THE INDIGO OR VAT COLORS
=History.=—Most of the colors of this group have been discovered
and put on the market within the last two years. Thus they form the
most recent as well as, in many respects, the most interesting and,
perhaps, the most important class of modern dyestuffs. On the other
hand, to this same group belong not only indigo itself, which has
been known and valued in the East from the earliest ages, but also
that most famous of all the ancient dyestuffs, Tyrian Purple.
Indigo itself does not exist as such in nature; but it is
easily formed by oxidation, or the exposure to air, of a
substance—Indican—which occurs as such, or can be produced by
a simple process of fermentation, in the juices of many widely
distributed plants. Accordingly, even quite barbarous races in
different parts of the world noticed the deep permanent blue stains
formed on their bodies and clothing when they crushed, accidentally
or on purpose, the leaves and stems of the various _Indigoferæ_.
Gradually they learned to extract the color in a solid and permanent
form so that they could dye with it, instead of using the juice of
the fresh plant itself—and then they took to cultivating the plants.
These plants—_Indigofera Anil_, _I. tinctoria_, and others originally
found wild have been, up to the last four or five years, extensively
cultivated in many tropical countries, notably in India (some of
the best qualities came from the province of Bengal, and hence the
common name for the natural dyestuff—Bengal Indigo), Japan, China,
Java, South and Central America, and Africa. From these plants the
indigo of commerce, in the form of dark blue granular lumps with a
characteristic coppery lustre, was prepared by a comparatively simple
process of fermentation, extraction, and oxidation.
Indigo may also be obtained, although in small quantities only, and
in an impure condition, from other plants. Notably among these is
_Isatis tinctoria_, or woad, which in early days was extensively
cultivated in England and the Continent, and which, even now, is used
in small quantities in some processes of indigo dyeing.
_Artificial Indigo._—The exact composition of indigo was first
determined some sixty years ago, and from that time on some of the
greatest chemists of the world have been attempting to prepare it,
artificially, from some comparatively inexpensive source, obtained
from coal tar or elsewhere. As early as 1875 the problem was solved,
at least from a scientific standpoint, but the process proved too
expensive for commercial purposes. During the last five years,
however, at least two of the great German firms have discovered
methods for making, in any desired quantities and at very reasonable
expense, absolutely pure indigo from some of the important coal-tar
derivatives. And since that time the cultivation of the indigo
plant has proved so unprofitable that it has been almost entirely
abandoned, and the land formerly used for this crop is being turned
over to other and, at present, more useful purposes.
This synthesis—i.e., chemical formation—of indigo from coal-tar
products has been justly regarded as one of the great triumphs of
modern science. Right here let me impress upon my readers this fact:
the real dyestuff, indigo, is absolutely the same material, whether
it comes mixed with a great mass of impurities, as in the woad; or
whether it contains from 5 to 25 per cent. of foreign matter of
little or no value, as in the Bengal or natural indigo; or whether we
get it from Metz or the Badische Company, chemically pure, either in
the dry state or, thinned with water, in the form of a 20 per cent.
paste. It is positively the same dye; and being absolutely without
contamination of any kind, the artificial or synthetic dyestuff
presents advantages in the matter of purity of shade, ease and surety
of manipulation, and permanence of the color produced, which could
never be obtained before its introduction.
=Application of Indigo.=—The principles of indigo dyeing are the same
now as with the Egyptians, the only difference being in the means
used to bring about the chemical changes involved. Indigo itself is a
blue solid, insoluble in water, acids, and alkalies, and practically
unaffected by sunlight. If, however, the element hydrogen be added to
it, or, as the chemist would say, if it is “reduced” by the action
of any one of numerous deoxidizing or reducing agents, the indigo
blue is changed to a new substance, indigo white, which is almost
colorless, and which dissolves, in the presence of alkalies, to a
bright yellow liquid. If cotton, wool, paper, wood, or indeed almost
any solid materials (noticeably the fingers and nails, as some of my
readers may find out), are immersed in the solution, they will absorb
some of this indigo white, and then, on exposure to the air, the
white indigo will rapidly take up oxygen, and become converted into
the insoluble blue coloring matter.
