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
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 |
|
|
Larch bark |
Pinus larix |
|
|
Mimosa bark |
Acacia sp. |
|
|
Babool bark |
Acacia abarica |
|
|
Cork-tree bark |
Quercus suber |
Laruch, |
|
Hemlock bark |
Abies Canadensis |
|
|
Sumach |
Rhus coriaria |
|
|
Valonia |
Quercus ægilops |
|
|
Divi divi |
Cæsalpina coriaria |
|
|
Myrobalans |
Terminalia sp. |
|
|
Algarobilla |
Prosopis pallida |
|
|
Terra japonica |
Acacia catechu |
|
|
Cutch |
Acacia catechu |
|
|
Kassu |
Areca catechu |
|
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.
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 |
|
|
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, |
— |
|
|
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.
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
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)