+------------+---+---------+-------+
| | | |Parts |
| Indigo. |No.| Price. |in 100.|
+------------+---+---------+-------+
| | |_s._ _d._| |
|Caraca flor.| 1 | 6 0 |54-1/2 |
|Guatimala | 2 | 5 0 |33-1/2 |
| ---- | 3 | 3 2 |19 |
| ---- | 4 | 4 6 |32-1/2 |
| ---- | 5 | 5 4 |50 |
| ---- | 6 | 5 0 |50 |
| ---- | 7 | 5 3 |35 |
| ---- | 8 | 4 8 |46 |
| ---- | 9 | 4 8 |33-1/2 |
| ---- |10 | 5 4 |50 |
+------------+---+---------+-------+
_Properties of Indigo._--It possesses a dark blue colour, passing into
violet-purple, is void of taste and smell, dull, but by rubbing with a
smooth hard body, it assumes the lustre and hue of copper. It occurs
sometimes less and sometimes more dense _apparently_ than water, which
circumstance depends upon its freedom from foreign impurities, as well
as upon the treatment of its paste in the boiling, pressing, and drying
operations. It is insoluble in water, cold alcohol, ether, muriatic
acid, dilute sulphuric acid, cold ethereous and fat oils; but boiling
alcohol and oils dissolve a little of it, which they deposit on cooling.
Creosote has the property of dissolving indigo.
Indigo is a mixture of several dye-stuffs, and other substances.
Berzelius found in it a matter resembling vegetable gluten or gliadine,
a brown, red, and blue pigment, besides oxide of iron, clay, lime,
magnesia, and silica.
1. Indigo gluten or gliadine is dissolved along with the calcareous and
magnesian salts by acids. If the powder be treated with dilute sulphuric
acid, if the solution be saturated with carbonate of lime, evaporated to
dryness, and its residuum treated with alcohol; the solution thus formed
leaves, after being evaporated, a yellow transparent extract, easily
soluble in water, more difficultly in acid liquids; showing that acids
extract only a portion of the gliadine from the indigo. It yields, by
dry distillation, much ammonia, a fetid oil, and comports itself in
other respects like vegetable gluten.
2. _Indigo-brown_, occurs in combination with lime, as also with
vegetable acid in considerable quantity, and more abundantly in the
coarser sorts of indigo than in the finer. Indigo purified by acids is
to be treated with hot strong caustic lye, which dissolves the
indigo-brown; the liquid part of the mixture passes with difficulty
through the filter, is black-brown, opaque, and holds some indigo-blue
in solution, or diffused in fine powder. The alkali being neutralized
with acetic acid, the liquor is to be evaporated, and alcohol poured on
the residuum, whereby the alkaline acetate is dissolved out from the
brown.
This pigment is a dark brown, almost black, but is not yet entirely
deprived of the other constituents of indigo. It is nearly tasteless, is
combustible, affords, by dry distillation, ammonia and fetid oil, forms
with acids combinations hardly soluble in water, with alkalis soluble
ones, but with earths hardly soluble. Lime possesses the property of
precipitating the indigo-brown completely from its alkaline solution.
Chlorine occasions a pale yellow brownish precipitate, which consists of
indigo brown and muriatic acid, but causes no further change. By drying,
it becomes again dark coloured. Indigo-brown seems to exist also in
woad.
3. _Indigo-red_, or more properly red resin of indigo. This may be
obtained by boiling alcohol of sp. grav. 0·830 upon some indigo which
has been previously treated with acids and alkalis; for the red
substance is hardly soluble in cold alcohol. The solution is dark red,
opaque, and leaves, by distillation, the indigo-red in the form of a
black-brown powder, or a glistening varnish, slightly soluble in alcohol
and ether. Alkalis do not dissolve it, but concentrated sulphuric acid
forms with it a dark yellow dye, from which water causes no
precipitation; wool extracts the colour from the acid solution, and
becomes of a dirty brown hue. Chlorine does not seem capable of
destroying the colour for though it makes it yellow, it becomes as dark
as ever on being dried. Indigo-red melts with heat, burns with a bright
flame, affords, when heated in vacuo, first a white crystalline
sublimate, and then unchanged indigo-red. That white matter is changed
by nitric acid into indigo-red.
