Wool is put into a net, and pressed down into the liquor with rods; but
cloth is smoothly stretched and suspended by hooks upon frames, which
are steadily dipped into the vat, with slight motions through the
liquor; yarn-hanks must be dipped and turned about by hand. All
unnecessary stirring of the liquor must however be avoided, lest the
oxygen of the atmosphere be brought too extensively into contact with
the reduced indigo, for which reason mechanical agitation with rollers
in the vat is inadmissible. The stuffs to be dyed, take at the first dip
only a feeble colour, though the vat be strong, but they must be
deepened to the desired shade by successive immersions of fifteen
minutes or more each time, with intervals of exposure to the air, for
absorption of its oxygen.
After the lapse of a certain time, if the fermentative power be
impaired, which is recognized by the dye stuffs losing more colour in a
weak alkaline test lye than they ought, the vat should be used up as far
as it will go, and then the liquor should be poured away, for the indigo
present is not in a reduced state, but merely mixed mechanically, and
therefore incapable of forming a chemical combination with textile
fibres. If cotton goods previously treated with an alkaline lye are to
be dyed blue, the vat should contain very little lime.
_Theory of the Indigo vat._--The large quantity of extractive matter in
woad and madder; as also the sugar, starch and gluten in the bran and
woad, when dissolved in warm water, soon occasion a fermentation, with
an absorption of oxygen, from the air, but especially from the indigo of
the woad, and from that introduced in a finely ground state. When thus
disoxygenated, it becomes soluble in alkaline menstrua; the red-brown of
the indigo being dissolved at the same time. When lime is added, the
indigo-blue dissolves, and still more readily if a little potash is
conjoined with it; but whatever indigo-brown may have been dissolved by
the potash, is thrown down by the lime. Lime in too large a quantity,
however, forms an insoluble combination with the reduced indigo, and
thus makes a portion of the dye ineffective; at the same time it
combines with the extractive. In consequence of the fermentative action,
carbonic acid, acetic acid, and ammonia are disengaged; the first two of
which neutralize a portion of the lime, and require small quantities of
this earth to be added in succession; hence also a considerable quantity
of the carbonate of lime is found as a deposit on the sides and bottom
of the vat. In the sound condition of the indigo vat, no free lime
should be perceived, but on the contrary a free acid. Yet when the
disengaged carbonic and acetic acids saturate the lime completely, no
indigo can remain at solution; therefore a sufficient supply of lime
must always be left to dissolve the dye, otherwise the indigo would fall
down and mix with the extractive matter at the bottom. Goods dyed in the
blue vat are occasionally brightened by a boil in a logwood bath, with a
mordant of sulpho-muriate of tin, or in a bath of cudbear.
Another mode of mounting the indigo vat without woad and lime, is by
means of madder, bran, and potash. The water of the vat is to be heated
to the temperature of 122° F.; and for 120 cubic feet of it, 12 pounds
of indigo, 8 pounds of madder, and as much bran are to be added, with 24
pounds of good potashes; at the end of 36 hours, 12 pounds more of
potash are introduced, and a third 12 pounds in other 12 hours. In the
course of 72 hours, all the characters of the reduction and solution of
the indigo become apparent; at which time the fermentation must be
checked by the addition of quick-lime. The liquor has a bright full
colour, with a beautiful rich froth. In feeding the vat with indigo, an
equal weight of madder, and a double weight of potash should be added.
The odour of this vat in its mild but active state is necessarily
different from that of the woad vat, as no ammonia is exhaled in the
present case, and the sediment is much smaller. The reduced indigo is
held in solution by the carbonated potash, while the small addition of
quicklime merely serves to precipitate the indigo-brown.
A potash vat dyes in about half the time of the ordinary warm vat, and
penetrates fine cloth much better; while the goods thus dyed lose less
colour in alkaline and soap solutions. This vat may moreover be kept
with ease in good condition for several months; is more readily mounted;
and from the minute proportion of lime present, it cannot impair the
softness of the woollen fibres. It is merely a little more expensive. It
is said that cloth dyed in the potash indigo vat, requires one third
less soap in the washing at the fulling mill, and does not soil the
hands after being dressed. At Elbœuf and Louviers in France, such vats
are much employed. Wool, silk, cotton, and linen may all be dyed in
them.
_Cold vats._--The _copperas_ or _common blue vat_ of this country is so
named because the indigo is reduced by means of the protoxide of iron.
This salt should therefore be as free as possible from the red oxide,
and especially from any sulphate of copper, which would re-oxidize the
indigo. The necessary ingredients are: copperas (green sulphate of
iron), newly slaked quicklime, finely ground indigo, and water; to which
sometimes a little potash or soda is added, with a proportional
diminution of the lime. The operation is conducted in the following way:
the indigo well triturated with water or an alkaline lye, must be mixed
with hot water in the _preparation_ vat, then the requisite quantity of
lime is added, after which the solution of copperas must be poured in
with stirring. Of this _preparation_ vat, such a portion as may be
wanted is laded into the dyeing vat. For one pound of indigo three
pounds of copperas are taken, and four pounds of lime (or 1 of indigo,
2-1/2 of copperas, and 3 of lime). If the copperas be partially
peroxidized, somewhat more of it must be used.