_Fermentation Method._—Until recently the methods used for reducing
the indigo—i.e., changing the solid blue into the soluble white—were
just about the same as those used by the ancients, and were based
upon some kind of fermentation, usually alcoholic. It was found
out at a very early date that if indigo, ground up with water to a
paste and rendered alkaline by the addition of wood ashes, lime, or
other simple alkali, were mixed with grape juice or any other sugary
liquid, and then kept warm and allowed to ferment, the resulting
fluid would contain the dyestuff dissolved in a form suitable for
dyeing. The vessel in which this process was conducted was known as a
vat, and the process of indigo dyeing is still called “Vat Dyeing.”
_Disadvantages._—At the very best this method is slow, uncertain,
and difficult to manage, especially on a small scale. In wool
dyeing, to this day, a few vats are still to be found where syrup,
ground madder root or, in some instances, woad, wheat bran, and
other materials which ferment readily in the presence of alkali, are
stirred up with warm water and soda, and then allowed to stand. In
two or three days they are in active fermentation, and the indigo, in
the form of paste, is added and well stirred in. After much further
delay, if all goes well, the indigo is finally “reduced,” and, if
the amount of alkali, the temperature, the concentration of the vat,
and various other factors are carefully attended to, the bath can be
used for several days, or even weeks, without being made over again;
fresh indigo and other ingredients being added, from time to time,
as needed. Cotton, linen, wool, and even silk can be dipped in this
bath, which should be light greenish yellow in color, with a blue or
bluish-green scum or coating, where the indigo is oxidized on the
surface. Goods immersed in this bath turn yellow, and then, when
taken out, wrung free from loose liquor, and exposed to the air, the
yellow color quickly changes to a permanent blue.
A serious drawback to all these various fermentation vats is that
a good deal of the dyestuff is always spoilt—i.e., decomposed into
colorless compounds which can never be regenerated or made useful.
Indeed, the loss from this cause frequently amounts to 20% or 25% of
all the dye used, and occasionally, especially in hot weather, and on
a small scale, to far more.
But, apart from the actual loss in valuable dyestuff, there is a much
more serious drawback to this method of indigo dyeing, namely, the
waste of time and energy involved. There is always a considerable
delay in getting a fermentation vat fairly started, even where all
the conditions are favorable; and when it is running smoothly,
the reducing process is a very slow one. Furthermore, the indigo,
not being dissolved in the liquid but only suspended in it, has
a constant tendency to sink to the bottom in the form of a blue
mud, and thus escape the chemical action of the fermentation gases
entirely.
A short time ago a teacher of handicraft dyeing was expatiating, in
my presence, upon the impropriety of using any of the new chemical
processes for dyeing, and insisted that the only way to dye indigo
was to set up a vat, and feed it, and work with it as our ancestors
used to. It was suggested to her that it would be at least two or
three days before cloth could be dyed in such a vat. “Eight or ten
days at the earliest,” was the reply. And when it was hinted that
the vat would have to be frequently stirred during all that time,
she proudly answered, “Stirred regularly and thoroughly every single
half-hour, night and day, during the whole period.”
“H—m,” remarked a bystander, “that’s a little worse than sitting up
with a baby sick of the croup.”
Somehow the great advantage of this particular process over the
modern ones, by which a proper bath can be prepared in perhaps five
minutes, failed to impress itself on some of her listeners.
_Modern Chemical Vats._—As soon as it was understood just what
chemical action was going on in the vats, and the object of it,
chemists began to find out methods for reducing the indigo without
the necessity of a long, tedious, and even nasty fermentation
process. They first introduced the “copperas-lime” vat, where the
reduction was done by the use of ferrous sulphate (green vitriol or
copperas), and slaked lime was the alkali used to keep the indigo
white dissolved.
Later they introduced zinc dust, a very powerful reducing agent,
in place of the copperas, avoiding in this way the large amount of
precipitated iron oxide which always forms in the copperas vat,
and leads to the loss of dye, and muddiness and dulness of color,
necessitating a special clearing bath of dilute mineral acid.
At present the most satisfactory method is to use the chemical known
as _sodium hydrosulphite_, as a reducing agent, in a bath made
strongly alkaline with caustic soda. Hydrosulphite is not expensive;
it acts very rapidly, leaving no sediment; it causes no loss or
waste of the indigo; and it does its work perfectly. Hence, with its
introduction, the dyeing of indigo has become extremely simple.