4. _Indigo-blue_, or pure indigo remains, after treating the indigo of
commerce with dilute acid, alkalis, and alcohol; it retains, however,
still traces of the matters thereby extracted, along with some earthy
substances. In order to procure indigo-blue in its utmost purity, we
must deoxidize the above blue residuum, thus form colourless indigo,
which again acquires a blue colour from the air, and constitutes the
pure pigment. For this purpose the above moist indigo is to be mixed
with slaked lime, green sulphate of iron, and hot water in an air-tight
matrass. The indigo when deoxidized by protoxide of iron being soluble
in lime-water, the clear yellow solution is to be poured off, and
exposed to the air. The indigo absorbs oxygen, and becomes again blue.
By digestion with dilute muriatic acid the foreign matters are
dissolved, and may then be washed away with distilled water, from the
_absolute_ indigo.
The indigo-blue obtained in this manner has a cast of purple red,
displaying the characteristic copper lustre in a high degree, but in
powder, it is blue. It is void of taste and smell, is by my experiments
of specific gravity 1·50, affords at 554° Fahr. a purple vapour, and
sublimes in shining purple scales, or slender needles in an apparatus
open to the air, whereby, however, much of it is destroyed. Some carbon
remains after the sublimation. A quick heat produces most sublimate.
These needles contain a brown oily matter, which may be dissolved out by
means of hot alcohol. Their specific gravity is 1·35, according to Mr.
Crum. The sublimate from common indigo does not contain any oil, but
some indigo-red and the above white crystalline matter. According to Mr.
Crum, indigo-blue consists of carbon, 73·22; oxygen, 12·60; azote,
11·26; hydrogen, 2·92; while according to Dumas, crystallized indigo
consists of carbon, 73·26; oxygen, 10·43; azote, 13·81; and hydrogen,
2·50: precipitated indigo consists of carbon, 74·81; oxygen, 7·88;
azote, 13·98; and hydrogen, 3·33: sublimed indigo, of carbon, 71·71;
oxygen, 12·18; azote, 13·45; hydrogen, 2·66. My own analysis
afforded--carbon, 71·37; oxygen, 14·25; azote, 10·00; hydrogen, 4·33. In
another analysis of Dumas, 3·93 parts of hydrogen were obtained. Hence
we must infer that considerable differences exist in the composition of
indigo in its purest state. Reagents act upon it much as upon common
indigo. Chlorine, iodine, and bromine convert it into a reddish brown
soluble substance. Concentrated sulphuric acid, especially the smoking
or anhydrous of Nordhausen, dissolves indigo-blue with the disengagement
of heat, but it makes it suffer some modification; for though it retains
an intense dark blue colour, it has become soluble in water, and may be
blanched by light, which does not happen with indigo itself. Nitric acid
destroys indigo-blue, forms indigotic (carbazotic) acid, carbonic acid,
artificial resin, and bitter principle.
Indigo-blue may be reduced by substances oxidized, with the co-operation
of alkalis or alkaline earths; for example, by such substances as have a
strong affinity for oxygen, and are imperfectly saturated with this
principle, as the sulphurous and phosphorous acids and their salts, the
protoxides of iron and manganese, the protoxide salts of tin, and the
corresponding compounds of chlorine, as the proto-chlorides of tin and
iron; and the solution of the former in potash. When in these
circumstances, in the presence of alkali, a deoxidation or reduction of
the indigo-blue takes place, the other bodies get oxidized by absorption
of the oxygen of the indigo-blue; the protoxides become peroxides, and
the acids in _ous_ become acids in _ic_, &c. Several metallic sulphurets
also reduce the indigo-blue in the same predicament, as the sulphurets
of potassium, of calcium, of antimony, and of arsenic (orpiment). A
similar influence is exercised by fermenting vegetable substances, such
as woad, madder, bran, raw sugar (molasses), starch, syrup, in
consequence of the formation of carbonic and acetic acids, by absorption
of the oxygen of the indigo-blue, for acetic acid and acetic salts are
found in the liquor of the warm blue vat, in which indigo has been
reduced by means of woad, madder, and bran.
