Application of Dyestuffs
To Textiles, Paper, Leather and Other Materials
by J. Merritt Matthews, Ph.D. 1920
CHAPTER XVIII - THE VAT DYES
1. Classes of Vat Dyes
2. Indigo
3. Methods of Dyeing Indigo
4. The Fermentation Vat
5. The Copperas Vat
6. The Zinc Vat
7. The Hydrosulphite Vat
8. Indigo Extract
9. Synthetic Indigo
10. Testing Indigo in the Fiber
11. Indigo Derivatives; Thio-Indigo Dyes
12. Substituted Indigo Derivatives
13. Anthraquinone Vat Dyes
14. The Carbazol Vat Dyes
15. Experimental (150-158)
THE VAT DYES
1. Classes of Vat Dyes. - The vat dyes are so called
because they are applied in a
special kind of a dyebath in which the dye is reduced to a soluble form by means
of a
strong reducing agent, such as hydrosulphite.
The vat dyes are to be divided into several groups, depending upon their
chemical
nature and origin, as follows:
(a) Indigo, including both natural and Synthetic
(b) Thio-indigo dyes, containing sulphur.
(c) Indigo derivatives, such as the brom-indigos; usually not derived
directly from indigo itself,
but built up synthetically.
(d) Anthraquinone derivatives, including the various Indanthrene, Marione,
Algol dyes, some Helindone, and others.
(e) Carbazol derivatives, of which Hydron Blue is the chief representative
In a broad sense, all of the vat dyes at present known appear to be members
of three distinct chemical groups:
(a) Indigoids including Indigo and its various derivatives such as the
Thio-indigos, Helindone and Ciba dyes, and some of the red Algol dyes. The
chemical constitution of tills class is similar to that of Indigo. They may be
applied in a neutral or slightly alkaline bath, and hence may be used for both
wool and cotton dyeing. Their reduction products are pale yellow or almost
colorless (similar to indigo), which exposure to the air re-oxidizes to the
original color. As distinguished from the next class they are sublimed as
colored vapors from the fiber when heated.
(b) Anthraquinone dyes, [ Of the different
classes of vat dyes in a general way it may be said that the anthraquinone dyes
are the fastest, the carbazol dyes are next and the indigoid dyes are the least
fast. On the other hand, however, the indigold dyes are easier to dye. Indigo is
faster on wool than it is on cotton.] including the Indanthrene,
the Marione, and most of the Algol dyes. They are complex derivatives of
anthraquinone and require a strongly alkaline vat in dyeing, consequently they
are only useful for the dyeing of cotton. In common -with all the dyes of the
anthracene class their reduced compounds are not colorless but have about the
same color as the original dye. When the dyed fiber is heated these dyes do not
sublime or form colored vapors as with the indigoids.
(c) Carbazol Dyes, or Hydron Blue.
With the exception of Indigo the vat dyes are all of comparatively recent
introduction. The dyes themselves are highly insoluble in water, but readily
yield products on reduction which are soluble in alkaline liquids. The dyebath,
therefore, consists of a mixture of the dyestuff, a strong reducing agent and an
alkali; and such a mixture is termed a " vat."
The vat dyes at present include quite a wide range of colors; Indigo is a
blue dyestuff, the thio-indigo dyes and their derivatives are mostly reds and
scarlets, the anthraquinone dyes include blue, yellow, brown, green, violet,
gray, orange, etc. The carbazol dyes are blue. The vat dyes are characterized in
general by great fastness to light, washing, acids, alkalies, and in many cases
to bleaching with hypochlorites. [Though the
vat colors in general are fast to bleaching with hypochlorite liquors (known as
fast to chlorine), they do not as a rule withstand kier-boiling with caustic
soda (an operation which usually precedes bleaching). Their fastness in this
respect has been found to be improved by boiling with sodium perborate. It has
also been found that by introducing a small quantity of potassiumbromate ('
?? oz. per gallon) into the kier liquor, the bleeding of the color may be
largely prevented. The use of a small amount of anthraquinone is also employed
for the same purpose.] This makes them very valuable products, especially
for cotton goods, and they have been coming more and more into use, and show
every indication of ranking
?? [*The different classes of vat dyes vary considerably in their ease of
application; the anthracene dyes are the most difficult to apply, while the
indigoid and carbazol colors are much easier to dye, especially with regard to
penetration and even colors. The halogenated anthracene dyes are much better in
this respect than the non-halogenated dyes The hydron series is the only one
which includes a navy blue at a reasonable price, the other classes of colors
have always been very high in price. In the dyeing of compound shades with the
vat dyes there is frequently considerable difficulty experienced, especially in
obtaining even colors. It may also be remarked that while the self shades of two
colors may be fast, when dyed in mixture the compound shade may not be as fast,
but on the other band the opposite may also be true; Anthraflavone, for example,
is not very fast as a self shade, but when dyed in combination with 1ndanthrene
Blue the compound shade is very fast. It must also be remembered that some of
the vat dyes are applied best at one temperature and some at another and in
dyeing mixtures a proper balance of temperature must, be maintained, as the
correct temperature in the dyeing of vat dyes is of the utmost importance if the
best results are to be obtained. The anthracene vat dyes require the use of more
alkali in tile vat than the dyes of the indigoid or hydron series (about four
times as much), but on the other hand they require much less hydrosulphite. The
amount of caustic soda to be used, strange to say, is to be calculated on the
volume of the liquor employed and not on the amount of dyestuff used, so that in
dyeing a light shade just as much caustic soda is required as for a heavy shade.
