thickness, but not in breadth, must become distended and finally
ruptured. In some cases the surface is renewed by fresh corky layers
constantly developed below it, and then the bark remains smooth and
unfurrowed, as in the beech and young oak, or in the birch, from which
thin corky layers are continually peeling; or it may produce a thick
layer of cork, as in the cork-oak. In many cases, and especially in
older trees, the outer or primary layer of cork-cambium ultimately dies
for want of nourishment, and a fresh cork-producing layer is developed
in the still living parenchym. As cork is practically air- and
water-proof, the new layer cuts off from its source of nourishment and
kills all the parenchym exterior to it. In some cases this peels off, as
in the Oriental plane (_Platanus_), but usually it forms a constantly
increasing coat of dead tissue forming the “ross” or “crap” (Ger.
_Borke_), which as it cannot increase in breadth, becomes deeply
fissured as the tree becomes old. In some cases the new growing layer or
secondary cork-cambium forms a complete coating parallel with the first,
but more often it consists of a series of arcs convex towards the tree
and cutting the primary cork-cambium at various places, so as to divide
the tissue outside itself into scales. Later on the process repeats
itself, new arcs forming inside the first, and cutting off further
portions of the parenchym. In this way the cork-forming layer gradually
sinks deeper and deeper into the bark, till it frequently passes even
into the bast-layer, and very complicated arrangements of tissue result,
in which corky layers from the secondary cork-cambium are interspersed
with bast-cells and sieve-tubes.
As a rule the outer and dead part of the bark contains but little
tannin, though to this there are exceptions, as, for instance, in the
hemlock and Aleppo pines. It always contains a large proportion of dark
colouring matters (reds, phlobaphenes, p. 297).
Cork consists of thin, and often roughly cubical cells, which are filled
with air, while tannin is usually contained in somewhat similar cells
with thicker walls. The walls of many vegetable cells are perforated
with fine holes, and become thickened by internal deposits of hard
ligneous matter which sometimes almost fill the entire cell
(“stone-cells”). Bark-cells often contain starch-granules, frequently of
peculiar and characteristic forms (which are easily recognised by the
blue colour produced on treating the preparation under the microscope
with a drop of a solution of iodine in potassium iodide), as well as
crystals of oxalate of lime and other matters. These, and the form and
arrangement of the cells as seen in sections under the microscope, form
useful marks of recognition of the various barks. Tannin is most easily
detected by staining, before cutting sections, with a solution of ferric
chloride in absolute alcohol.
Apart from microscopic characteristics, the external appearance of
barks, both to the naked eye and by the aid of a lens, forms a valuable
means of recognition. The arrangement of the bast and corky layers, the
remains of epidermis, or the form and character of the fissures, and of
the lenticels or small corky protuberances which take the place of
stomata in the epidermis, should be observed.
Space does not permit of any detailed account of the structure of
fruits, wood and leaves, which are also cellular structures in many
respects resembling the bark. The cuticles of leaves, and especially the
stomata or breathing pores, and the hairs are often very characteristic.
(Cp. Plates III. and IV., and p. 272.)
Valuable hints may also be obtained from the chemical reactions which
are described on p. 70 _et seq._, L.I.L.B.
BOTANICAL LIST OF TANNING MATERIALS.[135]
[135] The percentage of tannin given where the source of information
is not stated must in many cases be regarded as uncertain, many
analyses having been made before the introduction of modern methods,
but those quoted as having been done in the author’s laboratory are of
recent date and have been made by the latest methods.
CONIFERÆ, Pines, Cypresses, mostly containing Catechol tans, yielding
reds.
_Abies excelsa_, Lam. (_Pinus Abies_, _Pinus Picea_, _Picea vulgaris_,
Link.), Norway Spruce. Fr. _Faux sapin_; Ger. _Fichte_, _Rottanne_. The
source of the so-called larch-extract, and a principal tanning material
of Austria. Contains 7-13 per cent. of a catechol-tannin and much
fermentable sugar, and on this account is useful for swelling and
colouring, but does not tan heavily. English and Scandinavian bark does
not seem much utilised. Best bark 2-8 mm. thick; smooth, yellow inside,
with reddish-brown ross outside. For detailed description of structure
see von Höhnel, ‘Die Gerberinden,’ p. 35.
_Abies pectinata_, Silver Fir. Fr. _Sapin_; Ger. _Edeltanne_,
_Silbertanne_, _Weisstanne_. Used to a limited extent, but apt to be
confused with spruce. Contains 6-15 per cent. iron-blueing tannin. Used
in Styria, Austria, Russia. Without “ross,” but silver-grey and smooth
outside. (Von Höhnel, ‘Die Gerberinden,’ p. 40; ‘Gerber,’ 1875, p. 375.)
_Abies (Pinus, Tsuga) canadensis_, Hemlock Fir (Fig. 44). The principal
American tanning material, and source of hemlock extract; averages 8-10
per cent. of a catechol-tannin, but variable, 18 per cent. reported,
possibly from a different species. Abundant in Canada and the Northern
and North-western States of America. The bark of old trees, which is
principally used for tanning and extract-making, is 2-4 cm. thick,
smooth and yellow within, greyish and deeply fissured without. The
ross, which is red and thick, contains a considerable quantity of
tannin, with much dark-red phlobaphen. It does not differ in structure
from the inner living and yellow “flesh.” The bark is easily
recognisable by its well-marked concave lamellæ of cork, cutting off
successive layers of “ross” of several millimetres in thickness. (Von
Höhnel, _ibid._, p. 42.)
[Illustration: FIG. 44.[136]--Hemlock Fir (_Tsuga canadense_).]
[136] Bastin and Trimble’s American Coniferæ, American Journal of
Pharmacy.
_Abies alba_ (_Picea alba_), White Spruce, North America. In character
of tree and bark very similar to Norway spruce.
_Larix europæa_ D.C. (_Abies_ or _Pinus Larix_), Larch. Fr. _Mélèze_;
Ger. _Lärche_. Contains 9-10 per cent. pale catechol-tannin, mild and
suitable for light leathers. Used, especially in Scotland, for basil
tannage.
_Pinus halepensis_, Aleppo Pine. An important tanning material of the
Mediterranean coasts. The outer bark, stripped like cork from the living
tree (_Scorza_ or _Cortegia rossa_), is a deep red tannage, and contains
about 15 per cent. of tannin very similar to hemlock. It is largely used
in the island of Syra. The inner and fleshy part of the bark, only
obtained when the tree is cut, is _Snoubar_ or _Snobar_ bark, containing
up to 25 per cent. of lighter coloured tannin. This bark is reddish
brown, and pretty smooth on both sides, except for shell-like
depressions on the outer surface. The “scorza rossa” is dark red-brown
internally, grey and irregular outside, frequently very thick, and
divided into successive layers of 1-2 mm. thickness by cork lamellæ.
(Von Höhnel, _ibid._, p. 44.) In appearance the tree resembles the
Scotch fir.
_Pinus tæda_, America; _P. Laricio_, Austrian Pine; _P. maritima_,
Mediterranean; _P. Cembra_, Alps, Tyrol, 3-5 per cent.; _P. sylvestris_,
Scotch Fir. Ger. _Kiefer_; Fr. _Pin sauvage_, 4-5 per cent. _P.
longifolia_ Roxb., India, 11-14 per cent.
_Juniperus communis_, Juniper. Bark used in Russia.
_Podocarpus elongata_ and _Thunbergii_, Cape of Good Hope; _Geelhout_,
Yellow woods.
_Phyllocladus trichomanoides_, New Zealand; _Tanekahi_, _Tarsekahi_,
_Kiri-toa-toa_, “Golden Tan.” Used in dyeing glove-leather. Tannin, 30
per cent., gives green blacks with iron.
_P. asplenifolia_, Tasmania, Celery-topped Pine; 23 per cent.
_Phyllocladus_ belongs to Yew family.
LILIACEÆ.
_Scilla maritima_, Squill. Tannin stated from 2-24 per cent. More
valuable for pharmacy.
PALMÆ.
_Areca catechu_, Betel-Nut Palm of India. Yields a species of cutch of
no importance for tanning.
_Sabal serrulata_, Saw Palmetto of Florida (Trimble). (“Dwarf” palmetto
is _S. Adansonia_.) Palmetto root has been much talked of as a tanning
material; and makes a light-coloured leather.
An extract is now made from the roots of the Saw Palmetto, which grows
freely in the Southern States of America, and is especially abundant on
the east coast of Florida. The plant is an evergreen, the stem growing
flat along the ground, being held in place by numerous roots each the
size of a pipe-stem. The leaves are fan-shaped and ribbed, and two to
three feet in diameter. In its hardihood the palmetto resembles a weed,
as the leaves may be cut off quite close to the stem without damaging
the plant, which will grow freely on poor sandy land which is worthless
for other purposes. The average yield is stated to be about 10 cwt. to
the acre, but in good seasons and with rich land, over a ton per acre
has been obtained.
The air-dried leaves contain about 13 per cent. of tannin, but the
results obtained by different chemists vary from 5 to 20 per cent.
Possibly these variations are caused by the different amounts of
moisture in the various samples.
The leaves must be treated with a solution of caustic soda, to remove
the glossy siliceous shield which covers them and prevents their being
easily extracted. After the tanning matter has been extracted the
remaining fibre can be profitably disposed of to paper and rope
manufacturers.
As the supply of palmetto is very large it is likely that it will, to a
considerable extent, substitute the employment of gambier, and in the
United States the extract has already met with a considerable sale.
Samples of the extract examined by the Author analysed from 16-22 per
cent. of tanning matter, and several per cent. of mineral matter, and
produced a very soft and mellow leather of good colour. The extract
contains noticeable quantities of common salt, and organic salts of soda
which leave sodium carbonate on ignition.
_Cocos nucifera_, the Cocoa-nut Palm, also contains tannin in roots.
CASUARINÆ.
_Casuarina equisetifolia_ L. (_laterifolia_ Lam.); _Filao_ bark,
Reunion; _Tjamara laut_, Java; Casagha or Tinian Pine, Ceylon. Widely
distributed in Southern Asia, bark used for tanning and dyeing. Tannin
gives blue-blacks with iron. Several other species very similar in
structure and properties. (Von Höhnel.) Hooper found 11-18 per cent. of
tannin.
MYRICACEÆ.
_Myrica Gale_, Sweet Gale, or bog-myrtle.
_Myrica (Comptonia) asplenifolia_, U.S.A.; “Sweet Fern.” Covers millions
of acres in Michigan. Yields 40 per cent. of “extract.” Leaves 4-5 per
cent., roots 4-6 per cent. tannin, according to season (Trimble). Has
been much talked of, but in Prof. Trimble’s opinion is not likely to
prove of much importance.
_Myrica nagi_ (Hind. _Kaiphal_), India, contains 13-27 per cent. of
tannin in the bark, and a colouring matter, myricetin, identical with
that of sumach.[137] Leather tanned with it is of a somewhat reddish
colour which is much brightened by sumaching, and converted into a pale
yellow by treatment with alum. It promises to be a valuable tanning
material.
[137] Perkin and Hummel, Trans. Chem. Soc., 1896, p. 1287.
BETULACEÆ.
_Alnus glutinosa_, Common Alder. Fr. _Aulne_; Ger. _Erle_. Contains
16-20 per cent. iron-green tannin, with much red colouring matter; old
barks as low as 10 per cent. Colour develops during and after tannage.
Used alone it gives a red, hard and brittle leather, but with galls,
valonia, etc. it produces a satisfactory tannage. Its principal use is
to furnish gunpowder-charcoal, and it is possible the bark might be
obtained from powder-factories, if the use of gunpowder is not
superseded by nitro-compounds. (Von Höhnel.)
_Alnus maritima_, _Hannoki_, Japan; and _A. firma_, _Minibari_. Fruits
(_yashi_) contain 25 per cent. tanning matter (iron-blueing), and little
colouring matter. Used in Japan for dyeing and tanning. _A. nepalensis_
and _A. nitida_ used in India. Several other species of _Alnus_ contain
tannin.
_Betula alba_, White or Common Birch. Fr. _Bouleau blanc_; Ger. _Birke_.
Inner bark used in Scotland (in conjunction with larch for tanning
sheep-skins), Norway, Russia, etc. It contains only 2-5 per cent. of
iron-greening tannin, and much fermentable sugar. By far the most
important use of birch bark in tanning is to produce the birch-bark tar
used to give scent and insect-resisting power to “Russia” leather
(_Youft_; Ger. _Juchten_). The outside bark consists of thin layers of
cork, often white with a crystalline deposit of betulin, which when
distilled yields the odorous oil. The distillation is a dry one, and
tarry products accompany the true oil, and at first give a strong
empyreumatic smell to the leather, which it loses by keeping, while the
true “Russia” odour remains. This “ageing” may be hastened by hanging
the leather in a hot stove. If the oil is distilled in a current of
steam, or with petroleum ether, the tarry matter passes over, while the
matter giving the true odour remains in the retort (p. 372).