_Formation of colourless reduced indigo-blue, or indigotine._--Purified
indigo-blue is to be treated with copperas and slaked lime, as above
described; or the clear wine-yellow supernatant liquor of the cold
blue-vat mixture is to be taken, run by a syphon into a matrass, a few
drops of concentrated acetic or sulphuric acid, deprived of air, are to
be poured into it, and the vessel being made quite full, is to be well
corked. The reduced indigo soon falls in white flocks, or crystalline
scales. They must be edulcorated upon a filter with water deprived of
its air by boiling, then pressed between folds of blotting-paper, and
dried under the receiver in vacuo. Indigo-blue may likewise be reduced
and dissolved by solution of hydro-sulphuret of ammonia; and the
colourless indigotine may be precipitated by muriatic acid.
The reduced indigo is sometimes white at the instant of its elimination,
sometimes grayish, of a silky lustre, but becomes very readily greenish,
blue green, and blue, in the air; in which case it absorbs, according to
Berzelius, 4·2 per cent. of oxygen; but according to Liebig, 11·5 per
cent. It is void of taste and smell, is insoluble in water; well boiled
water free from air is not affected by it, but is turned blue by common
water. It dissolves in alcohol and ether into a yellow dye; not in
dilute acids, but in concentrated sulphuric acid, whereby probably a
portion of this is decomposed, and some hyposulphurous acid formed; the
colour of this solution is blue. Solutions of the caustic and carbonated
alkalis, even the alkaline earths, readily dissolve reduced indigo into
a wine-yellow liquid; but in contact with air, oxygen is absorbed, and
indigo-blue falls, while a purple-coloured froth, passing into
copper-red, appears upon the surface, just as in the indigo vats of the
dyer.
The reduced indigo may be combined, by means of complex affinity, with
other bases, with the exception of the oxides of copper, zinc, and
mercury, which oxidize it. These combinations are white, in part
crystallizable, become speedily blue in the air, and afford by
sublimation indigo-blue. Berzelius formed with lime a two-fold
combination; one easily soluble in water, and another difficultly
soluble, of a lemon colour, which contained an excess of lime; this is
formed both in the hot and the cold blue vat; in the latter it is
occasioned by an overdose of lime.
When pure indigo-blue is treated with concentrated sulphuric acid, and
particularly with six times its weight of the smoking _dry_ acid, it
dissolves completely, and several different compounds are produced in
the solution. There is first a blue sulphate of indigo; secondly, a
similar compound with the resulting hyposulphurous acid; thirdly, a
combination of sulphuric acid with the purple of indigo (called Phænicin
by Crum), a peculiar substance, generated from indigo-blue. These three
compounds are here dissolved in an excess of sulphuric acid. The more
concentrated the sulphuric acid is, the more blue hyposulphite is
formed. The solution in smoking acid, when diluted with water and
filtered, affords a considerable precipitate of indigo purple, which
that in oil of vitriol does not. The vapour of anhydrous sulphuric acid
combines with indigo-blue into a purple fluid.