In using a mixture of vat dyes for the production of compound shades it is
always best to first reduce each dyestuff separately and then mix them in the
dyebath.
In kier-boiling cotton pieces containing yarns dyed with the vat colors it
will sometimes be found that the goods at the bottom of the kier will be stained
by the marking off of the color. This is supposed to be due to the fact that the
alkali in the kier liquor together with the impurities removed from the cloth
form a sort of local vat which reduces and dissolves the color, thus allowing it
to run. Certain products, such as Ludigol (\meta-nitro-benzene-sulphonic acid)
have been recommended as additions to the kier to prevent the staining of the
goods, but it is a question as t w * they u ??
9 hether successfully accomplish this purpose.
as the principal cotton dyes of the future. As they are rather difficult to
manufacture and require complicated processes in their preparation, the vat dyes
are quite expensive, and on this account are used chiefly for the dyeing of raw
cotton or yarns to be used for colored stripes in otherwise white fabrics, so
that only a relatively small amount of dyed yarn is used in. the total fabric.
Though the vat dyes may be applied to all fibers they are more suited to the
dyeing of cotton, as most of them require a rather strongly alkaline dyevat.*
The material to be dyed is simply immersed in the " vat " or dyebath until the
goods are thoroughly impregnated with the solution. The material is then
squeezed and exposed to the air, which causes the oxidation of the reduced "
leuco " compound and the formation of the color. The temperature of the vat is
usually lukewarm for the purpose of
Fig. 211.- Indigo Mill. (Ball Form.) FIG. 215.- Indigo Mill. (Cone Form.)
facilitating the impregnation of the fiber with the solution. In some cases
the dipping and oxidation have to be repeated several times in order to build up
a heavy color.
The vat dyes have come to be very essential dyes for cotton, as it is only by
the use of these dyes that laundry-fast colors in cotton wash fabrics can be
obtained. They are necessary dyes for the production of colors in shirtings,
blouse material, cotton skirtings, and hosiery and such fabrics or garments that
require to be frequently laundered. No other class of colors will stand the
bleaching effect of the hypochlorite liquors used in whitening cotton goods in
the modern laundry. There are but few other dyes with this property (Chloramine
Yellow). It was on the great fastness of the vat dyes that the reputation of the
excellent quality and fastness of the German-made dyes became so strongly fixed
in the mind of the public. Many of the vat dyes are faster than Indigo, though
of course this latter dye is itself to be considered but a member of the general
class of vat dyes.
?? * [The vat dyes in some cases have been proposed for use with wool but
owing to the fact that the vat is strongly alkaline with caustic soda it is
difficult to apply properly the color to wool. Most of the vat dyes show no
affinity for wool below a temperature of 160' F., and glue or sulphonated oil
soap must be used in the bath to protect the fiber from the action of the
alkali; furthermore in order to obtain fast colors it is necessary to boil the
dyed goods in sulphuric acid to destroy the hydrosulphite, and this is a great
disadvantage.]
In the practical use of the vat dyes it is usually the custom to first
prepare a stock vat or solution of the reduced dyestuff, and this is used in
such quantities as may be necessary for there plenishing of the dyevat. In the
preparation of this stock solution the following is atypical method:*
100 lbs. of dyestuff (which is generally in the form of a paste containing 20
per cent of dry dye);
20 gallons of water at 160' F.;2-6 gallons of caustic soda solution of 76' Tw.;10-40
lbs. of hydrosulphite powder (anhydrous sodium hydrosulphite).
The exact amounts of hydrosulphite and caustic soda will depend on the
particular dyestuff employed. Very frequently some Turkey-red oil (or similar
sulphonated soluble oil) is also added.
The vat is best made up in a wooden barrel or tank fitted with a steam pipe
so that the contents may be maintained at a temperature of 160' F. until the
reduction is completed, which usually requires about one hour or somewhat less.
The solution may then be made up to 40 or 50 gallons with water and is ready for
use. It is important that the water employed for both the stock solution and the
dyevat should first have the dissolved air corrected by the addition of a little
hydrosulphite and caustic soda; 100 gallons of water will usually require about
4 ozs. of caustic soda and 3 ozs. of hydrosulphite. As far as possible soft
water should be used. The
Whittaker gives the following typical vats for the three classes of vat
dyes:
??
