Bourne Archive: Muspratt: Tannin

http://boar.org.uk/aaiwxw3MusprattL2Tannin.htm        Latest edit 27 Dec 2010


 

The Bourne Archive

 


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


Extracts Concerning Leather, 2: Materials Used in Tanning: 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.


Commentary

By 1859, Chemistry was a study conducted on scientific principles but it is clear from this text that it had still some way to go. It had evolved from its ancestor, Alchemy but appears to have remained as much a mystery as a science. For this reason, Muspratt is often unable to say that this or that is so: he has to write around the evidence. His writing can therefore be heavy going and seem long-winded. Keeping track of what he says is helped by having pictures of the molecules of tannic acid and gallic acid respectively – something he would have loved to have and which is available to us in Wikipedia. See Tannic acid and Gallic acid. RJP.


Vol. 2. pp. 493-508

Tanning Materials.—The second principal component of leather, such as that manufactured into boots and shoes, et cetera, is tannin or tannic acid, a substance of vegetal origin, and very extensively disseminated in plants and trees. For a long time the agent which gave such virtues to oak bark, in converting prepared skins into leather, was little known, and consequently, the peculiar relation of the proximate principles of this article could not be understood, notwithstanding that, in the absence of this knowledge, the leather manufactured was sometimes of prime quality. Lewis was the first who drew attention scientifically to the subject. He observed, during an investigation of the nut-galls, that they contained a substance which gave a black compound with a sesquisalt1 of iron and precipitated isinglass. Deyeux 2, who also studied the point, noticed this substance, but considered it a peculiar resinous matter. It was not, however, until Seguin 3 published the results of his labors, that any definite account of it was given. This chemist showed that the unknown body differed form the acid of galls—gallic acid; that it combined with animal matters, more particularly albumen and gelatine, and formed with them unalterable bodies, the analogues of which constituted the main bulk of leather. Since then, manufacturers have given great attention to this principle in the tanning material, and the consequences are that a grater variety of substances producing tannin, available for making leather, have been discovered, and that considerable expedition in the process has been attained. Although the forementioned chemists, with Davy, Chevreul, Pelletier, Legrange, Guillot, Hatchet 4, Tromsdorff, Richter 5, and Proust 6, may be justly said to have given a scientific exposition of tannic acid and its behaviour, and likewise pointed out its quantity in those plants that yield it most abundantly—still several more recent investigators  have contributed to the stock of information on this subject. According to Wahnlenberg 10, tannin exists only in perennial plants, and almost solely in the permanent parts. For example:—1. In the perennial roots of certain annual plants, as the septfoil and bistort or snakeweed. 2. In the bark of the trunks and roots of nearly all perennial trees, especially in the true cortical portion. 3. In the trunk and sap of many trees. 4. In the leaves of perennial plants—in small quantities. 5. In the leaves of different varieties of oak; of the rhus coriaria—sumac; of the arbutus uva ursi—bearberry; of the arbutus unedo—strawberry, et cetera, in large proportions. 6. In the capsule of the unripe fruit of the quercus suber ægilops—valonia oak; juglans regia—walnut; æsculus hippocastranus—horse chestnut; and terminalia chebula. 7, In the pulp of the fruit of the punica granatum—pomegranate; rosa pimpinellifolia; garcinia mangostana —mangosteen; and in the different species of anona.

Of the principal tanning materials imported into England, the following table exhibits the names, although a considerable quantity of oak bark—quercus pedunculata—is obtained in this country:—

Common title

Botanical name

Whence imported

Oak bark

Quercus pedunculata

Flanders, et cetera

Larch bark

Pinus larix

Scotland

Mimosa bark

Acacia sp.

New South Wales

Babool bark

Acacia abarica

Bengal

Cork-tree bark

Quercus suber

Laruch, Rabat, et cetera

Hemlock bark

Abies Canadensis

United States

Sumach

Rhus coriaria

Sicily

Valonia

Quercus ægilops

Smyrna, Trieste, Morea.

Divi divi

Cæsalpina coriaria

Maracaibo, Rio de la Hache, Savanilla

Myrobalans

Terminalia sp.