_Betula lenta_, American Black Birch. The bark and twigs distilled with
water yield an essential oil, which is almost pure salicylate of methyl,
and largely substituted for oil of wintergreen (_Gaultheria
procumbens_), with which it is chemically identical. Used for perfumery,
and as a rheumatism remedy. Often erroneously spoken of as the source of
“Russia” oil. A mixture of a trace of wintergreen oil with sandal-wood
oil considerably resembles the “Russia” scent (p. 373).
CUPULIFERÆ.
_Castanea vesca_, True or Spanish Chestnut. Fr. _Châtaignier_; Ger.
_Kastanie_. Abundant in Italy, South of France and Corsica, where it
forms great forests. Bark said to be nearly as strong in tannin as oak
(up to 17 per cent., de Lof), but not much used in tanning.
Wood only contains 3-6 per cent. tannin, but is the source of the
valuable chestnut extract, first employed for dyeing, and introduced as
a tanning agent by Aimé Koch. The strength of extract is of course very
variable, even for the same density (see p. 339), but it usually
contains from 28 to 32 per cent. of tannin.
The tannin gives blue-black with iron, but is not identical with either
oak-bark or gall tannins, but apparently a mixture, or possibly a
methylated derivative of the latter, and identical with oakwood tannin,
or so nearly so as to be indistinguishable; it may also be identical
with divi tannin. Decolorised chestnut extracts, sometimes mixed with
quebracho and other materials, are often sold as “oakwood” or
“oak-bark” extracts. The extract gives a firm leather, with a good deal
of bloom if used strong, and a more reddish tint than valonia. The
extract often contains dark colouring matters, and the colour of leather
tanned with it is readily darkened by traces of lime derived from
calcareous waters or imperfectly delimed hides. Like all wood-extracts
it tans rapidly, the colour penetrating first, and the tan following,
but, according to Eitner, it does not, alone, make full or solid
tannage, perhaps from want of acid-forming matters, but answers
particularly well in combination with spruce-bark. It is largely used in
England for sole-leather in combination with valonia, myrobalans and
other materials.
The higher the temperature of extraction, the more colouring matter is
contained in the extract in proportion to tannin matter and the greater
is its viscosity. Much colouring matter remains undissolved if the
extract is dissolved in cold water, but there is, in addition, a loss of
tanning power, the colouring matter being also capable of combining with
hide. It has in fact been used for tanning by dissolving it in solutions
of borax or alkaline salts. By improved methods of manufacture the
colouring matter has been much reduced.
The chestnut is an important food tree, the nuts forming a considerable
part of the food of the inhabitants of Corsica and Sardinia, and even of
Italy.
Oaks.
Almost all species of oak contain useful quantities of tannin in the
bark, and probably in the wood. Most if not all oaks yield
catechol-tannins with, probably, some mixture of ellagitannic acid.
_Quercus robur_, Common Oak. Fr _Chêne_; Ger. _Eiche_. It is frequently
separated into the two subspecies:--
_Quercus pedunculata._ Commonest oak of lowlands, England, Ireland and
Scotland. Acorns in bunches or spikes on a stalk ¹⁄₆ inch long, hence
Ger. name, _Stiel-Eiche_. Leaves sessile or short-stalked. In favourable
situations, _said_ to yield about 2 per cent. more tannin than _Q.
sessiliflora_, but this is doubtful. It is the commonest oak in
Slavonia, and the source of commercial oakwood-extract.
_Q. sessiliflora_, Ger. _Traubeneiche_. Common in hilly districts, and
scattered throughout the country. Acorns in bunch on the branch, or with
very short stalk; leaves on stalk ¹⁄₂-1 inch long.
Of English barks, Sussex and Hampshire are considered the best, and
contain up to 12-14 per cent. of tanning matter; a coppice bark from
Wastdale, Cumberland, is however, recorded to have yielded 19 per cent.
tanning matter (Hellon).
Probably each of the two varieties of oak gives best bark where it
thrives best (v. Höhnel).
Belgian bark is sometimes equal to English, and contains 10-12 per cent.
tanning matter. Dutch bark as exported is generally inferior and not
cleaned; Swedish is bright, but very poor.
Oak-bark contains a tanning matter, quercitannic acid, giving
green-blacks with iron salts, and possibly containing both catechol and
pyrogallol groups, but its constitution is not fully understood. It
yields both red anhydrides and ellagic acid; and gallic acid has been
obtained by the action of hydrochloric acid, though not by fermentation
in the tannery. The tannin is not a glucoside, but the fact that a
sugar, lævulose, is also present in the bark has led some observers to
erroneous conclusions regarding the constitution of the tannin. The
unpurified infusion of the bark of _Q. robur_ gives a blue-black with
iron-salts, from the presence of a colouring matter; but those of most
other oaks give green-blacks.
Most tannin is contained in the living part of the bark. The yield
diminishes in trees over twenty-five years, and coppice barks, from
absence of ross, are often strong, and also contain less colouring
matter and more fermentable sugar.
Warm and rich soils seem to yield the best barks.
The brighter the colour of the fresh cut “flesh,” the better the bark.
Dark brown inner side shows that bark has been exposed to rain, which
deteriorates strength and colour; but a very light colour is thought by
some to indicate poorness in tannin. White lichen is said to be a mark
of poor bark, and probably indicates a damp and unfavourable situation.
Oaks are generally cut when the sap is rising (15th April to 15th June),
and the buds open and new soft cells begin to grow, for the bark is then
more easily detached.
Experiments in France have shown that the bark of timber felled at other
seasons may be loosened by steaming, and it is said there is no
practical loss of tannin. Superheated steam, produced in a small boiler
in the woods, is used.
The bark is peeled with tools of various forms, the branch and knotted
places being loosened by beating with a mallet. The bark must be peeled
immediately the tree has been felled.
The peeled bark, in pieces up to three feet long, is laid on hurdles
sloped in such a way that the rain runs off as much as possible, and in
this way it is dried, but in wet seasons is much damaged. Bark so dried
in the woods often retains 40-50 per cent. water, and must be stacked or
stored so as to allow of further drying.
English bark is sometimes sold in “long rind,” and sometimes “hatched”
or chopped in pieces about four inches long. Belgian and Dutch barks are
generally hatched. Belgian tree bark is “cleaned” (and cleanings often
mixed back with bulk), Dutch bark is not cleaned. Much sand and dirt is
contained in most Continental bark: screenings of Belgian bark yielded a
black liquor, and contained so much sand that they would not even burn!
_Oak-bark extract_ is occasionally offered for sale, but is not usually
genuine or of good quality, except that of the American chestnut-oak,
_Q. prinus_, from which an excellent extract has been manufactured in
the Alleghanies. Factitious extracts often contain myrobalans and
quebracho.
[Illustration: FIG. 45.--Turkey Oak (_Quercus cerris_).]
_Oakwood_ contains only a very small percentage (from 2-4 per cent.) of
a tannin, practically identical with that of chestnut, but different to
that of oak-bark. It is stated by de Lof to reach 9-14 per cent. in old
heart-wood; but this is doubtful. The wood retains the tannin in its
interior for a long time. Wood of a Roman bridge built at Mainz 55 B.C.
is stated by de Lof to have still contained 2·14 per cent. tannin in
1881 A.D. A good deal of imitated oakwood extract is undoubtedly made
from chestnut wood, and unfortunately no very satisfactory way of
distinguishing it is known, though oak-bark extract can be distinguished
from oakwood by giving a precipitate at once, even in dilute solution,
with bromine-water, while the wood gives a brown precipitate only after
long standing. Precipitation by bromine-water is a general
characteristic of catechol tannins, and hence a mixture of quebracho (a
cheap catechol tan) with chestnut would simulate oak-bark in this
respect. If a few drops of the non-tannin solution or an alcoholic
extract from the “total soluble” of extracts containing quebracho or
other catechol tannins be treated with concentrated sulphuric acid in a
test-tube, a deep crimson will be produced, especially at the surface of
the acid, which remains pink on dilution with water. With pyrogallol
derivatives, such as genuine oakwood, a yellow or brown only is produced
(J. Hughes). The test is very delicate. Another distinction is that bark
extracts contain perceptible traces of manganese, but this cannot be
relied on as many wood extracts also contain some, probably derived from
the twig and branch bark which is used along with the wood. Oakwood
extract is now manufactured on an enormous scale in Slavonia, and is
used both by sole- and dressing-leather tanners, chiefly to increase the
strength of the layer liquors. The extract is also used to increase the
weight of leather after tannage by mopping on the flesh-side. All the
best oakwood extract manufacturers contract to sell on analysis and
colour estimation, and good Slavonian oakwood extract generally contains
26-28 per cent. of tanning matter, giving a tintometer measurement of
4-5° red, and 20-25° yellow, when a solution containing ¹⁄₂ per cent. of
tanning matter is measured in a 1 cm. cell. For particulars of the
manufacture of concentrated extracts see p. 337.
[Illustration: FIG. 46.--Cork Oak (_Quercus suber_).]
_Q. cerris_, Turkey Oak. Ger. _Zerreiche_. Common in southern Europe, a
fine tree, but bark inferior to _Q. robur_. Fig. 45.
_Q. pubescens._ Fr. _Chêne velu_; Ger. _Weiss-_ or _Schwarzeiche_. In
mountain districts and scattered in Southern Europe, about equal to _Q.
robur_.
_Q. ilex_, Evergreen Oak. Fr. _Chêne vert_, _Chêne yeuse_; Ger.
_Grüneiche_, _Steineiche_; Span. and Ital. _Encina_. South Europe,
Algeria. Said to be somewhat stronger in tannin than common oak,
yielding 5-11 per cent. of a rather darker coloured tannin, but well
adapted to sole-leather. Good bark is smooth outside, without fissures,
short in fracture.
_Q. Suber_, Cork Oak. F. _Chêne liège_; Ital. _Sughero_, _Suvero_.
(Figs. 46, 47.) The outer bark is cork; the interior bark contains 12-15
per cent. of tannin which is redder than that of ordinary oak. Trees at
first produce an irregular cork, sold as “virgin cork” for ferneries,
etc. After this is stripped, later growths are more uniform, and fit for
use; tanning bark is only obtained when the tree is cut down. Bark is
rough but pale-coloured on both sides and about 1 cm. thick; interior
like ordinary oak, but more strongly furrowed. Produced chiefly on
Mediterranean coasts, and formerly largely used in Ireland.
[Illustration: FIG. 47.--Section of Cork Oak, showing cork, inner bark
and wood.]
_Q. pseudosuber_, African Oak. Fr. _Chêne faux liège_. Algeria. Not
stronger than English oak, but with more colouring matter, hence strikes
quickly through leather. Bark very thick.
_Q. Mirbecki._ Fr. _Chêne Zeen_. Algeria. Rapid growth. Bark contains 8
per cent. of tannin.
_Q. Tozæ._ Fr. _Chêne tauzin_. Pyrenees and S. France. Bark contains 14
per cent. of tannin.
_Q. coccifera_, Kermes Oak. Fr. _Kermes_, _Garouille_ (Fig. 48). South
Europe and Algeria. Root bark is called “rusque” or “garouille”;
averages 10-18 per cent. tannin, but trunk bark does not exceed 11 per
cent. This tree is the food of the kermes insect, used for dyeing
scarlet before the introduction of cochineal. Garouille is principally
used in the south of France, giving a firm sole-leather of a
disagreeable odour and dark brown colour.
[Illustration: FIG. 48.--Kermes Oak (_Quercus coccifera_).]
_Q. Ægilops_ (and probably other species--_Q. macrolepis_, _græca_,
_Ungeri_, _coccifera_), Valonia. Fr. _Valonée_; Ger. _Valonea_,
_Ackerdoppen_, _Orientalische Knoppern_. Best Smyrna contains up to 40
per cent., Greek 19-30 per cent., Candia valonias up to 41 per cent.,
and Caramanian (probably not _Q. Ægilops_) 17-22 per cent. of tannins
which are at least principally pyrogallol derivatives and which give
blue-blacks with iron, no precipitate with bromine-water, and which
deposit a great deal of bloom consisting of ellagic acid.
[Illustration: FIG. 49.--Valonia Oak (_Q. Ægilops_).]
_Q. Ægilops_ (Fig. 49) is said to be most abundant in the highlands of
Morea, Roumelia, Greek Archipelago, Asia Minor and Palestine, while
_macrolepis_ forms large forests in many parts of Greece, and especially
on the lower slopes of Mount Taygetos. In Asia Minor the fruit ripens in
July-August, when the trees are beaten and acorns left on the ground to
dry. They are afterwards gathered, and carried on camels to stores in
the towns, and thence by camel and rail to Smyrna, where they are placed
in heaps 5-6 feet deep in large airy stores, and allowed to ferment and
heat for some weeks, when the acorn, which contains but little tannin,
contracts and falls from the cup, and is used for feeding pigs. This
fermentation is risky, and if carried too far the cups become
dark-coloured and damaged. The acorn contains a considerable amount of
fermentable sugar.