To still further shorten and simplify the process, the large
manufacturers not only furnish indigo already ground up to a fine
paste with water, but also supply it already reduced by hydrosulphite
or some other reducing agent, so that it is almost ready to dye with
as it is, and will dissolve almost instantaneously in an alkaline
bath with the addition of just a little more reducing agent. Such
products are the Indigo Vat III (_Metz_), and the Indigo Solution 20%
(_Badische_). By using either of these, the preparation of a vat
large enough to dye 3 or 3½ pounds of cotton is the task of but a
few moments. These special preparations, however, are more expensive
than the regular 20% pastes, and the hydrosulphite vat is so easy to
prepare that the saving of time is hardly worth the extra cost.
DYEING DIRECTIONS
For dyeing by the Vat method the dye-pot is two-thirds filled with
warm water, at about 120° F. (when the finger can hardly bear the
heat), and one or two tablespoonfuls of caustic soda are added—enough
to make the bath decidedly alkaline. The dyestuff, preferably first
mixed up with some hot water, to thin the paste, is stirred into the
liquid, and then to this is added sodium hydrosulphite, in powder, or
preferably dissolved in water, until the color of the bath changes
from blue, first to green, and then to greenish yellow, with a
bluish-green coppery scum. If the bath is bright yellow, too much
hydrosulphite has been used, and some more indigo should be added;
or, if this is not desirable for fear of getting too dark shades, the
bath should be exposed to the air and stirred frequently until the
color is right. If the bath, on scraping aside the scum, looks blue,
or even markedly green, it needs a little more hydrosulphite. If,
after reduction, the bath looks yellow but turbid, it probably needs
more alkali.
Into this bath the material is placed, and stirred around until
thoroughly saturated—the temperature being kept about 120° F. for
heavy goods, to assist penetration. Light goods can be dyed equally
well in a lukewarm, or even a cold bath. The goods are then taken
out, wrung lightly by hand, and are carefully passed two or three
times through the wringer, to get the color evenly distributed. They
are then shaken out and hung up in the air to oxidize. In fifteen or
twenty minutes, after the color has changed, they should be rinsed
well in two or three waters, to get rid of all traces of the caustic
alkali, and then boiled for several minutes in a soap bath, to wash
off the loose dyestuff and prevent rubbing. This after-treatment with
boiling soapsuds is of even more importance in the case of the other
Vat dyes than it is with indigo, for with most of them the oxidation
is not completed in the air—and so the color is developed as well as
brightened by the soap bath.
It is very important, when working with these Vat colors, to remember
that hot solutions of caustic alkali are very hard on the hands
and that, therefore, rubber gloves are extremely useful, if not
essential. Stains left on hands, clothes, and utensils, although
difficult to remove by washing, are almost instantly dissolved by
warm solutions of hydrosulphite with a little soda or other alkali in
them.
_Results._—Colors produced by synthetic indigo are clear and clean,
but not brilliant. If the slightly purplish shades of natural indigo
are desired, they can be obtained with special brands—Indigo R, or
Indigo RR, _Metz_—or by mixing small quantities of Algol Red B,
_Elberfeld_, or Thio Indigo Red B, _Kalle_, with the indigo before
reducing it. It is generally supposed that the characteristic shade,
the so-called “bloom” of natural indigo, was due to the presence of
small quantities of a reddish dyestuff, known as indigo red. As a
matter of fact, however, the method of dyeing has more to do with
this than the composition of the dyestuff.
For instance, if the indigo is very thoroughly reduced in the vat
before the goods are immersed, as is generally the case in the modern
hydrosulphite method, and the bath is made up with fresh reducing
agent for each dyeing, the resultant color will be a very clear,
rather greyish, shade of blue without any purple lustre. If, however,
the dyestuff is not very perfectly reduced, as was generally the case
with the old fermentation vats, and the bath, from standing in the
air, has a heavy scum on the top, and is greenish rather than clear
yellow in color below the surface, then the dyed fabrics will be apt
to show the marked purplish tone which is so characteristic of the
older indigo dyeings.
_Uses._—While of less importance than it used to be before the
discoveries of the last few years, the use of indigo for dyeing
cotton, especially for the craftsman, is not to be neglected. It
furnishes, easily and rapidly, in one bath, without either boiling,
mordanting, or after-treatment, exceedingly pleasant, soft shades
which are fast to both light and washing. For resist dyeing, such as
Tied and Dyed work, Resist Stencil work, and Batik, it will be found
particularly useful, because the fabric can be dyed in the cold.
Indigo possesses, however, certain disadvantages, especially for the
professional dyer, which it shares with the other Vat dyes described
below, and which prevent it, and the other Vat dyes, from being used
as widely as the Salt colors or even the Sulphur colors. In the first
place these dyes are all of them expensive. They cost more than most
others, pound for pound of the dry color, and full shades need much
larger proportions of them in the bath.