Bengal

Algarobilla

Prosopis pallida

Valparaiso

Terra japonica

Acacia catechu

East Indies

Cutch

Acacia catechu

Calcutta, Singapore

Kassu

Areca catechu

Ceylon

Besides the foregoing, which are partly well known in the trade, other astringent substances have been brought before the public, to be employed per se, as tanning materials, or with oak bark as an adjunct. These are various extracts from species of the acacia tribe: gambier, the extract of nuclea gambier, from Singapore; mangrove bark—rhizopora mangle—from Arracan, Malabar, and Singapore; turwar or cassia bark; avaraputtai, Tangada jigota—cassia auriculata—from Vazigapatam; saracondraputtai—cassia fistula—from Madeira and Tinnevelly; saul-tree bark from the Saul forests; pomegranate rind; darunka pucke—dadima jegota—from Kemaon and Vizagapatam; dalumka khola from the vicinity of Calcutta; jamoon-bark—eugenia jambolana—and pearl-bark, from Cuttack, samak-bark from Singapore, vangay or gum-kino—pterocarpus dalbergoides—from Malabar; kino or astringent extract of the Buchanania latifolia, from the district of Chota, Nagpore; majoophul or gall-nuts—ficus infectoria—from Nagore, et cetera; tamarisk call—tamarix indica—from Bombay and Lahore; several kinds of myrobalans; gaub, the fruit of the diospyros glutinosa, from Calcutta; hag-plum tree—spondias lutea—abundant on the banks of the river Berbice, and commonly used in British Guinea for tanning; courida bark—avicennia nitida—a tree very common on the Eastern coast of Demerara, et cetera.

Morfit remarks:—1. That tannin is never found to any extent on the interior of the trunk of trees. 2. That it does not exist in poisonous plants, nor in those with milky or viscid sap. 3. That its proportion is greater in young than in old plants. 4. That the tannin is converted to bitter principle, as the plant increases in age. 5. That it is most abundant in the cortical layers of the bark, and is usually absent altogether in the epidermis. 6. That the proportion of tannin in bark varies with the season, decreasing as the severity of the winter increases; and, 7. That the two extremes of quantity are attained in winter and spring.

These remarks admit of an important practical application with reference to the most suitable time for the barking of trees, et cetera. But before discussing this subject, it will be necessary to enter more minutely into the nature of the tanning principle.

Tannic Acid or Tannin.—By examining any of the numerous substances mentioned above, it will be found that they yield a product which is capable of precipitating a solution of gelatine, and also of giving a greenish of bluish-black one, with solutions of sesquisalts of iron. To matter exhibiting this reaction, and having an astringent taste, the title of tannin was given, irrespective of whatever difference might be exhibited upon further comparison. Now, many of those bodies, which in common with the true type of tannin—that which is extracted from galls—precipitate gelatine, and so far show that they are eligible for tanning materials, are known to differ in their characteristics; for instance, some possess acid properties, whilst others are devoid of them; and although towards gelatine their actions are similar, their atomic construction is widely different. Regarding their effect in converting skin into leather, this dissimilarity is not so very material as when the components of each are compared. A distinction is also made with respect to the physical appearance of the precipitate they afford with a sesquisalt of iron, and by this all the known tanning materials are divided into two classes. In the first of these are placed gall-nuts and the barks of the several oaks, and the second includes the quinquina, the cachou, gum-kino, pine barks, et cetera. The tannin afforded by the first throws down a bluish-black or purple combination, whilst the precipitate afforded by the second is greenish. Experiments have shown that such distinctions are not very marked, since the tannic acid from nut-galls of oak bark, in the presence of an alkali, will occasion a greenish when added to a sesquisalt of iron, and a bluish or violet compound when acids are present. Hatchett first observed that many plants contain a carbonaceous principle, which, when acted upon by nitric and sulphuric acids, according to the method of Chevreul, yield a substance which, like the tannins of the foregoing barks and fruit, throws down gelatine, and tenders it imputrescible. This variety has been designated artificial tannin; but, in the present state of chemical science, it could have been compounded with the natural products. Berzelius, Geiger, Waltl, and Cavallius, are of opinion that tannin, from whatever source it may be obtained, is identical, and that the difference of the reactions with the iron salt must be attributed to impurities which are accidentally present. Stenhouse, in his more recent researches, has shown that such is not the case, but that all those forming precipitates with sesquioxide of iron, varying from the color of that which the tannin of nut-galls affords, must be regarded as a distinct substance, in which case there seems to be an almost endless variety. Even when the iron reactions of some tannins agree, and also appear analogous in other chemical relations, the same chemists states that the composition is frequently diverse. Hence he distinguishes the various kinds of tannin by the source; thus, that obtained from the nut-galls he calls from gallotannic acid; from oak-bark, quercitannic acid; fustic—morus tinctoria—moritannic acid, and the like.