When ready for shipment, the valonia is hand-picked, the largest and
finest cups (prima) going to Trieste, the second selection to England
(Inglese), and the remainder, known as “natural,” also coming largely to
England. The “Inglese,” although inferior in appearance to the very
large selected cups, is, of course, less costly, and gives an equal
yield of tannin.
In 1887, Smyrna exported about 23,000 tons to England, and 16,000 tons
elsewhere, principally to Austria, Germany and Italy. The largest known
crop is stated at 70,000 tons in Asia Minor, and 14,000 in Greece, but
the average yield is considerably less than this.
The beard contains considerably more tannin than the cups, sometimes
over 40 per cent. It is often sold separately at the same or a lower
price, and in Smyrna is known by the Turkish name _tirnac_ (Ital.
_trillo_).
In Greece the best valonia is collected (in April?) before the cup is
matured and while it still encloses the acorn, and is known as _chamada_
(It. _camata_ and _camatina_). The colour of these kinds is excellent
and the percentage of tannin high. Mainly used by dyers, but often worth
attention for tanning where colour is important. In camatina the acorn
is completely covered in the cup, while in camata it is partly exposed.
The next quality, _rhabdisto_, is beaten down by sticks in
September-October (hence name), while after the first rains the fruit
falls and turns black, and is called _charcala_. It contains but little
tannin, and is not generally collected.
Sometimes valonia is attacked by a sort of honeydew, probably caused by
an aphis, which renders it very sticky, and perhaps more liable to heat,
but does not in itself damage its tanning properties.
The lighter the colour, the heavier the weight, and the thicker the
scales of the beard, the better the quality usually proves, but analysis
is the best guide. Caramanian valonia is very inferior.
The tannin contained in valonia is especially suitable in the
manufacture of sole-leather. It deposits much bloom, and if used as a
dusting material, has the characteristic of making the leather solid and
compact, but leaves the grain somewhat rough and hard to work. In
mixture with gambier and other materials, it is an excellent tannage for
dressing leather, and with proper management deposits little or no bloom
(cp. p. 231).
_Q. infectoria_ (Fig. 50) is the source of the “Turkish” or Aleppo
galls. Galls are caused by insects, principally of the genus Cynips, or
gall-wasps, which lay their eggs in different parts of plants, and in
some way cause an abnormal growth of the bud, leaf, or other part.
[Illustration: FIG. 50.--Gall Oak (_Q. infectoria_).]
_Aleppo galls_ are developed from the young shoot of the oak, are best
before the insect has escaped, and contain in this stage up to 50 or 60
per cent. of gallotannic acid. When the insect has developed and
escaped, the galls are of course perforated, much lighter, and more
porous. These galls and those of _Rhus semialata_ are the principal
sources of the tannin of commerce.
The _Q. infectoria_ also bears a large gall like an apple, “Apples of
Sodom,” or “rove,” caused by a different insect. This, in a crushed
condition, has been somewhat largely used as a tanning material, and
contains 24-34 per cent. gallotannic acid.
English oaks have several species of galls and oak-apples, but they do
not seem to be of much value for tanning.
[Illustration: FIG. 51.--Chestnut Oak (_Q. prinus_).]
_Knoppern_ are galls produced on the immature acorns of various species
of oaks, principally _Q. Cerris_ in Hungary, and were formerly largely
used there for tanning, as they contain up to 35 per cent. gallotannic
acid. They are now less abundant, and have been largely replaced by
valonia, sometimes called _orientalische Knoppern_. Like all purely
gallotannic materials, they naturally give a soft and porous tannage,
ill-adapted for sole-leather purposes, which has led to the Austrian
practice of drying, or rather stewing, the leather in very hot and damp
stoves, which make it hard and brittle.
_Chinese_ and _Japanese galls_ are the product of the action of an
aphis on a species of sumach, and will be mentioned again under sumachs
(_Rhus_).
_Djaft_, _dchift_, _jift_, or _jaft_ is a material apparently of Eastern
origin, and said to be derived from an oak of Kurdistan. Dark red scales
or fragments, origin uncertain, very astringent and darkish tannage,
liquor when spilt dries whitish, apparently from crystallisation of some
sort. It contains a large amount of tannin. It appears very irregularly
in commerce and the writer would be glad to obtain further samples and
details of origin. He once used 6 or 7 tons successfully in sole-leather
tannage. It has also been attributed to a shrub allied to the
_Cæsalpinias_ (p. 286).
The most important American oaks are--_Q. prinus_ (_castanea_,
_monticola_), the Chestnut or Rock Oak (Fig. 51). About equal to our oak
in strength, bark very thick, and infusion strongly fluorescent,
especially in presence of ammonia. Source of chestnut-oak extract. The
most important tanning oak-bark of the United States.
_Q. alba_, or “white oak,” is perhaps the most widely distributed and
abundant of any of the American oaks, and very closely resembles the
European _Q. robur_.
_Q. tinctoria_ or _nigra_, Black or Quercitron Oak. Poor as a tanning
material, but used for dyeing yellow, and for modifying the colour of
hemlock tannages. The dyestuff, _quercetin_, is closely allied to that
of fustic, and gives yellows with alum and tin mordants.
A good deal of information is given by Trimble[138] on American oaks and
other tanning materials.
[138] ‘The Tannins,’ vol. ii., Lippincott, Philadelphia, 1894.
Important Indian oaks are _Q. glauca_, _Q. lamellosa_ and _Q. incana_;
bark of last said to yield 22 per cent. of tannin.
SALICACEÆ, Willows.
The bark of various willows, especially _Salix arenaria_ and
_Russeliana_, is used for tanning in Russia, and for Danish
glove-leather. Some contain up to 12-14 per cent. of an iron-blueing
tannin. They impart a strong odour to leather, but different to that of
birch-tar oil, and the scent of genuine Russia leather is due to a
combination of both. In many cases the bark peeled off osiers for
basket-making is employed. A Russian willow (species unknown), in the
form of thin bark of osiers or small branches, gave 9·5 per cent. tannin
when examined in the Leather Industries Laboratory of the Yorkshire
College; and willow barks certainly demand more attention than they have
received in England as tanning materials for fine leathers. _Salix
caprea_ has been used in France for glove leathers, but is weaker than
_S. arenaria_.
Poplars belong to the same natural order, and have been used for
tanning, but their barks at the most contain 2-3 per cent.
POLYGONACEÆ, Docks.
Most members of this family contain tannin.
_Rumex hymenosepalum_, Canaigre, Gonagra (Cana agria), Red Dock, wild
pie-plant (Fig. 52). Common in sandy alluvial plains of Mexico and
Texas, and considerably resembling rhubarb. Its tuberous roots resemble
those of the dahlia, and contain, when air-dried, 25-30 per cent. of a
catechol tannin, probably allied to that of mimosa. Undried, the roots
contain about 68 per cent. of water and only 8 per cent. of tannin. When
well harvested by slicing thin and rapidly drying, it gives leather a
bright orange colour, and, it is said, considerable weight and firmness,
and is thus specially suitable for use in retanning and finishing light
goods and harness leather. Besides tannin, the root contains a yellow
colouring matter, and about 8 per cent. of starch, of which the granules
are very variable in form and size, but mostly oval or elongated. They
do not stain readily with iodine till they have been well washed, or
treated with dilute sulphuric acid. Both the starch and tannin are
contained in large and somewhat thin-walled cells, and the sliced
material is easily extracted at low temperatures. Greater heat
gelatinises the starch, and extracts a darker colour. The best
temperature for extraction is between 30° and 50° C. (see p. 348).
[Illustration: FIG. 52.--Canaigre (_Rumex hymenosepalum_). ‘New
Commercial Drugs and Plants,’ T. Christy.]
The root is most readily grown from tubers or portions including the
crown, as the plant seeds sparingly. Sandy soils, subject to inundation
or irrigation, seem best suited to its culture. In California and
Arizona the growth begins in October or November with the winter rains,
blooming about the end of January, while the leaves die down in May and
the roots remain dormant during the summer. It is not important at what
time the roots are harvested, and they seem to improve in percentage of
tannin up to the second year, after which they become darker and
deteriorate.
The harvested crop should be sliced into thin pieces and rapidly dried
at a low temperature, or still better, converted at once into extract.
This is already done on a considerable scale at Deming, New Mexico. The
residue after extraction is used in America as cattle-food; and might no
doubt be also applied to the production of alcohol.
Planting takes place in autumn, in rows, say 30 inches apart, with 10
inches between each root. Roots for “seed” should be kept in the ground
or stored in dry sand. This should yield a crop of 10 tons per acre in
an average season.
_References._--Report U.S. Commissioner of Agriculture, 1878, pp. 119
_et seq._; Trimble, Am. Jour. of Pharmacy, p. 395, 1889; Canaigre, Bull.
No. 7, Arizona Agr. Expt. Station, 1893; ‘Canaigre or Tanner’s Dock,’
Bull. No. 105, University of California, Berkeley, Cal.; ‘Canaigre
Tannin,’ Trimble and Peacock, Philadelphia, 1893; ‘Report to the German
Leather Trades Association,’ by V. Schroeder, 1894; ‘Il Canaigre,’ E.
Andrieis, Turin, 1899.
_Rumex maritima_, or _maritimus_. Central Europe, England, Ireland. It
is said by de Lof to be found in California, where it is used by the
Indians for tanning; but he probably confounds it with canaigre. De Lof
found its roots, wet, to contain 6 per cent. and after drying, 22 per
cent. of tannin, together with starch and an acid allied to malic.
Several English docks contain tannin; the writer had a sample of leather
tanned with dock-root (very possibly _R. aquaticus_), many years old,
but still soft and close in texture, and of excellent quality.
_Polygonum amphibium._ Said to grow on thousands of acres (?) on the
lower Missouri. Roots contain 22 per cent., branches 17 per cent. of
tannin. _P. amphibium_ is a common English and European plant, with
spikes of pink flowers, growing in marshes and ponds. Probably this is
the _Polygonum_ analysed by Fraas, who found 20-26 per cent. tannin.
_Polygonum Bistorta._ Common in damp places in England. Bistort,
Snakeweed, called “Eastermer giants” in Cumberland, where the young
leaves are used for making herb-puddings. Fraas found 16-21 per cent.
tannin in the roots.
Other species are known to contain much tannin. Perkin found a red
colouring matter in _P. cuspidatum_, a native of India and China,
commonly grown in gardens as a foliage plant (Journ. Chem. Soc., 1895,
p. 1084). _P. tinctorium_, used as a source of indigo in China and
Japan.
_Coccoloba uvifera_, Seaside Grape of West Indies; source of West Indian
kino. Whole plant rich in tannin.
LAURACEÆ, Bay Family.
_Persea_, or _Laurus lingue_. Bark used in Chili for tanning Valdivia
leather. (According to Arata, _Laurus caustica_.) A tree 25-30 feet high
and 2 feet in circumference. Bark rough outside, and whitish, with an
aromatic smell and taste, brittle and easily ground, contains 17-19 per
cent. of a catechol-phloroglucol tannin, greening iron salts (Journ.
Chem. Soc., 1881, p. 600). About 60,000 heavy hides are tanned yearly
with this bark in Valdivia and district, and mostly sent to Hamburg. The
hides are thick and scarcely tanned through, colour fair, leather soft
and porous.
_Persea Meyerina_ N. and _Laurus Pneumo_. Said to be also used in Chili.
SANTALACEÆ.
_Osyris compressa_ (_Fusanus compressus_, _Colpoon compressum_, _Thesium
Colpoon_), “Cape Sumach,” “_Pruim Bast_,” leaves and bark, Cape of Good
Hope. Leaves contain about 23 per cent. of tannin and form a useful
substitute for sumach; but the tannin is not identical, and is of the
catechol class, resembling gambier.
_O. arborea._ Northern India. Leaves rich in tannin.
_Fusanus acuminatus_ (_Santalum acuminatum_), “_Quandony_.” Australia.
18-19 per cent. tannin, dark coloured.
_Exocarpus cupressiformis._ Australia. Bark contains 15 per cent.
tannin.
DAPHNOIDÆ, Spurge Laurels.
_Daphne Cnidium_ L., “_Garou_.” Algeria. Used for dyeing and tanning.
PROTEACEÆ.
_Banksia serrata_, Heath Honeysuckle. Australia. Specimen examined
contained 11 per cent. tannin; according to Maiden it reaches 23 per
cent.
_Banksia integrifolia._ Queensland. Bark contains 11 per cent. tannin.
_Grevillia striata._ Australia. Bark contains 18 per cent. tannin.
_Leucospermum conocarpum. Kruppelboom._ Knotted Tree. Cape of Good Hope.
Said by de Lof to contain 22 per cent. of tannin; but a specimen
examined by the Author yielded 10·9 per cent. on analysis.
_Protea mellifera._ Sugarbush. _Suikerbosch._ Cape of Good Hope.
Contains 25 per cent. tannin, according to de Lof; but Palmer found 18·8
per cent.