....................10 per cent Chloranthrene Blue BD (10 per cent
paste) Anthracene Series
..30 per cent caustic soda (76' Tw.)
....................21 per cent hydrosulphite powder cone.
....................2 per cent Ciba Blue 2R powder Indigoid Series
....7 per cent caustic soda (76' Tw.)
....................7 per cent hydrosulphite powder cone.
....................6 per cent Hydron Blue G paste Carbazol Series
....6.6 per cent caustic soda (76' Tw.)
....................6 per cent hydrosulphite powder cone.
The color should first be stirred up to a smooth paste with the caustic soda,
hot water added, and then the hydrosulphite. In some cases it may be found
necessary to heat the liquor even to the boil in order to obtain a complete
solution.
?? purpose of the correction is to remove the dissolved oxygen and the
hardness in the water so as to avoid precipitation of the dyestuff.
The stock solution must be preserved from undue exposure to the air,
otherwise dyestuff Will be precipitated and cause bad shades. This same
precaution also applies to the dyevat. The latter is prepared by heating the
necessary volume of water to about 100'F., adding the necessary amount of
caustic soda and hydrosulphite required to counteract the dissolved oxygen, and
then adding the required amount of the stock dye solution. The vat is then
gently stirred and allowed to rest awhile before use for dyeing.
[In the preparation of the dyebath with the vat
dyes it is sometimes a question as to when the proper degree of reduction is
obtained. With some dyes the reduced or leuco-compound is of a different color
than the dye, and in this case it is easy to determine if the vat is completely
reduced; for instance, Chloranthrene Yellow gives a reduced vat which is blue in
color. On the other hand, some of the dyes give vats of the same general color
as that of the dye, and it is difficult to tell just when the color is reduced
by the appearance of the vat. Chloranthrene Blue, for example, gives a
reduced vat which is also blue in color. To determine if reduction in such a
case is complete Whittaker recommends drawing out some of the vat liquor in a
pipette arid allowing it to run slowly down the side of a clean test tube held
against the light; if the dye is reduced the liquor will show clear, but if it
is not completely reduced undissolved particles of color will be detected in the
liquid. With the blues of the indigold class, like Ciba Blue, the reduced vats
are bright golden yellow in color and consequently there is little difficulty in
ascertaining when reduction is complete.] When yarn is dyed it should
first be well boiled-out, and if open dye vats are used the yarn should be
entirely submerged beneath the liquor by being hung on bent iron rods similar to
those recommended for use in the dyeing of sulphur colors (see page 376). In
dyeing yarn it is also important that it should be evenly wrung out after
steeping in the vat, and more even results are always obtained if several dips
are given. After dyeing and wringing the yarn is then exposed to the air for
about thirty minutes to oxidize completely the leuco-compound to the dyestuff.
It is then boiled in a bath containing about 2 lbs. of soap per 100 gallons of
liquor in. ?? [Owing to the large amount of caustic
alkali used in the bath with the vat dyes ?? es, after dyeing the cotton
the goods must be soured in a weak acid bath to neutralize the alkali, then
washed, soaped well and washed again before drying. The soaping is a very
necessary operation, as it both develops and brightens the shade, besides
softening the goods and ensuring neutralization of all acid.] This is for
the purpose of completely developing the color and removing all unfixed
dyestuff, which would otherwise dull the shade and cause crocking.
On account of the difficulty of obtaining even shades and good penetration of
color, it is more satisfactory to dye vat colors in the loose stock rather than
on yarn or piece-goods. By dyeing yarn in the form of warps, however, very good
results can be obtained. When dyeing loose stock it is
?? indigo-brown, indigo-gluten and some mineral matters.
In the plant itself the coloring matter is supposed to exist in the
form of a glucoside called indican. The process of extraction of the dyestuff
from the plant is both interesting and complicated. The indigo plant, which is a
shrub growing 3 to 4 ft. in height, is cut, in summer. ~ The cut plants are tied
up in bundles and packed into long vats, which are then filled with water; in a
short time fermentation sets in which is allowed to continue for ten to fifteen
hours. This process converts the indican of the plant into soluble matters which
are extracted from the plant by the water and pass into solution. ++ The liquor
is then run into another vat, placed at a lower level, and here it is churned
and beaten up either by hand or mechanically for the purpose of exposing it to
the oxidizing action of the air, whereby the insoluble indigotine is formed and
precipitated out. This collects at the bottom of the vats in the form of a,
paste or mud which is washed, filtered, pressed into cakes, and dried. This
constitutes the raw indigo of trade, and comes in the form of large cubical
blocks. The best natural Indigo comes from Java and Bengal, and contains from 60
to 75 per cent of coloring matter. Madras Indigo is usually somewhat inferior,
while that from Guatemala, China, Africa, and Egypt is very variable.