Preparation of Tannic Acid.—It is very difficult to prepare tannic acid of absolute purity, so much so that none of the methods in use for its extraction can afford a product free from foreign matters. When extracting tannin on the small scale, nut-galls are selected, as they yield the largest product. A quantity of these is dried and reduced to powder, and subsequently exhausted of the tannin with water by moistening with this liquid, the subjecting to pressure. The extract thus obtained, if employed with fresh portions of the powdered gall successively, becomes highly saturated with tannic acid. Filtration cannot itself clarify the opaque inspissated liquid; but on mixing it with a little dilute sulphuric acid, and agitating, a precipitate of the suspended matter forms, and may be removed by filtering it through linen or paper. To the clarified liquid a solution of carbonate of potassa is cautiously added, till it almost ceases to deposit any matter from the menstruum. The crude tannate of potassa thus obtained is separated from the liquid, washed with ice-cold water, or with water at as low a temperature as possible, and dissolved in boiling dilute acetic acid. On cooling the latter, a brownish deposit containing acetic acid parts from it, and must be removed by passing the liquid through the filter. A solution of subacetate of lead is now added to the filtrate, and the resulting tannate of lead, removed and washed thoroughly, then disseminated in water, and a current of sulphide of hydrogen passed through till the whole of the metallic base is converted into sulphide. By evaporating the liquid after the separation of the sulphide of lead in the hollow of a flask, containing some carbonate of potassa, the tannin is procured in the form of gummy transparent scales of a yellowish color. In this state, however, the tannin is still impregnated wit a little gallic acid and brownish matter, which may be removed by treating the powdered substance with successive portions of Ether. The tannic acid is thus left in combination with the brownish matter; but by repetition of the process, this impurity, as well as the gallic acid, is dissolved out, and the tannic acid remains nearly colorless.

This substance may likewise be prepared by treating the clarified extract of gall-nuts, as above directed with ammonia till almost neutralized, and to the menstruum chloride of barium is added till it ceases to throw down a precipitate. The flask is then filled with water, and allowed to repose till the whole of the tannate of baryta has deposited. Any gallic acid which may have been formed, remains combined with the base in the supernatant liquid, and may, consequently, be removed by decantation and washing. During these operations, a small proportion becomes decomposed and colors the baryta salt greyish; on treating the latter with acetic acid, however, this impurity remains whilst the tannate of baryta is dissolved. To the liquid thus obtained, subacetate of lead is added, and the plumbous salt proceeded with, as directed above. When the substance is to be obtained in its greatest state of purity, recourse must be had to the process known as that of Pelouze, but which was first broached by Laubert. The latter chemist directed the formation of an extract of sixty grains of powdered nut-galls in one hundred and twenty of ether, the digestion to be continued during twenty-four hours, after which the ethereal extract was to be filtered and evaporated. Pelouze followed the foregoing method in principle by employing the apparatus of Robiquet and Boutron, as used for the preparation of amygdalin. Fig. 341 represents this digester. It is an elongated glass vessel, A having an orifice at the top, which is fitted with a ground-glass stopper, and connecting towards the other extremity, which fits tightly into the neck of a bottle or mattrass, B, which receives the extract. Sometimes the lower bottle has a second tubular opening for the purpose of receiving a cork furnished with a tube, to which a connector, C, of caoutchouc is appended, the other end being attached to a similar tube fitted in cork which closes the top orifice. This adaptation causes the filtration to take place much more readily, whilst it prevents contact with any further quantity of air beyond what is contained in the apparatus. The further process is such as already detailed.—See Vol. I. page 581. By careful evaporation—the better if under the exhausted receiver—the tannic acid is obtained in the form of a spongy iridescent, uncrystallizable mass, having a light straw-yellow color. It may be washed with repeated quantities of ether to remove any remaining gallic acid, after which treatment it may be desiccated as previously stated. So prepared, tannic acid is in its purest known state; but as sometimes it results almost colorless, doubtless it is not free from impregnations arising from decomposed tannin—apothema—or some other matter.

