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Title: Notes Upon Indigo
Author: Hayes, John
Language: English
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                         NOTES UPON INDIGO.

                                BY

                          JOHN L. HAYES,


 SECRETARY OF THE NATIONAL ASSOCIATION OF WOOL MANUFACTURERS, FELLOW
      OF THE AMERICAN ACADEMY, AND CORRESPONDING MEMBER OF THE
            ACADEMY OF NATURAL SCIENCES OF PHILADELPHIA.

From “The Bulletin of the National Association of Wool Manufacturers.”

                              BOSTON:

                   PRESS OF JOHN WILSON AND SON.

                               1873.


PART I.

NOTES UPON INDIGO.

PART I.

A publication devoted to the interests of the woollen manufacture,
while giving due prominence to its first raw material, wool, cannot
neglect the secondary materials which enter into finished fabrics. The
attractiveness and utility of the largest class of these fabrics are
due to the hue given them by the dyer; and of all the coloring
materials one of the most precious is indigo. In former times, as it
still does at the East, it occupied with madder the place of one of
the two most important of all dyeing materials. Forced of late years
to give way to the marvellous products of modern chemistry, it will
doubtless resume its place under the influence of a more enlightened
economy and a more subdued taste. To contribute to the hastening of
this return is one object of this essay. The most usual reproach
against American fabrics is the want of stability in our dyes,—a
reproach without justice, if applied to American fabrics alone; for
the cheapening of dyestuffs is practised in all the so-called
manufacturing nations, and is contemned alone in the East, from which
we have derived our arts, and by the people whom we despise as
barbarous. To remove this reproach from American fabrics would be
worthy of no little temporary sacrifice on the part of our
manufacturers.

The value of indigo as a dyeing material is due to the great stability
of the blue color, and the derivatives from blue, which it gives to
fabrics, especially of wool and cotton. It is not sufficient that a
dyed fabric should preserve its color when submitted to violent tests,
as when acted upon by vegetable or mineral acids or alkaline or soapy
baths: the only stable dyes are those which resist air and light, the
two destructive agents of vegetable colors. Indigo, from the
remarkable manner in which its color becomes fixed upon a fabric, to
be hereafter explained, possesses properties of resistance and
stability in a higher degree than any blue dye. And when we consider
that this blue has not only its own hue, but is the best foundation
for blacks, greens, purples, and even browns, the importance of these
properties cannot be over-estimated. Says M. de Kæppelin, a chemist
and manufacturer of Mulhouse, in one of a series of articles furnished
to the _Annales du gênie Civil_, 1864: “So high are the properties of
resistance and stability which indigo possesses, that it is perhaps to
be regretted for the art of the dyer and manufacturer of printed
calicoes, that the use of indigo becomes more and more rare, and that
the recent discoveries which modern science has placed at the service
of industry are daily eliminating it from our factories. I have
observed that whenever we have to dye stuffs of a high price, it is
indigo which always serves as a base for the foundation of all the
blue colors, or of those which are derived from blue. It is the same
for the fabrication of printed tissues, which serve for the poorer
classes, whose colors should have great stability without much
increase of cost. But of late years, especially, we find a tendency to
employ colors of little stability, and to prefer them, even in the
class of fabrics first referred to, to those which are more fast, on
account of their vivacity and freshness of tone. It is this tendency,
which the consumer partakes of even while complaining of it, that the
textile manufacturers ought to seek to combat. How often have I heard
the greatest manufacturers of Alsace deplore the obligation which they
felt that they were under of printing their tissues by means of colors
so fugacious and so little resistant as those composed from aniline.
We must hope, then, in the interest of that industry, that while
adopting the marvellous discoveries which science is every day making,
there shall be made a less general application of them, and that we
shall return to the fabrication of the styles which necessitate the
more constant employment of coloring materials,—less brilliant, it is
true, but more adherent to the tissues, and less alterable by air and
light. It seems to me, also, that taste would lose nothing; and that
printed stuffs, colored in a manner less brilliant, but more
harmonious, would be perhaps more appreciated, especially by those who
use them.”

The tendency to substitute the brilliant for the stable dyes prevails
too much in our own manufacture. A very considerable cloth
manufacturer replied to our inquiry as to the extent to which he used
indigo: “I hardly use it at all; the dye of the indigo blue is not
bright enough to be popular.” On the other hand, we have heard our
leading manufacturer of carpets, whose cultivated taste has led him to
partake of M. de Kæppelin’s views, deplore the introduction of aniline
dyes, as a positive calamity to the textile industry. It is the
influence of the trade, the immediate consumers of fabrics, rather
than the judgment of manufacturers, which promotes the use of the
modern fugacious dyes. The dealers desire not only to imitate the
fashionable colors of European goods, but to secure the utmost
cheapness. One of our largest manufacturers of woollen goods, who had
made a special study of the best processes abroad, and was desirous of
bringing better dyed goods into more general consumption, urged one of
his largest customers, an extensive dealer, to allow him to dye the
waterproof cloakings which he was furnishing for his house, in fast
indigo colors, assuring him that he would charge simply the additional
cost of the indigo, without profit. The offer, which involved the cost
of only a few cents a yard, which would have been gladly paid by the
last consumer if the difference of value had been made known, was
declined. It is not improbable that the inferior goods which the
manufacturer was compelled to furnish were sold to the public as fast
dyed. Our manufacturers, therefore, may not have been responsible for
the predicament in which the most enthusiastic defender of our
protective policy found himself, as we have it from his own lips.
Being about to make a speech in Congress in defence of American
industries, he put on, for the first time, a coat declared to have
been made of American cloth. Sitting down, heated and perspiring from
the excitement of his effort, he found that beneath the arms whose
gestures had enforced his eulogies of American industry, the pretended
fast blue of his coat had become _red_, literally _blushing_ for its
unmerited praise. That fast-dyed goods of the highest excellence can
be and are furnished by American manufacturers, is shown by our army
cloths. The government specifications, copies of which are published
elsewhere in this number, require that all the blue woollen cloth, cap
cloth, and flannels furnished for the army shall be “pure indigo
dyed.” The requisition is strictly enforced. The admirable effect of
this regulation may be witnessed at any dress parade of a battalion of
United States soldiers. The persistency and uniformity of the hue
under constant wear—the cloth of the common soldier in its superior
dye often favorably contrasting with the finer but fancy dyed cloth of
the officer—is one of the circumstances which justify the assertion,
that our army is the best clothed in the world. The contrast is more
remarkable still with the _quondam_ blue cloth, converted by sun and
rain-into every shade of shabbiness, which we purchased in Europe for
our soldiers at the commencement of our late war.


ORIGIN AND EXTRACTION OF INDIGO.

Indigo is a coloring material of vegetable origin, which owes its
color and its important applications to a direct blue principle, known
under the name of indigotine. It has been used as a dyestuff from time
immemorial, by the inhabitants of India; and it is from the East, the
cradle of the textile arts, that Europe has derived it. It was
probably received from India by the Greeks, among other products first
made known to them by the expeditions of Alexander the Great.
Dioscorides clearly refers to indigo in mentioning the two coloring
matters brought from India. Pliny mentions a coloring material, having
an admirable mixture of blue and purple, as coming from India, which
he calls _indicum_. That he refers to indigo is curiously manifest by
the test which he gives, by which the genuine drug might always and
_certainly be distinguished from the spurious_. This is by putting it
on live coals, when, says he, “the true _indicum_ will burn with a
flame of a most beautiful purple tint.” The purple vapor from burning
indigo is still a characteristic test. The Romans, it is apparent,
used indigo only as a pigment, not knowing what is still the most
important art connected with its use,—how to make it soluble so as to
be available in dyeing.

That indigo as a commercial product was first obtained from India is
not only proved by the testimony of Pliny, and other ancient writers,
but is confirmed by a variety of circumstances, and particularly by
its name, which is known to have been _nil_ in the Hindu language,
from the earliest times of which there is mention of it. This name is
still given by the Hindoos to the color blue, and to all the plants
producing indigo. The Arabs and Egyptians, who obtained a knowledge of
indigo from India, adopted the Hindu name, the Arabs calling it _nil_
or _nir_, and the Egyptians _nil_ or _niel_. The Portuguese preserved
the Indian name, with a slight modification, the substance being
called _aniliera_ in their language. The coloring substances
afterwards found in coal-tar having been first found in indigo, modern
science has adopted for them the name of _aniline_.

It has been asserted that this substance was not known in Europe until
the time of the discovery of the passage to India round the Cape of
Good Hope. But Dr. Bancroft has shown that indigo was brought by
merchants from India to Alexandria, and thence to Venice, when that
city was the _entrepôt_ of Europe and the East. It doubtless
contributed to the excellence which the Italian states first attained
in the wool manufacture. The drug was called _endigo_ in Venice, and
it is from that city that we have derived its name and use. It was
imperfectly known in England under its Spanish name in the sixteenth
century, for we find in Hackluyt “Voyages” his instructions to a
traveller who was going to Turkey to ascertain “if _anile_, that
coloureth blue, be a natural commodity of those parts, and if it be
composed of an herbe.”

The general introduction of indigo into Europe was impeded by
legislative enactments, prompted mainly by those employed in
industries which it threatened to displace. These were chiefly the
producers of and dealers in _woad_, formerly used exclusively for
dyeing blue, and the corporation of woad dyers. When dyers from Italy
and Flanders attempted to introduce the superior dyes of indigo, the
woad interests were sufficiently powerful to induce the Elector of
Saxony to denounce the use of the new dyestuff. It was pronounced in
the Diet of the Empire as “a corrosive color,” and “fit food only for
the devil,” _fressende teufels_. Similar propositions were made in
England and France, in which latter the free use of indigo was not
permitted until 1737.

Although indigo as known in the arts is a product of vegetable origin,
we must not omit to notice that one source of its production is the
human body. It was discovered some years since that the blue color
sometimes found in diseased urines, and in certain suppurations, is
due to indigo. Dr. Schunck, in some papers read before the Royal
Society, has shown that it is a frequent constituent of urine secreted
by persons in a healthy state, and that, in fact, it is produced
generally when persons do not take sufficient exercise; and he has
several times succeeded in producing it by taking in his food a rather
large excess of sugar. He has found this substance also in the urine
of beef cattle. It must also be observed that the chemical actions of
indigotine with oxidizing agents, showing indigo to have a very close
relation to aniline and carbolic acid, both products derived from
coal-tar, have produced in the minds of chemists the conviction that
indigotine, like alizarine, the coloring principle of madder, will one
day be artificially produced from coal-tar.

The plants which are known to furnish indigo are quite numerous, being
not less than sixty; they do not all belong to the same family, and
none of them contain the coloring principle already formed. The most
important belong to the leguminous family, from which most of the
vegetable dyes are derived, and to the genus _indigofera_. The species
cultivated and most esteemed are _Indigofera tinctoria_, _I.
disperma_, _I. anil_, _I. argentea_.

[Illustration: Drawing of the Indigofera tinctoria plant]

The principal source of the indigo of commerce is the _Indigofera
tinctoria_. The accompanying figure is a correct representation of the
plant, and we may dispense with a description of its botanical
characters, observing only that the plant has a half woody stem, and
rises to the height of from three to five feet. The plants exhale a
strong odor towards evening in the fields where they are cultivated.
The leaves have a disagreeable taste, and rapidly putrefy in water.
The plant originated in Campaja, or Guzerat, but is cultivated in
Hindostan, China, Java, and in the East Indies generally. It was
carried by the Spaniards to South America and the West Indies, and it
can be acclimated in all hot countries. The _Indigofera argentea_, or
indigo plant of Egypt, furnishes the indigo produced in that country
and Arabia.

The culture of the plant and the production of commercial indigo is
carried on a vast scale in Lower Bengal. We have before us a large
map, placed at our disposal by an India merchant of Boston, showing
the location of each of the hundreds of factories of that important
centre of production. These factories have been developed by British
enterprise; and India thus receives some slight compensation for the
ruin of her cotton manufacture by the same influence.

The propagation of the indigo plant in that country is made by sowing
in a thoroughly tilled silico-argillaceous soil. The seed of the plant
is sowed annually in the spring or autumn, according to the variety
used, some germinating more slowly, and requiring to remain in the
ground longer than others. The time of putting in the seed is also
governed by the nature of the soil and its position in respect to
neighboring rivers. In the lowlands subject to inundations, the indigo
ought to be all cut at the period of the rains and inundations, which
would destroy the crop in a brief time. Besides, during the rainy
period the planter has at his disposal sufficient water to commence
his operations of fabricating the indigo, which is the suitable time
for beginning the cutting of the plant. The time of cutting the
indigo plants is therefore regulated by the elevation of the land and
danger from floods. The high lands are always sowed several weeks
after those subject to inundations.

The Chinese prick out the young plants in parallel rows, always
preserving the land quite clear of weeds. By taking away the blossoms
of the plant before their development, they increase the growth of the
leaves, and, consequently, the return of indigo; for it is in the
leaves principally that the coloring material is found.

In certain localities the planters break off the leaves which have
acquired a bluish green tint. But more frequently the whole plant is
cut down close to the ground in the months of June or July, when the
flowers begin to open. The portion of the plant which remains pushes
up quite rapidly, and furnishes a second, and even third, and
sometimes, though rarely, a fourth cutting. The quality of the product
diminishes according to the number of the cuttings.

The plant called _nil_, cut down to the root and gathered up in
packages, is worked up the same evening. The package is formed from
the product of a space of land embraced by a chain about three yards
long. The value of the first material changes with the value of the
soil. Thus, one soil produces a plant which has many stems and few
leaves, while another gives many leaves and few stems. The richness in
coloring material depends upon the quantity of leaves, but varies also
with an equal weight of leaves with atmospheric influences. Thus
regular dealers in the article observe a marked difference in the
quality of indigo in different seasons.

M. A. Koechlin Schwartz has recently published some interesting notes
upon the preparation of indigo in Lower Bengal. In that country, which
furnishes excellent indigo, the factory includes, besides filters,
presses, a steam-engine, drying apparatus, and reservoir of water, two
lines of vats, arranged one above the other, from fifteen to twenty in
each line. These vats are built up with bricks, and covered with a
strong coat of solid and well made stucco. They are square, about six
yards on a side, and about a yard deep. The back row is about a yard
above the front one. The plant is fermented in the vats of the upper
row; when the operation of fermentation is terminated, a faucet is
opened, and the liquid is run into the lower vat. The water of the
Ganges, which is relatively pure, and thus well suited for this work,
is brought into basins of deposition, where it becomes clarified, and
is distributed by a common canal to the vats of the upper row. The
plants, cut in the morning and bound up into packages, come to the
factory after midday, and are thrown into the vat in the evening. A
vat contains one hundred packages carefully arranged, one beside the
other; heavy timbers are placed upon the plants, which are pressed
down by means of large wedges. It is necessary that the plants should
be pressed together very compactly, as without this the fermentation
does not take place to advantage. At nightfall the water is introduced
into the vats, and fills them so as completely to submerge the plants.
The fermentation is more or less prolonged according to the
temperature. Its duration varies from nine to fourteen hours. The
workmen judge as to the procedure of the operation by withdrawing a
little of the liquid in the lower vat. If it is of a clear pale yellow
when withdrawn, it will furnish a product less abundant but more pure
than if of a deep gold color.

At the moment of its issue from the fermenting vat the liquid is of a
yellow color, more or less deep. The liquid is allowed to remain
undisturbed for a brief period, when twelve naked men, armed with long
bamboos, enter the vat to beat the water while it is still warm.
During this time the upper vat is emptied and cleaned out for the
succeeding operation. One vat requires seventeen workpeople (twelve
men and five women). They thrash the water for two or three hours. The
liquid passes by little and little to a pale green, and the indigo is
found on suspension in the form of small floccules. The liquor is
suffered to remain undisturbed for half an hour; it is then gradually
decanted by opening, one after the other, the discharging holes placed
at different heights. The water returns to the river, and the
precipitate, under the form of a thin _bouille_, is turned into a
reservoir. This _bouille_ is pumped up into a vessel, and made to boil
for a moment to prevent a second fermentation, which would injure the
quality of the product, by turning it black. It is suffered to rest
about twenty hours, and the next morning it is again subjected to
boiling, the ebullition being kept up three or four hours. The boiling
deposit is then turned off upon a large filter, through which the
water drips. This filter is composed of a vat constructed of masonry,
covered with stucco, about eighteen feet long by six feet wide and
three feet deep. This is covered with bamboos, upon which is a grating
of smaller reeds, and above by a stout strained cloth. There remains
upon the cloth a thick paste, of a deep blue and nearly black color.
The water which is run into the vat deposits some indigo which has
pressed through the filter. This is decanted after being allowed to
rest, and the turbid liquid is boiled the next day with the fresh
indigo.

The paste of the filter is introduced into some small boxes of wood,
pierced with holes, and provided above and below with a strong cotton
cloth. The whole is again covered with a piece of stuff, and then with
a covering of wood, pierced with small holes, and it is placed under a
press, the force being gradually applied, so as to cause the water to
run out as much as possible. There is withdrawn from the box a cake of
the size of a cake of Marseilles soap. The water squeezed out flows
back into the filtering vat, to be boiled again with the fresh indigo.
The drying of the cakes ought to be done very slowly.

The dry-house is a large building of masonry, quite high, and pierced
with many openings, provided with narrow blinds, to prevent the direct
light of the sun from penetrating into the interior. Care is taken
also to surround the dry-house with large shade-trees. The cakes take
from three to four days to dry, after which they are packed in small
boxes and carried to Calcutta, the great market of Bengal.

The details above given apply to the factories managed by European
planters. The natives operate in nearly the same manner, but with less
care, and consequently their products are inferior. The average
product of indigo in Lower Bengal is stated at 4,000,000 kilograms, or
8,840,000 pounds per year. The most remarkable fact to be noticed in
these operations is, that the blue principle is developed by chemical
action from certain absolutely colorless principles existing in the
plant. The theory of the change effected is still somewhat in doubt,
because no chemist has studied the fresh plant, and observed upon the
spot the phases of the operation of the production of indigo on a
large scale. But the most accepted theory is that derived from the
researches of Dr. Schunck, upon the _isatis_ or woad-plant, which
produces indigotine in a much less degree than the true indigo plants;
viz., that the indigo exists in the plants combined with sugar,
forming a glucoside, to which he gives the name _indican_. This
compound, under the influence of fermentation in the manufacturing
process, is supposed to be unfolded into indigo and sugar.