The raw indigo of trade is a dark blue, earthy-looking substance. When
scratched with the finger nail good qualities will exhibit a coppery streak. At,
the present time a great, deal of the crude Indigo undergoes a refining process,
for the purpose of eliminating the many impurities liable to occur in the raw
product; it also comes into trade ready ground either as a powder or a paste in
order to facilitate its use by the dyer.
[In the first competition of synthetic
Indigo With the natural product, it was claimed that the presence of these other
bodies in the latter gave it more desirable properties than the synthetic.
Careful and unprejudiced examination of these claims, however, has demonstrated
the fact that these associated bodies must be regarded solely as impurities and
have little influence on the resulting color, as they are practically all
eliminated in the wash waters after dyeing or are decomposed into inert bodies
in the vat. This fact is also apparent in that refined Indigo (from which these
bodies have been removed) is preferred by the dyer to the crude material. It
seems to be amply demonstrated that the sole value of Indigo is in the
indigotine that it contains. Synthetic Indigo, being of a very high degree of
purity, usually gives somewhat clearer and brighter colors than the natural
dye.]
~ Two crops are usually gathered from the same plants each year.
2. The Indigo ?? is extracted chiefly from
the leaf of the plant; this contains, oil the average, about0.5 per cent of
coloring matter.
Before Indigo can be used by the dyer for purposes of reduction it must be
ground to a very fine impalpable powder. In former times where the natural
Indigo was bought in the form of blocks or lumps it had to be ground in special
indigo mills for a long time. This accounts for the fact that it is now so much
used in the form of a 20 per cent paste (with water or glycerin). Synthetic
Indigo is practically altogether marketed in the form of such a paste, as it is
then ready for direct use in preparing the stock solution of indigo-white.
The principle of indigo dyeing has always differed entirely from that of
other classes of dyestuffs, and has constituted an art by itself. Indigo is
perfectly insoluble in water, and hence cannot be applied in dyeing in this
form.
By the action of various reducing agents, however, it may be converted into a
substance known as indigo-white, which is soluble in alkalies; in this form, it
is applied to the fiber, and by subsequent oxidation by simple exposure to the
insoluble blue indigotine, which thus remains permanently fixed in the fiber.*
Indigo may also be converted into a soluble blue coloring matter by treatment
with strong sulphuric acid. This body, known as Indigo Carmine, or indigo
sulphonate, maybe classed as an ordinary acid dyestuff) being applied in the
usual form of acid dyebath; but it does not possess the great fastness and other
valuable properties of Indigo itself.
3. Methods of Dyeing' Indigo.-Indigo is extensively
used for both wool and cotton dyeing, though it is being used proportionately
less for the dyeing of wool since the introduction of the fast alizarine and
anthracene blue dyes. It is not much employed for the dyeing of silk. In calico
printing it has an extensive application, principally for discharge styles. The
vats used for cotton dyeing are generally more strongly alkaline than those for
wool, while the proportion of Indigo used in them is also higher. In cotton
dyeing, too, the vats are usually worked cold. Indigo dyeing is known as " vat "
dyeing because it is carried out in a specially prepared vat. According to the
character of the reducing agent employed, these vats are classified as follows:
Zinc vat, Fermentation vat, Hydrosulphite vat, Copperas vat.
The fermentation vat is the oldest form of indigo dyeing, and is still used
to a considerable extent for wool dyeing. Its action depends on the chemical
activity of certain ferments which reduce the indigotine to the soluble
indigo-white. This vat is used warm, while the other vats are usually worked
cold. ~
The copperas vat was the earliest form of chemical vat; it was exclusively
adopted for cotton. At the present time, however, it is almost obsolete. The
reducing agent employed was copperas, or ferrous sulphate.
The zinc vat was formerly the favorite one employed for cotton, and even at
the present time it is quite largely used. The reducing agent employed is an
alkaline solution of zinc dust.
Indigo is apparently fixed on the fiber mechanically; that, is to say, the
coloring matter is deposited in the fiber in a fine state of division; If the
color is deposited too rapidly it will lack fastness, especially to rubbing.
Hence it is not advisable to use concentrated vats for dyeing heavy shades of
Indigo, but to build up the color by means of several successive dips in weaker
vats.
~ Wool reacts somewhat differently with the reduced Indigo in the vat than
cotton. The former, on account, perhaps, of its somewhat alkaline character, has
considerable affinity for the acid indigo-white and consequently fairly deep
shades of good fastness can be obtained with one dip on wool. Cotton has much
less attraction for the reduced indigo and takes up a much smaller quantity of
the dye. It is customary, therefore, to dye wool in the warm (fermentation) vat,
while cotton is dyed in a cold vat as its affinity for the dye decreases as the
temperature rises.
The hydrosulphite 'vat is the one of latest origin; it is employed very
largely at the present time for all classes of indigo dyeing both on wool and
cotton. The reducing agent employed in this vat is sodium hydrosulphite, NaHS02,
prepared by the action of zinc dust on sodium bisulphite. It is gradually
replacing the other forms of vats as it is the most simple and scientific and
the most easily regulated.*
The alkali used for dissolving the indigo-white is the same for all forms of
vats; it maybe either lime or caustic soda, or a mixture of the two, depending
upon whether the vat is to be employed for wool or cotton dyeing. Ammonia does
not appear to dissolve indigo-white very readily, and the alkaline carbonates
are still less suitable.