_Protea grandiflora. Waagenboom._ Cape. Contains 25 per cent. tannin (de
Lof); 15·9 per cent. (Palmer); 15·6 per cent. (Procter).
_Protea speciosa._ Cape of Good Hope.
_Leucadendron argenteum_, Silver Tree, _Silverboom_, _Witteboom_ Cape of
Good Hope. Bark said to contain 16 per cent. tannin (de Lof); a specimen
examined by the Author yielded 9·2 per cent.
_Brabium stellatifolium_, _Wilde Amandelboom_, Wild Almond.
PLUMBAGINÆ.
_Plumbago Europea_, Leadwort. Fr. _Dentelaire_. A garden plant in
England, native in France; contains much tannin, especially in the
root-bark.
_Statice coriaria_, Marsh Rosemary. South of Russia. Roots up to 3
metres long and 2-12 cm. thick; used by Kalmucks for tanning
sheep-skins; contain 22 per cent. of tannin (de Lof).
_Statice limonum_, Sea Lavender. Coasts and salt marshes of Europe and
America. Richer in tannin than _S. coriaria_; used in France, Spain and
Portugal.
Several other species contain tannin. These plants are allied to
“Thrift” (_Armeria_).
MALPIGHIACEÆ.
_Byrsonima spicata_, Antilles, “Tamwood.”
_Byrsonima coriacea_, Jamaica, “Golden Spoon.”
_Byrsonima chrysophylla_, etc.
_Malpighia punicifolia_, Nicaragua, “Nancite”; “Mangrutta.” Bark
contains 20-30 per cent. of light-coloured tannin.
POLYGALACEÆ, Milkwort Family.
_Krameria triandria_, Rhatany, Peru.
The root is used in medicine, and is stated to contain 40 per cent. of
tannin.
Wittstein found only 20 per cent. of an iron-greening
catechol-phloroglucol tannin allied to tormentil tannin, in the
root-bark, the only active part of root.
ANACARDIACEÆ.
_Loxopteryngium Lorenzii._ Span. _Quebracho colorado._ South America,
especially Argentine Republic; the highest proportion of tannin
occurring in the wood from Gran Chaco district. Wood contains on an
average about 20 per cent. of a red, difficultly-soluble tannin,
yielding “reds,” and containing catechol and phloroglucol. The tannin is
not very soluble in water, and hence can only be used in weak liquors,
but is very astringent, and gives a firm, reddish leather. The wood also
contains a catechin[139] and a colouring matter, fustin, identical with
that of “young fustic.” It is imported into England, and more largely to
Havre and Hamburg, in logs, which are there chipped like logwood, and
either used direct for tanning, or made into extract. A very cheap tan.
With alum it gives a yellow colour. The extract usually dissolves to a
fawn-coloured turbid solution. Many quebracho extracts are now made
completely soluble by treatment with alkalies or sulphites (cp. p. 338).
[139] See P. Arata, Journ. Chem. Soc., 1878, A, p. 986; 1881, A, p.
1152; and Perkin and Gunnell, Trans. Chem. Soc., 1896, 1303.
“Quebracho” means “axebreaker,” and is consequently applied to a variety
of hard woods. Its specific gravity is 1·27-1·38, and it therefore sinks
in water.
_Pistacia lentiscus_, Ital. _Pistacio_, Fr. _Lentisque_. Sicily,
Cyprus, Algeria. Small myrtle-like leaves contain from 12-19 per cent.
of a catechol-tannin, and are very largely used in the adulteration of
sumach. Leather tanned with sumach adulterated with this material
darkens and reddens on exposure to light and air, and for this reason
its use in many cases is decidedly injurious. In Cyprus and the East it
is known as “_Skens_,” Ital. _Schinia_, Fr. _Poudre de Lentisque_, in
England, often called Cyprus sumach. (Cp. p. 272.)
_P. orientalis_, _terebinthus_, _vera_, etc., India, Mediterranean.
Various aphis galls, 30-40 per cent. tannin. A sample of galls of
_Pistacia vera_, “_Gool-i-pista_,” India, recently examined in the
Author’s laboratory, contained 30 per cent. of a light-coloured tannin.
_Schinus molle_, “Molle,” Buenos Ayres. Leaves only used; said to
contain 19 per cent. tannin.
_S. Aroeira_, Brazil. Said to contain 14 per cent. tannin.
_Rhus coriaria_, Sicilian sumach. Ital. _Somacco_. (Fig. 53.) A shrubby
bush, of which leaves and small twigs are used.
[Illustration: FIG. 53.--Sicilian Sumach (_Rhus coriaria_).]
Mostly propagated by suckers from older plants, which are planted in
rows about two feet apart in early spring, and pruned to 6-8 inches.
Bushes begin to bear the year after planting, though the strength is not
so good as from more mature plants. Cropping is either by pruning off
shoots, or gathering leaves by hand; in the latter case shrubs are
pruned in winter. The leaves are dried either in the fields, or on
covered threshing floors, where they are afterwards separated from the
stems by beating. Some is exported in this state, as “leaf” or “baling”
sumach, but most is ground to fine powder under edge-runners.
“Ventilated” sumach is winnowed to remove dust and sand, which often
contains iron. “_Mascolino_” is the best sumach from Palermo and
district; “_feminella_” consists of weaker sorts from other parts, and
is generally used for mixing.
The different varieties of sumach are classed as follows:--
Relative
Market Value.
Sumach for baling 2·5
„ „ grinding 2·3
„ from yearling plants 1·5
„ „ ends of branches collected in autumn 1·0
To prepare these different grades for ultimate consumption, they are
ground in mills similar to those employed for crushing olives, that is,
in which two large stone wheels follow each other, revolving upon a
circular bed, the whole construction being about the same as the Spanish
or Mexican _arrastre_. The sumach thus pulverised is passed through
bolting-screens to separate the finer from the coarser particles.
After the sumach leaf has been subjected to the first process of
trituration, the coarse remaining portions are re-ground and the product
added to that which has been already obtained. The still unpulverised
residue known as _peduzzo_ is sifted, and the coarser and ungrindable
parts are used as fuel, while the finer are mixed with the
partially-ground, small, leaf-bearing branches (_gambuzza_, _gammuzza_),
and ground again.
Palermo is the principal seat of the sumach trade. The material is
generally bought from the small growers by middlemen, who hold it till
market conditions are favourable. The quotations are always in _tarì_ of
42·5 centimes per _cantar_ of 79·342 kilos, which are obsolete even in
Sicily, and have to be reckoned into _lire_ (francs) and kilos.
Consequently 1 tarì per cantar equals 0·53565 lira per 100 kilos.
In 1894, the prices delivered at the mills were about 41-42 tarì for
mascolino, 37-38 tarì for femminello, 14-18 tarì for brusca, and 10 tarì
for stinco, per cantar; the lira being worth about 9_d._[140]
[140] Cf. ‘Kew Bulletin,’ No. 107, pp. 293-6.
Sumach has been introduced into Australia, and is said to thrive well in
the dry plains of the Wimmera district.
Sumach often contains much sand, and sometimes particles of magnetic
iron ore, which cause black stains, and may be collected by a magnet,
and which dissolve in dilute hydrochloric acid without evolution of
hydrogen, to a yellow solution. Metallic iron, which is also attracted
by the magnet, dissolves in hydrochloric acid with effervescence to a
colourless or green solution.
Good sumach contains at least 25-27 per cent. of tannin. The Author has
analysed samples of undoubted genuineness containing as much as 32 per
cent. of a tannin, principally gallotannic, with some ellagitannic acid,
and a colouring matter (myricetin) identical with that of _Myrica nagi_
(p. 250), which gives yellows with alumina and tin mordants, and is
fugitive to light.
Sumach is the best tanning material known for pale colour and soft
tannage, and is hence used for moroccos, roans, skivers, etc., and also
for brightening leathers of darker tannages, such as mimosa, gambier,
the colouring matters of which warm sumach liquors seem able to
dissolve.
In the report of the Society of Arts Committee on bookbinding
leathers,[141] it is stated on abundant evidence, that sumach-tanned
leathers are less affected by light and gas-fumes, and less liable to
decay than those of any other known tannage.
[141] Soc. Arts. Journ., 1901, p. 14.
Sumach is frequently adulterated with the ground leaves and twigs of
_Pistacia lentiscus_ (“schinia” or “skens”), _Coriaria myrtifolia_
(“stinco”), _Tamarix africana_ (“brusca”), _Ailantus glandulosa_, _Vitis
vinifera_ (leaf of the common grape vine) and some other species of the
Rhus family, but _Pistacia lentiscus_ is used to a much larger extent
than any of the others. _Pistacia_, _coriaria_, and _tamarix_ all
contain considerable quantities of tannin, though less than genuine
sumach, and of a different chemical constitution.
The most satisfactory method of detecting these adulterants is by
microscopic examination, none of the chemical methods proposed being
very satisfactory; though, as many of the added matters contain catechol
tannins, while those of sumach are purely pyrogallol derivatives, the
method proposed by Hughes for the detection of quebracho in oakwood by
the reaction of concentrated sulphuric acid (p. 296) might render good
service, and any sumach infusion which was rendered turbid by
bromine-water would at least be open to grave suspicion.
[Illustration: PLATE III.
_Ailantus glandulosa._
_Coriaria myrtifolia._
_Colpoon compressa._
_Rhus cotinus._]
[Illustration: PLATE IV.
_Pistacia lentiscus._
_Rhus metopium._
_Rhus coriaria._
_Tamarix Africana._]
The most important work on the microscopic structure of the tissues of
sumach and its adulterants was done by Andreasch, when during the later
stages of his last illness he was obliged to winter in Sicily.[142] His
work will well repay study, but unfortunately does not admit of useful
abstraction here. A very useful investigation was also made in the
Author’s laboratory by Messrs. M. C. Lamb and W. H. Harrison,[143] as
regards the treatment and examination of the leaf-cuticles, which
renders the detection of mixture comparatively easy. For details, the
original memoir must be consulted, but if the suspected sumach be gently
warmed for a few minutes with strong nitric acid, its more delicate leaf
structure is entirely destroyed, and after washing and neutralising with
sodium carbonate the strong cuticles of the leaves of the more common
adulterants, “schinia” (_Pistacia lentiscus_), “stinco” (_Coriaria
myrtifolia_), “brusca” (_Tamarix africana_), and _Ailantus glandulosa_
are uninjured, and easily recognised. Examination is rendered easier by
dyeing the cuticles; safranine, acid green, Bismarck brown, and naphthol
yellow being suitable for the purpose. Mr. Lamb’s photographs of the
cuticles are reproduced on Plates III. and IV., but if possible, it is
most satisfactory to compare the suspected sample direct with known
specimens of the adulterants.
[142] ‘Sicilianischer Sumach und seine Verfalschung,’ Wien, 1898.
[143] ‘Sumach and the Microscopic Detection of its Adulterants,’
Journ. Soc. Dyers and Colorists, March 1899.
[Illustration: FIG. 54.--American Sumach (_Rhus glabra_).]
_R. glabra_, Southern States, U.S.A. (Fig. 54). Very largely used in the
States to take the place of Sicilian sumach. A sample collected by the
late Professor Trimble, and analysed in the Leather Industries
Laboratory, contained 25 per cent. of tannin and produced a leather of
very much darker colour than Sicilian.
_R. typhina_, “staghorn” or Virginian sumach, contains 10-18 per cent.
of tannin. A sample from same source as above contained 13 per cent.
_R. cotonoides_, U.S.A. The analysis of a sample of this material gave
21 per cent. of tanning matter, and leather tanned with it was almost
equal in colour to that from _R. glabra_.
Other sorts found in States: _R. semialata_ (5 per cent. tannin); _R.
aromatica_ (13 per cent. tannin); _R. metopium_ (8 per cent.); _R.
copallina_, _R. pumila_, _R. canadensis_; _R. toxicodendron_ is the
well-known “poison ivy,” a climbing plant which causes a severe and
irritating eruption if touched.
_R. glabra_ and _R. copallina_ are chiefly recommended for extended
cultivation in the United States.
In Virginia, the leaves are collected and cured by the country people,
and sold and delivered to owners of mills for grinding. Their particular
object being to secure the largest possible quantity of product at the
lowest cost, little attention is given to the quality obtained, or the
manner of collecting. The most intelligent dealers in the raw material
urge upon collectors to observe the following particulars:--To ensure a
maximum value for tanning purposes, the leaf should be taken when full
of sap, before it has turned red, has begun to wither, or has been
affected by frost. Either the leaf-bearing stems may be stripped off, or
the entire stalk may be cut away, and the leaves upon it allowed to
wither before being carried to the drying shed; but care must be
observed that they are neither scorched nor bleached by the sun. When
wilted, they are carried to a covered place, and spread upon open
shelving or racks to dry, avoiding the deposit in any one place of a
quantity so great as to endanger the quality of the product by
overheating and fermentation. Sumach should be allowed to remain in the
drying-house for at least one month before sending to the market; in
case of bad weather, a longer period may be required. When ready for
packing for shipment, it should be perfectly dry and very brittle,
otherwise it is likely to suffer injury in warehouses from heating and
fermentation.