Without dwelling upon this question, which is beyond our province, we
observe that the plants of the genus _indigofera_ are used for the
production of commercial indigo, on account of the greater richness in
the coloring principle. Other plants, which furnish the same coloring
principle, indigotine, are more frequently used directly in dyeing to
furnish the blue principle than they are for the production of indigo.

The most important of these plants, although there are others, such as
the _Polygonum tinctorium_ and the _Nerium tinctorium_, is the _Isatis
tinctoria_, which produces pastel, or woad. This, plant belongs to the
family of cruciferæ, and is a biennial. It is represented in the
accompanying figure.

[Illustration: Drawing of the Isatis tinctoria plant]

The leaves which surround the stem are collected in May or June of the
second year, when they begin to turn yellow. The wasted and dried
leaves are sometimes used directly for dyeing, but more generally the
leaves, after being cut and dried, are carried to a mill, and then
ground to a paste, after which it is formed into a mass or heap, and
being covered to protect it from rain, is left to undergo a partial
fermentation for about a fortnight. The heap is then well mixed and
formed into balls, which are exposed to the sun and wind to dry, and
thereby prevent the putrefaction which would otherwise take place.
Being afterwards collected in heaps, these balls again ferment, become
hot, and emit the odor of ammonia, which Hume tells us, in the History
of England, gave such offence to Queen Elizabeth that she issued an
edict to prohibit the cultivation of this plant. After the heat has
continued for some time, these balls fall into a dry powder, in which
form the woad is usually sold to the dyer. The best French woad comes
from Provence, Languedoc, and Normandy. In Germany, the pastel of
Thuringia is used almost exclusively; the packages have the trade-mark
of three towers, with the numbers 4, 5. In this country, owing
probably to the prejudices of practical dyers, who have generally come
from England, the Lancashire woad is almost exclusively used. The very
little imported of late years, ranging from two thousand to twelve
thousand dollars annually in value, is used for mixing with indigo in
the so-called woad vat, to be hereafter described.


COMMERCIAL INDIGOES.

The following description of the indigoes of commerce is taken
principally from Schutzenberger’s excellent treatise on coloring
materials. It coincides very nearly with that given by Napier from
Dumas and Chevrueil. Indigoes are classed, according to their origin,
into three groups.

1. Indigoes of Asia (from Bengal, Oude or Coromandel, Manilla, Madras,
and Java).

2. Indigoes of Africa (Egypt, Island of France, Senegal).

3. Indigoes of America (Guatemala, Caraccas, Mexico, Brazil, and the
West Indies).

The three varieties in most esteem are those of Bengal, Java, and
Guatemala.

_Indigoes of Java_.—These are distinguished by the great purity of
their coloring material. They contain the minimum of extractive
organic matter. If, in spite of this, they do not give a high yield of
indigotine; this is owing to a mixture of silicious mineral substances
with their paste. The paste is soft. It adheres strongly to the
tongue, and its density is feeble. They are generally of a pure blue,
light or ash colored in the kinds which are less rich, and of a
magnificent violet blue in the superior qualities. The last take a
beautiful copper color when scratched by the nail. They are placed in
the very first rank among all indigoes in respect to fineness and
beauty, if not in richness in the blue coloring principle. Their
purity, complete absence from carbonate of lime, and the small
quantity of foreign organic materials which they contain, cause them
to be much sought for, for the preparation of _carmine_ of indigo. The
consumption of the Javan indigoes in this country is so small as not
to be appreciated.

_Bengal Indigoes_.—These are the indigoes _par excellence_, for in
them are found the most varied qualities, from the most beautiful and
rich to the most ordinary. The superior qualities are of a deep violet
blue, with a fine and uniform paste; they adhere to the tongue, are
easily pulverized, and take a beautiful coppery tint when scratched by
the nail. The fresh fracture shows a magnificent purplish blue
reflection. Their yield in indigotine does not surpass seventy-two per
cent.

After these come the reddish-violet indigoes with a purplish hue, and
a fracture more uniform and shiny. They are also more dense and hard
than the superior qualities. The reddish hue does not proceed from the
greater or less amount of coloring material contained, but from the
presence of a greater quantity of brown and red extractive matter.
These qualities are not to be despised, for the kinds which give the
best results in the dyeing vat are found in these indigoes. It would
seem, in fact, says the author whom we are following, “that the browns
and reds of indigo play an important part in vat dyeing, that they are
able to become dissolved and to fix themselves upon the tissues at the
same time as the indigotine, and thus operate to reinforce the hue.
The fact is, that dyers generally prefer the reddish indigoes to the
other varieties.” Among the Bengal indigoes there is found a clear
blue variety, less rich in coloring matter, but also more exempt from
organic substances. The impurity is constituted by mineral matters. It
is less dense, adheres strongly to the tongue, and does not take a
coppery hue, like the other varieties, when scratched by the nail.

The worst qualities of the Bengal indigoes, as in all the species, are
the clear blues, shading on to gray or green. This coloration denotes
a great quantity of extractive matter different from the indigo brown
which characterizes the red varieties, and completely inert. These
indigoes are hard, dense, adhere little or none to the tongue, and do
not show coppery reflections when scratched.

The most skilful connoisseurs distinguish forty-three varieties of
Bengal indigo. The most important are the following:—

1. _Superfine blue, light or floating_.—Color bright blue; light,
friable, and spongy; adherent to the tongue, soft to the touch,
showing coppery reflections when rubbed by the nail; paste uniform and
pure.

2. _Fine blue_.—Like the preceding, but the color a little less vivid.

3. _Violet blue_.—A little less light and friable. Has a violet blue.

4. _Superfine violet_.

5. _Superfine purple_.

6. _Fine violet_.

7. _Good violet_.

8. _Red violet_.

9. _Ordinary violet_.

10. _Good soft red_.

11. _Good red_.

12. The indigoes, _fine coppery, good coppery, ordinary coppery, and
low coppery_.

_The Indigoes of Oude and Coromandel_.—These are made in the interior
of Hindostan. Those of the best quality correspond to the middling
Bengal indigoes, and are met with in square masses, having an even
fracture, but are more difficult to break; the inferior qualities are
heavy, of a sandy feel, having a blue color, bordering on green or
gray, or even black; often in large squares, and covered with a slight
crust or rind of a greenish color. They are the most difficult to
break of all the indigoes of commerce.

_Madras Indigoes_.—They have a grained fracture, and are of a cubical
figure. The superior qualities have no rind. The qualities are fine
blue, mixed violet blue, and ordinary. They are all lighter, and less
rich in coloring matter than the Bengal indigoes.

_Manilla Indigoes_.—These occur in cubical blocks, flat squares, or in
irregular pieces. They are light, with a fine paste, and of a clear
blue. They effervesce with acids, showing the presence of carbonate of
lime incorporated in their paste. They are consequently poor in
coloring material, and are hence almost exclusively used as a bluing
material in washing fabrics.

_American Indigoes_. _Guatemala_.—These indigoes are produced now
altogether in Hunduras, although they still retain in commerce the
name of Guatemalan. They are generally found in small pieces,
irregular in form and size, and come in envelopes of skin containing
about half as much as the Bengal chests. Putting aside the difference
in exterior form, these indigoes approach very closely to those of
Bengal. The same qualities are found, only they are more frequently
mixed. The clear blue is more rare, and, when it is found, it is
poorer in coloring matter. In purchasing these indigoes it is
necessary to beware of the reds, which often contain a strong
proportion of the brown extractive matter. It is not rare to find
among the Guatemalan indigoes beautiful specimens of the blue violet,
equal to the richest Bengal variety. Unfortunately, this superior
variety is generally mixed with inferior kinds, as to have less value.
The American indigoes are classified as follows:—

_Guatemala floro_.—Bright blue, paste uniform, soft and light. This
variety, in Bancroft’s time, was the most esteemed of all indigoes.

_Guatemala sobresaliente_.—Less light, the paste firmer and the blue
less beautiful.

_Guatemala corte, or copper-colored_.—Paste less firm and heavier,
coppery red.

_Caraccas_.—These resemble very much the Guatemala varieties. The
qualities are designated by analogous names, but they are, in general,
less esteemed than the preceding.

_Mexican_.—They hold an intermediary rank between the Caraccas and
Mexican.

_Brazil_.—These indigoes are in small rectangular parallele-piped
masses, or in irregular lumps of a greenish gray color externally, and
having a smooth fracture, a firm consistency, and a copper-colored
tint of greater or less brilliancy.

_The indigoes of Africa and Egypt_.—These have only been manufactured
within the last twenty years; they are in flat squares. The paste is
fine and quite light, and the color pure blue or bordering on violet.
The varieties are distinguished as fine blue and good violet and red.

_Indigoes of the Isle of France and Senegal_. Rare in commerce, but
of good quality.

The indigoes of the inferior qualities, characterized by a salt-like
color, bordering more or less upon green; by a coarse, uneven, and
very dense paste; by not adhering to the tongue, and by not showing a
coppery color when scratched,—can never be employed to advantage,
notwithstanding their low price. The purchaser of these qualities must
be guided solely by the results of analysis; for an article is found
in commerce whose richness in indigotine does not exceed twelve to
fourteen per cent. The presence of so high a proportion of foreign
matter prevents the chemical change which the indigo ought to undergo
in the dyeing vat; and this foreign matter, added to the deposits of
the dyeing vats, causes great loss of the coloring matter. These
indigoes should be used as little as possible, especially in the cold
vats used for dyeing cotton and linen. The middle varieties of the
Bengal and Guatemala indigoes, and, above all, the red varieties,
produce in the cold vats the most advantageous results. The lower
qualities above spoken of present less inconvenience in the hot vats
used for dyeing wool; and it is for this purpose that they are
generally used. In considering the previous observations, the wool
manufacturer may arrive at this conclusion: that while he can, with
less loss than the maker of cotton fabrics, make use of the lowest
qualities of indigo, he will obtain _the best results from the middle
qualities of the reddish Bengal indigoes_.

The skilled dealers in indigo recognize not only the above
distinctions, founded upon the country of production, color, and
physical qualities, but they observe whether the article has any of
the following defects, which are designated by certain well-understood
terms: such as whether the indigo is _sandy_,—when brilliant points
are observed in the interior, which are in reality particles of sand;
_spotted_, that is to say, of unequal tint, and marked by small
blackish points; _ribboned_, marked by transversal bands of a paler,
and sometimes red color; _burnt_, the pieces having a scorched
appearance, due to rapid drying, and separating into small black
fragments under the pressure of the hand; _crumbly_, when in pieces of
irregular figure, proceeding from fractures of the squares; _cold_,
when the indigo does not adhere to the tongue. The above
classification is presented with a full knowledge that these
distinctions are by no means recognized in the ordinary commerce in
this article. It is not, however, without interest as an illustration
of the minute attention given to this subject in Europe, where a
higher manufacture requires a nicer investigation of the qualities of
materials employed.


DETERMINATION OF THE RICHNESS AND PURITY OF INDIGOES.

It is evident that the commercial form and the high price of this drug
favor fraud, and the desire to illicitly introduce foreign substances
into the paste. It is important, therefore, that the purchaser should
carefully ascertain the actual value of the article which he is to
use. He should know not only the proportion of indigotine contained,
which varies in the commercial indigoes from twelve to seventy-five
per cent, but the hardness and density. A good indigo ought to have
qualities which can be recognized by the eye and touch alone. The
first and the only examination ordinarily made by purchasers is in
respect to the physical qualities of the article. Different pieces are
selected, and their fresh fracture is attentively observed. The
purchaser observes whether the squares are like each other, and if the
parts of the same piece present the same tint. He determines the
porosity by the simple means of applying his tongue to the fresh
fracture. The more rapid the adherence of the tongue, the more porous
the indigo. By scratching the piece with his finger-nail, he
determines the extent of the coppery reflection,—an important test.

From all these characters, taken together, the purchaser can form
quite a correct idea of the value of indigoes in general; and the
greater number of dyers, both in Europe and this country, are
satisfied to make their purchases with only this physical examination.
The most experienced dealers in this country make no other examination
than the physical one. An eminent indigo broker in Boston has
permitted me to copy the following memoranda for the physical
examination of indigo from his notebook.

The chief signs of good indigo are its lightness, feeling dry when
touched, and, when broken, appearing of a beautiful violet blue. Good
indigo swims in water; if thrown upon burning coals it emits a
violet-colored smoke, and leaves but little ashes.

In selecting indigo the large regularly formed cakes should be
preferred,—those of a fine, rich blue color, extremely free from the
white adhesive mould[1], and of a clean, neat shape. When broken, it
should be of a bright purple cast, of a close and compact texture,
free from specks or sand, and when rubbed with the nail should have a
beautiful shiny coppery appearance; when burnt in a candle it should
fly like dust; that which is heavy and dull colored should be
rejected. Indigo is estimated and classed in commercial language, as
follows: fine blue, ordinary blue, fine purple, inferior purple, and
violet, strong copper, and ordinary copper. It is purchased by the
factory maund (74⅔ lbs. The Bazaar maund is 82²⁄₅₀ lbs.), packed in
cases containing on an average 2¼ cwt., dammered (pitched) and covered
with gunny bagging.

  -------------------------------------------------------------
  [1] Many experienced purchasers in this country pay no regard to
  this mould, as it weighs scarcely any thing.—Ed.
  -------------------------------------------------------------

Still, in making large purchases, as a measure of wise precaution the
chemical test should be added. This is used to ascertain the
proportion per cent of indigotine which a given indigo has. The
determination of the quality of indigotine contained is not alone
sufficient to fix the value of an indigo. With an equal yield of
indigotine, the indigoes are always to be preferred which have a light
and soft paste; and for the preparation of the indigo vat the
preference should be always given to the violet red rather than to the
clear blue indigoes.

The chemical works which treat of this subject give elaborate details
of a great number of processes for determining by chemical tests the
amount of indigotine, or the coloring material in indigoes. To give
these numerous processes would only confuse the reader. In our own
confusion upon this subject we submitted the descriptions of these
various processes to one of the most eminent and practical of American
chemists, Dr. Charles T. Jackson, an official State Assayer for the
State of Massachusetts, who has had much experience in testing indigo,
with a request that he would describe the process which he approves
and practises. He has obliged us by the following communication:—

                                         Boston, Nov. 21, 1872.
                                         No. 47 Court Street, Room 4.

John L. Hayes, Esq.

Dear Sir,—In reply to your inquiry as to the simplest method of
analyzing indigo, I would say that I first ascertain the amount per
cent of earthy matters and metallic oxides, in the samples brought to
me, by burning a weighed quantity in a counterpoised platinum
crucible, until all organic matters are removed or consumed, and then
weighing the ashes obtained. The ash is then subjected to analysis in
the usual way, and lime, alumina, peroxide of iron, and some other
earthy impurities are separated.

Then, to determine the amount of coloring matter, or indigotine, I
make use of a standard sample of pure reduced indigo, which is
dissolved in the most concentrated sulphuric acid, and diluted with
water after solution. Then I ascertain how much bleaching powder
(chloride of lime) is required to dissolve the solution. This is the
quantity required for absolutely pure indigo.

Now, the indigo of commerce does not contain more than say from forty
or fifty per cent of pure indigotine, and of course will require a
smaller quantity of bleaching powder to decolor it; or the quantity of
bleaching powder to decolor a given weight of pure indigo may be
weighed out, and the sample to be compared having been dissolved in
strong sulphuric acid, and diluted with water, is to be poured in and
stirred or shaken well until the point of decoloration is ascertained.
In this case it is best to weigh out at least twice as much of the
sample to be tested as was used of pure indigo, and to measure the
solution in a graduated glass vessel,—an alkalimeter, for example,—so
that by measure we may know exactly how much of the sample we add to
the solution of bleaching powder. Thus the relative coloring values of
the samples may be readily ascertained.

If you have no purified indigo on hand, you can make a good
comparative trial of your samples against a perfectly good sample of
Bengal indigo, which may be kept for a standard of comparison. Very
useful practical results may thus be obtained.

It is well, however, to keep on hand a standard sample of pure indigo,
prepared from reduced or white indigo, as directed by Berzelius (vol.
vi. page 3, French ed., 1832), and in Muspratt’s Chemistry applied to
the Arts (Dyeing, Indigo).

In the analysis by reduction of indigo, the process is simply as
follows: Reduce the indigo to fine powder, and weigh it; weigh out an
equal quantity of pure quicklime (made from pure white marble).
Measure in a graduated vessel a certain volume of water. Slack the
lime with a portion of this water. The rest of this water is to be
used in rubbing up the indigo in a mortar. Then the slacked lime is to
be mixed with the indigo, rubbing the substances well together.
Introduce the whole into a large flask; 1½ to 2 litres (about 3 to 4½
pints) of water is required for 1 gramme (or about 15 grains of
indigo). The flask and contents are then to be exposed to a heat of
from 176° to 190° F. for some hours. This is best effected in a water
bath. By this digestion the lime is made to combine with the indigo
brown, and the coloring matter is set at liberty. Dissolve in the
liquor a little protosulphate of iron, exempt from copper, and reduced
to a fine powder. The flask is to be corked and well shaken, and
allowed to cool. When the sediment is settled, decant the clear
solution by means of a syphon into a graduated glass. The coloring
matter oxidizes by exposure to the air; and to favor this oxidation
and to keep the lime in solution, add muriatic acid to the liquor.
When the liquor has become clear, filter and collect the precipitate
on a weighed filter, which wash with hot water, and dry at a
temperature of 212° F. Thus we can learn, by weighing the filter
again, how much indigotine is contained in the sample.

If we make use of 200 measures of water, and have drawn off 50
measures of the solution to oxidate, and this 50 measures has produced
10 grains of indigo, the whole sample evidently contained 40 grains of
indigo blue.