Indigo-white behaves like a very weak acid, and it requires an excess of
rather strong caustic alkali to bring it into solution, and it is readily
precipitated again by the addition of any acid. On this account the vat must
always be kept alkaline.
Before Indigo is introduced into the vat (of whatever variety) it must be in
a very finely divided state, otherwise the reduction NN-111 always be
incomplete. The grinding of Indigo is a rather important consideration; it is
usually first ground in the dry state, and then ground a second time with a
little water (to which a small amount of alkali may be added) to the form of a
paste, Indigo paste of this character may be purchased ill the market by the
dyer, and may be added to the vat directly; it should contain 20 per cent of
indigotine.
During the reduction of Indigo in the vat, the process is usually accompanied
with secondary chemical reactions varying ill their nature and degree with the
character of the vat. This results in the conversion of smaller or larger
amounts of the dyestuff into substances other than indigo-white and a resultant
loss of coloring matter. This is especially large in the copperas vat, it also
amounts to considerable in the fermentation and zinc vats; in the hydrosulphite
vat it is reduced to a minimum of about 2 per cent.
Indigo vats when used for dyeing should not have a concentration of more than
3parts of indigotine per 1000 parts of liquor. More than this tends to the
production of shades which are liable to crock and also lose in washing. The vat
must also possess an excess of reducing agent. This may act in several ways. It
prevents the premature oxidation of the indigo-white arising from the vat liquor
coming in contact with the air or water in the pores of the material being dyed,
and so prevent or retard the
* The time required for the reduction of the Indigo in the various vats is
about as follows:
Copperas vat ...................two to three hours Zinc-lime vat
..................four to five hours Hydrosulphite vat ..............one-half to
one hour
penetration of the indigo-white. Again, the more thoroughly the indigo white
in the vat is reduced the more completely will it work its way into the material
and the faster will be the color. A certain excess of reducing agent is also of
advantage in the subsequent oxidation of the indigo-white to indigo-blue, as
then the action of the oxygen is slower and more uniform, giving better
penetrated colors and also causing the dyestuff to be precipitated in a finer
state of division, which results in faster and better colors. If there is not
sufficient excess of reducing agent in the vat, on washing after dyeing and
oxidizing a large part of the color will be removed, whereas if more reducing
agent were present, the loss on washing should be little.
FIG. 217.- Machine for Dyeing Loose Stock with Indigo and Vat Colors
The fastness of Indigo is said to be improved by an after-treatment with
bluestone and acetic acid. The use of glue in the vat also has the same effect.*
Indigo is frequently bottomed by first dyeing with certain substantive or
sulphur dyes; and indigo blue on cotton may be topped by dyeing with basic
colors; the goods after dyeing in the vat being mordanted with tannin, fixed
with tartar emetic and dyed.
Redder shades may be obtained with Indigo on cotton by steaming after dyeing
in the vat, but this somewhat decreases the fastness to washing. Heavier shades
may be obtained by first mercerizing the cotton
* Treatment, with bluestone causes the shade to become somewhat greener. In
using the glue treatment it is recommended to pad the cloth previous to dyeing
with a solution of glue (11 to 21 ozs. per gallon). This causes the shade to be
brighter and redder and increases the fastness to rubbing.
By passing the cloth before dyeing through a solution of Turkey-red oil and
alumina the fastness to alkali and chlorine is much increased.
by treatment with a strong solution of caustic soda.* The mercerized fiber
shows a greater attraction for the Indigo than the untreated cotton. In order to
save Indigo it has been suggested to mercerize only one side of the cloth to be
dyed, and when this is run through the vat the mercerized side will dye up much
darker than the other. ~
With improvements in mechanical devices it has become possible to dye Indigo
on cotton in the form of cops, tubes, cheeses, beamed warps, etc. In such
machines the material remains stationary and the indigo vat liquor is forced
through the fiber. Only the hydrosulphite vat can be used for this purpose as
there must be no sediment or undissolved particles, because the cotton material
in this case acts as a filter to the liquid. Therefore great care must be taken
in preparing the vat for this method of dyeing. Special apparatus must be used
for dyeing Indigo (and the other vat dyes as well) differing from that employed
for the ordinary dyestuffs, as provision must be made to draw air through the
dyed material in order to oxidize the color.
In piece dyeing two forms of indigo vats are used: (a) immersion vat, and (b)
continuous vat. In the first form of vat sinking frames are used on which the
goods are spirally attached by means of hooks. These frames are immersed in the
vat for the required time, then lifted out, and exposed to the air for
oxidation, when another dip is given until the required depth of color is
obtained. Usually between each dip the frame is turned bottom up so as to get
even dyeings. Immersion vats are chiefly used for heavy goods that do not
readily dye through, for the frame may be left in the liquor for any length of
time necessary, whereas in continuous dyeing machines this is not possible.