Another modification of the method just detailed was introduced by Dominé, and offers the advantage of yielding a larger product, while it admits also of being applied on the large scale for purposes of the manufacturer. He places the powdered galls in a damp cellar for several days, during which they absorb moisture; the powder is next transferred to a wide-mouthed jar, and made into a paste with ether—specific gravity 0.75—after this the vessel is hermetically close, and the contents allowed to digest for twenty-four hours. At the expiration of this time the pasty contents are transferred to a strong linen bag and subjected to gradual pressure, when the ethereal extract of tannin, having a dark sirupy consistence, flows off into the receiver. This liquid must be evaporated to dryness at a gentle heat—100° to 110°—by which the tannic acid is left in the form of light-colored resin-like scales. The compressed residue is further treated with ether, to which six per cent. of water is added in the same manner as when preparing for the first extract, and, on expelling the fluid by a gentle heat, a residue of tannin is obtained. It is, indeed, more impure than the product resulting from the method of Pelouze, as it contains portions of chlorophyl,  volatile oil, and gallic acid; still the tannin thus prepared answers very well for medicinal and other uses, for which it is applied in the arts.

Properties of Tannic Acid.—In its purity, tannic acid is colorless, in odorous, possesses a highly astringent taste, and is very soluble in water. It is, however, devoid of all bitterness, notwithstanding its astringency is so powerful on testing the aqueous solution with litmus, the reaction evidences the acid nature of the tannin; and further, by treatment with an alkaline carbonate, carbonic acid is freely displaced, showing that a combination of the alkaline base and the tannin has taken place. With the oxides of the heavy metals it in a great measure combines, giving rise to precipitates which are the tannates of the respective bases, and generally possess a color more or less remarkable. Thus an infusion of nut-galls reacts in the following manner:—

With a neutral solution of salts of

 

Precipitates given

 

Brandie.

Dumas.

Protoxide of manganese,

Dirty-yellow

Protoxide of iron,

Purple tint

Sesquioxide of iron,

Black

Blue-black

Oxide of zinc,

Dirty-yellow

Oxide of tin,

Straw-yellow

Yellowish

Binoxide,

Fawn-yellow

Yellowish

Oxide of cadmium,

Oxide of copper—protochloride,

Yellow-brown

Grey

Oxide of copper—protonitrate,

Green

Grey

Oxide of lead,

Dingy-yellow

White

Teroxide of antimony—from emetic tartar,

Straw-yellow

White

Teroxide of bismuth,

Yellow

Orange

Oxide of cobalt,

Yellowish-white

Salts of cerium,

Yellowish

Salts of titanium,

Blood-color

Salts of tellurium,

Isabelle-yellow

Salts of chromium,

Brown

Salts of tantalum,

Orange

Salts of molybdenum,

Brown

Salts of uranium,

Brownish-red

Salts of silver,

Pale-yellow

Salts of platinum,

Greenish-shade

Salts of gold,

Brown

Salts of osmium

Bluish-purple

In making the foregoing experiments, it is necessary to employ neutral solutions of the salts, otherwise any excess of the acid may not only alter the shade of the precipitate, but prevent it altogether. This is especially the case with salts of iron and titanium. When a solution of tannic acid is added to another of protosulphate of iron, according to Dumas, no precipitate occurs; but if one of the sesquisulphate be substituted, there is an abundant deposit of a blue-black color. By exposing an aqueous solution of tannic acid to the air, at a moderately elevated degree of heat, oxygen is freely absorbed, and an equal volume of carbonic acid is disengaged. Upon examination, the matter in solution will be found to be wholly transformed into two other substances, gallic and ellagic acids, the former of which is in excess, and is dissolved in the liquid, whilst the latter remains insoluble in the form of a yellowish powder.

This transformation takes place in the aqueous extract of gall-nuts much more readily than in one of pure tannin; this is particularly the case if the marcs 7 or insoluble residue of the galls be left in contact with the liquid, as it induces a fermentative action, which brings about the transformation the more readily. When boiled with sulphuric or hydrochloric acid, gallic acid is also formed, together with grape sugar, which, as Strecker states, remains in the mother liquid after the newly-formed acid is removed by crystallization. The change here induced may be represented thus, according to Miller:—

C24 H32 O34

+

10 HO

=

3 (C14 H3 P7, 3 HO)

+

C12 H12. 2aq.