This method serves both for an assay of the sample and the production
of a standard sample of pure indigotine. The operation may be carried
on upon a larger scale for the manufacture of a standard sample.

Yours truly,

C. T. Jackson.

Dr. Jackson adds the following note:—

In the processes given I have not referred to the qualitative analysis
or testing for all the kinds of adulterations, but have given only
valuation of the coloring power of indigo.

I have had occasion to search indigo for Prussian blue, an occasional
adulterant. This is ascertained by caustic potash, which becomes in
part an oxide if Prussian blue is present. This acidulates with
muriatic acid, and, tested with sulphate of iron, proves, by formation
of Prussian blue, the presence of the ferrocyanide of potash in the
solution, and hence Prussian blue in the indigo. Lime and clay are the
usual adulterants, and oxide of iron is often present accidentally or
from the clay adulterants. Starch and flour are rarely used, as they
add little to the weight.

C. T. J.


COMMERCE IN INDIGO.

Before proceeding to a consideration of the practical applications of
indigo in manufacturing, we must pause to make some general
observations upon the commerce in indigo.

The first European impulse given to this commerce was made by the
Spanish and Portuguese. They not only imported indigo from the Indies,
but established its fabrication in their colonies. To them we owe its
production in Guatemala, Caraccas, and Brazil. The French exported
from the Island of San Domingo, only, in 1774, 2,350,000 pounds weight
of this commodity. British influence was exerted in favor of the
development of this article in the American colonies, and, in 1773, in
the space of twelve months, over a million pounds of indigo were
exported from South Carolina. The production in India was at that time
of little importance. It was not until 1783 that the attention of the
English was directed to the culture of indigo in India for European
consumption, that produced by the natives being all consumed in their
own manufactures. In the hands of the English this product rapidly
rose to be the most important of India, in a commercial view, except
that of rice. The small cost of a factory, and the comparatively small
capital required for this production, caused the indigo culture to be
preferred to sugar planting. The importation and sale of this
commodity at the East India House, in 1792, amounted to 581,827 lbs.,
while the importation into Great Britain from other parts of the world
amounted to 1,285,927 lbs. In 1806 the importation from the East
Indies, and sales at the East India House, amounted to 4,811,700 lbs.,
and produced in sterling money £1,685,275. In the year 1862–63, the
export from India, and the destination of supplies, were as follows:—

  Destination.               Quantity.         Value.
  ----------------------------------------------------
  United Kingdom            8,537,133 lbs.  $1,627,035
  America                     134,064           26,949
  Arabian and Persian Gulfs   343,037           33,385
  France                    1,922,120          371,396
  Germany                      85,680           15,504
  Suez                        295,269           51,730
  Other places                  9,577              815
  ----------------------------------------------------
  Total                    11,326,880 lbs.  $2,126,814

The value of exports in 1866 was £1,861,501. In the same year the
imports of indigo from the whole of Central America, including
Honduras, was 672,480 lbs. The consumption of indigo in Great Britain
did not increase during the ten years ending with 1867. This
stationary demand, notwithstanding the fall in the price of the drug
and increase of population, is attributed by McCulloch principally to
the decreasing use of blue cloth. It is more probably due to the
substitution of cheaper dyes. The average home consumption in Great
Britain for seven years ending in 1867, was 1,675,072 lbs. per year.

The importation into this country for the twenty years last past is
shown by the following table, kindly prepared at our request by the
chief of the Bureau of Statistics:—

_Statement of Imports of Indigo into the United States during the
Fiscal Years ended June 30, 1853–1872._

                   INDIGO.
  Fiscal
  Year
  ended
  June 30   FREE OF DUTY.          DUTIABLE.
  --------------------------------------------------
          Pounds.   Dollars.    Pounds.    Dollars.
  1853                         1,387,847    947,367
  1854                         1,965,789  1,282,367
  1855                         2,097,397  1,151,516
  1856                         1,732,290  1,063,743
  1857                         1,533,037  1,010,509
  1858                         1,647,767    945,083
  1859                         1,773,868  1,441,429
  1860                         1,707,116  1,413,790
  1861    185,039    160,138     719,563    505,766
  1862  2,501,052  3,281,441
  1863    885,834  1,008,187     178,364    219,169
  1864    684,813    623,406     897,821    671,899
  1865    741,438    601,283     415,575    324,207
  1866    798,855    609,160      44,660     41,268
  1867  1,069,506    816,974
  1868    870,164    775,751
  1869  1,574,449  1,649,550
  1870  1,270,579  1,203,664
  1871  1,994,172  2,052,222
  1872  1,526,869  1,484,744

  EDWARD YOUNG, _Chief of Bureau_.
  Bureau of Statistics, Nov. 16, 1872.

The extraordinary quantity imported in 1862, we hardly need remark,
was due to the demand for consumption in army cloths. Indigo imported
directly, was made free of duty in 1861. The duty which appears by the
above table to have been charged since that period, was upon indigo,
the product of India, imported by way of England, which was subject to
an extra duty of ten per cent.

The indigo consumed in the United States is generally supplied by the
Boston and New York Calcutta houses, who have either an American
partner resident in Calcutta, or who employ a resident American as
agent. Indigo, like other Calcutta goods, is sold through the agency
of brokers, who receive on this article a commission of one per cent.
The value of the article is known almost daily in these cities by
telegrams, giving exact information of the state of the trade,
transmitted from Calcutta as often as every five days. Some of the
brokers publish monthly circulars, showing the stock of indigo with
other Calcutta goods on hand in our market. The regular trade reports
issued by the India merchants show that The higher qualities of indigo
do not come to our market. The following is an extract from a report
of Whitney, Brother, & Co., of 1871:—

  Indigo for Continent      fine             350 to 362 rupees.
    „     „     „           good             330  „ 345   „
    „     „     „           middling         310  „ 325   „
  American consuming        fine             280  „ 300   „
      „       „             good             250  „ 275   „
      „       „             middling         200  „ 240   „
      „       „             low and ordinary 150  „ 170   „

At the present moment there is great depression in the trade in this
article. The last telegrams show a decline of price in the Indian
trade in this article of from fifty to seventy-five per cent from the
prices of last year; and the apprehension is even entertained that
indigo is going out of use, the dreaded competitors being the aniline
dyes, and particularly the Nicholson blue. We maybe presumptuous in
giving our opinion on the question, but we hazard the prediction that,
notwithstanding the temporary popularity of the cheap substitutes, a
reaction will take place in favor of that “wonderful and most valuable
production,” whose importance as a dye has been held in India for
thousands of years and Europe for two centuries, “greatly to exceed
any other.”[2]

  -------------------------------------------------------------
  [2] The “Dictionnaire Universel du Commerce,” &c., published in
  1861, contains an exhaustive article on the commerce in indigo,
  by M. S. Beekrode. From the statements of this writer, it appears
  that the consumption of indigo was estimated, in 1835, as follows:—

    Great Britain            1,214,380 kilograms (2,683,779) lbs.
    France                     912,915     „     (2,017,542)  „
    United States              130,000     „       (277,300)  „
    Other countries          2,435,473     „     (5,382,395)  „
                             ---------           -----------
    Total                    4,692,768 kilograms (10,362,016) lbs.

  The approximate consumption in 1859 is stated as follows:—

    Great Britain              800,000 kilograms (1,768,000) lbs.
    France                     800,000     „     (1,768,000)  „
    United States              400,000     „     (  884,000)  „
    Russia                     860,000     „     (1,900,600)  „
    The Zollverein           1,250,000     „     (2,762,500)  „
    Switzerland                150,000     „     (  331,500)  „
    Austria                    400,000     „     (  884,000)  „
    Other countries            300,000     „     (  663,000)  „
                             ---------           -----------
    Total                    4,960,000 kilograms(10,961,600) lbs.

  The average production in 1859 is estimated as follows:—

    Bengal, Madras, &c.      3,500,000 kilograms (7,735,000) lbs.
    Java                       550,000     „     (1,215,500)  „
    Central America            300,000     „     (  663,000)  „
    Other sources              100,000     „     (  221,000)  „
                             ---------           -----------
    Total                    4,450,000 kilograms (9,834,500) lbs.

  As the maximum annual consumption in 1859 is set down at
  5,000,000 kilograms, the author concludes that the average
  production at that time did not surpass the requirements of the
  dyers of the whole world.
  -------------------------------------------------------------


FORMER PRODUCTION IN THE CAROLINAS.

As pertinent to the commercial branch of our subject, we must briefly
notice the remarkable facts of the sudden growth and equally sudden
and extraordinary extinction of the production of indigo in the
Carolinas. Indigo was for many years the second great staple of South
Carolina. So highly was this staple estimated that the historian of
the State declares that “it proved more really beneficial to Carolina
than the mines of Mexico or Peru are or ever have been to Old or New
Spain.” Its introduction was the happy result of a woman’s culture and
energy. In the early part of the last century, the indigo plant had
been extensively cultivated in the West India Islands, which then
furnished the chief supply of Europe. The governor of Antigua, George
Lucas, whose home plantation was at Wappoo in Carolina, having
observed the fondness of his daughter, Miss Eliza Lucas, afterwards
the mother of General Charles Cotesworth Pinckney, for the culture of
plants, was in the habit of sending to her tropical seeds to be sowed
on his plantation at Wappoo. Among others, he sent her some seeds of
the indigo plant cultivated in the West Indies. She planted them for
two years; but the seeds failed to germinate, or were killed by the
frost. On the third year’s trial, in 1741 or 1742, she was successful.
Governor Lucas, on hearing that the plants had ripened and produced
seed, sent from Montserrat a person skilled in making indigo. Vats
were built on Wappoo Creek, and there the first American indigo was
manufactured. The attempts of the expert to conceal his processes were
defeated by the vigilance of Miss Lucas. The process of manufacture
was made known. Seeds from the Wappoo plantation were freely
distributed and successfully planted; and the culture of indigo became
common. In 1747, a considerable quantity of indigo was sent to
England, which induced the merchants trading with Carolina to petition
parliament for a bounty on Carolina indigo. In 1748, an act of
parliament was passed granting a bounty of sixpence per pound on
indigo raised on British-American plantations and imported directly to
Britain from its place of growth. This act stimulated the planters of
Carolina to double vigor in the production of this new material for
export. “Many of them,” says Dr. Ramsay, “doubled their capital every
three or four years by planting indigo.” In the year 1754, the export
of indigo from the province amounted to 216,924 lbs., and in the years
1772 and 1773 the export had risen to 1,107,661 lbs. The production
was greatly checked by the war of the Revolution. Near the close of
the century the large importations from India lowered the price, so as
to make the planting unprofitable. In the mean time, the culture of
cotton had sprung up under the protective tariff of 1789. The grounds
suitable for indigo planting were equally fitted for cotton, and were
for the most planted with the new staple. It is curious to observe how
the former was displaced by the latter staple. The export of indigo
from Charleston in 1797 was 96,121 lbs.: in 1800, it fell to 3,400
lbs. During the same years, the exports of cotton rose from one
million to six and a half million pounds. The production of American
indigo appears to have revived from time to time up to 1829. A writer
of that period in Silliman’s Journal of Science estimates—although it
would seem on doubtful authority—the production of indigo in the
United States at 20,000 lbs. The price of the American article had
fallen, owing to the great quantity of extractive which it contained,
to fifty cents per pound, while the Bengal indigo was worth $1.15 per
pound. We have no data as to its production at the present time, but
infer, from the fact that no reference has been made to this product
in the Government Agricultural Reports for many years past, that the
production, if any, is too unimportant to be noticed.


INDUSTRIAL APPLICATION OF INDIGO.

All the applications of indigo require that the material should first
be reduced to an impalpable powder. It is better to grind it with
water, to prevent the loss of material in the form of powder, although
the dry pulverization is necessary when the indigo is to be used for
the manufacture of the sulphate. To facilitate the grinding the
material into a paste, it should be previously soaked in hot water
from one to three hours. The grinding on a small scale may be done by
a very simple apparatus. This is a hemispherical vessel of copper or
cast-iron, eighteen inches in diameter, furnished at the edge with two
handles. The workman, sitting astride a bench, places the vessel
before him, in which he places three heavy cast-iron balls, the indigo
which has been softened, and a sufficient quantity of water. Holding
the basin by the handles, he gives it a circular oscillatory movement,
in such a manner that the balls, following this movement, crush the
indigo which surrounds them; after which the contents are poured into
another vessel, water is added, and the material is stirred. The
portions incompletely ground are made to reunite themselves at the
bottom by means of regular blows with a hammer on the rim of the
vessel. The upper liquid is decanted, and the deposit is submitted to
a new manipulation in the basin.

In large establishments the grinding is done by machinery. An
apparatus highly recommended, consists of two circular plates of
cast-iron, arranged horizontally and slightly separated, one from the
other, which are rapidly rotated by power, in inverse, directions. The
interior surfaces of these disks are provided with deep grooves
radiating in a curved line from the centre to the circumference, and
diminishing in depth in the same direction. The indigo which has been
previously softened enters between the two plates by an opening in the
centre of the upper one, and escapes in a thin paste by the
circumference.

The application of indigo to the coloring of textile fabrics requires
the complete dissolving of the substance, for which the mechanical
division is only a preliminary. There are only two known means of
dissolving this substance: 1. By reduction; 2. By the action of
concentrated sulphuric acid. The first means allows indigotine to be
regenerated; and, when the dyeing is completed, it is pure indigotine
which adheres to the colored fibre. By the second means, or dissolving
by sulphuric acid, the coloring material enters into a new
combination, from which it can never be separated: it becomes a new
substance, endowed with new and special properties.

_The fixing of Indigotine by means of Reduction_.—In this method the
operator avails himself of one of the most remarkable qualities of
indigotine: this is the facility with which this body takes up
hydrogen, and becomes transformed into a colorless substance, which is
soluble in favor of alkaline or alkaline-earthy bases, and is
susceptible of reproducing indigotine by simple oxidation in contact
with air. This hydrogenized substance is called white indigo. Blue
indigo, or indigotine, is insoluble except by concentrated sulphuric
acid; and this insolubility gives it its superiority to all other blue
dyes. Not being soluble, it cannot, as blue indigo, attach itself to
the material to be dyed; but in the soluble form of white indigo it
can perfectly penetrate the fibre. If by any means of oxidation we can
transform the white indigo into blue indigotine, the latter becomes
insoluble, and is imprisoned in the pores of the fibre. This is,
briefly, the whole theory of the use of indigo in dyeing or printing,
although the reaction may be applied in different ways to the coloring
of fibres, such as—

1. The indigo is dissolved by means of an alkaline reduction in a vat,
and the fibre is immersed in the bath. This is the common blue vat.

2. The solution prepared beforehand is painted by a hair pencil and
printed by a stamp or roller upon only certain parts of the tissues.
This is the pencil blue.

3. The white indigo is precipitated under the form of a paste, in
combination with a metallic oxide having strong reducing power, such
as hydrated protoxide of tin, which prevents the too rapid reoxidation
of the indigotine. The thickened paste is printed, and the tissue is
placed in an alkaline bath (lime or soda), which, displacing the oxide
of tin, forms a soluble combination of white indigo. The latter can
then penetrate the fibre, and afterwards become fixed by reoxidation.
This is the printer’s solid blue.

4. The finely ground, but not dissolved indigo, is placed upon the
tissue in such conditions that it can be dissolved and reduced in
place. This done, the fixing of the indigotine is effected by
oxidation. This is the method for China blue or _bleu faïence_.

Without dwelling upon the details of these methods, we hasten to a
consideration of the most important of all the applications of indigo:—


DYEING BY THE INDIGO VAT.

_The Copperas Vat_.—For dyeing cotton, the method of reduction found
by experience to be the most convenient and practical is founded upon
the action of the hydrate of the protoxide of iron in the presence of
lime. The hydrated protoxide of iron is obtained from sulphate of iron
(green vitriol, or copperas) with freshly burned lime. Certain
precautions should be observed in the use of these materials. The
copperas used for the preparation of these vats should be free from
sulphate of copper, because the oxide of copper which would be formed
in the vats rapidly oxidizes the reduced indigo, and causes its
precipitation in the bath. The copperas ought not to contain red oxide
of iron, nor sulphate of alumina. The coppery or oxidized vitriol may
be purified by boiling the solution with pieces of iron, which
precipitates the iron and neutralizes the oxide. The lime ought to be
pure, containing no magnesia; when slacked lime has been exposed to
the air, even for a short time, it absorbs carbonic acid, and becomes
converted into chalk. The lime, therefore, should always be newly
slacked. The ingredients, then, of a copperas vat are water, pure or
purified green vitriol, indigo ground into a homogeneous impalpable
paste, and pure and freshly slacked lime. The proportions used in
different establishments are exceedingly variable. Those which answer
for a laboratory vat, or a small vat used for precipitating the white
indigo immediately for printing, are: indigo, one part; sulphate of
iron, two parts; slacked lime, three parts. These proportions are not
enough for the large vats used in dyeing pieces. In them it is
necessary to make the quantities of lime and sulphate of iron larger
than the theory of the vat requires. The excess of lime and hydrate of
iron serve the purpose, whenever the vat is stirred, to repair the
losses of indigo caused by its oxidation from contact with the air.
Schutzenberger gives the proportions generally used by the dyers of
France, as follows:—

  Indigo                                         1 part.
  Crystallized sulphate of iron                  3 „
  Freshly slacked lime                           3 „

Others, he says, use more lime than copperas, as in the following
proportion:—

  Indigo                                         2 parts.
  Sulphate of iron                               5.5 „
  Quicklime                                      6.5 „

M. de Kæppelin, who is especially familiar with the cotton dyeing in
Mulhouse, describes the ordinary vats for cotton dyeing as bound with
iron, and placed on the level of the ground. They hold from 3,000 to
4,000 litres (1,055 gallons) of liquid. In preparing them the dyer
fills them about three-quarters full of water, and pours in a milk of
lime, prepared with 45 kilograms (100 lbs.) of freshly slacked lime; a
fine liquid paste having been previously made from 15 kilograms (33
lbs.) ground in water. This is added to the lime in the vat by
portions, the liquid in the vat being stirred up by a rake after each
portion of the indigo paste has been added. The indigo becomes
dissolved in about twenty-four hours, when the vat can be used. After
describing the manner in which the frame, or _champignon_, containing
the goods to be dyed is arranged and immersed in the vat, this author
continues: “It will be understood that the vat is composed according
to the degree of intensity of the color which is sought to be
obtained, and that hues more or less deep may be obtained by means of
more or fewer repeated immersions of the fabric to be dyed. After each
immersion the _champignon_ is lifted out of the vat, and the fabrics
are left to _ungreen_ themselves by contact with the air. (It must be
observed that, although soluble indigo is called white, because it is
without color when carefully prepared in the laboratory, the goods,
when first taken from the ordinary vat, are of a green color.) Exposed
to the air, the soluble indigo is precipitated in the state of blue
indigo upon the fibres of the tissue. This oxidation, or
_dehydryzation_, may be hastened by plunging the tissue into a vat
containing a solution, very much diluted with water, chloride of lime,
bichromate of potash, or sulphuric acid. The first two act as
oxidizing agents; the last facilitates the restoring of the blue
indigo by depriving it of the lime which is in excess in the solution
of indigo which the tissue has imbibed from the vat.”