Then it is difficult to dye to shade with them, because the color,
as a rule, alters so much when exposed to the air. In practice, when
dyeing large quantities of goods to the same shade, it is customary
to divide the materials into several lots of the same weight; and
to make a strong “stock solution” of the dyestuff, properly reduced
with alkali and hydrosulphite. By making up a fresh vat for each
lot of goods, using exactly the same volume of water and of “stock
solution,” and working each lot for the same length of time and at
the same temperature, even results can be produced with much less
trouble than by dyeing to shade by the eye.
Another drawback is that indigo-dyed goods, especially of the heavy
full shades, are apt to “rub.” This can best be avoided by always
using a well-reduced bath; by washing with boiling soap after each
dip; and by building up the deep shades by successive dippings in
moderately weak vats, rather than by obtaining the shade, once for
all, by using a very strong, concentrated dye-liquor.
For many hundreds, and even thousands, of years, indigo has been
universally recognized as the most permanent and most valuable blue
dyestuff for cotton and indeed for woolen goods. For the latter
purpose it is now but little used, thanks to the introduction of the
exceedingly fast dyestuffs of the Acid and Mordant classes. But for
cotton it is still considerably used, for fast shades.
THE MODERN VAT COLORS
Up to a very recent date indigo was the only dyestuff, of any
importance at any rate, that was used in the manner just described,
and produced colors fast to light and to washing. During the past
three or four years, however, the attention of the dyeing chemists
has been directed to this question, and at least five of the great
dye houses have introduced dyestuffs covering a great range of colors
which, when dyed in the same way as indigo, not only rival but
distinctly surpass that color in permanence as well as beauty.
=Names.=—These dyestuffs, while known generally as the Vat colors,
have been given special class names by their manufacturers, as
follows: Algol (_Elberfeld_); Ciba (_Klipstein_); Helindone (_Metz_);
Indanthrene (_Badische_), and Thio Indigo (_Kalle_). The Cassella
Company are just introducing the first members of their series, to be
known as Hydrons.
=List of Selected Dyestuffs=:—
Badische— Indanthrene Claret, B, Extra
Indanthrene Yellow, G
Indanthrene Blue, G C D
*Indigo pure
Cassella— *Hydrone Blue, R
*Hydrone Blue, G
Elberfeld— Algol Red, 5 G
Algol Yellow, 3 G
Algol Blue, 3 G
Kalle— *Thio Indigo Red, B G
*Thio Indigo Scarlet, S
Thio Indigo Brown, G
*Indigo, K G
Klipstein— Ciba Red, G
Cibanone Yellow, R
Ciba Green, G
Ciba Blue, 2 B
Ciba Violet, R
Metz— Helindone Red, 3 B
Helindone Fast Scarlet, R
*Helindone Yellow, 3 G N
*Indigo M L B, 6 B
N.B.—The dyestuffs marked * will dye in a lukewarm or even cold bath.
=Properties and Uses.=—These Vat dyes are not all of equal value,
but as a class they are, distinctly, the fastest of any as yet
introduced; and the best of them may properly be considered as the
most permanent coloring agents of any sort or kind that have ever
yet appeared on the earth. They not only far surpass in this respect
the best of the vegetable dyestuffs, with the possible exception
of the very best qualities of Turkey red, but in resistance to
chemicals and outside agencies of various sorts, are much better
than the best mineral colors. This is so much the case that the
modern specifications for dyed cloth for Government purposes, as for
instance the khaki uniforms for soldiers in active service, which up
to a year or two ago were dyed with iron buff modified with oxide of
chromium, have been raised, in one country after another, until they
exclude every class of dyestuffs except these new Vat colors.
During the last year or two these dyes have been introduced, though
with some difficulty, into commerce, and it is possible to obtain
shirtings and other printed goods, dyed in permanent colors, so
permanent indeed that the cloth will wear completely out before the
color changes in the slightest. The extra cost of the dyestuffs, and
the comparative difficulty of dyeing to shade, furnish an excuse for
increasing the price of the goods. And the perhaps not unnatural
disinclination of the shopkeepers to push the sale of materials
which, in their opinion, are quite unnecessarily fast, has combined
with the cost to delay the general adoption of these remarkably
valuable coloring agents.