Gallotannic acid

 

Water

 

Gallic acid

 

Glucose

A concentrated solution of caustic potassa as the boiling point has also the  effect of transforming tannic into gallic acid; and if air or oxygen be admitted, of converting the latter into ulmin-like substance. Hydrochloric, nitric, phosphoric, and arsenic acids, occasion in strong solutions of tannic acid white precipitates, which are, as stated by Berzelius, compounds of the tannin with those acids respectively. All these precipitates are, however, readily decomposed, and their tannin changed into gallic acid, by seething them with dilute sulphuric acid or with an excess of alkali. If a cold aqueous solution of tannin be mixed with another of gelatine, isinglass, bone-size, or glue in excess, an opaque white precipitate is obtained of tanno-gelatin, that is soluble by the aid of heat in supernatant fluid. When, however, the conditions are reversed, and an excess of tannin is taken, it happens that instead of dissolving, the precipitate agglutinates, and forms a membranous matter of a greyish hue, that possesses considerable elasticity, and thus approximates to caoutchouc. The solutions filtered from these bodies strongly tinge those of the sesquisalts of iron. It appears that tannin solutions react in a similar way with protein compounds in general.

It was thought for a long time that the insolubility of the tanno-gelatin offered a means for ensuring the purity of tannic acid, and of detecting the absence or presence of gallic acid in the sources affording tannin; but this insolubility, excepting in water alone, does not answer the purpose. Pelouze employed a piece of skin, cleaned in the ordinary way from hair and epidermis; this was immersed for some hours in the extract, and the whole agitated occasionally, and finally the liquid filtered. When pure tannic acid was operated upon, the whole combined with the integument, so that the fluid had no astringent taste, nor did it color a solution of a sesquisalt of iron; but if gallic acid had been mixed with it, the bluish-black tint would present itself in the latter case.

Freshly precipitated alumina, agitated with a solution of tannin, absorbs the latter rapidly, and forms with it an insoluble composition; gallic acid behaves in a similar manner. At the temperature of an oil bath, tannic acid is decomposed, water and carbonic acid being formed; but when the heat is increased, other products are generated, and nothing remains but charcoal, which, if heated on platinum wire, burns, leaving no residue. Strong ether dissolves tannin, and forms a sirupy liquid, which does not combine with fresh ether. Alcohol behaves with tannin in the same way, the solvent power of the spirit decreasing in reference to it as it approaches purity or dehydration.

From the analysis of Berzelius, it appeared that tannin was composed centesimally as follows:—

 

Berzelius

Pelouze

Carbon

52.69

51.40

Hydrogen

3.86

3.51

Oxygen

43.45

45.09

 

100.00

100.00

These numbers nearly correspond with the formulæ, C18 H8 O12 = C18 H5 O9. 3 HO; but Strecker in his later researches gives the equation for gallo-tannic acid thus, C54 H22 O34, showing that it contains less oxygen and hydrogen that is indicated above.

Varieties of Tannin.—As already stated, recent investigations have a tendency to the conclusion that tannin from the various vegetal products manifests such differences in its reactions with several substances, as to justify the recognition of many varieties of this compound, though classed under the above generic name. The impurity of this body was long considered to affect its behaviour with reagents as to vary the reaction; but, latterly, Stenhouse and others have arrived at other conclusions, and regard the tannins from the different sources, and which give unlike reactions with reagents, as distinct chemical bodies. Doubtless, the other extractive matters in the solutions of tannin modify to a great extent the color of the precipitate or other characteristics, from those afforded by the pure tannin, but all the difference cannot be attributed to this cause. Wit gelatine many of the tannins give well-marked indications, which would go far to show the dissimilarity in the nature of the tannin; yet it is upon their behaviour wit a solution of sesquisalt of iron that the discrimination is based. To ascertain this point, Stenhouse prepared a reagent which affords indications only with the variety of tannin contained in galls and with gallic acid, and which consists in the production of pyrogallic acid when the body to be tested is submitted to the destructive distillation. In this way he observed that many varieties, which comported themselves exactly similar with sulphate of iron and in their general reactions, are by no means identical. Taking the behaviour with solutions of sesquisalts of iron as the criterion, the following varieties are detected:—

1.                  Tannin which forms a bluish-black precipitate in solutions of sesquisalt iron:—Tannin from nut-galls, oak, poplar, beech, chestnut, maple, cherry, apricot, plum, elder,, and other barks; sumach and logwood; from the roots of lithrum salicana, iris pseudacorus, geum urbanum; polygonum bistortum—bistort; alchemilla vulgaris—ladies’ mantle; from the leaves of the arbutus uva ursi—bearberry; amothera coriaria; from lentils, et cetera.