Buyers of sumach leaves for grinding depend largely upon colour for the
determination of the value; the leaves should, therefore, when ready for
market, present a bright-green colour, which is evidence that they have
suffered neither from rain after being gathered, nor from heating during
the process of drying. Leaves having a mouldy odour or appearance are
rejected. The Virginian crop reaches 7000-8000 tons, and is collected
at any time between July 1st and the appearance of frost.
There is an important difference in the value of the European and
American products. The proportion of tannic acid in the latter is
generally lower than that found in the former, which is much preferred
by tanners and dyers. By using Sicilian sumach it is possible to make
the finest white leathers, while by the employment of the American
product, the leather has a disagreeable yellow or dark colour,
apparently due to a colouring matter, which exists in larger quantity in
the American variety than in the Sicilian.
Experiments upon the presence of colouring matters made by treating an
infusion of sumach with a solution of gelatine, gave the following
results:--
Virginia, mixed, collected in June, gave A nearly
white
precipitate.
„ „ „ July, „ A decidedly
yellowish-
white
precipitate.
„ _R. copallina_ „ August, „ A dirty-
yellow
precipitate.
„ _R. glabra_ „ „ „ A very dirty-
white
precipitate.
Fredericksburg, mixed „ „ „ A dirty-
yellow
precipitate.
Sicilian „ „ „ A slightly
yellowish-
white
precipitate.
For the purpose of tanning white and delicately coloured leathers,
therefore, the collection should be made in June; while for tanning
dark-coloured leathers, and for dyeing and calico-printing in dark
colours, where the slightly yellow shade will have no injurious effect,
the collection may be made in July. It appears that for all purposes,
the sumach collected after the 1st of August is inferior in quality.
Experimental results as regards percentage of tannin obtained by
collecting sumach at different seasons showed:--
Per Cent. of
Tannic Acid.
Virginia, mixed, collected in June, gave 22·75
„ „ „ July, „ 27·38
„ _R. glabra_ „ August, „ 23·56
„ _R. copallina_ „ „ „ 16·99
Sicilian, _R. coriaria_ „ „ „ 24·27
It is evident, therefore, that in order to secure the maximum amount of
tannic acid, the sumach should be collected in July, but the colouring
matter of the leaves has an important influence upon the value of the
product. The leaves of the upper extremities of the stalks are always
richer in tannic acid than those of the base; and the increase of age of
the plant is accompanied by a general diminution of this acid.
The mill used for grinding sumach leaves consists of a heavy, solid,
circular, wooden bed, 15 feet diameter, with a depression around the
edge a few inches deep and 1 foot wide, for the reception of the ground
sumach from the bed, and two edge-rollers, weighing about 2500 lb. each,
5-6 feet diameter, and provided with numerous teeth of iron or wood,
thickly inserted. In Europe and in some parts of the Southern States,
sumach is still ground by stones revolving on a stone bed, and the
sifting is often done by hand.
[Illustration: FIG. 55.--Venetian Sumach (_Rhus cotinus_).]
_R. cotinus_, Venetian sumach. Fr. _Arbre à perruques_; Ger.
_Perrukenstrauch_ (Fig. 55). More important as a dyeing than as a
tanning material, its twigs and wood, “young fustic,” containing a large
proportion of a colouring matter (fisetin), which with tin and alumina
mordants dyes bright yellows; and much resembles, but is not identical
with the myricetin present in _R. coriaria_.[144] Its leaves, known as
Turkish or Venetian sumach, contain about 17 per cent. of tannin, and
are used for tanning.
[144] Perkin and Allen, Trans. Chem. Soc. 1896, 1299.
_R. pentaphylla_, “Tezera,” Algeria, is used by the Arabs for tanning
goat-skins.
_R. Thunbergii_, _Kliphout_, Cape of Good Hope. A sample of the bark
analysed in the Author’s laboratory contained 28 per cent. of tanning
matter. A valuable tanning material, of reddish colour. The tannin is of
the catechol class.
Several other species of Rhus are used in tanning. _R. semialata_
yields Chinese and Japanese galls, containing up to 70 per cent.
gallotannic acid. They are caused, not by a fly, but by the attack of an
aphis, as are those of the allied Pistacia.[145] The aphides pass their
asexual stage inside the gall, which is large and thin-walled. A similar
aphis-gall is found on the American sumach. A specimen of the leaves
examined at the Yorkshire College yielded only 5 per cent. of tannin.
[145] See Flückiger and Hanbury, ‘Pharmacographia.’
_Mangifera indica_, Mango, widely distributed in the Tropics. Bark and
leaves rich in tannin, which gives green-blacks with iron.
CORIARIACEÆ.
_Coriaria myrtifolia_, French sumach (of which there are four
kinds--_fauvis_, _douzère_, _redoul_ or _redon_, and _pudis_). A
poisonous shrub of South of France; leaves used for tanning, and as a
sumach adulterant under the name of “stinco”; contain about 15 per cent.
tannin. (Cp. p. 272.)
_Coriaria ruscifolia_ bark, the _tutu_ of New Zealand, contains 16-17
per cent. of tannin.
Other Coriarias merit examination, and are known to contain much tannin.
RUBIACEÆ.
_Rubia_, Madder, allied to Galiums, which are almost the only English
representatives of the family. The coffee- and cinchona-plants are
foreign representatives.
_Nauclea_, or _Uncaria gambir_. East Indies. (Fig. 56.) A climbing
shrub, source of “gambier,” or “Terra Japonica”; also called “Catechu,”
in common with several other solid extracts. Gambier is first described
by the Dutch trader Couperus, in 1780; plant introduced in Malacca,
1758; plantations established in Singapore in 1819.
Culture is mainly by Chinamen, and is very rude; it yields rapid return,
but under the treatment to which it is subjected a plantation is worn
out in ten to fifteen years. Cropping commences three years after
planting, and is continued two to four times annually, with little
regard to fitness of shrubs, the plant being cropped till it has barely
leaves left to support existence. It is found advantageous to combine
pepper-culture with that of gambier, the spent leaves form a good
protection for the pepper-plant roots, but they have little actual
manurial value.
[Illustration: FIG. 56.--Gambier Shrub (_Nauclea gambir_).]
Cropping is done with a knife called a _parang_, while a larger knife is
used for chopping the leaves and twigs before they are put in a boiler,
in which they are heated with water till the liquid, which is constantly
stirred during the operation with a wooden five-pronged stirrer, becomes
syrupy. The leaves are then brought out with a wooden fork, and allowed
to drain on a tray, so that the liquor runs back into the boiler. The
coarser matter still remaining in the boiler is removed with a strainer
like a racquet, and the finer by straining the liquor through a
perforated cocoanut shell into small shallow tubs, where it is allowed
to cool with constant stirring with a cylindrical wooden bar, which is
worked up and down with a rotary motion until the catechin
crystallises. When quite cool the pasty mass is turned out of the tub,
cut into cubes with sides 1 inch long with a hoop-iron knife, and dried
on bamboo trays in racks under sheds, or sometimes smoke-dried with wood
fires.
Good cube gambier is an earthy-looking substance and is dark outside,
but pale within from crystallisation of catechin. Catechin is not itself
a tanning material, but is apparently converted into a tannin by drying
at 110°-126° C., when it parts with a molecule of water. It is very
probable that a similar change occurs in the tannery. The tannin is a
catechol-phloroglucol derivative, less astringent than most of this
series, and of pale colour. (See p. 297.)
A commoner quality, called “block-gambier,” instead of being cut into
cubes, is run into large oblong blocks of about 250 lb. weight, which
are wrapped in matting and exported in a pasty condition. These contain
35-40 per cent. of tannin, as estimated by the hide-powder method, while
the best cubes reach 50-65 per cent. Besides the forms named, various
others are made, principally for native use in chewing with betel-nut in
the form of small biscuits, or in thin discs (“wafer gambier”) by
running the pasty mass into bamboos and cutting the cylinder so formed
into thin slices. These forms are usually light in colour, and very rich
in catechin.
For details of the chemistry and employment of gambier, see pp. 228,
231, 239, etc.
APOCYNACEÆ.
_Aspidospermum quebracho_. Sp. _Quebracho blanco_. Brazil. Bark contains
aspidospermin, an alkaloid used in medicine, but both bark and wood are
poor in tannin.
_Quebracho colorado_, see ANACARDIACEÆ, p. 269.
ERICACEÆ, Heath Family.
_Arctostaphylos_ (or _Arbutus_) _uva-ursi_, Bearberry. Used in Russia,
Finland; twigs and leaves said to contain 14 per cent. tannin. Often
adulterated with leaves of _Vaccinium vitis-idæ_ or Cowberry.
_Arbutus unedo_, Common Arbutus. Leaves, fruit and bark used on
Mediterranean coasts.
VACCINIÆ.
_Vaccinium Myrtillus_, Bilberry. Used in Piedmont.
SAXIFRAGEÆ.
_Weimannia glabra_ L., “Curtidor” bark. Venezuela.
_Weimannia macrostachys_ D.C. Reunion.
_Weimannia racemosa_, New Zealand Towai or Tawheri bark.
These species contain 10-13 per cent. of iron-blueing tannin, and have
been practically used, but are not of much importance.
TAMARISCINIÆ.
Most of the members of this group are poor in tannin, but several
species have galls which are rich.
_Tamarix africana_; Egypt, Algeria. Galls containing 26-56 per cent.
tannin. The small twigs are collected in Tunis, and when dried and
ground are imported into Sicily to be used for the adulteration of
sumach under the name of “Brusca,” and contain about 9 per cent. of
tannin. (Cp. p. 272.)
_T. articulata_, Morocco, yields galls produced by aphides, called in
Arabia _Takout_, and stated by Vogel to contain 43 per cent. of tannin.
_Tamarix gallica_, used in Spain and Italy.
OXALIDEÆ.
_Oxalis gigantea_, source of _churco_ bark, Chili. A thin, brittle, dark
red bark, mostly about 2 mm. thick, cork and ross entirely absent. The
bark is brittle, and the cells thin. It contains about 25 per cent. of
an easily extracted, dark red tannin, giving green-blacks with iron. The
bark has been incorrectly attributed to _Fuchsia macrostemma_. (Cp. Von
Höhnel, ‘Die Gerberinden,’ p. 125, and this book, p. 284.)
COMBRETACEÆ.
Several families of this genus contain trees rich in tannin, but most
important are the Myrobalans (often, but incorrectly, written Myrabolams
or Myrabolans), the unripe fruit of various species of Indian
_Terminalia_.
[Illustration: FIG. 57.--Myrobalan Tree (_Terminalia Chebula_).]
_T. Chebula_ (Fig. 57), a tree 40-50 feet high, and yielding good
timber, is the source of all the ordinary varieties, which differ only
in the district from which they are obtained, and the state of maturity
of the fruit. The nuts contain from 30 to 40 per cent. of tannin. Of the
various sorts, probably those known as Bombays are least unripe, while
“lean greens” are the most so. The unripe fruit is the richest in
tannin. “Bombays” have a smooth skin in coarse wrinkles, and when cut
are porous and light coloured. “J’s” (Jubbalpores) and “V’s”
(Vingorlas), have finer and shallower wrinkles, and are harder, solider
and consequently darker looking, but do not give a darker liquor, while
“lean greens” are greener, have less yellow colouring matter, and
consequently more nearly approach in character to sumach, which the
tannin in many respects resembles, though probably containing more
ellagitannic acid in proportion to gallotannic acid than the latter.
The “nuts” should be bright in colour, not worm-eaten, nor “waxy” or
soft. If kept in a damp place they rapidly absorb moisture, and fall
into the “waxy” condition, in which they are very difficult to grind,
sticking to and choking the cutters or beaters of the mill.
Neither the large hard stones nor their kernels contain tannin, but the
latter have an oil which gives a peculiar odour to leather. The tannin
exists in large and rather thickly-walled cells, and is not very easily
extracted; the skin is wrinkled, but the uncrushed nuts swell up to
their original plum-like form when placed in water for some time. The
bark is almost as rich as the fruit, and the tree also yields galls.
_T. Belerica_ yields Beleric or “Bedda nuts,” which are downy, rounder
and larger than ordinary myrobalans, and contain about 12 per cent. of
tannin, used as adulterant of ground myrobalans. A sample of solid
extract made from the bark of T. Belerica contained 70 per cent. of
tannin.
_T. tomentosa_ has downy nuts, containing about 10 per cent. of tannin,
bark stated by de Lof to contain 36 per cent. of tannin. A sample of
solid extract contained 56 per cent. of tannin. The bark contains about
11 per cent.
There are several other Indian species.
_T. Catappa_, “Badamier bark” of Mauritius, contains 12 per cent. of
tannin.
_T. mauritiana_, “Jamrosa bark,” said to contain 30 per cent. of tannin.