Heavy linens, moleskins, and such fabrics are often left in the vat overnight,
or even for several days in order to obtain proper penetration. Immersion vats
are also used for goods to be dyed on one side of the piece only; in such a case
two pieces are fixed back to back on the frame. When the dyed pieces are exposed
to the air only the outer sides are oxidized and the Indigo is chiefly developed
there, the other side being dyed a considerably lighter shade. For dyeing on
immersion frames the zinc vat is more suitable than the hydrosulphite vat, as
the latter contains hydrosulphite and caustic soda, which cannot be squeezed out
in this case after dyeing, and consequently uneven colors are liable to result.*
To produce full shades of blue the Hoechst Co. recommend passing the goods,
before dyeing in the vat, through a solution containing 1 to 11 lbs. of starch
to 100 gallons of water.
~ For producing very heavy shades of Indigo cotton is sometimes first dyed
with a weak Aniline Black, as follows: For 100 lbs. of cotton yarn, work for,
one hour at 100' F. in a bath containing 31. lbs. aniline salt, 312 lbs. sodium
bichromate, and 7 lbs. of hydrochloric acid; wring out well and treat in a fresh
bath with 141 lbs. of soda ash for one-half hour at 100' F. Then rinse twice and
hydro-extract and dye in the indigo vat. In this way very coppery shades of blue
maybe obtained with very little Indigo. Instead of using Aniline Black a light
shade of manganese bronze may also be employed as a bottom.
In continuous machines the pieces are run through successive vats and exposed
to the air for oxidation between the dips. Usually four to six vats are employed
in one range soas to obtain heavy shades. The hydrosulphite vat liquor is most
generally employed for this form of continuous dyeing, as there is no sediment
and the vat is easily regulated. The
FIG. 218.- Dyeing Machine for Indigo and Vat Dyes. (Zittauer.)
depth of color may be regulated by varying the speed of running and the
number of immersions. It is always preferable to enter the goods in the wet,
state. By drying the goods first after dyeing and then souring and washing
heavier shades of blue are obtained.
4. Fermentation Vat.- The
essential ingredients of the fermentation vat are: Indigo, lime, woad, bran, and
madder. The woad furnishes the proper kind of ferment for the reduction of the
Indigo, the bran and madder serve as nourishment for the growth of the ferment,
while the lime serves to neutralize the acids liberated during the fermentation
and also furnishes the alkali necessary for the solution of the reduced Indigo.
In dyeing carbonized wool or shoddy in the indigo vat care must be had to
have the goods thoroughly neutralized with soda before entering the vat, a s any
acid in the wool may cause, disturbances in the vat by neutralizing the alkali.
There are, however, a large variety of substances used in the preparation of
the fermentation vat, and almost every indigo dyer has his own special formula,
but the essential ingredients are those given. According to the make-up of its
constituents, the fermentation vats are classified as follows:
Woad vat, constituted as above outlined.
Urine vat, containing urine as an active source both of fermentation and
alkalinity; atpresent almost obsolete.
Potash vat, in which potash is used as the chief alkali.
Soda vat, also known as the German vat, in which soda is the chief alkali
used.
In Eastern countries all manner of substances are added to the indigo vat
for purposes of aiding the fermentation or supplying nourishment to the ferment;
among some of these substances may be enumerated dates, raisins, honey, plant
seeds, glucose, etc.*
(a) Saxon vat.-This is one of the earliest forms of Indigo dyeing in Europe,
and is still practiced in the same primitive manner by the peasants of Saxony,
where the celebrated Saxon blue is dyed. The following experiment will
illustrate this method: Take10 grams Indigo paste (20 per cent) and mix with 10
grams potash dissolved in 50 cc. water; place 50 grams raw unscoured wool in a
wooden or earthenware vessel, and pour over it .the above solution. sufficiently
diluted to just cover the wool. Set aside in a warm place for a week or ten
days. A moderate fermentation sets in which causes the reduction of the Indigo,
which is absorbed by the wool, and thus the dyeing is accomplished. When
sufficiently colored, remove the wool, squeeze, allow to oxidize in the air, and
finally wash in a soap solution The shades obtained in this manner are
especially beautiful, and they are highly prized on account of their fastness to
rubbing.