He adds further: “To facilitate the formation of blue indigo in the
interior of the fabrics, the stuff to be dyed may be previously
impregnated by a saline solution, which has the property of
precipitating the white indigo from the alkaline solution, and of
fixing itself more rapidly upon the tissue. Oxide of copper and oxide
of manganese possess these properties in a high degree, and are used
in many establishments to hasten the dyeing process, and produce an
economy in raw materials. The pieces of cloth are placed in a solution
of sulphate of copper, in the proportion of 15 to 20 grams to the
litre (2.11 pints), and lightly thickened with starch. The fabrics,
thus impregnated by a kind of mordant, before receiving the blue dye
are first passed through a weak bath of milk of lime, which fixes the
oxide of copper upon the tissue. The blues thus obtained are more
intense, and have a peculiar lustre. This process is used in Austria
and Germany, where cotton fabrics are printed on both sides of the
tissue.”

Coming to the English authorities, Dr. Grace Calvert, in his recent
lectures before the Society of Arts, speaking of the cold vat for
dyeing cotton, says: “The oldest, and still most generally employed
method of preparing cold vats, consists of putting into a vat
containing about 2000 gallons of water 60 lbs. of indigo, very finely
powdered, 180 lbs. of slacked lime, and 120 lbs. of sulphate of
protoxide of iron, or green vitriol (free from any trace of copper
salt), the two latter substances being added from time to time. The
greater part of the lime used unites with the sulphuric acid of the
iron salt, to produce sulphate of lime or gypsum; and the liberated
protoxide of iron removes the oxygen from the indigo, becoming
converted into saline oxide, whilst the reduced indigo dissolves in
the excess of lime employed.”

He adds the following facts, which may be of practical value:—

“Messrs. R. Schloesser & Co., of Manchester, have introduced within
the last year or two a marked improvement, in the preparation of cold
vats, which removes the great objections of the bulky precipitate of
sulphate of lime, the formation of an oxide of iron, and the loss of
indigo by its combination with the oxide of iron. The bath remaining
much more fluid, the pieces are less apt to be spotted, and a better
class of work is produced. To carry out their process, they add to the
ordinary 2,000 gallon vat 20 lbs. of ground indigo, 30 lbs. of iron
borings, 30 lbs. of _their remarkable powdered zinc_, and 35 lbs. of
quicklime; the whole is stirred up from time to time, for twenty-four
hours, when it is ready for use. If the bath is not considered
sufficiently strong, a little more lime and zinc are introduced. The
chemical theory of the process is, that the zinc, under the influence
of the lime, decomposes the water, combining with its oxygen, and the
hydrogen thus liberated removes oxygen from the indigo which then
dissolves in the lime.”

An excellent description of the processes employed at Manchester,
England, in preparing and working the copperas, or cold vat, is given
in Ure’s “Dictionary of Manufactures.” “The ingredients necessary for
setting the vat are copperas, newly slacked quicklime, and water.
Various proportions of these ingredients are employed, as, for
instance: 1 part by weight of indigo (dry), 3 parts of copperas, and 4
of lime; or, 1 of indigo, 2¹⁄₂₃ of copperas, and 3 of lime; or, 8 of
indigo, 14 of copperas, and 20 of lime; or, 1 of indigo, ¾ of
copperas, and 20 of lime; or 1 of indigo, 4 of copperas, and 1 of
lime. The sulphate of iron should be as free as possible, from red
oxide of iron, as well as sulphate of copper, which reoxidize the
reduced indigo-blue. The vat, having been filled with water to near
the top, the materials are introduced, and the whole, after being well
stirred several times, is left to stand for about twelve hours. The
chemical action which takes place is very simple. The protoxide of
iron, which is set at liberty by the lime, reduces the indigo-blue;
and the indigo, which is then dissolved by the excess of lime, forming
a solution, which, on being examined in a glass, appears perfectly
transparent and of a pure yellow color, and becomes covered, whenever
it comes in contact with the air, with a copper-colored pellicle of
regenerated indigo-blue. The sediment at the bottom of the vat
consists of sulphate of lime, peroxide of iron, and the insoluble
impurities of the indigo, such as indigo-brown in combination with
lime, as well as sand, clay, &c. If an excess of lime is present, a
little reduced indigo-blue will also be found in the sediment in
combination with lime. The dyeing process itself is very simple.
The vat having been allowed to settle, the goods are plunged into the
clear liquor, and, after being moved about in it for some time, are
taken out, allowed to drain, and exposed to the action of the
atmosphere. While in the liquid, the fabric attracts a portion of the
reduced indigo-blue. On now removing it from the liquid, it appears
green, but soon becomes blue on exposure to the air, in consequence of
the oxidation of the reduced indigo-blue. On again plunging it into
the vat, the deoxidizing action of the vat does not again remove the
indigo-blue which has been deposited within and around the vegetable
or animal fibre, but, on the contrary, a fresh portion of the reduced
indigo-blue is attracted, which, on removal from the liquid, is again
oxidized like the first, and the color thus becomes a shade darker. By
repeating this process several times the requisite depth of color is
attained. This effect cannot, in any case, be produced by one
immersion in the vat, however strong it may be. The beauty of the
color is increased by finally passing the goods through diluted
sulphuric or muriatic acid, which removes the adhering lime and oxide
of iron. After being used 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 to settle again, so as to leave
the liquor clear. The indigo-blue, however, is in course of time
gradually removed, and by degrees the vat becomes capable of dyeing
only pale shades of blue. When the color produced by it is only very
faint, it is no longer worth while using it, and the contents are then
thrown away. In dyeing cotton with indigo, it seems to be essential
that the reduced indigo-blue should be in contact with lime. If potash
or soda are used in its place, it is impossible to obtain dark shades
of blue.”


FERMENTING VATS FOR WOOL DYEING.

The application of indigo-blue to wool and woollen tissues is always
made by means of vats, which have special names; as, the pastel or
woad vat, the urine vat, German vat, molasses vat, &c. The reduction
or _hydrogenation_ of the indigo-blue is the result of a peculiar
fermentation, which is developed within an alkaline liquor by means of
nitrogenized substances and bodies rich in sugar or hydrocarbonized
substances. It is known that in these conditions, especially where the
temperature is slightly raised, the sugar is converted into butyric
acid, and at the same time carbonic acid and hydrogen are set free. We
find here the source of the nascent hydrogen which fixes itself upon
the indigo-blue, and transforms it into white indigo, which is soluble
in the alkalies of the vat. It has recently been observed that the
butyric fermentation proceeds from the development of minute
infusoria. These animalculæ live without any supply of oxygen, and, in
fact, are killed in its presence. They therefore live at their ease in
the vat of reduced indigo, where no oxygen is permitted to enter.

The ingredients most usually employed for furnishing the
hydrocarbonaceous substances for fermentation are bran and ground
madder, although molasses is sometimes used. The nitrogenized material
is found in the woad or pastel, which is often added in very large
proportions to the fermenting vats. It is observed by the chemists who
have studied this subject most carefully, that the preparation of
vats, founded upon the principle of fermentation, does not repose upon
principles so sure and constant as those of the copperas vat, and that
many unforeseen accidents interpose to disturb the work of an
inexperienced dyer. The phenomena in fermentations are often complex.
It is admitted that in these phenomena theory has not said its last
word, and that empiricism is often more fortunate than science. In
conducting the operations of the warm fermenting vat, the conceit of
the practical dyer, so often remarked upon, is not without foundation.
By practical experience and the traditions of his art he has acquired
a knowledge of the almost insensible modification in conditions which
can change or arrest the chemical reaction. It is the knowledge of the
workman, a knowledge almost instinctive, which can never be
communicated to the books, and which is most respected by those most
profoundly informed in theory.

_The Woad or Pastel Vat_.—In former times woad, already referred to,
was the only material known to the dyers of Europe for producing the
blue color of indigo. For dyeing wool, the use of woad, now abandoned
wholly in cotton dyeing, has been retained to the present day,
generally for the purpose of exciting fermentation, and without regard
to its effect in imparting color to the material to be dyed; for the
woad grown in England, and used in the dye-houses of that country,
contains no trace of coloring matter. The woad, or pastel, grown in
the warmer districts of France contains about two per cent of
indigotine, which is regarded in that country as an important addition
to the coloring material, especially for improving the tone of the
color. Various substitutes, such as rhubarb leaves, turnip and carrot
tops, and weld, have been tried, but without advantage, with the
exception, perhaps, of weld, which is still used by some dyers. Some
chemists regard the use of woad as the remnant of a prejudice; but the
better opinion is, that this material possesses peculiar
fermentiscible qualities, whose exact action science has yet to resolve.

According to Schutzenberger, the most recent and highest French
authority, the dimensions of the pastel vat are about 6½ feet in
diameter, by 9 in depth. 100 kilograms (221 lbs.) of pastel, in balls,
is placed in the vat, which is then filled with boiling water. To this
is added 10 kilograms (22 lbs.) of madder, 3 to 4 kilograms (about 6½
to 8¾ lbs.) of bran, and 4 kilograms of quicklime, which has been
slacked, and in the form of a _bouilli_. Sometimes weld is also added.
After three hours of rest, the vat is well raked, and the operation is
repeated every three hours. There is gradually developed a
characteristic ammoniacal vapor, and a blue scum, with veins of deeper
blue, forms on the surface; and the liquid, when agitated in the air,
rapidly becomes blue. These symptoms indicate the dissolution of the
indigotine of the woad; then there is added 10 kilograms (22 lbs.) of
indigo which has been previously ground in water, and the vat is
stirred. If the fermentation appears to be proceeding too actively,
which is recognized by the disengagement of gases, it is checked by
the addition of a proper dose of lime. On the other hand, the
fermentation is made more active by increasing the dose of bran. The
first dyes are not so good as those subsequently obtained, as the woad
absorbs from the bath certain brown or yellow materials, kept in
solution, and furnished as well by the pastel and madder as by the
indigo itself. 100 kilograms of wool require from 8 to 12 kilograms of
indigo. The vat is kept up by successive additions of indigo and lime,
made in the evening.

Another kind of pastel vat, prepared much like the last, receives an
addition of a dose of potash. M. de Kæppelin describes it as the one
at present in general use in France. Into a vat containing from 3,000
to 4,000 litres (791 to 1,055 gallons) there is placed 75 kilograms
(166 lbs.) of pastel in loaves, or which has undergone a kind of
fermentation; or, what is preferable, 80 to 100 kilograms (176 to 221
lbs.) of pastel or woad gathered without fermentation, and 10
kilograms (22 lbs.) of indigo, ground to a paste with water. This
mixture is well stirred, and there is added 4 kilograms (about 9 lbs.)
of Avignon madder, and the same quantity of carbonate of potash. After
the vat has been well raked there is added 2 kilograms (4½ lbs.) of
slacked lime, and some pails of bran. The vat is well covered, either
with a wooden lid, or woollen cloths. The fermentation is allowed to
proceed, and after five or six hours the vat is uncovered and raked
with much care for half an hour. This operation is repeated every
three hours, until it is recognized that the indigo is well dissolved.
In this case the bath ought to be of a beautiful yellow color, and be
covered with a light blue irised film, veined with yellow at the least
movement given to the liquid. If the fermentation proceeds too
rapidly, a little lime is added to moderate it.

For keeping up a vat like this, and to obviate the different
inconveniences to which it is subject, the dyer sometimes adds lime,
or sugar, and carbonate of ammonia, sometimes madder, or bran, or even
tartar-lees. These last additions are made to saturate the excess of
lime which the vat contains. In this case the yellow veins and the
beautiful blue scum which cover the surface disappear, or become pale;
a piquant odor is disengaged, and the liquor becomes blackish. When
lime or sugar are added, it is for the purpose of retarding the
fermentation of the woad. Sugar might even entirely take the place of
pastel for effecting the reduction of the indigo, and many
establishments in France are commencing to use it for this purpose. A
good vat, well supplied with successive additions of indigo, pastel,
bran, and madder, in proportions necessary to effect and prolong the
fermentation necessary for the dissolution of the indigo, may be kept
up many years.

Schutzenberger observes that the vats of fermentation are subject to
certain maladies, the two most frequent of which are due, one to an
excess, and the other to an insufficient quantity of lime. “In the
first case, the liquid takes a tint more and more free of color, loses
its _fleurée_ (surface scum) and odor; the fermentation is then
arrested by the precipitation of the active matters. This
inconvenience is remedied, if seen in time, by adding sulphate of
iron, which eliminates the too great excess of lime. In the second,
the fermentation becomes too active, passes into a putrid
fermentation, and the liquid assumes a reddish tint; a fabric dyed
with indigo in this state becomes very soon discolored. The sole means
of safety is to heat the bath up to 90° and to add lime. If this does
not accomplish the purpose of arresting the putrefaction the vat is
lost.”

The following account of the method of dyeing woollen goods with
indigo by means of the woad vat is given by Dr. Ure, as that carried
on in Yorkshire, the great centre of the woollen manufacture of England.

“The dye-vats employed are circular, having a diameter of six feet six
inches, and depth of seven feet, and are made of cast-iron
five-eighths of an inch in thickness. They are surrounded by
brickwork, a space of three inches in width being left between the
brickwork and the iron, for the purpose of admitting steam, by means
of which the vats are heated. The interior surface of the brickwork is
well cemented. In setting a vat the following materials are used: 5
cwt. of woad, 30 lbs. of indigo, 56 lbs. of bran, 7 lbs. of madder, 10
quarts of lime. The woad supplied to the Yorkshire dyers is grown and
prepared in Lincolnshire. It is in the form of a thick, brownish
yellow paste, having a strong ammoniacal smell. The indigo is ground
with water in the usual manner. The madder acts in promoting
fermentation, but it also serves to give a reddish tinge to the color.
The lime is prepared by putting quicklime into a basket, then dipping
it in water for an instant, lifting it out again, and then passing it
through a sieve, by which means it is reduced to a fine powder, called
by the dyers _ware_. The vat is first filled with water, which is
heated to 140° F., after which the materials are put in, and the whole
is well stirred until the woad is dissolved or diffused, and it is
then left to stand undisturbed overnight; at six o’clock the next
morning the liquor is again stirred up, and five quarts more of lime
are added; at ten o’clock five pints of lime are again thrown in, and
at twelve o’clock the heat is raised to 120° F., which temperature
must be kept up until three o’clock, when another quart of lime is
introduced. The vat is now ready for dyeing. When the process of
fermentation is proceeding in a regular manner, the liquid, though
muddy from insoluble vegetable matter in suspension, is of a yellow or
olive yellow color; its surface is covered with a blue froth or
copper-colored pellicle, and it exhales a peculiar ammoniacal odor; at
the bottom of the vat there is a mass of undissolved, matter of a
dirty yellow color. If there is an excess of lime present, the liquor
has a dark green color, and is covered with a grayish film, and, when
agitated, the bubbles which are formed agglomerate on the surface, and
are not easily broken. Cloth dyed in a liquid of this kind loses its
color on being washed. This state of the vat is remedied by the
addition of bran, and is of no serious consequence. When, on the other
hand, there is a deficiency of lime, or, in other words, when the
fermentation is too active, the liquor acquires first a drab, then a
clay-like color; when agitated, the bubbles which form on its surface
burst easily, and when stirred up from the bottom with a rake it
effervesces slightly, or _frits_, as the dyers say. If the
fermentation be not checked at this stage, putrefaction soon sets in,
the liquid begins to exhale a fetid odor, and when stirred evolves
large quantities of gas, which burns with a blue flame on the
application of a light. The indigo is now totally destroyed, and the
contents of the vat may be thrown away. No further addition of woad is
required after the introduction of the quantity taken in first setting
the vat, the fermentation being kept up by adding daily about four
pounds of bran with one quart or three quarts of lime. Indigo is also
added daily for about three or four months. The vat is then used for
the purpose of dyeing light shades, until the indigo contained in it
is quite exhausted, and its contents are then thrown away.”

This author adds: “Woollen cloth, before being dyed, is boiled in
water for one hour, then passed immediately under cold water. If it be
suffered to lie in heaps after being boiled it undergoes some change,
which renders it afterwards incapable of taking up color in the vat.
In dyeing, the cloth is placed on a net-work of rope attached to an
iron ring, which is suspended by four iron chains to a depth of about
three feet beneath the surface of the liquor. The cloth is stirred
about in the liquor by means of hooks for about twenty or thirty
minutes. It is then taken out and well wrung. It now appears green,
but, on being unfolded and exposed to the air, rapidly becomes blue.
When the vat has an excess of lime the cloth has a dark green color
when taken out. It is then passed through hot water, and dipped again
if a darker shade is required.”