_T. Oliveri_, Malay Archipelago, yields “Thann leaves,” from which an
extract is made as a cutch substitute. A sample of the extract from
Burmah examined recently in the Author’s laboratory, contained 62 per
cent. of tannin. The tannin is a catechol derivative, differing from
that of _Acacia catechu_ in containing no phloroglucol (p. 297).
A sample of bark from Mandalay contained 31 per cent. of tannin, while
the leaves from the same tree contained 14 per cent.
Emblic myrobalans, see p. 293.
RHIZOPHORACEÆ, Mangles or Mangroves.
_Rhizophora Mangle_, and other allied species, Mangrove or Mangle,
_Manglier_, _Paletuvier_, _Cascalote_, grows on tropical coasts all
round the world. The barks vary much in strength, from 15 up to 40 per
cent. in different species (see _Ceriops_). Leaves, used in Havana, are
said to contain 22 per cent. tannin. According to Eitner, the younger
plants contain the highest proportion of tannin. _R. Mangle_ seems to
yield a bark inferior to several other species.
All trees growing in swamp, and of the same character of growth as
mangrove, are called “Bakau” in the East Indies (Anglice, mangrove) and
various species of _Ceriops_ yield the best tanning bark. A tidal
mangrove swamp at low water is a tangle of arched roots like inverted
branches on which the trees are supported.
The catechol-tannin, which is easily extracted, is of deep red colour
and allied to that of the mimosas. In admixture with other materials the
red colour has a much smaller effect, and mangrove bark is now largely
used in combination with pine, oak and mimosa.
Several other species are also rich in tannin, and used in different
parts of the world under the name of mangle, as are also several species
of _Conocarpus_ belonging to the _Combretaceæ_.
_Rhizophora mucronata_. India and Burmah. Bark varies considerably;
David Hooper, Indian Museum, Calcutta, gives 26·9 per cent. of tannin.
Dr. Koerner (Deutschen Gerberschule, Freiberg) analysed two samples in
1900, one containing 48 per cent. and the other 21 per cent. of tannin;
two samples from the British Imperial Institute recently examined by the
Author showed only 4·5 and 6·1 per cent. of tannin respectively.
_Ceriops Candolleana_, _Bakau_ or _Tengah_ Bark, East Indies. _Goran_,
Bengal. Contains up to 27 per cent. of tannin and yields an extract
which promises well as a substitute for cutch, to which, for dyeing
purposes it is nearly or quite equal. The solid extract contains up to
65 per cent. tannin, making a good but dark red leather.
_Ceriops Roxburghiana_, a somewhat larger tree, also growing in the
Sunderbans, bark very similar in strength and character to the above.
ONAGRACEÆ, the Œnothera Family.
_Fuchsia excorticata_, the only deciduous tree of New Zealand. Contains
5 per cent. tannin.
_Fuchsia macrostemma_, Chili. Yields Tilco or Chilco bark. Churco bark
has been incorrectly attributed to this plant, but it is certainly
derived from an _oxalis_, as stated by the Kew authorities. (Cp. von
Höhnel, ‘Die Gerberinde,’ p. 125.)
GUNNERACEÆ.
_Gunnera scabra_ (_Pangue?_), _Pauke_, Chili. Used occasionally in the
tanning of goat-skins.
MYRTACEÆ.
_Eucalyptus globulus_, and other species of _E._ common in Australia,
and introduced into Algeria and Southern Europe (gum-trees), are more or
less rich in catechol-tannins, their sap being the source of Botany Bay
or Australian kinos, which contain up to 79 per cent. tannin. Several
species of _Eucalyptus_ afford astringent extracts; those from the
“red,” “white,” or “flooded” gum (_E. rostrata_), the “blood-wood” (_E.
corymbosa_), and _E. citriodora_, being quite suitable for replacing the
officinal kind. The gum is chiefly obtained by woodcutters, being found
in a viscid state in flattened cavities in the wood, and soon becoming
inspissated, hard and brittle. Minor quantities are procured by incising
the bark of living trees; a treacly fluid yielding 35 per cent. of solid
kino on evaporation is thus obtained. The gum is imported from
Australia, but there are no statistics to show in what quantity.[146]
[146] Compare Journ. Soc. Chem. Ind., 1902, p. 159.
_Eucalyptus longifolia_ bark, the “woolly-butt” of Australia, contains
8·3 per cent. of tannic acid, and 2·8 of gallic. The “peppermint” tree
contains 20 per cent. of tannic acid in its bark. The “stringy-bark”
(_E. obliqua_) gives 13¹⁄₂ per cent. of kinotannic acid. The Victorian
“iron-bark” (_E. leucoxylon_) contains 22 per cent. of kinotannic acid,
but is available only for inferior leather.
_Myrtus communis_, and several other myrtle species, contain a
considerable amount of tannin in the bark and leaves.
GRANATACEÆ.
_Punica Granatum_, Pomegranate. Peel of fruit employed in Spain and the
East as substitute for sumach, containing up to 25 per cent. of tannin.
Bark said to contain 22 per cent. tannin. _Balaustines_, wild
pomegranates, East Indies. Fruit, said to contain 46 per cent. tannin.
ROSACEÆ.
_Tormentilla erecta_, _Potentilla tormentilla_. Root variously stated to
contain 20-46 per cent. tannin. Red coloured leather, formerly used in
Orkneys, Shetland, and Faroe Islands, and in some parts of Germany.
_Sorbus_ or _Pyrus Aucuparia_, Mountain Ash. Bark said to be stronger
than oak.
Many other plants of the family contain tannin, among others the
strawberry.
PAPILIONACEÆ.
_Butea frondosa_.[147] This (with _Pterocarpus marsupium_)[148]
furnishes East Indian kino. The flowers are used in India as a dye,
under the name of Tesu. Bark fairly rich in tannin.
[147] ‘Dictionary of Economic Products,’ I.B., p. 944; Hummel and
Cavallo, Proc. Chem. Soc. 1894, p. ii.
[148] Agricult. Ledger, 1901, No. 11, Gov. Printing Office, Calcutta.
_Pterocarpus_ or _Drepanocarpus senegalensis_ is the source of African
kino, which contains up to 75 per cent. of tannin.
_Cæsalpinia coriaria_, Divi-divi. A tree of 20-30 feet, native in
Central America, introduced successfully in India, but principally
imported from Maracaibo, Paraiba and Rio Hache. The dried pods contain
40-45 per cent. of a pyrogallol-tannin, mainly ellagitannic acid, and
would be a most valuable tanning material, but for a liability to
fermentation and sudden development of a deep red colouring matter. The
causes are not well understood, but apparently the risk can be
materially lessened by use of antiseptics. If used in strong liquors it
gives a heavy and firm leather, but is principally employed as a partial
substitute for gambier on dressing leather. Used in rapid drum-tannage
for light leathers, an excellent colour may be obtained. It is said to
give an especially firm and glossy flesh. Leather tanned with it, even
when of outwardly good colour, has often a blueish-violet shade within,
perhaps due to the development of a colouring matter allied to that of
logwood. The seeds do not contain tannin, which lies almost free in the
husk of the pod. The pods are about 3-4 cm. long, dark outside, and curl
up in drying to an S-shape.
_C. digyna_, _Tari_ or _teri_ pods. Occurs in Prome, Toungoo, Bassin,
Mynang and other parts of India and Burmah, where it is used as a drug.
The pod-case is said to yield over 50 per cent. of tanning matter. A
sample from Burmah, kindly sent by the Imperial Institute, examined by
the Author in 1900, contained 24 per cent. of tannin, but after removing
the seeds the remaining pod-cases yielded 44 per cent. of tannin on
analysis. _C. digyna_ promises to become a valuable tanning material if
it proves free from the tendency to ferment which is so troublesome in
divi-divi. It has been introduced into England under the name of “white
tan,” which yields a leather quite as white as sumach; but the supply
seems at present uncertain.
_C. cacolaco_, Cascalote, Mexico. Pods rich in tannin (up to 55 per
cent., Eitner). Pods larger and fleshier than divi, seeds smaller,
tannin similar.
The pods of several other Cæsalpinias are used in tanning, sometimes
under the name of “Algarobilla,” which is simply a diminutive of
Algaroba, the carob, or locust-bean, derived from Arabic _al Kharroba_,
and applied to several small pods. (See _Balsamocarpon_ and _Prosopis_.)
_C._ (or _Balsamocarpon_) _brevifolia_, Chili, ordinary Algarobilla.
Fig. 58. One of the strongest tanning materials known, containing an
average of 45 per cent. of a tannin very like that of divi, but less
prone to discoloration. The tannin lies loose in a very open skeleton of
fibre, and is easily soluble in cold water; the seeds contain no tannin.
If not allowed to ferment it produces a very bright-coloured leather.
Algarobilla has been attributed to _Prosopis pallida_, but this appears
incorrect. Several species of _P._ are known to yield tanning pods;
those of _P. Stephaniana_ of the desert of Kaschan, in Persia, are
_dschigh dschighe_, perhaps identical with _dchift_ or _jaft_. (See p.
263.) Bark of _P. spicigera_ used in Punjab.
_C._ (or _Hæmatoxylon_) _campechianum_, Logwood, Central America. In
addition to colouring matter, and a glucoside which it yields on
oxidation, this wood contains about 3 per cent. tannin. Its principal
use is in dyeing blacks with iron or chrome mordants. (See p. 413.)
[Illustration: FIG. 58.[149]--Algarobilla (_Cæsalpinia brevifolia_).]
[149] ‘New Commercial Drugs and Plants,’ No. 5, T. Christy.
_C. echinata_ yields “Brazil-wood.” (See p. 413.)
_C. Sappan_, Sappan-wood, India.
_Cassia auriculata_, _Turwar_ or _Tanghadi_ bark, Southern India. Used
for tanning so-called “Persian” sheep- and goat-skins, contains about
17 per cent. of a catechol tannin. Leather tanned with it is of a pale
yellow colour, but rapidly reddens in sunlight. Cp. p. 235.
_C. fistula_, India. Husk of pod, 17 per cent. tannin. The pulp of pod
is used as an aperient.
_C. elongata_ and _lanceolata_. Senna leaves. Upper Egypt.
_C. Sophora_, “_Bali-babilan_.”
[Illustration: FIG. 59.--Babool (_Acacia arabica_).]
MIMOSEÆ, a Tribe of Leguminosæ.
_Acacia arabica_, “Babool,” “Babul,” India, Egypt. Fig. 59. Bark
contains about 12-20 per cent. of catechol tannin; one of the principal
Indian tanning materials, used for kips and heavier leathers. Pods, used
in India for bating, contain about same amount of tannin as bark, but of
a different kind, that of the bark being a catechol-tannin, with a good
deal of red colouring matter, while the pods contain a paler tannin
allied to divi, which is not precipitated by lime-water. In Egypt the
pods are called _bablah_, a name which is also applied to pods of _A.
cineraria_ and _A. vera_, and others. They are used for dyeing
glove-leathers.
_A. nilotica_, Egypt. Pods called _neb-neb_ or _bablah_.
[Illustration: FIG. 60.--Cutch Tree (_Acacia catechu_).]
_A. catechu_, India. The wood yields cutch or “dark catechu.” A lighter
coloured variety called _kath_, containing much crystallised catechin,
is also made in India, and principally used for chewing with betel. _A.
catechu_ is a tree 30-40 feet high, common in India and Burma, and also
in tropical East Africa, where, however, it is not utilised. In Southern
India, _A. suma_ is also used for the same purpose.
Trees of about 1 foot diameter are cut down, and the wood (some state
the heart-wood only) is reduced to chips, and boiled with water in
earthen jars over a mud-fireplace. As the liquor becomes thick and
strong, it is decanted into another vessel, and the evaporation
continued till the extract will set on cooling, when it is poured into
moulds made of leaves or clay, the drying being completed by exposure to
the sun and air. “Kath,” or pale cutch, is made in Northern India, by
stopping the evaporation at an earlier point, and allowing the liquor to
cool, and crystallise over twigs and leaves thrown into pots for the
purpose. It contains a large proportion of catechin, apparently
identical with that of gambier, but its tannin is much redder. Good
cutch contains about 60 per cent. tanning matter, but is principally
used for dyeing browns and blacks with chrome and iron mordants. It
contains quercetin, a yellow colouring matter (p. 263).
_A. leucophlea_, India and Java “_Pilang_.” Pods and bark equal to _A.
arabica_.
Australia abounds in acacias (mimosas), many of which are used in
tanning, but vary greatly in strength, not only according to species,
but probably also by situation and growth. Probably the best information
is to be found in a pamphlet on ‘Wattles and Wattle-Bark,’ by J. H.
Maiden, F.L.S., published by the Department of Public Instruction at
Sydney, 1890. His analyses were made by the Löwenthal process, and can
only be roughly compared with those by the hide-powder method. The
analyses given are by the I.A.L.T.C. method, and mostly on samples
furnished by Mr. Maiden.
A peculiarity largely developed in the mimosa family is the tendency for
the true leaves to be suppressed, and their place taken by the flattened
and expanded midrib (phyllode). Thus leaves of two very distinct forms
are common in the genus, and some acacias, as _A. heterophylla_, may
have both forms on the same branch. Compare _A. pycnantha_ and _A.
decurrens_.