In order to obtain from the dark blue solution each of these blue acids
in a pure state, we must dilute it with forty times its weight of water,
and immerse in the filtered liquor, well washed wool or flannel, with
which the blue acids combine, while most of the sulphuric acid and some
other foreign substances remain free in the liquor. The wool must be
then scoured with water containing about half a per cent. of carbonate
of ammonia, or potash, which neutralizes both of the blue acids, and
produces a blue compound. This being evaporated to dryness at the
temperature of 140° F., alcohol of 0·833 is to be poured upon the
residuum, which dissolves the blue hyposulphite, but leaves the blue
sulphate undissolved. From either salt, by precipitating with acetate of
lead, by acting upon the precipitate with sulphuretted hydrogen water,
and evaporation, either of the two blue acids may be obtained. They may
be both evaporated to dryness, especially the blue sulphate of indigo;
they both become somewhat moist in the air, they are very soluble in
water, and the blue sulphate also in alcohol; they have a not unpleasant
smell, and an acid astringent taste.
From these habitudes, particularly in reference to the bases, it appears
that indigo-blue does not comport itself like a saline base towards the
acids, but rather like an acid, since it enters into the salts, just as
the empyreumatic oil of vinegar and oil of turpentine do into resin
soaps. The blue pigment of both acids is reduced by zinc or iron without
the disengagement of hydrogen gas; as also by sulphuretted hydrogen,
tepid protochloride of tin, while the liquor becomes yellow.
_Indigo-blue sulphate of potash, or ceruleo-sulphate of potash_, may be
obtained by extracting the blue colour from the wool by water containing
1 per cent. of carbonate of potash, evaporating nearly to dryness,
treating the extract with alcohol to remove the _indigo-blue
hyposulphite_, then with acetic acid and alcohol to remove any excess of
carbonate of potash. It is found in commerce under the name of
precipitated indigo, indigo paste, blue carmine, and soluble indigo. To
prepare it economically, indigo is to be dissolved in ten times its
weight of concentrated sulphuric acid; the solution after twenty-four
hours is to be diluted with ten times its weight of water, filtered, and
imperfectly saturated with carbonate of potash; whereby a blue powder
falls down; for the resulting sulphate of potash throws down the
ceruleo-sulphate, while the hyposulphite of potash remains dissolved. It
is a dark blue copper shining powder, soluble in 140 parts of cold
water, and in much less of boiling water. It is made use of as a dye,
and to give starch a blue tint. When mixed with starch into cakes, it is
sold under the name of _blue_ for washerwomen.
Ceruleo-sulphate of ammonia may be formed in the same way. It is much
more soluble in water. Ceruleo-sulphate of lime is obtained by
saturating the above dilute acid with chalk, filtering to separate the
undyed gypsum, and washing with water till the purple colour be
extracted. This liquor evaporated and decomposed by alcohol, affords a
bluish flocky precipitate, which is more soluble in water than common
gypsum, and dries up in a purple-blue film. Ceruleo-sulphate of alumina
may be obtained by double affinity; it is dark blue while moist, but
becomes black-blue by drying, and is soluble in water.
The blue present in all these salts of _ceruline_ is destroyed by
sunshine, becomes greenish-gray by caustic alkalis; and turns
immediately yellow-brown by alkaline earths. But when the solution is
very dilute, the colour becomes first green, then yellow. The carbonates
of alkalis do not produce these changes. Nitric acid decomposes the
colour quickly. Mr. Crum considers ceruline to be a combination of
indigo-blue with water.
_Phenicine_, or indigo-purple combined with sulphuric acid, is obtained
when the solution of indigo-blue in concentrated sulphuric acid, has
been diluted for a few hours with water, and then filtered. It seems to
be an intermediate body into which the indigo-blue passes, before it
becomes soluble _ceruline_. Hence it occurs in greater quantity soon
after digesting the indigo with the acid, than afterwards. It is dark
blue, dissolves gradually in water, affords after evaporation a blue
residuum, of the same appearance as the above blue acids. When a salt is
added to it a purple precipitate ensues, which is a compound of
indigo-purple, sulphuric acid, and the base of the salt. Indigo-purple
is reduced by bodies having a strong attraction for oxygen, if a free
alkali or alkaline earth be present, and its solution is yellow, but it
becomes blue in the atmosphere. According to Mr. Crum, _Phenicine_
contains half as much combined water as ceruline.