2.                Tannin which forms a green precipitate in solutions of sesquisalts of iron:—Tannin from catechu—juice of the mimosa catechu; gum-kino—juice of the coccoloba uvifera; most of the cinchona barks; horse-chestnut bark; the roots of the rumex aquaticus—water dock; tormentilla erecta—septfoil; krameria triandria—ratany; polypodium filex mas—male fern; tea and badian; a species of anise; the leaves of salvia officinalis—sage; and of arnica montana—leopard’s bane, and in date-stones.—Morfit.

Artificial Tannin.—Besides the foregoing, chemists distinguish other varieties of tannin which result from the chemical reaction of several powerful agents upon many substances; as for instance, from the action of nitric acid upon charcoal and indigo, and likewise from the effects of sulphuric acid upon resins and camphor. The first notice of artificial tannin was published by Hatchett in the Transactions of the Royal Society in 1801: he discovered it whilst making experiments upon the slow carbonization of plants. This tannin is different, however, from other varieties of this substance at present known. When one part of charcoal, reduced to an impalpable powder, is digested with five parts of nitric acid of spec. grav. 1.40, and ten of water at a moderate heat, the fluid being added portionwise to the charcoal, the latter after a time disappears, and a transparent deep-brown liquor remains, which, on evaporation, leaves the artificial tannin slightly impregnated with acid. By repeated solutions in water, and evaporations, this impurity may be removed. From the above materials about 1.2 if artificial tannin is obtained.

Artificial tannin prepared in this way is a brown, inodorous substance, having a vitreous fracture, and very soluble in water. Thomson states that alcohol of spec. grav. 0.80 dissolves it, and so far it manifests a difference of behaviour from natural tannin. The aqueous solution of this substance precipitates gelatine of a brown color, but the shade deepens as the liquids are more dense. This precipitate is a compound of tannin and gelatine, in the proportion of thirty-six parts of the former to sixty-four of the latter. It is insoluble both in hot and cold water. With the alkaline bases it combines, forming more highly-colored solutions, which, in the course of some time, get cloudy. The ammoniacal solution of artificial tannin, on being evaporated to dryness and redissolved, will not precipitate with gelatine till after the alkali which it retained is neutralized with an acid. The alkaline earths precipitate it, but the precipitates are slightly soluble in water; the heavy oxides throw it down in combination as a puce-colored substance. On treating the precipitate with nitric acid, the artificial tannin dissolves without change, whereas the natural product is altered by this agent. Heated alone in a retort, it yields water and nitric acid, ammonia, carbonic acid, and another gaseous substance, probably nitrogen. According to Chevreul, this variety of tannin is a compound of nitrous or nitric acid with a carbonaceous matter.

A second kind of tannin is produced when indigo, or similar compounds rich in carbon, are treated with nitric acid, as in the foregoing method. A very bitter, orange-colored residue is obtained, which is soluble in water, precipitates gelatine and metallic bases, and affords, when decomposed, nitrous or nitric acid. If sulphuric acid be made to react upon resin or camphor till these dissolve, and heat be applied till the liquor blackens, and then cold water added, a black powder falls which, when treated with alcohol and the spirituous extract evaporated, yields a residue which is a third kind of tannin compound, this variety precipitates gelatine and some of the bases. Chevreul considers that this compound contains sulphide of hydrogen.

Uses of Tannin.—The range of the application of tannic acid is not very extensive. It will be already understood to be of primary importance in the manufacture of leather; but, besides this application, it serves another purpose in wine-growing countries, namely of precipitating the glaiadine from the fermented must, especially of white wines. The removal of this principle is of very great importance to the vintner, for if it be suffered to remain, it is apt to induce a secondary viscous fermentation, which would destroy the wine. Tannic acid seems to be the best antidote in this instance, since it coagulates the glaiadine, and forms a tannate with it. Tannic acid is also extensively employed as a medicine on account of its powerful astringent properties, which are readily explained by what has been said regarding its action on albumen, fibrin, and gelatine. Its effects are to check hæmorrhages, profuse secretions, and to constringe relaxed fibres; and for either purpose it may be administered either internally of externally. When taken into the system, it would appear that it is capable of being converted into gallic acid, and a humus-like substance, all of which may be detected in the urine; but this change does not seem to have been effected till the tannin has exhibited its peculiar astringent action. In the laboratory, tannic acid is occasionally employed as a test.