The Australian mimosas have been naturalised in India, and grow freely
in the Nilgiri Hills, but the bark does not appear to be utilised.
The most important species are as follows:--
_A. pycnantha._ (Fig. 61.) “Broad-leaved” or “Golden Wattle,” South
Australia. One of the strongest tanning barks known. A sample marked
“special,” analysed in the Yorkshire College, contained 50 per cent. of
tannin; another sample marked “ordinary” contained 40 per cent.
[Illustration: FIG. 61.--Broad-leaved Wattle (_Acacia pycnantha_).]
[Illustration: FIG. 62.--Green Wattle (_Acacia decurrens_).]
_A. longifolia_, the Golden Wattle of New South Wales, only contains
half as much tannin as _A. pycnantha_.
_A. mollissima_, with its two varieties _A. decurrens_ (Fig. 62) and _A.
dealbata_, are among the most important of the Wattle family
commercially. Two samples of the former marked “Green Wattle” showed
36-39 per cent. of tanning matter; another sample marked “Sydney Green
Wattle,” contained 41 per cent. A sample of _A. decurrens_, the second
variety, was much weaker, showing only 12 per cent. on analysis.
_A. penninervis_ (Hickory bark) is said to be particularly hardy, but
its strength seems to vary. A sample from Bateman’s Bay contained 38 per
cent. of tanning matter.
_A. binervata_, another “Black Wattle” contains up to 30 per cent.
tanning matter, as does also the “Weeping Willow,” _A. saligna_. The
latter is poisonous, and is said to be used for killing fish.
_A. prominens_, the bark of which resembles that of the Golden Wattle,
_A. longifolia_, in appearance contains only 14 per cent. tannin.
The cultivation of wattles in Australia has been somewhat neglected, but
would render possible the utilisation of many acres of land lying waste,
or which have already been exhausted and rendered unfit for the growth
of cereals. It requires so little attention as to make it very
profitable, and wattle-growing and sheep-grazing can be combined
satisfactorily after the first year, when the young trees in the
plantation have reached the height of 3-4 feet. In Natal the Australian
wattles, especially _A. mollissima_, have been acclimatised and
cultivated with success, and large quantities of excellent bark are now
exported to England. African wattle-barks usually contain about 30 per
cent. of tannin.
Wattles grow in almost any soil, even the poorest, but their growth is
most rapid on loose, sandy patches, or where the surface has been broken
for agricultural purposes. When the soil is hard and firm,
plough-furrows should be made at a regular distance of 6-8 feet apart,
and the seeds dropped into these. The seed should be sown in May, having
been previously soaked in hot water, a little below boiling temperature,
in which they may be allowed to remain for a few hours. It should be
dropped at an average distance of 1 foot apart along the furrow, in
which case, about 7200 seeds would suffice for one acre of land. The
seed should not be covered with more than about ¹⁄₄ inch of soil.
On loose sandy soil, it might even be unnecessary to break up the ground
in any way; the furrows may be dispensed with, and the seed sown
broadcast after the land has been harrowed. After the plants have come
up, they should be thinned so that they stand 6-8 feet apart. When the
young trees have attained the height of 3-4 feet, the lower branches
should be pruned off, and every effort afterwards made to keep the stem
straight and clear, in order to facilitate the stripping, and induce an
increased yield of bark. It is advisable that the black and broad-leaved
kinds should be grown separately, as the black wattle, being of much
larger and quicker growth, would oppress the slower-growing broad-leaved
one. Care should be taken to replace every tree stripped by re-sowing,
in order that there should be as little variation in the yield as
possible. In Victoria, the months of September-December are those in
which the sap rises without intermission, and the bark is charged with
tannin. Analysis proves that the bark from trees growing on limestone is
greatly inferior in tannin to that obtained from other formations,
differing 10-25 per cent.
The following are South American mimosas:--
_A. cavenia_, Espinillo. Bark, contains 6 per cent., pods, 18-21 per
cent., or more of tannin.
_A. cebil_, Red Cebil. Bark, contains 10-15 per cent.; leaves, 6-7 per
cent. tannin. Argentine Republic.
_A. Guarensis_, Algarobilla of Argentine Republic. Bark, pods and
flowers said to be used for tanning.
_A. timbo_, Buenos Ayres.
_A. curupi_, Curupy bark.
_A. angico_, or _Piptadenia macrocarpa_, Brazil, yields “angica bark,” a
sample of which contained 20 per cent. of tanning matter when analysed
recently in the Author’s laboratory.
“White Bark,” South America, probably an acacia, bark internally very
similar to angica, if not identical.
_A. horrida_, “_Doornbosch_,” Cape of Good Hope, contains 8 per cent. of
tannin.
_Inga feuillei_, “_Paypay_,” Peru. Pods said to contain 12-15 per cent.
of tannin (doubtful). Several other species of _Inga_ known to contain
tannin.
_Elephantorrhiza Burchellii_, _Elandsboschjes_, _Tugwar_, or _Tulwah_,
South Africa; a papilionaceous plant. The air-dry root contains 12 per
cent. of tannin, and a great deal of red colouring matter. The roots are
several feet long, and about 2 inches in diameter, growing by the sides
of rivers.
The following additions may be made to the above list:--
EUPHORBIACEÆ.
_Cleistanthus collinus_, “_Kodarsi_,” Deccan. Bark stated to contain 33
per cent. of tannin.
_Phyllanthus emblica_, India, yields emblic myrobalans, which in
immature condition contain considerable tannin. Leaves (18 per cent.)
and bark used for tanning.
_Phyllanthus distichus_ and _nepalensis_ both yield tanning barks.
COMBRETACEÆ.
_Anogeissus latifolia_, India. Bark and leaves rich in tannin.
GUTTIFERÆ.
_Garcinia mangostana_, India. The rind of the mangosteen fruit contains
much tannin.
CHAPTER XIX.
_THE CHEMISTRY OF THE TANNINS._
The essential constituents of tanning materials are members of a large
group of organic compounds known as “tannins” or “tannic acids,” which
are widely distributed throughout the vegetable kingdom, and said to
have one representative among animals, in the body of the corn-weevil.
Their use in vegetable physiology is as yet uncertain, and indeed they
appear in some cases to be waste products of organic change. The
tannins, though varying considerably in their chemical constitution, and
in many important characteristics, are all marked by the power of
precipitating gelatine and some allied bodies from their solutions, of
converting animal skin into the imputrescible material known as leather,
and of forming dark-coloured compounds with ferric salts which are often
utilised as inks. They are also precipitated by lead and copper
acetates, stannous chloride, and many other metallic salts, and form
insoluble compounds with many organic bases, such as quinine, and with
the basic aniline colours. They are possessed of feeble acid character.
All tannins are soluble in water to a greater or less degree; they are
also soluble in alcohol, in mixtures of alcohol and ether, in ethyl
acetate, acetone, and a few similar solvents, but are not dissolved by
dry ether alone, nor by chloroform, petroleum spirit, carbon disulphide,
nor benzine.
As the tannins are uncrystallisable, and incapable of being distilled
without decomposition, they are exceedingly difficult to obtain in a
state of purity, and, owing to the considerable differences in their
character, no one method is equally applicable to the whole group. As
their successful separation requires considerable chemical training, and
experience, detailed description is outside the scope of the present
work, but some particulars of the more important methods employed are
given on p. 43, L.I.L.B.
Their chemical constitution is complex and in most cases imperfectly
understood, but all the natural tannins which have been investigated
prove to be derivatives of the trihydric phenol, pyrogallol, or of the
dihydric phenol, catechol, the latter of which is often accompanied by a
trihydric phenol, phloroglucol, which is isomeric with pyrogallol. The
phenols are, themselves, a class of derivatives of benzene, C₆H₆, in
which one or more of the hydrogen atoms are replaced by OH groups.
Common phenol or “carbolic acid” is their simplest representative. Many
of them, including pyrogallol and catechol, are used as photographic
“developers.” The phenols on replacing another hydrogen by carboxyl
(CO.OH) form true acids, of which salicylic corresponds to common
phenol, protocatechuic to catechol, and gallic to pyrogallol; and the
tannins are apparently complicated acids, in which one of the two latter
acids is linked to a second molecule of the same or another acid as an
anhydride, in some cases possibly with the addition of phenols or other
organic groups. For more detailed information, see L.I.L.B., p. 45.
Gallotannic acid is apparently digallic acid, in which two molecules of
gallic acid are linked together after giving up the elements of a
molecule of water. Natural gallotannic acid and many other tannins are
glucosides, or at least contain glucose, which in many cases can be
removed by purification.
From what has just been said, it is obvious that a classification of the
tannins according to constitution, is at present impracticable, not only
from our imperfect knowledge, but from the difficulty of separating and
determining the products of their decomposition. It is not, however,
difficult to distinguish the catechol- from the pyrogallol-tannins by
their chemical characteristics, apart from actual separation of the
phenols, and the division is important as it is marked by certain broad
differences in their properties which affect their use in tanning.
The catechol-tannins, dissolved in water, yield a precipitate when
bromine-water is added till the solution smells strongly of it. The
precipitate is occasionally crystalline, but generally amorphous, and of
yellowish or brownish colour. When the infusion of tannin is very weak,
the precipitate is sometimes only slight, or forms slowly.
Pyrogallol-tannins give no precipitate with bromine-water. Another
reaction, which is generally characteristic of catechol-tannins, is that
if concentrated sulphuric acid is added to a single drop of the infusion
in a test-tube, a dark red, or crimson ring is formed at the junction of
the two liquids, and on dilution with water, the solution is generally
pink. Pyrogallol-tannins on the other hand give a yellow, or at most a
dark brown ring, which dilutes to a yellowish solution. This reaction is
of great delicacy, which may be further increased by the use of an
alcoholic instead of an aqueous extract. It is often given also by the
non-tannin residue of catechol-tannins which is left after treatment
with hide-powder, in which case it is probably due to the presence of
catechins allied to the tannins. With ferric salts (preferably a
solution of iron-alum), pyrogallol-tannins give blue-blacks, while
catechol-tannins generally give greenish blacks, though the reaction is
apt to be rendered uncertain by the presence of colouring matters, or
perhaps in some cases by the constitution of the tannin. Thus aqueous
infusions of common oak-bark (_Quercus robur_) give a decidedly blue
black with iron, though the tannin is a catechol one, and the purified
tannin gives a green-black. Most of the barks of American oaks, such as
_Q. prinus_, give green-blacks without purification. The Australian
mimosas generally give dull purple-blacks with iron-salts, though they
all contain catechol-tannins. The iron test was first proposed by
Stenhouse as a means of classification. Trimble has shown that while the
purified pyrogallol-tannins only contain about 52 per cent. of carbon,
the catechol-tannins have about 60 per cent.[150]
[150] ‘The Tannins,’ ii. p. 131.
Only two tannins of the pyrogallol group have been definitely
distinguished, though it is very possible that more exist. These are
ordinary tannic acid of gall-nuts (probably digallic acid), which yields
gallic acid when heated with dilute acids, or by the action of certain
unorganised ferments or zymases (p. 16) which are generally present in
tanning materials; and ellagitannic acid (usually present in greater or
less proportion in mixture with the gallotannic acid), which, under the
same conditions yields “bloom” (an insoluble deposit of ellagic acid),
as one of its products. Hence it happens that most pyrogallol tannins
deposit “bloom” on leather, though in very different proportions,
gall-nuts and sumach giving very little, and myrobalans, valonia and
divi-divi a great deal. English oak-bark deposits a good deal of “bloom”
on leather, though it is certain that its principal tannin is a catechol
one, but it is possible that the blue-black which it gives with iron
salts may be due to the presence of ellagitannic acid, though
gallotannic acid is known to be absent. The tannins of oakwood, chestnut
and valonia are principally if not entirely pyrogallol derivatives,
closely allied to, if not identical with the two just named, but, if so,
very difficult to obtain in a pure condition. It is noteworthy that so
wide a difference exists between the various products of the oak; galls,
bark, fruit and wood yielding tannins of very varied properties. The
tannin of other galls, such as those of the sumach and pistacio,
generally contain gallotannic acid, even when, as in the last case, the
remainder of the plant yields catechol-tannins.
The tannins of the catechol group appear to present much more variety
than the pyrogallol-tannins, though it is possible that many apparent
differences may be due to the presence of impurities. It is, however, at
least certain that the tannins of gambier and cutch contain phloroglucol
as one of their constituents, while it is absent from most other tanning
materials. Its presence is easily detected by moistening pine-wood (a
deal shaving) with an infusion of the tannin in question, and applying a
little concentrated hydrochloric acid, when after a few minutes, a
bright red or purple stain is produced. The catechol-tannins, on boiling
with acids, yield no gallic acid, or bloom, but generally a deposit of
“reds,” insoluble in water but soluble in alkaline liquids, and in
alcohol, and which are closely allied to resins, and especially to the
red resin known as “dragon’s blood.” These reds are anhydrides of the
tannins, that is, are produced from them by the abstraction of water;
and are consequently formed by any agency which tends to remove water,
such as long boiling or high temperature. The lower anhydrides (that is,
those from which least water has been abstracted) are not wholly
insoluble, but form the “difficultly soluble” tannins which are
naturally present in many materials. They are much more readily soluble
in hot than in cold water, which is one of the causes why liquors made
by the aid of heat generally give darker colour to leather than those
extracted cold. They exist in large quantity in hemlock extract and
quebracho. Attempts have been made to utilise their alkaline solutions
for tanning, but without much success; though alkalies or alkaline
sulphites are frequently used to obtain “soluble” quebracho extracts (p.