_The Indian Vat_.—This presents much analogy to the woad vat, as the
fermentation of vegetable matters effects the transformation of the
indigo-blue. According to Dr. Calvert, the Indian vat, probably so
called from its origin in the East, is taking the place in England of
the old woad vat for dyeing wool and woollens. He describes its
preparation as follows: 8 lbs. of powdered indigo is added to a bath
containing 3½ lbs. of bran, 3½ lbs. of madder, and 12 lbs. of potash,
which is maintained for several hours at a temperature of 200° F. It
is then allowed to cool to 100° F., when fermentation ensues. After
about forty-eight hours the indigo is rendered soluble, being reduced
by the decomposition of the sugar and other products contained in the
bran and the madder root during the process of fermentation. The
distinguishing feature of this vat is the use of _potash_. The Indian
or potash vats are spoken of by the best authorities as more easy to
manage than the woad vat. They are less subject to accidents, and
yield their coloring material more readily to the fibre, while three
times as much wool can be dyed in the same time. On the other hand,
they do not last so long, and require to be renewed at the end of
twenty-five or thirty days. Besides, the fibres dyed in the potash vat
have a darker shade than those dyed in the woad vat, owing to the
large quantity of the coloring matter of the madder dissolved by the
potash, which becomes fixed on the stuff with the indigo-blue.

_The Urine Vat_, but little used except for domestic dyeing, is
founded upon the same principles as the other fermenting vats. This
excretion, when putrefied, contains at the same time the nitrogenized
principles which work as ferments and the alkali in the form of
ammonia necessary for dissolving the indigo.

According to Dr. Calvert, improvements have been made of late years in
the fermenting indigo vats by which the expense of madder is avoided.
They are now prepared by adding to water, at a temperature of 200° F.,
2 buckets of bran, 26 lbs. of soda crystals, 12 lbs. of indigo, and 5
lbs. of slacked lime. After five hours the bath is allowed to cool to
100° F., when fermentation ensues, and the indigo is dissolved in the
alkali. This is, in fact, the German vat, soda taking the place of the
potash, and the only fermenting material consisting of bran.

_The German Vat_ is largely used by the dyers in the north of France,
and is considered as more advantageous than the Indian vat, because
the employment of soda is more economical than that of potash, while
the vat can be maintained as long as two years. The vats used by them
are prepared as follows: The water is heated to a temperature of 95°,
and receives 20 pails of bran, 11 kilograms (about 24 lbs.) of
crystals of carbonate of soda, 5.5 kilograms (11 lbs.) of indigo, and
4½ lbs. of slacked lime. After twelve hours, the temperature having
been kept at 40° or 50°, fermentation commences, the liquid becomes of
a greenish blue color, and disengages bubbles of gas. Indigo, soda,
and lime are put in from time to time in the proportions above
indicated, and also from six to eight pounds of molasses. At the end
of the third day the vat is fit for use.

M. de Kæppelin, writing in. 1864, informs us that the reduction of
indigo by means of molasses, is at present largely employed in the
great establishments for dyeing woollen cloth at Sedan, Louviers, and
Elbœuf.

The vat used is of very large dimensions, and from twenty-two to
twenty-six pounds of indigo are dissolved in it; an equal weight of
molasses is used, and three or four times the same weight of potash
made caustic by a proportionate addition of lime.

The space reserved for this subject in our present paper will not
permit us to enter upon a description of the processes used in the
American dye-houses. This, as well as the applications of indigo in
printing, and the uses of sulphate of indigo, must be deferred to
another number.

Let us, in concluding the first part of our paper, at the risk of
repetition, bring out in bolder relief a statement which presents the
philosophy of all the various processes of the indigo vat, and at the
same time, a conclusive argument for the use of this material, in
preference to all cheaper substitutes. Indigo cannot enter into a
fibre until it is dissolved. It cannot be dissolved so long as it is
in a blue state. When reduced by any of the processes above described
to the white state, it is easily dissolved, and can enter the pores of
the fibre. Upon exposure to the oxygen of the air it takes up an
equivalent of oxygen; it returns to the blue state, and, being then
insoluble, it cannot be washed away from the fabric, and being
saturated with oxygen it cannot be changed by air or light. This
theory of the application of indigo involves a lesson to
manufacturers, dealers, and consumers, especially of woollen fabrics.
The theory, as well as experience, dating back to the dawn of the
textile arts in the East, establishes that this material is
incalculably superior to any other, in permanence at least, for
imparting to woollen fibre a blue color, or as a foundation for most
of the darker colors. By far the largest proportion of all cloths are
of dark colors,—blue, black, green, brown, gray, or mixed,—and can
advantageously receive in all or a portion of the fibre constituting
them a direct dye or bottom for other dyes from indigo. It may be
safely stated that, as a whole, no cloths in the world are
manufactured from such good wool as those produced in the United
States. We might expect that the shoddy goods of Yorkshire should be
further falsified by fugacious dyes; but is it not a shame that our
admirable wool should be deprived of half its value by parsimony in
dyeing? The slightest shortcomings in dyeing are revealed in wear. The
writer cannot forbear referring to an illustration directly before his
eyes. He is wearing a garment, reduced now to the retired service of
an office coat, made of an admirable cheviot cloth of American
manufacture. The cloth originally was selected not only for its
excellent texture, but as an illustration of philosophical principles
applied in the formation of color. The tissue was made by weaving
three yarns of distinct colors,—blue, yellow, and red. Either of those
hues alone would have been glaring and conspicuous, but, by the law of
color, the combination of blue, red, and yellow makes black, and the
new cloth at a distance had the effect of a dark mixture. Upon
exposure to ordinary wear, the yellow and red have retained their
pristine hues; the blue, not being indigo dyed, has faded; and the
original dark mixture, although sound in fabric, has become of a
yellowish brown. The extra expense of a permanent dyeing material
forms so small a proportion of the whole cost of a finished garment,
that it ought not to be generally spared. The reform cannot be made by
the manufacturers; it must be made by the dealers, and especially by
that class of producers which has risen in our day into such great
importance,—the manufacturers of ready-made clothing. If they would
demand of the manufacturers, and furnish to their customers cloths
more permanently dyed, it would be another step in the direction to
which these establishments are tending,—the supply of the chief
portion of the woollen clothing of the people. The manufacturers would
gladly aid them; for it is the growing sentiment of American
manufacturers that all their productions should be, in the proverbial
phrase adopted from the dye-house, as expressing the highest
excellence,—_true blue_.


BIBLIOGRAPHY.

Citations of authorities having been but partially made in the
preceding article, the writer, for the purpose of giving his sources
of information, and for the convenience of those who wish to pursue
the subject further, appends a list of the more important works which
he has consulted:—

Schutzenberger’s Traité des Matières Colorantes, t. ii. (the most
recent and best modern authority); Bancroft’s Philosophy of
Permanent Colors, vol. i.; Edinburgh Encyclopædia; Berzelius,
Traité de Chimie, t. vi; Chevrueil, Leçons de Chimie Appliquée à
Teinture, t. iii.; Dumas, Chimie Appliquée aux Arts, t. viii;
Wurtz, Dictionnaire de Chimie, 1872, art. Indigo; Indigo et son
Emploi, par De Kæppelin; Annales du Génie Civil, 1864, t. iii.;
Lectures of Dr. Grace Calvert, Chemical News, Aug. 9 and 23, 1872;
O’Neill’s Dictionary of Dyeing and Printing; Napier’s Chemistry
Adapted to Dyeing; Muspratt’s Chemistry Applied to the Arts,
articles Indigo and Dyeing; Ure’s Dictionary of Manufactures, ed.
of 1860; Proceedings of Royal Society, vol. xvi.; Proceedings of
Literary and Philosophic Society of Manchester, vol. iv.; McCulloch’s
Dictionary of Commerce, ed. 1869; Dictionnaire Universel du
Commerce, &c., ed. 1861; _South Carolina Production_.—Ramsay’s
History; Drayton’s South Carolina; Silliman’s Journal, vol. xviii.
A more complete bibliography is given in Schutzenberger’s work.


PART II.

PART II.

We entered upon the subject of indigo, which we have treated at some
length in our last issue, as much in the interest of the people as of
manufacturers, for we were deeply impressed with the conviction that
no improvement in our manufacturing processes would confer more
benefit upon the masses than imparting stability of color to the
clothing of the people. When one has a deep conviction upon a subject,
upon which others have equal opportunities for judging, he may be sure
that he is not alone in his impressions. He is moved by one of those
waves of thought which, operating simultaneously upon many minds,
gives that uniformity to public opinion at which we so often wonder.
We are gratified to find, from responses to our last article, that we
are not alone in our conviction of the importance of reviving “true
blue” dyes. The head of a mercantile house, the extent of whose
_clientèle_ in mills both of wool and cotton is hardly surpassed, has
assured us that we have not overstated the reform in dyeing which we
have advocated. He had long shared in our convictions. Pointing to the
throng of men in the crowded street, where we were conversing, he
remarked that there was hardly a man in the crowd whose clothing would
not have been improved by indigo dye. “The failure to use indigo
dyes,” he emphatically said, “costs the laboring people of this
country millions of dollars every year. The fault is not to be charged
to our own manufacturers alone; for the blue coat which I wear, and
which I bought in Paris, annoys me by the crocking caused by its
aniline dye.” In one very large mill of which he is director as well
as selling agent, he is putting his principles in practice. All the
heavy blue cloths intended for popular consumption are faithfully
dyed, and each bears a stamp, “Warranted indigo dyed.” The ready-made
clothing establishments which largely consume these goods have already
found their advantage in purchasing them, and a similar stamp is
attached to each article made from this cloth.

Some of our most celebrated cotton fabrics have won and still retain
their reputation by the use of indigo dyes. The ginghams are a signal
illustration. The blue check is formed by weaving cotton yarns dyed
blue in the cold indigo vat with undyed yarns. These goods can be
washed indefinitely without change.

Another illustration is the famous A.B.A. Amoskeag tickings, an
article of such excellence that the question of the right to use
trade-mark A.B.A. gave rise to the leading American case in this
branch of law.[3] A prominent feature in these goods was and still is
the permanence of the dye in the blue stripe, produced by the cold
indigo vat. Still another illustration is the blue and white “shirting
stripe” first made by Mr. Samuel Batchelder, at the Hamilton Mills,
now so generally adopted for sailors’ shirts. The indigo dye enables
the color to resist the roughest possible usage.

  -------------------------------------------------------------
  [3] See the case stated at length in our article on Trade-marks,
  Bulletin, vol. i. p. 102.
  -------------------------------------------------------------

To recur to the application of indigo dyeing to wool and woollens. We
have been unable, although we have written more than fifty letters of
inquiry upon the subject, to learn of any peculiarity or improvements
in the American processes of wool dyeing with indigo.[4] Our dyers are
for the most part foreigners. For this reason, or because the art of
indigo dyeing has long since reached perfection in the best
establishments abroad, they rigidly pursue the old European methods.
The best dyers regard the successful management of the warm fermenting
vats for wool as the highest test of their art. We have already spoken
of the complicity of the phenomena in fermentations. Practical dyers
endow the fermenting vat with a sort of personality. “An indigo vat,”
says one to us, “is more like a sick man than any thing in the world:
you have to watch it as you would a sick patient, and give it physic
or ferments to stir up the system and purify it.” [5] The diagnosis of
a sick vat requires that sort of instinctive knowledge which
experience gives to the practised physician. The impatience of our
young Americans will not permit them to serve the long apprenticeship
necessary to acquire the proper experience. The artisans not
thoroughly trained will naturally prefer the dyes and processes
introduced by modern science, which require but little skill in their
application. It is a curious fact that the influence of the national
government has been largely instrumental in preserving the old system
of indigo dyeing. Thanks to the Quartermaster-General’s Bureau, or the
man of science, General Meigs, who presides over it, indigo dyed
cloths have been persistently insisted upon for the army. The late war
gave a new impulse to indigo dyeing. A skilled dyer, whom we have
consulted, was constantly employed in Connecticut, on a tour of
professional inspection of a dozen or more different establishments
making army goods. No doctor, he says, ever found in hospital practice
more complications of disease than he found in the ailing vats. Among
other difficulties there was a deficiency of imported woads, although
the cultivation of excellent woad immediately sprung up in
Connecticut. In the mean time carrot and rhubarb tops were used as
substitutes for the fermenting material of the woad. Carrot-tops grown
expressly for that purpose brought as high as twenty-five cents per
pound. Since the war the requisitions for indigo dyed woollen goods
have not relaxed, and the art is not likely to be lost.

  -------------------------------------------------------------
  [4] A reply by Mr. D. R. Whitney, an extensive indigo importer,
  to a letter of inquiry, enables us to correct some errors in our
  former article, under the head of “commerce in indigo.” The value
  of export from India in 1862–63, stated in dollars, through a
  typographical error, should have been pounds sterling; thus,
  instead of $2,126,814, read £2,126,814. It is stated in our first
  article that the telegrams show a decline of price of indigo in
  the Indian trade of from 50 to 75 per cent; “per cent” should
  read “rupees,” which would make a decline of from 25 to 30 per
  cent. The reason for the decline, as stated by Mr. Whitney, is
  the unusually large crop of this year. The average crop of indigo
  in Bengal is about 100,000 maunds. The crop of this year is
  135,000 maunds, about 30 to 35 per cent above the average.

  According to Mr. Whitney, the consumption of Bengal indigo in the
  United States was 2,458 cases of 270 lbs. to a case on an
  average, in 1871; and in 1872, 1,802 cases. Guatemala indigo,
  3,132 serroons in 1871, and 2,578 serroons in 1872.

  [5] See notes on “sickness” of vats in Appendix.
  -------------------------------------------------------------

With the real difficulties which attend the process, it is hard for
indigo dyeing to sustain itself in the face of cheap substitutes of
easy application, such as the Nicholson blue. It is exceedingly
difficult to piece dye with indigo and preserve a uniform hue upon the
cloth. Hence indigo dyes are generally given in the wool. The wool
absorbing the foreign material of the dye is more difficult to work in
the operations of carding and spining. In other words, a finer and
costlier wool is required. A great _desideratum_ therefore is a means
of piece dyeing with indigo so as to preserve a perfect uniformity of
hue throughout the piece. This, we are happy to say, has been recently
successfully accomplished by one of the largest and most faithful of
our cloth-making establishments. It would be premature, before the
patents are secured for this invention, to explain the ingenious and
expensive apparatus devised for this purpose, which constitutes in
fact a battery of vats so arranged that the operation may be
continuous. The experiments authorize the statement that bottom dyes
of indigo, so desirable for a great variety of colors, can be applied
with no other additional cost than that of the dyeing material. When
this establishment, as it proposes, stamps upon the cards which
designate goods, already so admirable in material and texture,
“Warranted indigo dyed,” we shall regard it as an era in the American
card-wool manufacture.

The old European woad vat process is that used in all our
establishments. Mr. Henderson of the Washington Mills, whose
experience as a practical dyer of wool is exceptionally large, informs
us that he has found no work so instructive upon this process as
Napier’s “Chemistry of Dyeing” (published by Henry Carey Baird, of
Philadelphia, 1869). Napier’s description of the process is extracted
from Dumas’s “Lectures on Dyeing.” The appreciation expressed by so
competent a judge induces us to reprint Dumas’s description in an
appendix to this article.

That we may give at least a general view of the whole subject, we will
proceed to consider indigo in some relations not yet adverted to.

In Part I. of our notes we have treated only of the application of
this substance in dyeing by means of reduction through the indigo vat.
Indigo may be applied by means of reduction in the printing of
fabrics, as well as in dyeing them. A true scientific arrangement
would compel us next in order to consider this other application of
indigo by means of reduction. But the more natural and practical order
is to pursue the subject of dyeing, and to consider next the
applications of the derivatives from indigo in dyeing proper.


SULPHURIC DERIVATIVES,—SAXON BLUE, &C.

The powerful action of sulphuric acid upon indigo, and the bright and
lively blue color thereby produced, had been observed by chemists long
ago; but no person appears to have applied this color upon cloth,
until it was done about the year 1740, by Counsellor Barth, at
Grossenhein, in Saxony. The vividness of the dye, and the facility
with which it was applied, brought it into great vogue under the name
of Saxon blue, from its origin. Its popularity in former times is
evinced by the words of the old song, “The Blue Bells of Scotland:”—

  “In what clothes, in what clothes is your Highland laddie clad?
  His bonnet’s of the Saxon blue, his waistcoat of the plaid.”[6]

  -------------------------------------------------------------
  [6] First sung by Mrs. Jordan, about the year 1799.
  -------------------------------------------------------------

The Saxon blue consists simply of a solution of indigo, the Guatemala
blue indigo being preferred, in sulphuric acid suitably diluted with
water. The result of this reaction is not a single chemical substance,
but two acids giving different tints, one called _sulpho-purpuric_
acid or _phenicine_, and the other _sulpho-indigotic_ acid; the first
giving to wool a reddish-violet color, and the other a pure blue. A
third compound has been indicated by Berzelius, the nature of which
has not been determined. Whether one or the other of the two named
acids, or the two combined, shall be produced by the reaction between
the sulphuric acid and the indigo, depends upon the duration of the
contact, the temperature of the mixture, and the nature and proportion
of the acid used.

Persoz gives the following general receipt:—

  “1 part by weight of indigo, finely rubbed.
   1  „    „    „    „ Nordhaussen acid.
   1  „    „    „    „ ordinary sulphuric acid.
Leave for forty-eight hours, then heat until a drop turned into
water will dissolve without producing a precipitate. Leave to cool,
and dilute with water till the strength is brought to 18 Beaumé.”

Napier says that he has found the following method of preparing
sulphate of indigo, in quantities for use, very satisfactory: “The
indigo is reduced to an impalpable powder, and completely dried by
placing it on a sand bath or flue for some hours at a temperature of
about 150° F. For each pound of indigo six pounds of highly
concentrated sulphuric acid are put into a large jar, or earthen pot,
furnished with a cover. This is kept in as dry a place as possible,
and the indigo is added gradually in small quantities. The vessel is
kept closely covered, and care taken that the heat of the solution
does not exceed 212°F. When the indigo is all added, the vessel is
placed in such a situation that the heat may be kept up at about
150°F., and allowed to stand, stirring occasionally, for forty-eight
hours. These precautions being attended to, we have uniformly found
that any failure occurring was clearly traceable to the impurity of
the indigo or weakness of the acid used.”