For craftsmen, however, where the price of the dyestuffs constitutes
such a small percentage of the cost of the finished article,
and where the absolute permanence of the color is of the utmost
importance, these colors are most useful. They are not to be used,
excepting under special circumstances, for animal fibres—wool,
silk, leather, feathers, etc.—for fear of injuring the materials by
the action of the caustic alkali. But on cotton and linen, both in
direct or resist dyeing, and for stencil work, there are no colors to
compare with them in fastness, not excepting even the very best of
the Sulphur colors.
DYEING DIRECTIONS
These dyes are all applied, just like indigo, in an alkaline
hydrosulphite vat. The colors are applied in paste form, usually 20%
strong, or at any rate equivalent in strength to a 20% paste of pure
indigo. Care must be taken to thoroughly mix and stir up this paste
with a glass rod, in the original package, each time it is used, so
as to keep its composition uniform.
The proper amount, to be determined only by experience, is first
thinned with a little hot water, and then stirred into the dye-pot,
two-thirds full of hot water, about 140° F. (This is well below a
boil, and yet hot enough to slightly scald the tips of the fingers.)
To this is added caustic soda, in the proportion of two to three
spoonfuls to each one of the color, the amount of soda being
proportionately greater for light shades than where large amounts of
color are used.
After this has been dissolved the dyestuff is reduced by adding
slowly, with constant stirring, spoonful after spoonful of the
powdered sodium hydrosulphite until the bath clears and generally the
color changes. In most cases, as with indigo, the completion of the
reducing change can be told by a marked alteration in the shade of
the bath.
Thus, in general, the blue dyes, like indigo, turn yellow or
orange when the proper amount of hydrosulphite is added. For the
other colors there is no general rule. Thus Indanthrene Yellow
(_Badische_), when reduced, is blue—while the Helindone Yellow
(_Metz_) is blood red. Helindone Scarlets (_Metz_), when reduced,
appear green, while the Thio Indigo Red and Scarlet have about the
same color, when reduced, that they have when oxidized.
The best way to tell whether the bath is in proper condition is to
dip a piece of white blotting paper into it, and notice, on taking
it out, whether the color is in specks or is dissolved. On standing
in the air for a few minutes the color should become oxidized, and
firmly fixed to the paper. As a rule these Vat colors should be
reduced warm, because, in many cases at least, the reduced color does
not dissolve in a bath of cold alkali. In most cases, however, after
having been reduced at a temperature of about 140° F, the bath may be
allowed to cool considerably, before it loses its dyeing value. This
enables these colors to be used for Batik, or other processes where
the temperature must be kept below 80° or 90° F. The dyestuffs which
can be thus used will be found marked with an asterisk in the list of
selected dyestuffs above.
The well-wetted materials are placed in the reduced dye-bath, and
stirred and worked for five or ten minutes, or longer, according to
the depth of shade experienced. For full shades, however, as in the
case of indigo, it is much better to build up the color by successive
dippings than to try to put it all on in one bath. For heavy goods
the addition of a little Turkey red oil, about half a tablespoonful
to the gallon, is an advantage, though not absolutely necessary. When
thoroughly impregnated with the dye-liquor, the goods are taken out,
wrung carefully, two or three times, to remove the waste liquor as
evenly as possible, and then shaken out and exposed to the air for
fifteen or twenty minutes. They are then boiled in a soap bath for
about twenty minutes, and then well rinsed, and dried. This hot soap
bath, as before mentioned, is of great importance in most of these
colors, not only for getting rid of loosely fixed dyestuff, but for
oxidizing and fixing the color itself.
For dark shades it is well, as in the case of the Sulphur colors, to
add salt—three or four tablespoonfuls per gallon of dye-liquor—to
the bath, taking care to have it well dissolved before the goods are
entered. This is always done when dyeing with Helindone Yellow 3GN,
_Metz_.
The shades of these new Vat colors are extremely bright and clear,
and, by combining these properly, any desired effects may be
produced. The splendid series of reds and scarlets for the first
time allow the characteristic shades of Turkey red to be obtained,
in one bath, and of at least equal, if not of superior fastness
to the original. One peculiarity of these colors is their extreme
fastness, not only to light and washing, acids and alkalies, but also
to various oxidizing agents, such as chloride of lime or bleaching
powder. Accordingly goods properly dyed and finished with these
dyestuffs can be entrusted with safety, so far as the color goes, to
agencies which would speedily ruin fabrics dyed in any other manner.