338).
Many catechol tanning materials, and especially gambier, cutch and
quebracho, contain in addition to the tannin, considerable portions of
colourless bodies called catechins, which are only slightly soluble in
cold water, but readily in hot, and which crystallise out on cooling.
These bodies do not tan, but are in a sense the source of the tannins,
which appear to be their first anhydrides, the reds being formed by the
successive loss of further molecules of water. These bodies very
probably ultimately become converted into tannins by changes in the
tanyard. The change may be brought about very rapidly by heating to a
temperature of 100 to 120° C.[151] The catechin of gambier, by
crystallising on and in the leather, is the cause of a trouble known as
“whites,” which is common where gambier is largely used.
[151] Some doubt exists as to the exact temperature at which catechins
become converted into anhydrides, and Perkin puts it higher than that
stated.
An unfortunate peculiarity, apparently common to all catechol-tannins,
is that, however light-coloured the leather produced by them, it darkens
and reddens rapidly by exposure to strong light, and ultimately becomes
quite friable and rotten.[152] Cp. pp. 234, 272.
[152] Cp. Report of Committee on Leathers for Bookbinding, Journ. Soc.
of Arts, 1901, p. 14.
Wagner, a German chemist, attempted to classify the tannins into
“physiological” tannins, which were produced in the natural growth of
the plant, and “pathological” which were caused by the attack of insects
such as the gall-wasps, and he further ventured the assertion that only
the former class were capable of producing leather. It has since been
shown that the tannins produced in galls are identical with some of
those found in healthy plants, and galls themselves have been used in
tanning from very ancient times. It is only necessary to remind the
reader of the use of Turkish gall-nuts, in place of sumach, which was
common in the East in the tannage of moroccos, and of the “Knoppern,” or
oak-galls formerly so largely used in Austria as a tanning material for
sole leather. It is true that the tannin of galls is not very suitable
for the latter purpose, consisting as it does mostly of gallotannic
acid, which, giving no solid deposit of bloom or reds, is incapable of
making a heavy or solid leather. Pure gallotannic acid itself produces a
very white and soft leather.
The class to which the tannins of the different tanning materials belong
is mostly mentioned in the Botanical List (Chap. XVIII.), but it may be
well here to specify a few of the most common. Galls and sumach contain
gallotannic acid with a little ellagitannic; myrobalans, valonia,
divi-divi, algarobilla, oakwood and chestnut are all pyrogallol-tannins
giving ellagic and gallic acid among their decomposition products. All
the pine barks, including the American hemlock, and the larch, all the
acacias and mimosas, including the Indian Babul (_Acacia arabica_), the
oak barks (though not the oak wood, fruits, or galls), quebracho wood,
cassia[153] and mangrove barks, canaigre, cutch and gambier are
catechol-tannins, and the two last contain phloroglucol, of which minute
traces are also present in many other catechol-tannins (p. 297).
[153] _Cassia auriculata_, or “turwar” bark, is the ordinary tannage
of the East Indian or “Persian” sheep- and goat-skins, largely used in
bookbinding, but which redden and decay very rapidly.
Gallotannic acid, and several artificial tannins with the characteristic
reactions of the class have been produced in the laboratory, but there
is no present prospect of their manufacture at prices which can in the
faintest way compete with those of natural production.
Tanning materials frequently contain mordant colouring matters, often
derived from the same phenols as the accompanying tannins. They also
usually contain gums, starch and glucose. Oak bark contains lævulose
which is not combined with the tannin. Many tannins, however, exist in
nature in combination with the sugars as glucosides, which are easily
decomposed by the action of acids or by fermentation. These sugary
matters are important as furnishing by fermentation the acetic and
lactic acids of tanning liquors.
CHAPTER XX.
_THE SAMPLING AND ANALYSIS OF TANNING MATERIALS._
Although the analysis of tanning materials falls more properly within
the scope of a book for chemists than one intended primarily for
tanners, and though it has been treated at considerable length in the
‘Leather Industries Laboratory Book,’ a slight sketch must now be given
of the methods in general use, since it is of great importance that at
least the principles on which they are based should be understood by all
to whom they are of practical interest, and also because an approximate
analysis of a tanning material by the hide-powder method is within the
scope of any intelligent tanner who will provide himself with the
necessary implements. Much attention has been paid to the subject area
by the International Association of Leather Trades Chemists, and also by
the American Official Association of Agricultural Chemists, and as the
methods prescribed by one or other of these are with very little
exception employed by all qualified chemists throughout the world, their
directions, corrected up to date, are given in Appendices A and C. As,
however, these directions are addressed to chemists already familiar
with the usual course of analysis, a somewhat fuller explanation must
here be given.
It must specially be insisted on, that absolute adherence to the methods
given is essential to obtaining concordant results, and little points of
manipulation which appear in themselves unimportant, are frequently the
result of long experience and careful discussion. The members of the
International Association, especially, are bound by their rules to make
note in their analytical reports of any deviation, however small, from
the prescribed process.
The first step in the analysis of any material is to draw a sample
truly representing the bulk, which is often by no means easy, while
failure to accomplish it is probably the cause of more errors and
disputes than any inaccuracy of the method of analysis itself. In very
many cases, chemists are blamed for discrepancies which really exist in
the samples supplied to them. The chemists of the International
Association only hold themselves responsible for the accuracy of their
analyses when the sampling has been done strictly according to the rules
prescribed by their Association. On this account, all important samples
should be drawn in the presence of a principal, or some other
responsible person.
_In liquid extracts_, the thorough mixing of the liquid is of the
greatest importance. Most extracts contain a portion of “difficultly
soluble” tannins (see p. 297), which slowly settle to the bottom, or
adhere to the sides of the cask; from which such expedients as merely
rolling a full cask are quite inefficient to dislodge them. In fact
nothing but taking the heads out of a sufficient number of casks, and
actually stirring them with a suitable plunger, which should be
specially applied to the sides and bottom, or emptying the entire
contents of the casks into a tank in which the whole can be adequately
mixed, is really thoroughly reliable, though at times it is necessary to
be content with less satisfactory methods. In any case, when it is
probable that samples must be submitted to more than one chemist, the
whole should be drawn at once, thoroughly mixed and divided, and sealed
in separate bottles, and in dividing a sample the same care must be
taken to ensure complete mixture, as in drawing the original sample.
_Solid and pasty extracts_, such as quebracho, cutch and gambier, are
still more difficult to sample fairly, as the outside is almost
invariably much drier than the interior. Generally the only way is to
select such portions as are thought fairly to represent the bulk, to
chop them into moderately small pieces, mix and seal in an air-tight
tin, leaving it to the chemist to draw from these the smaller sample
required for analysis. Gambier is best sampled with a tubular tool like
a cork-borer, designed by Mr. Kathreiner, Fig. 63, which should be
passed completely through the bale, or the cylindrical sample of gambier
cannot be withdrawn. The same tool may also be used for sampling sumach
in bags, if the damage to the bag is not objected to. If such a tool is
not available, the only fair way to sample gambier is to cut slices
completely through the bale with a clean fleshing knife. In any case it
is of the utmost importance that the sample once drawn, should be mixed
as rapidly as possible, and at once enclosed in an _air-tight_ box or
jar, sealed and labelled.
[Illustration: FIG. 63.--Kathreiner’s Sampling Tools. A, strong
cross-handle; B, guard-disc; C C´, brass tube sharpened at C´; D, brass
or wooden plunger.]
_Dry tanning materials_, such as bark and valonia, require judgment in
selecting samples which fairly represent bulk. If they are of a nature
which do not readily separate into dust and fibre, a good method is to
grind a sufficient quantity in an ordinary bark-mill, and after well
mixing, to draw the sample from the ground portion. In other cases it is
best to empty a sufficient number of bags one upon another in layers on
a smooth floor, and to take out a section down to the floor. In such
materials as valonia and divi-divi, the dust or beard is usually much
stronger than the average of the pods or cups.
The same sort of precautions are required in drawing the still smaller
sample required for analysis from the larger original sample, but these
are sufficiently detailed in the directions of the I.A.L.T.C. given in
the Appendix. As materials usually require finer grinding than can be
managed with the mills employed in the tannery, a suitable mill must be
provided, and one of the simplest, at a moderate price, is a No. 4
drug-mill made by A. Kenrick and Sons, Limited, West Bromwich, Fig. 64.
Coffee mills are seldom strong enough for the purpose, but if nothing
better is available, the sample must be _thoroughly_ dried before
grinding, and its loss of weight noted, and taken into account in
calculating the analysis, care being taken that the sample after
grinding is so preserved that it cannot re-absorb moisture. Valonia,
myrobalans and even barks, may before grinding be broken with a
flat-faced hammer, on a thick cast-iron plate, with raised edges to
prevent loss from flying fragments.
[Illustration: FIG. 64.--Kenrick’s Drug-Mill.]
_Preparation of solution for analysis._--As the method of analysis only
gives satisfactory results when the quantity of tanning matter in the
solution is within certain limits, the International Association
prescribes that it must be such as to contain between 3·5 and 4·5 grms.
of tanning matter per liter, or as near as possible, on the average, to
4 grms. If, as will rarely happen, the strength of a material is quite
unknown, it may be necessary to make a trial test to ascertain the
quantity of substance to be used, but the following table gives the
quantity with sufficient accuracy for most ordinary materials.
TABLE SHOWING THE AMOUNT OF DIFFERENT MATERIALS TO BE WEIGHED OUT FOR
ANALYSIS TO MAKE UP ONE LITER OF SOLUTION.
_Barks, etc._
Grams.
Algarobilla 9
Canaigre 15
Divi-divi 9
Hemlock bark 16
Mimosa bark 11
Myrobalans 15
Oak-bark 30
Oak-wood 100
Quebracho wood 20
Sumach 15
Valonia 15
Valonia beard 11
_Extracts._
Oak-wood, sp. gr. 1·2 or over 15
Chestnut ditto 14
Quebracho (solid) 6
„ (liquid) 9 to 13
Gambier (block) 10
„ (cube) 7
The best method of weighing out exact quantities may be here described
for those to whom it is not already known, as much time may be wasted by
attempting it unsystematically. The material is of course weighed in a
basin, which together with the weight which is desired of the material,
is exactly counterpoised by weights in the other pan. Where many
weighings of the sort have to be made, it saves time to keep one
particular basin for the purpose, which should be properly marked[154];
and to make a counterpoise of lead or brass exactly equal to it in
weight, so that it is only necessary to add weights corresponding to the
quantity required to be weighed out. Supposing now, that it is a liquid
extract which is to be weighed, a sufficient quantity is introduced into
the basin with a pipette, to slightly exceed the required weight. The
pipette is now emptied, and a small quantity is withdrawn with it from
the basin. If the basin is still too heavy, the pipette is emptied, and
the process repeated until the basin is too light. The true weight now
lies between that in the basin and the small quantity retained in the
pipette, from which extract is added till the basin is again
over-weighed, and the same process is repeated, each time reducing the
margin, till a sufficient approximation is obtained. It is not necessary
in weighing out the sample, to be accurate to a single milligram; but
with practice, this amount of accuracy is easily attained. If the
material is solid, a spatula is substituted for the pipette. The
weighing of liquid or pasty extracts should be as rapid as possible, as
they lose weight on the balance by evaporation.
[154] Porcelain basins may be indelibly marked by writing on them with
an ordinary iron ink, and heating strongly with a blowpipe.
Liquid extracts are most easily dissolved by placing a large funnel in
the neck of a liter flask, and after pouring a little boiling water into
the flask, holding the basin inclined in the funnel, and washing out its
contents with boiling distilled water from a glass wash-bottle, or a
perfectly clean copper kettle, till the flask is filled to the mark. The
flask is now covered with a small beaker, which must hang loose on its
neck, without resting on its shoulders, and is rapidly cooled by placing
it under a cold water tap, to a temperature as little above 15° C. as
possible, and is then filled up to the mark on the neck with cold water,
and well mixed by shaking very thoroughly.
Solid or pasty extracts are dissolved in a beaker by stirring with
successive quantities of boiling water, which are poured off into the
flask, leaving the undissolved matter in the beaker. When the flask is
nearly full, if any small portions remain undissolved or insoluble, they
may be rinsed into it with the last portions of hot water, and the flask
is now cooled and mixed as already described.
[Illustration: FIG. 65.--Procter’s Extractor.]