The table which I published in 1830 (as given above) shows very clearly
how much the real quality and value of indigo differ from its reputed
value and price, as estimated from external characters by the brokers.
Various test or proof processes of this drug have been proposed. That
with chlorine water is performed as follows. It is known that chlorine
destroys the blue of indigo, but not the indigo-red or indigo-brown,
which by the resulting muriatic acid is thrown down from the sulphuric
solution in flocks, and the chlorine acts in the same way on the
gliadine or gluten of the indigo. Pure indigo-blue is to be dissolved in
10 or 12 parts of concentrated sulphuric acid, and the solution is to be
diluted with a given weight of water, as, for example, 1000 parts for 1
of indigo-blue. If we then put that volume of liquor into a graduated
glass tube, and add to it chlorine water of a certain strength till its
blue colour be destroyed by becoming first green and then red-brown, we
can infer the quantity of colour from the quantity of chlorine water
expended to produce the effect. The quantity of real indigo-blue cannot,
however, be estimated with any accuracy in this way, because the other
colouring matters in the drug act also upon the chlorine; and, indeed,
the indigo itself soon changes, when dissolved in sulphuric acid, even
out of access of light, while the chlorine water itself is very
susceptible of alteration. Perhaps a better appreciation might be made
by avoiding the sulphuric acid altogether, and adding the
finely-powdered indigo to a definite volume of the chlorine water till
its colour ceased to be destroyed, just as prussian-blue is decoloured
by solution of potash in making the ferro-cyanide.
Another mode, and one susceptible of great precision, is to convert 10
or 100 grains of indigo finely powdered into its deoxidized state, as in
the blue vat by the proper quantity of slaked lime and solution of green
sulphate; then to precipitate the indigo, collect and weigh it. The
indigo should be ground upon a muller along with the quicklime, the
levigated mixture should be diluted with water, and added to the
solution of the copperas. This exact analytical process requires much
nicety in the operator, and can hardly be practised by the broker,
merchant, or manufacturer.
_Employment of indigo in dyeing._--As indigo is insoluble in water, and
as it can penetrate the fibres of wool, cotton, silk, and flax, only
when in a state of solution, the dyer must study to bring it into this
condition in the most complete and economical manner. This is effected
either by exposing it to the action of bodies which have an affinity for
oxygen superior to its own, such as certain metals and metallic oxides,
or by mixing it with fermenting matters, or, finally, by dissolving it
in a strong acid, such as the sulphuric. The second of the above methods
is called the warm blue, or pastel vat; and being the most intricate, we
shall begin with it.
Before the substance indigo was known in Europe, woad having been used
for dyeing blue, gave the name of woad vats to the apparatus. The vats
are sometimes made of copper, at other times of iron or wood, the last
alone being well adapted for the employment of steam. The dimensions are
very variable; but the following may be considered as the average size:
depth, 7-1/2 feet; width below, 4 feet, above, 5 feet. The vats are
built in such a way that the fire does not affect their bottom, but
merely their sides half way up; and they are sunk so much under the
floor of the dyehouse, that their upper half only is above it, and is
surrounded with a mass of masonry to prevent the dissipation of the
heat. About 3 or 3-1/2 feet under the top edge an iron ring is fixed,
called the _champagne_ by the French, to which a net is attached in
order to suspend the stuffs out of contact of the sediment near the
bottom.
In mounting the vat the following articles are required: 1. woad
prepared by fermentation, or woad merely dried, which is better, because
it may be made to ferment in the vat, without the risk of becoming
putrid, as the former is apt to do; 2. indigo, previously ground in a
proper mill; 3. madder; 4. potash; 5. slaked quicklime; 6. bran. In
France, weld is commonly used instead of potash.