The processes for producing and separating the two acids derived from
the combination of sulphur and indigo are minutely given by Berzelius,
in vol. i. of his “Traité de Chemie,” who states this curious fact
illustrative of the peculiar affinities of wool with certain dyeing
substances. Wool or flannel thoroughly scoured, when immersed in the
blue solution of indigo with sulphuric acid, acts as a base: it
combines gradually with the acid blue, and becomes itself colored of a
deep blue. When saturated with color, it is withdrawn. Fresh wool is
introduced until the bath yields no more color. If sublimed or
perfectly pure indigo is used, there remains in the bath nothing but
free sulphuric acid. The wool thus plays the part of a base with which
the blue acids combine. The dyed wool is afterwards washed and treated
in feeble alkaline bath (ammonia), which redissolves the blue. This
method of purifying the Saxon blue is still practised by French
manufacturers.

The combination of indigo with sulphuric acid, sometimes improperly
called sulphate of indigo, is known by the dyers here and in England
under the name of _chemic_. The name of _chemic_ blue or green is also
given the dyes formed from the indigo extract hereafter spoken of. It
is largely used for making certain greens required in Scotch plaids.

The old Saxon blue or simple solution of indigo with sulphuric acid is
now seldom prepared by the manufacturers themselves. It is now
generally prepared for them, and furnished commercially under the name
of _indigo extract_. The finer qualities used for fine dyeing and
printing are known under the name of _carmines_ of indigo, _neutral
extract_, _soluble indigo_, _ceruline_, &c.

The production of indigo carmines, which are simply alkaline
sulphindigotates or sulpho-purpurates, is founded upon their
insolubility in a liquid charged with a salt.

If, for example, we dissolve one part of indigo in four parts of
fuming acid, and dilute the liquid with sixty or eighty times its
weight of water, it will contain, besides the _sulphindigotic_ acid,
an excess of sulphuric acid. By adding one part of crystals of soda so
as to neutralize the bath, there will be formed not only
sulphindigotate of soda, but sulphate of soda: as the former is
insoluble in the saline liquid, the presence of the sulphate of soda
causes the precipitation of the sulphindigotate in deep blue
floccules. These are collected on woollen filters and washed to remove
the sulphate of soda and a green coloring material, probably a
modified chlorophyl, which the paste often contains, and which has the
singular property of fixing itself on silk, but not on wool.

The carmines are divided according to their richness in indigo into
simple carmine (4.96 per cent of indigo, water 89, saline materials
57), double carmine (10.2 per cent indigo, water 85, salts 4–8),
triple carmine (12.4 per cent indigo water, 73.7, salts 13.9). A
species of solid carmine known as Boiley blue or purple is in high
repute in France.

The carmines may be tested by dyeing a specimen of wool in an
acidulated bath to which cream of tartar has been added. The presence
of the green matter, so objectionable to silk-dyers who make much use
of these carmines, is detected by rubbing a small quantity of the
carmine on a piece of glazed paper, which, when the color dries, gives
a color varying from blue to a rich copper color: if any green
coloring matter is left, it shows itself by a green aureola around the
blue color. The method of applying the carmines in dyeing wool and
silk,—for they are not adapted to cotton fabrics,—as given by M. de
Kæppelin, is as follows:—

The operation is conducted in small wooden vats, provided with
openings for manipulation, and pipes for inducting steam to heat the
baths to the proper temperature. It consists of two parts, that of
mordanting and dyeing. The former is thus conducted.

For each kilogram of tissue which has been previously scoured and
bleached, there are provided 200 grammes of cream of tartar and 250
grammes of alum. These are dissolved in the bath of water of the vat,
the temperature is raised to boiling heat, and the tissue is immersed
in the bath f of an hour while it is worked over through the opening
for manipulation. The pieces are then taken from the bath, to which is
added a solution of the carmine in water containing a quantity of
coloring matter proportionate to the intensity of the blue sought for.
The solution ought to be prepared with care and passed through a silk
sieve, so that the small insoluble grains which might have been left
through bad fabrication may be left on the sieve. After the pieces
have been manipulated in the colored bath, so as to exhaust the color
and obtain the required blue, they should be rapidly washed in running
water and dryed in the shade. Silk stuffs are dyed in the same way;
but the alum should be previously applied cold by means of a saturated
solution of alum, in which the stuffs should be immersed for an hour.


COLORS NOT FAST.

In regard to all the combinations of indigo with sulphuric acid,
including the carmines, it must be observed that their application
does not constitute true indigo dyeing: the colors are not fast. It is
not pure indigotine which is fastened on the tissues as in the vat
dyeing, but another compound of indigo with the sulphur. Berzelius
observes that “the color of soluble indigo is fully as alterable and
fugacious as that of the colors extracted by the decoction of
vegetable materials. By a long exposure to the sun the indigo blue is
destroyed: it becomes green during evaporation, and changes its
nature.” The carmines as well as the sulphur acids are easily
decolorized by reducing agents, such as hydrogen and sulphuretted
hydrogen, although they gradually assume their original color when
exposed to the atmosphere. We are informed by some of the older
dealers that imported cloths and merino stuffs known as “Saxony” were
formerly largely sold in our shops, but that, notwithstanding their
attractiveness to purchasers, they were objectionable on account of
the instability of their color.


APPLICATION OF INDIGO IN PRINTING STUFFS.

Our notes would be incomplete without some reference to the uses of
indigo in printing fabrics. In pursuing this branch, we are
embarrassed on the one hand by the consideration that the subject is
too technical for the general reader, and on the other by the
consciousness that it would be presumption in us to attempt to
instruct those skilled in the art. It may not, however, be without
benefit in producing a higher appreciation of science for the general
reader to observe how science comes in play, even in the printing of a
single color; while to the skilled reader our notes may possibly be of
value as a vehicle for conveying some receipts taken from works not
easily accessible.


PRINTING STUFFS OF WOOL AND SILK, AND STUFFS WITH COTTON WARPS.

This branch of our subject is directly allied to the one last
considered, the application of the compounds of sulphur and indigo;
for indigo is applied to printing wool and silk principally in the
form of indigo carmines. These applications are less numerous than
they were formerly, since they have been replaced by Prussian blue,
and more recently by the aniline blues, which are now generally used.
When the carmines are used, it is for making sky blues, and they enter
into the composition of some greens and browns. The salts of alumina
and vegetable acids are used to fix the indigo carmine upon tissues of
wool and silk. Some receipts recommended by M. de Kæppelin, himself a
practical printer, are given in a note.[7]

  -------------------------------------------------------------
  [7] BLUE NO. 1.
  Indigo carmine                                   400 grammes.
  Alum                                             100    „
  Oxalic acid                                      150    „
  Boiling water                        1¼ litre
  Gum water prepared in proportion of
    1 kilogram to the litre            1¼ litre

  GREEN NO. 1.
  Gum water as above                  12 litres.
  Cuba lac                            12   „
  Alum                                 1 kilogram, 500 grammes.
  Oxalic acid                          2   „
  Indigo carmine                       4   „

  BOUILLON FOR THE GREENS AND BLUES.
  Boiling water                       12 litres.
  Alum                                             600 grammes.
  Oxalic acid                                      750    „
  Gum water                           12   „

  SKY BLUE FOR WOOLLEN STUFF WITH COTTON WARP.

  First solution.—Boiling water      4 litres.
                    Cyanuret of iron and potash    800 grammes.
  Second solution.—Boiling water     2   „
                    Tartaric acid                  300    „

  Third solution.—Cold water         3   „
  Sulphuric acid                                   300    „

  Pour in the first solution, then the second and third, agitating the
  color with a spatula after each new addition.

  The following mixture is afterwards applied to the stuff:—

  Gum water                           12 litres.
  Water                                6   „
  Blue No. 1 for wool                  3   „
  -------------------------------------------------------------

In printing tissues of wool with cotton warp, the carmines are not
used alone. They are combined in certain proportions with cyanites of
iron and potash, to obtain upon the cotton a blue color of equal
intensity with that produced by the carmines upon wool. It is also
necessary to previously mordant the fabrics by means of a solution of
oxide of tin or caustic soda which is precipitated on the fibres by
passing through a bath of water, to which sulphuric acid has been added.


APPLICATIONS OF INDIGO IN PRINTING COTTON FABRICS.

Before entering upon methods used in large establishments, it may not
be without interest to observe the processes still used in Java for
printing calicoes, which the natives prefer to any imported from
Europe. In Java there are no factories, and the women in each family
make and dye or print all the cotton cloths required for their own
consumption. They apply by means of a brush or pencil, which they use
with great skill, to the cotton tissue which they wish to cover a thin
coating of wax mixed with a little resin, the wax being applied to all
the parts where the design, which has been first traced upon the
cloth, requires that the fabric should remain uncolored. They then
immerse the stuff several times in an indigo vat until they have
obtained the desired tint. The stuff is afterwards washed and dried
for a new application of the wax, carefully applied with a pencil as
before. The cloth is then immersed in a bath of a different color,
made with madder or catechu, but always of some dye which is perfectly
stable; and the operation is repeated according to the number of
colors desired. By these successive applications of wax and immersions
into different vats, they succeed in producing very complicated and
harmonious colors, while no European goods compare with them in
stability of dye.

In the European, and our own manufacture, the blue bottoms upon
vegetable fibres, made by immersion in the indigo vat, are combined
with white impressions, or others variously colored, by two distinct
methods. Sometimes there is printed upon the cloth before dyeing in
the indigo vat a preparation called a reserve or resist, which
prevents the indigotine from being deposited in the places where it is
applied. Sometimes, on the contrary, the indigo, which has been
uniformly fixed upon the fabric, is destroyed in certain places marked
out by printing upon them certain chemical agents, called _discharges_.

The _reserves_ are mechanical, resisting the penetration of the dye,
such as wax and pipe clay, or chemical. The last, through these acid
or oxidizing properties, cause the precipitation of the indigotine
before it has touched the fibre or penetrated into its pores. Such are
the salts of copper and bi-chlorate of mercury. Other bodies perform
the part both of mechanical and chemical reserves. The salts of zinc
or alumina, for instance, which are frequently used, produce at the
same time a deposit of indigo white and a gelatinous covering of
hydrated oxide of zinc or aluminium. The composition of a good reserve
is declared to be principally a question of good proportions of the
constituent parts, varying with the strength of the vat and the
intensity of the blue which is desired to be reserved. The first
condition is that it hardens immediately after immersion in the vat:
if it softens, on the contrary, it will cause the running of the
color. In other words, the acidity of the impression should be
proportionate to the strength and alkaline character of the vat. The
white reserve, that most generally used, is composed of pipe clay,
gum, verdigris, and sulphate of copper. The styles of work produced by
dipping with reserves are generally of a cheap and low class. The
system is clumsy and expensive, and is only tolerated because of the
want of a method of directly applying indigo, which will yield the
deepest shades.

Certain styles, formerly in great vogue, called _Lapis_, and forming
one of the richest branches of the cotton-printing industry, are
founded upon the use of reserves; and in these styles, by very simple
means which we shall not attempt to describe, different colors
produced from madder, catechu, &c., are produced upon the fabric so
perfectly surrounded by blue that the eye cannot detect the slightest
want of continuity. This fabrication has the greatest perfection in
Russia. The imitation cashmere fabrics of cotton imported from that
country, formerly much in fashion for dressing-gowns, are specimens of
this fabrication. The great stability of the colors is a remarkable
feature of these goods.

The system of resists or reserves possesses the inconveniences of not
producing impressions of great firmness, and of requiring very strong
vats. When the strength of the vat is partially exhausted, they may be
thrown aside. These inconveniences are obviated by the system of
discharges (_enlevages_). In this system the cloths are vat dyed of a
uniform blue. The strength of the vat is of less importance, and it
can be used until the indigo is quite exhausted. The means of
destroying the indigo which has been fixed upon the fibre are founded
on the use of active oxidizing agents, which transform the insoluble
indigotine into soluble isatine. The agent generally used is chromic
acid. As this acid cannot be incorporated with the thickening to be
printed, as the thickening would produce oxide of chrome, the cloth is
passed through a strong solution of chromate of potash, and dried in
the shade. The required pattern is then printed on the cloth with a
mixture whose principal elements are acids which are susceptible of
setting free the chromic acid on the tissue, which then acts upon the
indigo producing a white pattern. The acid generally employed for
freeing the chromic acid is oxalic acid, thickened with British gum,
dextrine, or starch, with the addition of pipe clay. To prevent
running, nitric, sulphuric, or tartaric acid are sometimes used.[8]

  -------------------------------------------------------------
  [8] Schutzenberger gives the following receipts:—

  PREPARATION FOR DISCHARGE.

  Water                                         2 litres.
  Yellow chromate                               500 grammes.

  WHITE DISCHARGE ON BLUE BOTTOM.

  Tartaric acid                                 3 kilograms.
  Oxalic acid                                   250 grammes.
  Burnt starch                                  4 kilograms.
  Nitric acid                                   500 grammes.
  Water                                         4 litres.

  De Kæppelin gives the following:—

  WHITE DISCHARGE ON BLUE.

  Water                                         2 litres.
  Starch                                        1 kilogram, 800 grammes.
  Oxalic acid                                   500 grammes.
  Tartaric acid                                 250 grammes.
  Sulphuric acid                                375 grammes.

  The pieces, having been dyed blue, are then placed in a solution of
  bichromate of potash in water, which is prepared in the ratio of 50 to
  60 grammes to the litre, according to the intensity of the blue. The
  pieces thus prepared must be dried away from direct solar light or too
  much heat. In fact, under the action of these agents, the bichromate
  would be decomposed and the tissue altered. The pieces are often
  rolled up to prevent this effect. After the pieces are printed, they
  are passed into a vessel containing water and holding chalk in
  suspension in sufficient quantity to give it a milky aspect. The
  temperature of the bath is raised to 60° R. The excess of acid of the
  color applied is saturated by the chalk, and the excess of bichromate
  of potash with which the tissue is impregnated is dissolved in the
  bath. The pieces are afterwards washed and passed through slightly
  soapy water.
  -------------------------------------------------------------

By the method of discharges the white designs upon blue are brought
out with a distinctness which it is impossible to obtain by resists,
while the most delicate work of the graver can be exactly reproduced
upon the tissue.


APPLICATION OF INDIGOTINE BY PRINTING.

The first step in the art of printing indigotine upon calicoes was the
application of what is called pencil blue. Instead of immersing the
fabrics in an indigo vat, the indigo white formed in a very strong
indigo vat was thickened and applied locally to certain places on the
cloth. The preparation was painted upon the cloth by means of pencils
made of willow sticks, the ends of which were broomed up into a kind
of brush. The style was hence called pencil blue. The methods now used
to apply white indigo locally are of two kinds. The china blue
process, and the solid blue process, sometimes called fast or
precipitated blue. The china blue process derives its name from the
resemblance of its color to the blue on the old china ware. It has
great depth of tint, and permanency. It is scarcely used now, except
for certain articles requiring great depth of color, such as certain
furniture goods, and by the Germans and Swiss for the manufacture of
calicoes for exportation to India.

We do not venture to condense the descriptions at our hand of the
processes for applying the china blue and the solid blue, and
translate those furnished by chemists of high authority. After the
method indicated by Darwin in his recent works, we present them in
smaller type, with the perhaps unnecessary suggestion that they may be
passed over by the general reader.

_China blue_.—The theory of this printing blue, says Schutzenberger,
is very simple. The indigo, reduced to an impalpable powder and
thickened, is printed by a plate or roller. After drying, the tissue
seems dyed blue, more or less deep, according to the proportion of
coloring material used; but it is only a blue of application, which
can be removed with the thickening, by the slightest washing. The
object is now to reduce and redissolve the indigotine in place to
enable it to penetrate the fibre at the end of a consecutive
oxidization, and without producing a running of the color or altering
the purity and distinctness of the contours of the design. I owe to M.
Ed. Schwartz some valuable hints upon the fabrication of this style,
which is also described with much care and details in the treatise on
printing by M. Persoz.

The reduction of the indigo is obtained by alternate passages of the
printed tissue into vats containing,—the first, quicklime slacked; the
second, sulphate of iron; the third, soda. The operation is terminated
by a passage through a bath of sulphuric acid, which removes the oxide
of iron and precipitates the indigo white by hastening its oxidation.

The success depends upon the composition of the color printed, and
above all upon the strength of the vats of immersion and the duration
of the treatment.

The operator uses six vats,—for instance, two lime vats, provided each
with 12 kilograms of lime; a copperas vat at 70 Beaumé; a caustic soda
vat marking 140 Beaumé; a sulphuric acid vat with 500 grammes of acid
(_par mesure d’eau_); and finally a vat of pure water.

The receipts for printing are:—

1. THE BLUE PREPARATION.

  Ground indigo                             4 kilograms.
  Acetate of iron                          10 litres.
  Sulphate of iron                          1 kilogram.
  Water                                    10 litres.
  Gum Senegal                               6 kilograms.

Pass through a sieve; leave some time at rest, and stir whenever used.
Caraccas indigo is preferred because it can be broken into a finer
powder and gives a finer paste.

2. COLORS FOR ROLLER PRINTING NOS. 1, 2, 3, 4.

The blue preparation above                              1, 1, 3, 4.
Acetate of iron containing 700 grammes of gum per litre 2, 1½, ½, ½.
Gum water at 600 grammes per litre                      16, 2½, ½, ½.

These proportions can be varied according to the tint desired.

The piece is treated a quarter of an hour in the first lime vat by
giving it a light movement from above to below; it is left a quarter
of an hour in repose in the sulphate of lime vat; a quarter of an hour
in the second lime vat; a quarter of an hour in the copperas vat; five
minutes in the caustic soda; half an hour in the sulphuric acid, and
then thoroughly rinsed.