Gallic Acid.—The tendency of tannic to be transformed into gallic acid has been already mentioned; but it may be well to show more in detail the circumstances under which this change is effected, and to point out the properties of gallic acid generally, as also with regard to the operations of the tanner. Its name has been derived, as is evident, from galls, on account of those excrescences containing it ready-formed; it likewise exists in the cypress-nut, arnica flowers, white hellebore, meadow saffron—colchicum autumnale—and in a number of astringent barks, together with tannic acid, but in small quantities. Its most prolific source is tannic acid, which by oxidation, yields it abundantly.  The conditions affecting the change, such as exposure of the solution of tannin to air and at an increased temperature, have been already mentioned, as likewise the decomposition of the same body by the mineral acids. All the methods recommended for the production of gallic acid from tannin, are based upon these properties of the latter. Thus Braconnot prepares it from powdered nut-galls, by drenching this substance with water and maintaining a temperature of 70° to 75°. After a while a quiet action succeeds, and when this subsides the pasty mass is to be expressed and afterwards heated with boiling water, which dissolves out the gallic acid, and yields it upon evaporation in impure crystals. These, by re-solution in water and filtration of the liquid through animal charcoal, afford a menstruum which gives the gallic acid in a state of purity when concentrated. Withell prepares it by treating thirteen drachms of dry tannin with two fluid ounces of sulphuric acid of 1.84°, diluted with four times its volume of water and the application of a boiling heat. Gallic acid in crystals is deposited to the amount or eighty per cent. of the tannin employed.

It would appear from various researches, that the change of tannin to gallic acid, on exposing the extract containing the former to air and moisture, is due to a fermentative process, which is considerably expedited if the vegetal matter in the bark or gall, or whatever else the source may be, is permitted to remain in contact with the liquid. Such is the case, especially, with extract of gall-nuts; and it might be inferred that the tannin from all other sources, and which is capable of being transformed into gallic acid by oxidation in the above manner, is similarly affected. M. Laroche, has shown by comparative experiments the effect of the marcs of gall-nuts, after the extraction of the tannin with ether in the ordinary way, in operating this change. By inclosing an aqueous solution of tannin and a portion of this residue in a bottle or flask, and allowing it to remain some time, he found that the tannin was entirely converted into gallic acid, whilst a strong extract of tannin, freely exposed to the air at the same time, retained nearly the whole of its tannin unaltered. Hence, doubtless, the insoluble matter in galls in the presence of moisture, is capable of acting on tannin as a ferment, and that the change of tannin into its derivative is the result of a fermentation. It is well known that fermentation by contact of yeast, blood, and albumen, effects the same transformation of the astringent extract as the residue of the galls which have been digested in ether, and also that the latter will excite the vinous fermentation in solutions of grape-sugar, and, therefore, these reactions may be taken as mutually corroborative. Further, the gallic fermentation may be intercepted by antiseptics, like the vinous. Protochloride of mercury—corrosive sublimate—completely prevents the decomposition; and pyroligneous acid, or carbonic acid, as shown by the investigation of Calvert, on account of some empyreumatic matter which it contains, likewise considerably retards the fermentation.

It has been shown, that sulphuric and hydrochloric acids at a boiling heat rapidly change tannin into gallic acid, and without the intervention of air; but if the mixture be kept at a common temperature, M. Antoine has shown by his researches that the decomposition of the tannin is retarded by a small portion of those acids. On the contrary, tartaric, malic, and vegetal acids in general, expedite the decomposition of the tannin. The rapidity with which sumac ferments, may be referred to the quantity of malic acid in the leaves.—Parnell.