(b) Woad vat.-It is very difficult to obtain any very satisfactory results
on a small experimental scale with the fermentation vat, but the following will
illustrate the method of setting this vat: Place 6 liters of water in a wooden
or stoneware vessel and heat to about160' F.; add 50 grams of woad previously
broken up and soaked for several hours in a little warm. water; next stir in 20
grams bran, 8 grams soda ash, 3 grams lime, 20 grains madder, and 12 grams
Indigo paste (20 per cent). Stir well, and then cover with a cloth and allow to
stand in a warm place for twenty-four hours. During this time the fermentation
has become quite active; the liquor should be yellowish in color and be covered
with a light blue froth. [Cotton dyed in the
fermentation vat acquires a peculiar " indigo smell " which is insisted upon by
buyers in some countries.] It should now be stirred up well, and if any
large quantity of gas is given off, a little lime should be added; after which
it is again covered and left to ferment for a few hours more. When the reduction
of the Indigo is complete, the liquor of the vat will be yellow in color with
the surface covered with a dark blue layer, which if skimmed off should be
granular in appearance. During dyeing the vat should be maintained at a
temperature of 120' F. If the vat does not have a satisfactory appearance, a
little more lime should be added, the liquor stirred up, and then covered and
allowed to stand for a couple of hours.
When the vat has been brought to a proper condition, steep a handful of
well-scoured wool in the liquor for a few minutes. On being taken out the wool
should be of a greenish yellow color; squeeze and expose to the air until the
blue color is completely developed, then wash in a warm soap bath. The latter
treatment should cause the wool to lose but a small amount of color. In dyeing,
care should be taken not to disturb the sediment in the vat, otherwise streaked
and uneven colors will result. The vat may now be used for dyeing a variety of
woolen material (loose wool, tops, yarn, and 1170Vell pieces). Heavier shades
may be produced by giving several dips in the vat, squeezing and oxidizing in
the air after each dip. Care should be taken not to agitate the liquor too much,
as otherwise it will rapidly oxidize and turn blue, and no longer be fit for
dyeing. The vat should be contained in a tall-shaped vessel, as about one-third
of the vat is made up of the sediment which it is not desirable to disturb while
dyeing.
The vat may be maintained continuously for a long period of time. After being
worked for some time it becomes partially exhausted and oxidized; then a little
glucose (syrup), bran, madder, and lime may be added together with more Indigo
paste. It is well stirred up, covered over, and allowed to stand for several
hours or overnight, when it is again ready for dyeing. For good results the
amount of Indigo in the vat should not rise above 3parts per 1000.
The woad vat is also known as the " bastard " vat, and the proportion of its
ingredients may vary considerably. On a large scale the following
proportions are recommended:
Content of vat 600-800 gallons.
Woad ........................................
50 lbs. Bran ........................................
20 lbs. Soda ash ....................................
8 lbs. Lime ........................................
3 lbs. Madder ......................................
20 lbs. Indigo paste (20 percent) ..................12 lbs
If solid Indigo is used only about 2 to 4 lbs. should be used; but in this
case the dvestuff should be very carefully ground in a ball or roller mill
prepared from stale mine, salt, madder, and Indigo, the alkali being supplied
by the ammonium carbonate present in the stale urine.
Properly to prepare and maintain a fermentation vat requires considerable
skill and experience, especially with regard to the proper amounts of and the
proper times for adding the lime. The fermentation must be regulated in such a
manner as to reduce the Indigo sufficiently by the generation of the proper
amount of hydrogen, and yet kept sufficiently under control as to prevent the
danger of putrid fermentation setting in, which will result in the rapid
destruction of the Indigo. W lien putrid fermentation starts, the vat is said to
have. " gone sick," and lime must be added and the vat well stirred up. If the
secondary fermentation, however, has gone too far and cannot be stopped in this
manner, the vat must be boiled up in order to prevent a total loss of the Indigo
therein. After this, of course, the vat must be set all over again. The addition
of lime always tends to reduce the fermentation, if too much is added the
fermentation may be lessened beyond that point necessary for the complete
reduction of the Indigo. If the fermentation is proceeding too slowly it may be
increased by the addition of bran. If too little lime is present, the acids
liberated by the fermentation will throw the Indigo out of solution, hence the
vat will become weak, and bluish in color.
In dyeing heavy shades with Indigo it is best to build up the color with
several successive dips in weaker vats, rather than to dye it to the full shade
by a single dip in a very strong vat. In this manner the pigment is More
thoroughly absorbed by the fiber and will not be so liable to crock off as
otherwise.
In using synthetic Indigo the following fermentation vat is recommended. Use
25 lbs. of Indigo paste (20 per cent), 12 lbs. of bran, 12 lbs. of soda ash, and
8 lbs. of madder. The dye will be reduced in about twenty-four to thirty-six
hours. The liquor at first has a muddy appearance, this gradually becomes
greenish, and after the addition of lime shows a golden yellow color. The fresh
vat has a sickly smell, but this gradually disappears, giving place to a pungent
odor. In order to keep up the fermentation in the vat after use an addition of 5
ozs. of molasses to each pound of indigo used is made.
5. The Copperas Vat.-This form of indigo
vat is not much used at the present time, as it is not very suitable for
continuous dyeing on account of the large amount of sediment it contains.* The
essential ingredients of this vat are ferrous sulphate and slaked lime; these
react in the following manner:
FeS04 + Ca(OH)2 = Fe (OH)2 + CaS04-
The copperas vat was chiefly used for dyeing skein yarn. Its chief advantage
was that it was easily set and kept in condition. A considerable amount of
Indigo is always lost in the copperas vat, due to over-reduction and combination
of the dye with tile hydrate of iron.