To each lime vat there is given 2 kilograms of lime per piece of
cloth. To the vitriol vat there is added 50 kilograms of sulphate of
iron for each dozen pieces. The soda vat is renewed after 5 pieces by
the addition of 12 kilograms of salt of soda, which has first been
made caustic. The acid vat receives 25 kilograms of acid after 5
pieces, and ought to be renewed whenever it becomes saline. The other
vats must be cleared out whenever the deposit becomes too great for
success.

M. Ed. Schwartz recommends as important conditions, (1) the perfect
causticity of the tissue, and an average strength of 140 Beaumé; (2)
the neutrality of the sulphate of lime vat. For this end old iron
should be boiled in it.

After leaving the sulphuric acid vat the pieces are rinsed in the
water vat, then in river water, and afterwards should be soaked in a
sulphuric acid bath at 40 Beaumé, for the purpose of dissolving the
last traces of the peroxide of iron adhering to the fibre. The fabric
is then washed in water and finally passed through a soapy water at
40° R.

_Solid or precipitated blue, Schutzenberger’s receipt_.—The process
consists in printing indigo white precipitated in a vat, in a thick
paste to dissolve it on the tissue by a passage through an alkaline
bath (lime or soda), and of reprecipitating it by oxidizing it as soon
as it has entered the fibre.

It is then the china blue process, minus the reduction which is made
before printing, and consequently minus the sulphate of iron vat.

Indigo white is too alterable to be printed with success, so it is
generally precipitated in combination with a stannic hydrate (hydrate
of a salt of tin), which gives it body and preserves it from a too
rapid oxidation.

The stannic indigotate in paste, or as it is generally called
precipitate of indigo, is prepared by turning into the clear portion
of a strong copperas vat an acid solution of protochlorate of tin, and
filtering it upon woollen filters,—as much as possible away from the
air. It would be better to prepare a strong tin vat by heating a
mixture of indigo, caustic soda, and protochlorate of tin, and to
precipitate by chlorohydric acid.[9]

  -------------------------------------------------------------
  [9] Mr. T. P. Shepard gives in his valuable “Receipts for Calico
  Printing,” published in 1872, the following:—

  NO. 52. INDIGO PRECIPITATE FOR FAST BLUE AND GREEN.

  10 pounds quicklime, slacked with
  6½ gallons water; then
  2 pounds ground indigo finely rubbed in water are stirred in; then add
  6 pounds copperas dissolved in 5 gallons of water; then add
  5 gallons hot water and
  15 gallons cold water.

  Stir well from time to time, until the liquid has assumed a yellow
  color and deep blue veins or streaks appear on its surface. When this
  moment arrives, draw off the clear liquor, and precipitate every ten
  quarts of it with

  ½ pound tin crystals, dissolved in ½ pound muriatic acid.

  To the remainder of the mixture of lime and indigo, 15 gallons of
  water may be added, and the whole stirred; and when settled, the
  indigo may be precipitated from the clear liquor as before. This
  operation may be repeated a second time before all the indigo is
  exhausted.

  The indigo precipitate is to be collected on a muslin filter, and
  well squeezed out.
  -------------------------------------------------------------

The deposit is made into a paste with gum water; a salt of tin is
often added to prevent oxidation. It is important to prevent the
transformation of the indigo white into indigotine before printing.
This indigotine would not fix itself on the fabric. Moreover, after
printing, it is necessary to hasten the dissolution of the indigo
white to enable it to penetrate the fibre. It is sufficient for this
end to pass it through milk of lime. The stannic combination is
immediately destroyed; the colorable matter unites itself with the
lime, and the color passes into a pale gray with apple green. The
indigo white becomes momentarily soluble; but the presence of the
excess of lime and the thickening, as well as the attractive affinity
of the thickening, prevent any running.

The piece on issuing from the lime water is placed in running water,
when reoxidation commences, which this time fixes the color. The piece
is finally passed through a sulphuric acid bath to absorb the lime,
and washed.

By adding to the color a salt whose base precipitates in the milk of
lime and oxidizes in the running water, and replacing the simple acid
bath by an acid bath with yellow prussite, the intensity of the blue
is increased through the formation of Prussian blue.

Although we have seen beautiful effects from the application of the
solid blue of indigo on prints at our Pacific Mills, the colors
produced by Prussian blue and aniline are so much more brilliant and
easy of application that the use of indigo in printing goods for
ordinary consumption is likely to decline rather than increase. It
will be otherwise if we should ever manufacture for the East India
markets. Here is a field still open for our manufacturers. Mr. Watson,
in his beautiful work on “The Costumes of the People of India,”
remarks that “British manufacturers have hitherto failed to appreciate
Oriental tastes and habits, and hence supply but an insignificant part
of the clothing of the two hundred million persons that form the
population of what is commonly spoken of as India.” The great defect,
he observes, is the want of stability of color in the cotton fabrics
introduced,—this stability being an imperative demand in the Oriental
markets.

The applications of indigo to cotton fabric are altogether secondary,
in our mind, to its relations to the woollen manufacture. If we have
felt called upon to say a word in behalf of the most ancient and best
ally which the fibre of wool has ever had, it is because the vividness
of color of the new products of coal, and the fascination which the
application of the recent discoveries of science always possesses, is
threatening the eclipse of the more ancient sober and solid dyes. Let
the new colors have their place as auxiliaries, not as substitutes for
the ancient dyes. Let them serve to give a bloom[10] to goods, but let
the foundation be the good old dyes which the experience of ages has
proved to be the most unalterable by light and air. The recent
wonderful discovery of alizarine, or artificial madder, in coal tar
products, has led practical men to expect too much from science. The
opinion is quite prevalent among manufacturers that artificial
indigotine has already been obtained from the same source. And some
manufacturers are sanguine that the difficulties of indigo dyeing will
thus be resolved. It is not improbable—for what is impossible to
modern chemistry?—that this result will yet be partially obtained. But
we have looked over all the recent foreign chemical reviews, and
personally consulted some of our best chemists, and we can find no
authority for the prevailing opinion that artificial indigotine has
been produced. If the production of artificial indigotine should be
realized, the only benefit would be the possible cheapening of the
material. The difficulties of the indigo vat would still remain; for
we cannot too often repeat, that in the very difficulties of the
process, or in the insolubility of blue indigotine by ordinary agents,
consists the excellence of the dye.

  -------------------------------------------------------------
  [10] _Guernsey Blue_.—The darkest of the Nicholson Fast Blues. On a
  bottom of barkwood, camwood, madder, or inferior _indigo_, produces
  an indigo blue which will stand all the acid tests the same as
  colors made from indigo.

  _Serge Blue_.—It will be found very serviceable to give bloom to
  goods dyed with indigo, and by itself shows a very good indigo test
  with nitric acid.—_Instructions for Working the Atlas Works Aniline
  Dyes_.
  -------------------------------------------------------------


APPENDIX.

DYEING WOOL IN THE WARM INDIGO VAT.

[_Extract from Dumas’s Lectures on Dyeing_.]

The value attached by practical wool-dyers to the following induces us
to publish it without condensation:—

Indigo Blue.—We give a solid dye of indigo blue to wool by plunging it
into an alkaline solution of indigo white, and then exposing it to
contact with the air. The solution of indigo white is prepared in a
vessel usually from eight to nine feet in depth, and six to seven feet
in diameter. This size is very convenient for the requisite
manipulations, and presents a large volume of water, which, when once
heated, is capable of preserving a high temperature for a long time.
This vessel should be made of wood or copper. It always bears the name
of vat. These vats are covered with a wooden lid, divided into two or
three equal segments. Over this lid are spread some thick blankets.
Without this precaution the bath would come in contact with the
atmospheric air, a portion of the indigo would absorb oxygen and
become precipitated. There would also be a great waste of heat.

A most necessary operation, and one which has to be frequently
repeated, consists in stirring up the deposit of vegetable and
coloring matter which is formed in the vat, and intimately mixing it
in the bath. For this purpose we employ a utensil called a _rake_,
which is formed of a strong square piece of wood, set on a long
handle. The workman takes hold of this with both hands, and, dipping
the flat surface into the deposit at the bottom of the vessel, he
quickly draws it up until it nearly reaches the surface, when, giving
it a gentle shake, he discharges the matter again through the liquor
of the bath. This manœuvre is repeated until the whole of the deposit
seems to be removed from the bottom of the vessel. Before the tissue
is dipped into the dye-bath, it should be soaked in a copper full of
tepid water; it is then to be hung up and beaten with sticks. In this
state it is plunged into the vat; it thus introduces less air into the
bath, while it is more uniformly penetrated by the indigo solution.
The cloth is now kept at a depth of from two to three feet below the
surface of the liquid, by means of an open bag or piece of network
fixed in the interior of an iron ring, which is suspended by cords,
and fixed to the outside of the vat by means of two small iron hooks;
the bag is thus drawn backwards and forwards without permitting it to
come in contact with the air. When this operation has been continued
for a sufficient length of time, the cloth is wrung and hung up to dry.

Flock wool is also, for the purpose of dyeing, enclosed in a fine net,
which prevents the least particle from escaping, and which is fixed in
the bath in the same way as in the foregoing case.

The many inconveniences attending the use of wooden baths, which
necessitate the pouring of the liquor into a copper for the purpose of
giving it the necessary degree of heat, have led to the general
employment of copper vessels. These are fixed in brickwork, which
extends half way up their surface, whilst a stove is so constructed at
this elevation that the flame shall play around their upper part. By
this means the bath is heated and kept at a favorable temperature
without the liquor being obliged to be removed.

The potash vats are usually formed of conical-shaped coppers,
surrounded by a suitable furnace. These may be constructed with less
depth, inasmuch as there is less precipitation induced in the liquor.
By using steam for heating the vats, we might dispense with the
employment of copper vessels, and so return to those of wood.

The vats employed for dyeing wool are known under the names of the
pastel vat, the woad vat, the potash vat the tartar-lee vat, and the
German vat.

Pastel Vat.[11]—The first care of the dyer in preparing the vat should
be to furnish the bath with matters capable of combining with the
oxygen, whether directly or indirectly, and of giving hydrogen to the
indigo. We must, however, be careful to employ those substances only
which are incapable of imparting to the bath a color which might prove
injurious to the indigo. These advantages are found in the pastel, the
woad, and madder. This latter substance furnishes a violet tint when
brought into contact with an alkali, and by the addition of indigo it
yields a still deeper shade.

  ----------------------------------------------------------------------
  [11] The distinction between pastel and woad is not very clear.
  Schutzenberger says: “Pastel, woad, and satis tinctoria is a _plant_
  of the family of the crucifera. It would seem, however, that the term
  _pastel_ as used by the old French dyers is applied to the leaves of
  the woad which have been fermented, formed into paste, and afterwards
  into balls, and which contain much blue coloring matter. And the term
  _woad_ as distinguished from _pastel_ is applied to the unfermented
  plant.”
  ----------------------------------------------------------------------

In preparing the Indian vat, we ordinarily employ one pound of fine
madder to two pounds of indigo. The madder is here especially useful,
by reason of the avidity of some of its principles for oxygen.

The pastel vat, when prepared on a large scale, ordinarily contains
from 18 to 22 lbs. of indigo; 11 lbs. of madder would suffice for this
proportion, but we must also bear in mind the large quantity of water
which we have to charge with oxidizable matters. I have invariably
seen the best results from employing 22 lbs. to a vat of this size.
Bran is apt to excite the lactic fermentation in the bath, and should
therefore not be employed in too large a quantity; 7 to 9 lbs. will be
found amply sufficient.

Weld, which is often used, is rich in oxidizable principles; it turns
sour, and passes into the putrid fermentation with facility. Some
dyers use it very freely; but ordinarily we employ in this bath an
equal quantity of it to that of the bran. Sometimes weld is not added
at all.

In most dye-houses the pastel is pounded before introducing it into
the vat. Some practical men, however, maintain that this operation is
injurious, and that it interferes with its durability. This is an
opinion which deserves attention. The effect of the pastel, when
reduced to a coarse powder, is more uniform; but this state of
division must render its alterations more rapid. When the bath has
undergone the necessary ebullition, the pastel should be introduced
into the vat, the liquor decanted, and at the same time 7 or 8 lbs. of
lime added, so as to form an alkaline lye which shall hold the indigo
in solution. Having well stirred the vat, it should be set aside for
four hours, so that the little pellets shall have time to become
thoroughly soaked, both inside and out, and thus be prepared for
fermentation. Some think coverings are to be spread over the vat, so
as to preserve it from contact with the atmosphere. After this lapse
of time, it is to be again stirred. The bath at this moment presents
no decided character; it has the peculiar odor of the vegetables which
it holds in digestion; its color is of a yellowish-brown.

Ordinarily, at the end of twenty-four hours, sometimes even after
fifteen or sixteen, the fermentative process is well marked. The odor
becomes ammoniacal, at the same time that it retains the peculiar
smell of the pastel. The bath, hitherto of a brown color, now assumes
a decided yellowish-red tint. A blue froth, which results from the
newly liberated indigo of the pastel, floats on the liquor as a thick
scum, being composed of small blue bubbles, which are closely
agglomerated together. A brilliant pellicle covers the bath, and
beneath we may perceive some blue or almost black veins, owing to the
indigo of the pastel which rises towards the surface. If the liquor be
now agitated with a switch, the small quantity of indigo which is
evolved floats to the top of the bath. On exposing a few drops of this
mixture to the air, the golden-yellow color quickly disappears, and is
replaced by the blue tint of the indigo. This phenomenon is due to the
absorption of the oxygen of the air by the indigogen from the pastel:
in this state we might even dye wool with it without any further
addition of indigo; but the colors which it furnishes are devoid of
brilliancy and vivacity of tone, at the same time that the bath
becomes quickly exhausted.

The signs above described announce, in a most indubitable manner, that
fermentation is established, and that the vat has now the power of
furnishing to the indigo the hydrogen which is required to render it
soluble,—that contained in the pastel having been already taken up;
this, then, is the proper moment for adding the indigo, which should
be previously ground in a mill.

We stated above that the liquor of the vat should be previously
charged with a certain quantity of lime; we also find in it ammonia
generated by the pastel; but a part of these alkalies become saturated
by the carbonic acid gas along with the proper acids of the madder and
of the weld, as well as by the lactic acid produced by the bran during
fermentation. The ordinary guide of the dyer is the odor, which,
according to circumstances; becomes more or less ammoniacal. The vat
is said to be either soft or harsh; if soft, a little more lime should
be added to it. The fresh vat is always soft; it exhales a feeble
ammoniacal odor accompanied with the peculiar smell of the pastel; we
must, therefore, add lime to it along with the indigo; we usually
employ from five to six pounds, and, after having stirred the vat, it
is to be covered over. The indigo, being incapable of solution except
by its combination with hydrogen, gives no sign of being dissolved
until it has remained a certain time in the bath. We may remark that
the hard indigoes, as those of Java, require at least eight or nine
hours, whilst those of Bengal do not need more than six hours, for
their solution. We should examine the vat again three hours after
adding the indigo. We ordinarily remark that the odor is by this time
weakened; we must now add a further quantity of lime, sometimes less,
but generally about equal in amount to the first portion; it is then
to be covered over again, and set aside for three hours.

After this lapse of time, the bath will be found covered with an
abundant froth and a very marked copper-colored pellicle; the veins
which float upon its surface are larger and more marked than they were
previously; the liquor becomes of a deep yellowish-red color. On
dipping the rake into the bath, and allowing the liquid to run off at
the edge, its color, if viewed against the light, is of a strongly
marked emerald-green, which gradually disappears, in proportion as the
indigo absorbs oxygen, and leaves in its place a mere drop rendered
opaque by the blue color of the indigo. The odor of the vat at this
instant is strongly ammoniacal; we also find in it the peculiar scent
of the pastel. When we discover a marked character of this kind in the
newly formed vat, we may without fear plunge in the stuff intended to
be dyed; but the tints given during the first working of the vat are
never so brilliant as those subsequently formed; this is owing to the
yellow-coloring matters of the pastel, which, aided by the heat,
become fixed on the wool at the same time as the indigo, and thus give
to it a greenish tint. This accident is common both with the pastel
and the woad vats; it is, however, less marked in the latter.

When the stuff or cloth has been immersed for an hour in the vat it
should be withdrawn; it would, in fact, be useless to leave it there
for a longer time, inasmuch as it could absorb no more of the coloring
principle. It is therefore to be taken from the bath and hung up to
dry, when the indigo, by attracting oxygen, will become insoluble and
acquire a blue color. Then we may replunge the stuff in the vat, and
the shade will immediately assume a deeper tint, owing to renewed
absorption of indigo by the wool. By repeating these operations, we
succeed in giving very deep shades. We must not, however, imagine that
the cloth seizes only on that portion of indigo contained in the
liquor required to soak it. Far from such being the case, experience
shows that, during its stay in the bath, it appropriates to itself,
within certain limits, a gradually increasing quantity of indigo. We
have here, then, an action of affinity, or perhaps a consequence of
porosity on the part of the wool itself.

Woad Vat.—These vats are extensively employed at Louviers, and in the
manufactories of the north of France. The bath is prepared in the same
manner as in the foregoing case; the finely cut root is introduced
into the copper along with 2 lbs. of pounded indigo, 9 lbs. of madder,
and 15½ lbs. of slaked lime. The liquor is, after the necessary
ebullition, poured upon the woad. This substance contains but a very
small quantity of coloring principle; we must, therefore, add some
indigo when preparing the vat, so as to indicate the precise instant
when the mixture arrives at the point of fermentation so necessary for
imparting hydrogen to the coloring principle, and for rendering it
soluble. We must also use a larger quantity of lime, since the woad
contains no ammonia resulting from previous decomposition, such as we
find to be the case with the pastel of the south. When the vat is in a
suitable state of fermentation, a rusty color becomes manifest, in
addition to the signs already described in speaking of the pastel vat;
besides the ammoniacal odor, the bath always retains the peculiar
smell of the woad. The pounded indigo is now added, and we proceed, in
the manner already detailed, to reduce it to a state of solution fit
for dyeing.

The vats prepared by means of pastel have greater durability than
those made with the woad; but it is thought that the colors given by
the latter are more brilliant than those obtained from the former dye.

