as in tanning, and determine the amount of the acquired weight. For this purpose, strips of the best ox hides, shaved as thinly as possible, are washed in water, thoroughly dried and weighed, then soaked in water as second time till they become soft and porous, and immersed in the extract of the example of tan under examination, at a temperature of 90°. In the course of eight or ten hours, the whole of the tannin will have combined with the skin, so that after this period, by abstracting the strips, drying and weighing, noting the increase as the amount of tannin in the sample operated upon, the per centage in the bark may be calculated. This mode is likewise tedious, and fails to give satisfactory results.

Warington proposes to estimate the tannin, not only in barks, but in all other astringent substances volumetrically, by determining what volume of a standard solution of gelatine is required to precipitate the tannin from the extract prepared from the sample submitted to examination. In preparing the test solution, the above chemist recommends the long staple isinglass, as that is the most constant in its quality, and the least liable to undergo change. The solution is made of such a density, that a degree on the burette will represent one-tenth or one-fourth of a grain of tannin. Besides the preparation of the test liquor, the only other novelty in the operation, is the method employed for filtering off portions of the menstruum in order to determine whether the whole of the tannin be precipitated. As the passing of the liquor through bibulous paper, would not afford a clear filtrate, a piece of glass tubing, a foot in length, and half and inch in internal diameter, was selected; into the lower extremity a piece of wet sponge was introduced, and when it was desired to abstract a portion of clear liquid from the assay, in order to test if further precipitation took place, the sponge end of the tube was submerged, and the fluid filtered by ascending through the sponge. Part of the clear menstruum thus obtained, was transferred to a test tube, and a drop of the test solution added, till the point of saturation had been accurately arrived at. By this method, which is as applicable to the substances rich in tannin as to bark, accuracy and expedition are secured.

The annexed table from Morfitt, indicates the per centage of tannic acid contained in the various substances specified:—

 

table of the average quantity of tannin in different substances

Substance

Per centage of tannin

Authority

Catechu—Bombay,

55.0

Davy.

Catechu—Bengal,

44.0

Davy.

Ratany root,

42.6

Peschier.

Ratany root,

38.3

C. G. Gmelin.

Kino—tannin and extractive

75.0

Vauquelin.

Butea gum,

73.2

E. Solly.

Nut-galls—Aleppo,

65.0

Guibort.

Nut-galls—Chinese,

69.0

Bley.

Nut-galls—Istrian,

24.0

Roder.

Old oak—white inner bark,

21.0

Cadet de Gassincourt

Old oak—white inner bark,

14.2

Davy.

Young oak—white inner bark,

15.2

Davy.

Young oak—colored or middle bark,

4.0

Davy.

Young oak—entire bark,

6.0

Davy and Geiger.

Young oak—spring-cut bark,

22.0

Davy and Geiger.

Oak kermes—bark of the root,

8.9

Davy and Geiger.

Terra japonica, or gambir,

40.0

Esenbeck.

Avens root—geum urbanum.

41.0

Tromsdorff.

Squill—bulb,

24.0

Vogel.

Statice of South Carolina,

12.4

Parrish.

Birch bark,

1.6

Davy.

Birch bark,

1.4

Biggers.

Beech bark,

2.0

Davy.

Larch bark,

1.6

Davy.

Hazel bark,

3.0

Davy.

Chestnut, American rose,

8.0

Cadet de Gassincourt

Chestnut, Carolina,

6.0

Cadet de Gassincourt

Chestnut, French,

4.0

Julia de Fontenelle.

Chestnut, Spanish—white inner bark,

1.3

Davy.

Chestnut, Spanish—colored or middle bark,

0.3

Davy.

Chestnut, Spanish—entire bark,

0.5

Davy.

Chestnut, horse,

2.0

Julia de Fontenelle.

Lombardy poplar,

3.5

Julia de Fontenelle

Blackthorn,

3.3

Davy.

Ash bark,

3.3

Davy.

Sassafras—bark of the root,

58.0

Reinsch 25.

Elm bark,

2.9

Davy.

Sumac, Sicily,

16.2

Davy.

Sumac, Malaga,

16.4

Davy.

Sumac, Malaga,

10.4

Franck.

Sumac, Carolina,

5.

Cadet de Gassincourt.