The ferrous hydrate thus formed acts as a reducing agent in the presence of
water:
2Fe(OH)2+2H20= Fe2(OH)6+H2.
The indigo-white formed by the reduction dissolves in the excess of lime
present.
The vat employed should be narrow and deep to accommodate the large amount of
sediment formed. The temperature of the vat should be kept at about 70 to 750 F.
To prepare the copperas vat proceed as follows:* 36 grams of quicklime are
slaked to a thin paste with water; while warm stir in 30 grams Indigo paste (20
per cent). Then add 30 grams ferrous sulphate (copperas) dissolved in about 100
cc. water at 140' F. Then dilute with water to 500 cc.
Have this solution in a covered flask; allow to stand for four to six hours
with occasional stirring, in which Wine the liquid should have become yellow in
color with a coppery-looking bead. Before adding the stock vat to the dyevat, 1
lb. of ferrous sulphate and 11 to 2 lbs. of quicklime should be added per 100
gallons of water.
The copperas vat is also known as the vitriol vat. As a rule it is not
replenished, but is worked three times a day, being well stirred after each
dyeing. In about ten days the vat should be exhausted. It is mostly used for
yarn dyeing and " resist " dyeing.
When cotton is dyed in a vat containing lime and which has considerable
sediment, the material must always be washed with acid (I to 2 per per cent of
sulphuric or hydrochloric acid is used) after dyeing in order to remove all
particles of ?? hnie from the fiber, which would otherwise tender the cotton on
drying. After the acid treatment the cotton must be thoroughly washed.
Darker shades are obtained in this vat if the yarn is dried before
acidifying, and redder shades can be produced by drying at a high temperature.
Also by steaming the shade is made more violet and bloomy. These remarks hold
true for cotton dyed in any form. of vat.* This coppery appearance, however, is
changed by washing towards black.
In practical dyeing various proportions have been suggested by different
authorities, as follows:
?? Indigo, Natural Indigo, Synthetic60 per cent. Quick Lime. Ferrous
Sulphate. Vat.20 per cent. Pounds. Pounds. Pounds. Pounds.Gallons..............
80 so 400............. 25-50 30-40 400 a ............ 70 70 40020 ... 15-19
40024 6016 40
25 20
It is probable that Indigo forms a chemical compound with ferrous sulphate
and lime, and this entails a considerable loss of dyestuff, for under the most
favorable conditions only 75 to 80 per cent of the Indigo placed in the vat can
be found again. A part of the Indigo remains in the sediment probably combined
with ferrous hydrate.
In setting the copperas vat it is customary to put the Indigo and copperas
into the bath first, and then to add the milk-of-lime. To save time, however,
and to obtain a better reduction of the Indigo it is advisable to prepare a
stock vat. This may be prepared conveniently by mixing 25 lbs. of Indigo paste
(20 per cent) with 20 lbs. of copperas previously dissolved in hot, water, and
then add 25 lbs. of lime in the form of a thin cream. Have the temperature of
the Vat at about 120' F., stir up well and allow to stand until fully reduced,
which will require about three hours. In practice the vat is usually prepared in
the evening and allowed to stand overnight.
The dyevat is usually a stone or wooden circular vat 6 to 9 ft. deep and 21
to 5 ft. in diameter, and generally sunk into the floor of the dyehouse so as to
make it convenient for working. In starting a new vat the necessary amount of
water is run in, and then for each 100 gallons 1 lb. of copperas and 2 lbs. of
lime are added in order to counteract the effect of the oxygen in the water. The
necessary amount of the stock vat is then added, the liquid stirred up and left
for two to three hours. The liquor should then be clear and of a brownish amber
color, and on gently stirring it, dark blue streaks should appear with a coppery
scum or flurry float on the surface. Before entering the goods to be dyed this
flurry should be skimmed off and added to the stock vat.
If the liquor is greenish it indicates that part of the Indigo is not
reduced, and more copperas has to be added. If it has a darkish appearance more
alkali is needed and additional lime is added. An excess of either copperas or
lime, however, should be avoided. After a day's working the vat should be well
raked up and if necessary replenished by additions from the stock vat. The
sediment in the copperas vat contains a considerable amount of Indigo, hence
this should be saved and the Indigo recovered by treatment with hydrochloric
acid.
According to the Badische Co. bright reddish shades may be obtained by
previously treating the cotton goods with bone glue. For this purpose the goods
are run in a solution containing 2 to 5 parts of glue per 1000 parts of water,
squeezed and dyed. Better results are said to be obtained if the goods are dried
before dyeing. This method of treatment is especially recommended for dyeing in
the hydrosulphite vat.
[ More excerpts
from Matthews Book are coming. ]