Modified Pastel Vat.—This vat is about 7 feet in depth, and 6½ feet in
diameter. It is made of copper, and heated by steam. The lid is
composed of three segments, each of which is formed of two planks,
about an inch thick, and strongly secured together by bolts.

The beating is performed in the usual way, with sticks before the
first dipping, after having moistened the cloth in tepid water. This
operation is not subsequently repeated.

This vat is prepared with 13 lbs. of indigo, 17½ lbs. of madder, 4½
lbs. of bran, 9 lbs. of lime, and 4½ lbs. of potash. Having filled the
vat, we heat it to about 200° Fah., and, as soon as the water is
tepid, introduce 441 lbs. of pastel. The liquor becomes of a
yellowish-brown color; small bubbles appear upon its surface,
ordinarily at the end of four hours if the vat be heated by steam, but
not until after eight or twelve hours where heat is applied by the
common fire; in the latter case the mixture should be stirred every
three hours. When the liquor displays the signs of fermentation, we
add the above-mentioned ingredients, and cover the vat over; it is
then to be set aside, stirring it every three hours, or oftener if the
fermentative action be very rapid. Each time that it is stirred we are
to add from 2 to 4 lbs. of lime; if fermentation proceed quickly we
even use more, but in the contrary case less. After about eighteen
hours, we plunge into the vat three pieces of common cloth, measuring
from twenty to twenty-five ells in length each; when they have
received six or seven turns, they are to be taken out again. The
object of this is to remove the excess of lime from the bath. The vat
is then set aside for three hours, when it is to be stirred, and 13
lbs. of indigo, with 2 lbs. of madder, added to it. We now again apply
heat to the mixture.

If the vat contains a superabundance of lime, it will be unnecessary
to add more; otherwise we throw in a further quantity. During the
night it should be covered with a cloth, and a workman left to watch
it. It is usually stirred once before the morning; but if it be
deficient in lime, it will require this manipulation to be more
frequently repeated, and also fresh lime added to it. On the following
day the stirring should be continued every three hours, and so on for
the next thirty hours, taking care to heat the vat from time to time.
On the morning of the fourth day the dyeing may be commenced.

The temperature should be maintained at a pretty uniform point; if it
be too hot, the blue takes a red reflection, by reason of the madder
contained in the liquid. A vat thus prepared will last three months;
we may even work it for double that period, but after the third month
it appears to lose some of its indigo.

We maintain the power of the vat by introducing every night 2¼ lbs. of
madder. Some indigo is also added twice or three times a week. These
additions are made in the evening. After the former, the vat is left
at rest for forty-two hours; with the latter only for twenty-four, at
the same time observing the precautions already indicated. At the end
of three months, or sooner when we wish to stop the working of the
vat, we exhaust the indigo; for this purpose we continue to charge it
every night for the space of a month with madder, and dip into it
white cloths, or more particularly woollen tissues, which become more
or less loaded with the indigo. We must continue this plan until these
matters take up no further color. The dippings are to be performed
twice a day at first, but once only towards the termination. Many
dyers make use of this bath for preparing a new vat, but it is better
to throw this away and make it up afresh with common water.

Indian Vat.—These vats are of more simple and of more ready
construction than the pastel or woad vats. We are to boil in water a
quantity of madder and of bran, proportioned to the weight of indigo
which we wish to employ. After two hours’ ebullition, we turn into
this bath some tartar-lees, which are also to be boiled for an hour
and a half or two hours, so as to charge the bath with whatever
soluble matter they may contain; after this ebullition the bath should
be allowed to cool, and the indigo, which has been previously ground,
is then to be introduced. Supposing that we wish to employ 21 lbs. of
indigo, the following would be the proportions used in preparing this
vat: 41 lbs. tartar-lees, 13 lbs. of madder, and 5 lbs. of bran. These
vats are usually mounted in coppers of a conical shape; a small fire
should be kept up around them, so as to maintain a moderate and
uniform heat. The indigo will usually be found dissolved at the end of
twenty-four hours, often even after twelve or fifteen hours. The
liquor has a reddish color in the new vats, and a green tint in those
which are in a working state. The frothy surface, as well as the
brilliant-colored pellicle, becomes manifested in this as in all other
preparations of a like kind.

This species of vat has to be renewed much more frequently than the
woad and pastel vats, from the indigo being more difficult to dissolve
after a certain lapse of time. A moderate heat should be maintained in
all these vats.

Potash Vat.—This species of vat is extensively employed at Elbœuf for
the dyeing of wool in the flock. It presents in all respects a perfect
analogy with the Indian vat; in fact, the action of the tartar-lee in
the latter preparation depends entirely on the carbonate of potash
which it contains. The ingredients used in the preparation of the
potash vat are bran, madder, and the subcarbonate of potash of commerce.

We obtain the deep shades in this species of vat with greater celerity
than in all others, a fact which undoubtedly depends on the greater
power which potash has of dissolving indigo than is possessed by lime.
Experience proves that the potash vat has the advantage in point of
celerity of nearly a third; but this is balanced by the inconvenience
resulting from the darker shade, which we must attribute to the large
quantity of coloring matter of the madder dissolved by the alkaline
lee, and which becomes fixed on the stuff with the indigo.

To render this vat in its most favorable state, the indigo should be
made to undergo a commencement of hydrogenation before turning it into
the mixture; for this purpose we prepare in a small copper a bath
analogous to that in the vat, to which the pounded indigo is added.
This bath is maintained for twenty-four hours at a moderate heat,
taking care to stir it from time to time. The indigo assumes a
yellowish color, becomes dissolved, and in this state is turned into
the vat; we thus avoid many delays and losses in its preparation, and
indeed it would be desirable if a similar plan were adopted with all
these compounds.

German Vat.—This vat is of nearly similar dimensions to that used for
the woad, being three times the size of the potash vat. Its diameter
is about 6½ feet, and its depth 8½ feet. Having filled the copper with
water, we are to heat it to 200° Fah.; we then add 20 pailsful of
bran, 22 lbs. of carbonate of soda, 11 lbs. of indigo, and 54 pounds
of lime, thoroughly slaked, in powder. The mixture is to be well
stirred, and then set aside for two hours; the workman should
continually watch the progress of the fermentation, moderating it more
or less by means of lime or carbonate of soda, so as to render the vat
in a working state at the end of twelve, fifteen, or, at the most,
eighteen hours. The odor is the only criterion by which the workman is
enabled to judge of the good state of the vat, he must therefore
possess considerable tact and experience.

In the process of dipping we introduce 84 lbs., 106 lbs., or even 130
lbs. of wool, in a net bag, similar to that used in the woad vat,
taking care that the bag is not allowed to rest against the sides of
the copper. When the wool has sufficiently imbibed the color, we
remove the bag containing it, and allow it to drain for a short time
over the vessel. We operate in this way on two or three quantities in
succession; we then remove the vat, and set it aside for two hours; we
must be careful, from time to time, to replace the indigo absorbed by
the wool, as also to add fresh quantities of bran, lime, and
crystallized carbonate of soda, so as constantly to maintain the
fermentation at a suitable point.

The German vat differs, then, from the potash vat by the fact that the
potash is replaced by crystallized carbonate of soda and caustic lime,
which latter substance also gives to the carbonate of soda a caustic
character. It presents a remarkable saving as compared to the potash
vat; hence the frequency of its employment; but it requires great
care, and is more difficult to manage. It also offers considerable
economy of labor; one man is amply sufficient for each vat.

The army cloth is usually dyed by means of the pastel vat, which gives
the most advantageous results. We here make use of vats about 8½ feet
in depth, and 5 feet in diameter, into which we introduce from 361
lbs. to 405 lbs of pastel or of woad, after previous maceration. The
vat is to be filled with boiling water, and we then add to the bath 22
lbs. of madder, 17½ lbs. of weld, and 13 lbs. of bran. The mixture is
to maintained in a state of ebullition for about half an hour; we next
add a few pailsful of cold water, taking care, however, not to lower
the temperature beyond 130° Fah.; during the whole of this time a
workman, provided with a rake, keeps incessantly stirring the
materials of the bath. The vat is then accurately closed by means of a
wooden lid, and surrounded by blankets, so as to keep up the heat. It
is now put aside for six hours; after this time it is again stirred by
means of a rake, for the space of half an hour; and this operation
should be repeated every three hours until the surface of the bath
becomes marked with blue veins; we then add from six to eight pounds
of slaked lime.

The color of the vat now borders on a blackish-blue. We immediately
add the indigo in a quantity proportioned to the shade which we wish
to obtain. The pastel in the foregoing mixture may last for several
months; but we must renew the indigo in proportion as it becomes
exhausted, at the same time adding both bran and madder. In general we
employ—

  11 to 13 lbs. of good indigo for 100 lbs. of fine wool.
  9 to 11 lbs. of good indigo for 100 lbs. of common wool.
  9 to 11 lbs. of good indigo for 131 yards of cloth dyed in the piece.

Management of the Vats.—A good condition of the vat is recognized by
the following characters: The tint of the bath is of a fine
golden-yellow, and its surface is covered with a bluish froth and a
copper-colored pellicle. On dipping the rake into the bath, there
escapes bubbles of air, which should burst very slowly; when they
vanish quickly, it becomes an indication that we must add more lime.
The paste which is found at the bottom of the vat, green at the moment
of its being drawn up, should become brown in .the air; if, however,
it remains green, this is a further sign that more lime is required.
Lastly, the vat should exhale the odor of indigo. We usually complete
the assurance of the vat being in a good state by plunging into it,
after two hours’ respite, a skein of wool, which, on being withdrawn
after the lapse of half an hour, should present a green color, but
change directly to blue. We then once more mix the materials of the
vat, and two hours after it may be considered ready for dyeing.

These vats, like those already described, are provided with a large
wooden ring, the interior of which is armed with a kind of network,
for the purpose of preventing the objects which are intended to be
dyed coming in contact with the materials at the bottom of the vat;
we, moreover, take the precaution of enclosing the wool or cloth in
bags. These tissues, when plunged into the bath, should remain there
for a longer or shorter time, according to the shade which we wish to
obtain; one dipping, however, will never suffice for this object;
usually we leave in the stuff for half an hour only; it is then to be
taken from the bath, wrung, and exposed to the air. This operation is
repeated until we have succeeded in procuring the desired shade; we
ordinarily suffer three hours to elapse between each dipping. The heat
of the vat should never be allowed to fall below 130° Fah. After each
operation the bath must be well stirred, and fresh lime added;
generally speaking, a pound a day will suffice. We re-establish the
indigo about every second day. When once this vat is well mounted, and
we are careful to examine its working, we may dye from two to four
batches a day with it.

When the stuffs have acquired the desired shade, they are first to be
washed in common water, and then in a very weak solution of
hydrochloric acid (about one part in a thousand); after this they are
again rinsed in pure water.

The Indian vat is much more easily managed than the foregoing; it
presents less danger of failure, from the fact that it is quickly
exhausted, and also from the fermentative process, which is so
difficult to govern in the pastel vat; this vat not having time to
change in character. It is prepared by first introducing an equal
quantity of madder and of bran, and a triple quantity of potash; this
is to be gradually heated until it reaches a temperature of 167° Fah.,
and we then add to it the indigo, thoroughly agitating the matters for
half an hour. The vat is maintained at a temperature of 86° to 100°
Fah., by keeping it closely covered, and at the same time the mixture
is to be stirred occasionally at intervals of twelve hours. It should
by this time present a beautiful green shade, the liquor being
surmounted by a copper-colored pellicle and a purplish froth. We may
now commence the dyeing, following the same course as with the pastel
vat; but the stirrings being here repeated much more frequently than
with the other mixture, we can dye a larger quantity of wool within a
given time. When the vat ceases to give a brilliant blue, we must
altogether renew it; if it be merely weakened, we add to it a small
quantity of freshly prepared liquor containing a few pounds of potash,
and a little less bran and madder. In giving the dark and the clear
sky blues, we must be careful to employ a quantity of indigo
proportioned to the color which we wish to obtain, or, better still,
we may use the previously exhausted vat for the dark blue.

When exposed to the influence of the putrid fermentation, indigo is
decomposed and loses its color. If rendered soluble, it obeys the
impulse communicated to the azotized matters with which it is brought
into contact, although, if macerated in pure water at the ordinary
temperature, it is itself decomposed with great difficulty.

The pastel and the woad are very prone to the putrid fermentation, by
reason of the large quantity of azotized matters which they contain,
as do all the cruciferæ; they require therefore considerable care in
their employment.

When a vat is mounted, if the fermentation be allowed to continue
unchecked, after the appearance of the blue froth and the other signs
already indicated, the liquor will acquire a yellow color similar to
that of beer; the froth will become white; it will give out a stale
smell and lose its ammoniacal odor; after a few days it will turn
whitish, and exhale a smell at first similar to that of putrefied
animal substances; then it will acquire the odor of rotten eggs, and
set free sulphuretted hydrogen. The lime in the pastel and the woad
vats, and the tartar-lee and potash in the other mixtures, are used
for the purpose of preventing these accidents.

Besides the oxygenated compound, which is formed by the combination of
oxygen with the extractive matters of the plants held in digestion,
there is a production of carbonic acid which saturates the alkaline
lee, and forms a carbonate of lime in the pastel vat. We find this
attached to the sides of the vat in such quantity that the inside of
these vessels becomes incrusted with it to a considerable depth. It is
this product which dyers call the tartar of the vat; it effervesces
with acids, and gives on analysis carbonic acid, lime, and a few
particles of indigo. In the potash vat the solubility of the carbonate
of potash prevents its deposition; but it is very probable that we
have even here a formation of some carbonated products, perhaps in
part formed at the expense of the carbonic acid of the air.

The soluble extractive principle being the only matter which remains
in solution in the bath with the indigo, the lime, &c., we have formed
deposits which, varying both in their volume and in the greater or
less facility with which they are precipitated during the various
periods of fermentation, lead to a more or less considerable waste of
time. If we plunge a piece of woollen tissue into a vat which has been
recently stirred, it will acquire a dark color, and will be found
covered with brown stains which are with difficulty removed. When the
woad or paste vat has been stirred, it need be left two or three hours
only before plunging in the stuff, at least during the early months of
its working, inasmuch as the pastel, being but slightly divided and
attenuated, is readily precipitated; but when, by reason of its
extreme division, in consequence of repeated operations, it is thrown
down with less facility, the dipping should not be performed oftener
than three times in the day.

The Indian vat requires less time than the others; we may even dye
with it an hour after stirring the mixture. The potash, being soluble,
forms no precipitate; while the ligneous fibre of the madder and the
pellicles of the bran become deposited with great facility. We can
also dip with these vats much oftener than with those made by pastel
or woad.


SICKNESS OF THE WARM VAT FOR DYEING WOOL, AND ITS REMEDIES.

We have to thank that excellent practical magazine, “The American
Chemist,” for the following notes on the sicknesses of the warm vat,
by F. W. Kugler, translated from Reimann’s Färberzeintung:—

In the wool indigo vat, among the principal “sicknesses” is the
blackening of the vat, or “sharpening.” This arises from the presence
of too much lime. When “sharpened,” the liquor, instead of having a
waxy yellow color with a dense blue film on its surface, has no film;
while the liquor is a dark blackish-green, and on being stirred shows
a gray or white scum on its surface, while it emits at the same time a
pungent odor. If the vat is only slightly affected, it is sufficient
to add some bran and madder and to let it stand over night. If it has
not quite recovered by morning, it may be necessary to heat it up,
agitate it, and let it stand for a couple of hours, after which
perhaps the addition of a little lime will be necessary.

If the vat is much sharpened, it is recommended to sink in it a bag of
bran, and leave it over night, when the fermentation will have
restored the vat in a considerable degree; but it will be necessary to
add lime cautiously and by degrees, to bring it to a proper state for
working.

The theory of the souring of the vat is given. Butyric fermentation
takes place under certain circumstances, butyric acid being formed;
and hydrogen is set free, which reduces the indigo. The addition of
lime makes the vat too strongly alkaline, and sets ammonia free, which
gives the pungent odor of the soured (_verschäften_) vat.
Simultaneously the lime with the white indigo forms a difficultly
soluble compound, which settles, and thus interferes with the working
of the vat. The excess of lime must be removed, which is accomplished
by introducing bran, which causes a lactic fermentation; and the
lactic acid neutralizes the excess of lime, and destroys the lime
compound with indigo which had been formed. The lime may be
neutralized by the use of mineral acids, but there is danger in that
case of precipitating the indigo.

A second “sickness” is “becoming too sweet.” The symptoms are,—the
blue veins and surface film disappear on stirring, the foam gives a
rustling sound, the bath assumes a reddish-yellow color, blue goods
placed in the bath lose their color, and the vat has an unpleasant odor.

The vat when “too sweet” needs to be brought to the regular
temperature, and lime to be added cautiously until the vat is brought
to its normal state. It is safer to add an excess of lime and “sour”
the vat, and then bring it back according to the directions under that
head, than to add too little, as less indigo is lost. To use up all
the dye and to dye a light blue, as little lime should be present as
is consistent with the workings of the vat.

The cause of the “falling away” of the vat is a too active
fermentation, which produces considerable lactic acid, from which
butyric acid forms, setting free hydrogen, thereby making white
indigo, which, if the action is allowed to continue, changes to a
compound from which the indigo cannot be recovered. If lime is added,
the lactic and butyric acids unite with it and precipitate it, while
the excess precipitates the white indigo, which is slowly recovered,
as fermentation progresses, which forms lactic acid, which, taking the
place of the white indigo, sets it free. Besides the sicknesses, there
are various results of mismanagement, of which the first is
overwarming, which causes the bath to turn brown, which is the
beginning of the souring.

When the bath begins to sour from overheating, some logwood should be
added and then bran, and the vat left to itself over night. The reason
of it is that the temperature is too high for the desired fermentation
to operate. The vat sometimes suddenly turns green, and even when
indigo and the other necessary ingredients are added it remains of
this color. This is called the “breaking up of the vat.” The reason is
that the temperature is too low; to remedy it, it is necessary to add
logwood and bran, warm it up, and stir, when it should stand for some





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