Properties of Gallic Acid.—When pure, this acid crystallizes in long silky needles, which are unalterable in the air; its taste is acrid and styptic. Branconnot states that it dissolves in a hundred parts of cold, and in about three parts of hot water; four or five parts of alcohol in the cold dissolve it, but one part of the hot spirit will take it up. It is soluble in ether, but in less quantity. The aqueous and spirituous solutions, but the former more readily, undergo decomposition, so that the surface of the liquid becomes mouldy, and a brownish matter is produced, which Döbereiner considered ulmin 8. Added to a solution of gelatine, no precipitation occurs—a behaviour which at once distinguishes gallic from tannic acid, and which indicates that it is unavailable in converting skin into leather. This being so, and, as has been already pointed out, tannin being readily convertible into this body, it becomes of the utmost importance to the tanner to guard against this transformation, which, in the usual way of tanning, is so apt to take place. The researches of Laroque have shown that, in regard of the tannin from calls, the decomposition is completely and readily effected, provided the insoluble residuary matter be allowed to remain in contact with the liquor containing the tannic acid, and it is equally well known that the tannin from sumac is liable to decomposition, as well when it remains in contact with the insoluble portion of this body, as when it is separated from it; but independently of the action of the residue, the malic acid here plays an important part. Though the evidence proving that the insoluble portion of the barks, et cetera, employed in tanning, when allowed to remain in the tan-liquor, does not operate in the rapid conversion of tannin into gallic acid, thereby rendering the former valuable body useless, is not definite,—still, knowing that in the waste tan-liquor, a large quantity of gallic acid is found, it may be safely stated that its production is from the tanning, and also is owing more to the effects of the insoluble matter of the source of the tannin, than to any oxidation which might take place during the tanning processes. Even those kinds of tannin which are incapable of producing gallic acid by decomposition are subject to a metamorphosis which destroys the tanning agent, and this fermenting action id more readily produced when the insoluble matter is left in contact, than if the liquor were retained by itself. The only benefit which the tanner derives from gallic acid is, that it aids in swelling the hides, and thus facilitates the absorption  of the tannin; but when it is considered that a dilute solution of sulphuric or vegetal acids effect the same thing quite as well, it will be evident that he who relies on the swelling by gallic acid, and sacrifices a corresponding portion of tannin in its production, suffers  a considerable loss, and is not fit to compete with those who can obviate such a sacrifice.

At 410° to 420°, according to the observation of Pelouze, gallic acid is entirely volatized, producing a sublimate of beautiful white crystals and pure carbonic acid; if the heat be urged to 464° or 482°, instead of the foregoing change, another takes place, in which pure carbonic acid is likewise developed. But in addition to this a quantity of water is produced, and which runs along the sided of the retort, in the bottom of which remains a considerable mass of brilliant-black insoluble matter, which at first sight might be taken for charcoal. It is, however, a true acid, and is called by Pelouze, meta-gallic acid.

According to the above-named chemist, crystallized gallic acid is composed of—

 

Centesimally

Carbon

50.10

Hydrogen

3.64

Oxygen

46.24

 

100.00

Numbers which agree with the formula C7 H3 O5 HO; Strecker, however, regards it as a tribasic acid, thus C14 H6 O10 2HO=3 HO, C14 H3 O7, 2 aq.,—and capable of forming salts which correspond with the general formula, MO, 2 HO, C14 H3 O7; 2 MO, HO, C14 H3 O7, and 3MO, C14 H3 O7; MO being here the representative of a metallic oxide.

As pyrogallic acid, C7 H3 O3 or C14 H6 O6, meta-gallic acid, C7 H2 O2 or C14 H6 O6, ellagic acid, C14 H2 O7, HO, and other derivatives from gallic acid and, therefore, remotely from tannic acid, are of no importance in the making of leather, they will not be further dwelt upon. Ellagic acid is said, however, to be the chief agent that gives the bloom to tanned hide.

Next Page: Sources of Tannin.


Footnotes

1. ^    Sodium sesquicarbonate is an example of a sesquisalt.

2. ^   Nicolas Deyeux (1745-1837) was Napoleon’s pharmacist but he also took much interest in the wider aspects of chemistry. See French Wikipedia.

3. ^   Marc Seguin (1786-1875) was a grand nephew of Joseph de Montgolfier. Although he was primarily notable for civil engineering, he had a wide range of technological business interests. It seems likely that he is the man in question. See Seguin on the www.art-et-histoire.com site (in French).

4. ^   This is probably David Macbride (1726-1778), from County Antrim. He was a physician and chemist who studied anatomy in Edinburgh and midwifery in London. He was a medical officer in a Royal Navy hospital ship, before setting up in medical practice in Dublin. He was interested in finding a cure for scurvy and in 1774, he discovered that lime-water was more efficacious than plain water in part of the tanning process. For this, he was made an honorary member of the Dublin Royal Society (DNB).

5. ^   Hieronymus Theodor Richter (1824-1898) had begun publishing articles in the early 1860s, when he was working ain Freiburg University of Mining and Technology. He is perhaps the man concerned.

6. ^   Joseph Proust (1754-1826) was a French chemist who showed that the components of compounds were always present in the same proportions, though others showed that there were exceptions to his rule.

7. ^   Marc – This is a French word which may mean the residue from a pressing of fruit to obtain its juice, or the dregs of an infusion such as coffee. Here, the analogy would be with the latter.

8. ^   Ulmin – a dark-coloured product of the decay of wood or vegetable matter, or the action of certain chemical agents on sugar. (OED)


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