A vat containing a considerable excess of lime is called a _sharp_ vat,
and is not well adapted for dyeing. A _soft_ vat, on the contrary, is
that which contains too much copperas. In this case the precipitate is
apt to rise, and to prevent uniformity of tint in the dyed goods. The
sediment of the copperas vat consists of sulphate of lime, oxide of
iron, lime with indigo brown, and lime with indigo blue, when too much
quicklime has been employed. The clear, dark wine yellow fluid contains
indigo blue in a reduced state, and indigo red, both combined with lime
and with the gluten of indigo dissolved. After using it for some time
the vat should be refreshed or fed with copperas and lime, upon which
occasion, the sediment must first be stirred up, and then allowed time
to settle again, and become clear. For obtaining a series of blue tints,
a series of vats of different strengths is required.
Linen and cotton yarn, before being dyed should be boiled with a weak
alkaline lye, then put upon frames or tied up in hanks, and after
removing the froth from the vat, plunged into, and moved gently through
it. For pale blues, an old, nearly exhausted vat, is used; but for deep
ones, a fresh nearly saturated vat. Cloth is stretched upon a proper
square dipping frame made of wood, or preferably of iron, furnished with
sharp hooks or points of attachment. These frames are suspended by cords
over a pulley, and thus immersed and lifted out alternately at proper
intervals. In the course of 8 or 10 minutes, the cloth is sufficiently
saturated with the solution of indigo, after which it is raised and
suspended so as to drain into the vat. The number of dippings determines
the depth of the shade; after the last the goods are allowed to dry,
taken off the frame, plunged into a sour bath of very dilute sulphuric
or muriatic acid, to remove the adhering lime, and then well rinsed in
running water. Instead of the dipping frames some dyers use a peculiar
roller apparatus, called _gallopers_, similar to what has been described
under CALICO PRINTING; particularly for pale blues. This cold vat is
applicable to cotton, linen and silk goods.
When white spots are to appear upon a blue ground, resist pastes are to
be used, as described under CALICO PRINTING.
The _urine vat_ is prepared by digestion of the ground indigo in warmed
stale urine, which first disoxygenates the indigo, and then dissolves it
by means of its ammonia. Madder and alum are likewise added, the latter
being of use to moderate the fermentation. This vat was employed more
commonly of old than at present, for the purpose of dyeing woollen and
linen goods.
The mode of making the china blue dye has been described under CALICO
PRINTING; as well as the _pencil blue_, or blue of application.
A blue dye may likewise be given by a solution of indigo in sulphuric
acid. This process was discovered by Barth, at Grossenhayn in Saxony,
about the year 1740, and is hence called the Saxon blue dye. The
chemical nature of this process has been already fully explained. If the
smoking sulphuric acid be employed, from 4 to 5 parts are sufficient for
1 of indigo; but if oil of vitriol, from 7 to 8 parts. The acid is to be
poured into an earthen-ware pan, which in summer must be placed in a tub
of cold water, to prevent it getting hot, and the indigo in fine powder,
is to be added with careful stirring, in small successive portions. If
it become heated, a part of the indigo is decomposed, with the
disengagement of sulphurous acid gas, and indigo green is produced.
Whenever all the indigo has been dissolved, the vessel must be covered
up, allowed to stand for 48 hours, and then diluted with twice its
weight of clear river water.
The undiluted mass has a black blue colour, is opaque, thick, attracts
water from the air, and is called _indigo composition_ or _chemic blue_.
It must be prepared beforehand, and kept in store. In this solution,
besides the _cerulin_, there are also indigo-red, indigo-brown, and
gluten, by which admixture the pure blue of the dye is rendered foul,
assuming a brown or a green cast. To remove these contaminations, wool
is had recourse to. This is plunged into the indigo previously diffused
through a considerable body of water, brought to a boiling heat in a
copper kettle, and then allowed to macerate as it cools for 24 hours.
The wool takes a dark blue dye by absorbing the indigo-blue sulphate and
hyposulphite, while at the same time the liquor becomes greenish blue;
and if the wool be left longer immersed, it becomes of a dirty yellow.
It must therefore be taken out, drained, washed in running water till
this runs off colourless, and without an acid taste. It must next be put
into a copper full of water, containing one or two per cent. of
carbonate of potash, soda, or ammonia (to about one third the weight of
the indigo), and subjected to a boiling heat for a quarter of an hour.
The blue salts forsake the wool, leaving it of a dirty red brown, and
dye the water blue. The wool is in fact dyed with the indigo red, which
is hardly soluble in alkali. The blue liquor may now be employed as a
fine dye, possessed of superior tone and lustre. It is called distilled
blue and _soluble blue_. Sulphuric acid throws down from it the small
quantity of indigo red, which had been held in solution by the alkali.
When wool is to be dyed with this sulphate of indigo blue, it must be
first boiled in alum, then treated with the blue liquor, and thus
several times alternately, in order to produce an uniform blue colour.
Too long continuance of boiling is injurious to the beauty of the dye.
In this operation the woollen fibres get impregnated with the
indigo-blue sulphate of alumina.
With sulphate of indigo, not only blues of every shade are dyed, but
also green, olive, gray, as also a fast ground to logwood blues; for the
latter purpose the preparatory boil is given with alum, tartar,
sulphates of copper and iron, and the blue solution; after which the
goods are dyed up with a logwood bath containing a little potash.
STATISTICAL TABLES of INDIGO; per favour of James Wilkinson, Esq., of
Leadenhall-Street.