CHAPTER VI
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.