[Illustration: 579 580]
The indigo mill is represented in _figs._ 579. and 580. _a_ is a
four-sided iron cistern, cylindrical or rounded in the bottom, which
rests upon gudgeons in a wooden frame; it has an iron lid _b_,
consisting of two leaves, between which the rod _c_ moves to and fro,
receiving a vibratory motion from the crank _d_. By this construction, a
frame _e_, which is made fast in the cistern by two points _e´ e´_, is
caused to vibrate, and to impart its swing movement to six iron rollers
_f f f_, three being on each side of the frame, which triturate the
indigo mixed with water into a fine paste. Whenever the paste is
uniformly ground, it is drawn off by the stopcock _g_, which had been
previously filled up by a screwed plug, to prevent any of the indigo
from lodging in the orifice of the cock, and thereby escaping the action
of the rollers. The cistern is nearly three feet long.
The vat being filled with clear river water, the fire is to be kindled,
the ingredients introduced, and if fermented woad be employed, less lime
is needed than with the merely dried plant. Meanwhile the water is to be
heated to the temperature of 160° Fahr., and maintained at this pitch
till the deoxidizement and solution of the indigo begin to shew
themselves, which, according to the state of the constituents, may
happen in 12 hours, or not till after several days. The first characters
of incipient solution are blue bubbles, called the flowers, which rise
upon the surface, and remain like a head of soap-suds for a considerable
time before they fall; then blue coppery shining veins appear with a
like coloured froth. The hue of the liquor now passes from blue to
green, and an ammoniacal odour begins to be exhaled. Whenever the indigo
is completely dissolved, an acetic smelling acid may be recognized in
the vat, which neutralizes all the alkali, and may occasion even an acid
excess, which should be saturated with quicklime. The time for doing
this cannot be in general very exactly defined. When quicklime has been
added at the beginning in sufficient quantity, the liquor appears of a
pale wine-yellow colour, but if not, it acquires this tint on the
subsequent introduction of the lime. Experience has not hitherto decided
in favour of the one practice or the other.
As soon as this yellow colour is formed in the liquor, and its surface
becomes blue, the vat is ready for the dyer, and the more lime it takes
up without being alkaline, the better is its condition. The dyeing power
of the vat may be kept up during six months, or more, according to the
fermentable property of the woad. From time to time, madder and bran
must be added to it, to revive the fermentation of the sediment, along
with some indigo and potash, to replace what may have been abstracted in
the progress of dyeing. The quantity of indigo must be proportional, of
course, to the depth or lightness of the tints required.
During the operation of this blue vat two accidents are apt to occur;
the first, which is the more common one, is called the _throwing back_,
in French the _cuve rebuté_, and in German, the _Scharf_ or
_Schwartzwerden_ (the becoming sharp or black); the second is the
_putrefaction_ of the ingredients. Each is discoverable by its peculiar
smell, which it is impossible to describe. The first is occasioned by
the employment of too much quicklime, whereby the liquor becomes neutral
or even alkaline. This fault may be recognized by the fading of the
green, or by the dark green, or nearly black appearance of the liquor;
and by a dull blue froth, owing to a film of lime. The remedy for a
slight degree of this vicious condition, is to suspend in the liquor a
quantity of bran tied up in a bag, and to leave it there till the
healthy state be restored. Should the evil be more inveterate, a
decoction of woad, madder, and bran must be introduced. Strong acids are
rather detrimental. Sulphate of iron has been recommended, because its
acid precipitates the lime, while its oxide reduces the indigo to the
soluble state.
The decomposition or putrefaction of the blue vat is an accident the
reverse of the preceding, arising from the transition of the acetous
into the putrid fermentation, whereby the dyeing faculty is destroyed.
Such a misfortune can happen only towards the commencement of working
the vat, whilst the woad is still powerful, and very little indigo has
been dissolved. Whenever the vat is well charged with indigo, that
accident cannot easily supervene. In both of these distemperatures the
elevation of the temperature of the vat aggravates the evil.