Sumac, Virginia,

10.0

Cadet de Gassincourt.

Willow, Leicester—white inner bark,

16.0

Davy.

Willow, Leicester—colored, or middle bark,

3.0

Davy.

Willow, Leicester—entire bark,

6.8

Davy.

Willow, Leicester—bark of the trunk,

1.4

Biggers.

Willow, weeping,

16.0

Cadet de Gassincourt.

Sycamore bark,

16.0

Cadet de Gassincourt.

Sycamore bark,

1.4

Biggers.

Elder,

2.3

Davy.

Plum-tree,

1.6

Biggers.

Cherry-tree,

24.0

Cadet de Gassincourt.

Cherry-tree, Cornish,

19.0

Cadet de Gassincourt.

Tormentil root,

46.0

Cadet de Gassincourt.

Cornus sanguinea of Canada,

44.0

Cadet de Gassincourt.

Alder bark,

36.0

Cadet de Gassincourt.

Apricot,

32.0

Cadet de Gassincourt.

Pomegranate,

32.0

Cadet de Gassincourt.

Bohemian olive,

14.0

Cadet de Gassincourt.

Tan shrub with myrtle leaves, bark of,

13.0

Cadet de Gassincourt.

Service-tree bare—June berry,

18.0

Cadet de Gassincourt.

Cloves,

15.0

Davy.

Winter’s bark,

9.0

Henry.

Next Page: Bark mills.

Commentary

1. ^    This is really a Middle English word, derived from Mediaeval Latin (OED). The modern one would be vegetable. The text includes several examples of Latin influence such as the use of the Latin-based spelling of color instead of the usual French-based one: colour. On the other hand, it is possible to detect the influence of the French sources which Muspratt has used.

2. ^   Clicking on the hyperlinked word in the first column of each row of the table will take you to Muspratt’s text dealing with that subject.

3. ^   Thickened or condensed (OED).

4. ^   I have not transcribed Muspratt’s internal references.

5. ^   There were two botanist brothers of this name: Christian Gottfried Daniel (1776-1858) and Theodor Friedrich Ludwig (1787-1837).

6. ^   From the way he expresses it, Muspratt probably has the Malacca Strait in mind.

7. ^   Actually, in Burma.

8. ^   Catechin.

9. ^   I am afraid I have not been able to track him down.

10. ^ The Riau Islands are now part of Indonesia and lie off the southern end of the Malay Peninsula, some of them just across the Singapore Strait from Singapore. Map. In 1819, they formed part of Stamford Raffles’ diplomatic dealings.

11. ^  Maceration is a softening and breaking down of a body by soaking in a liquid. It is used in several operations but here it will mean soaking until the tannin comes out into the water.

12. ^ Muspratt writes of bloom as something desirable. In more modern thought, it seems less so. See James Hewit & Sons’ web site.

13. ^  Smyrna is the Greek name of what is now the Turkish city of İzmir.

14. ^ The only possibility I have traced is the very early, Johan Franck. His dates are out of keeping with those of Muspratt’s other sources.

15. ^  The English Wikipedia article is very brief. The French one is better. See a picture of a morocco bound book.

16. ^ This is probably Priego de Córdoba.

17. ^  The colour of cloth which as been neither dyed nor bleached; i. e. light brown or yellowish brown. The colour of new hempen cloth (OED).

18. ^ A little above room temperature.

19. ^ In 1859, this will have meant the stem of a clay tobacco pipe. Briar pipes became general later.

20. ^ 212° is the normal boiling point on the Fahrenheit scale.

21. ^ Peschier’s figures amount to only 99.3%. Presumably one must add 0.7 in the loss row.

22. ^ Oil bearing the burnt smell imparted by fire (OED). It is a concept which really belongs to Alchemy, in the form of the philosophers’ oil.

23. ^ Charcoal itself, was a very useful commodity. It formed part of gunpowder, was much in demand for smelting iron and at some times and places, was used for cooking and heating. It is almost fume-free and free of chemical content which would spoil chemical processes such as smelting.

24. ^ Dried up (OED). For example, exsiccation is part of the process of mummification.

25. ^ The German chemist, H. Reinsch is mentioned by French Wikipedia, in connection with pyrocatéchol (catechol).

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