Fiber -  Dyes - Textile History from Straw Into Gold

Application of Dyestuffs
To Textiles, Paper, Leather and Other Materials

by J. Merritt Matthews, Ph.D. 1920

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  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.