PART II.
We entered upon the subject of indigo, which we have treated at some length in our last issue, as much in the interest of the people as of manufacturers, for we were deeply impressed with the conviction that no improvement in our manufacturing processes would confer more benefit upon the masses than imparting stability of color to the clothing of the people. When one has a deep conviction upon a subject, upon which others have equal opportunities for judging, he may be sure that he is not alone in his impressions. He is moved by one of those waves of thought which, operating simultaneously upon many minds, gives that uniformity to public opinion at which we so often wonder. We are gratified to find, from responses to our last article, that we are not alone in our conviction of the importance of reviving “true blue” dyes. The head of a mercantile house, the extent of whose _clientèle_ in mills both of wool and cotton is hardly surpassed, has assured us that we have not overstated the reform in dyeing which we have advocated. He had long shared in our convictions. Pointing to the throng of men in the crowded street, where we were conversing, he remarked that there was hardly a man in the crowd whose clothing would not have been improved by indigo dye. “The failure to use indigo dyes,” he emphatically said, “costs the laboring people of this country millions of dollars every year. The fault is not to be charged to our own manufacturers alone; for the blue coat which I wear, and which I bought in Paris, annoys me by the crocking caused by its aniline dye.” In one very large mill of which he is director as well as selling agent, he is putting his principles in practice. All the heavy blue cloths intended for popular consumption are faithfully dyed, and each bears a stamp, “Warranted indigo dyed.” The ready-made clothing establishments which largely consume these goods have already found their advantage in purchasing them, and a similar stamp is attached to each article made from this cloth.
Some of our most celebrated cotton fabrics have won and still retain their reputation by the use of indigo dyes. The ginghams are a signal illustration. The blue check is formed by weaving cotton yarns dyed blue in the cold indigo vat with undyed yarns. These goods can be washed indefinitely without change.
Another illustration is the famous A.B.A. Amoskeag tickings, an article of such excellence that the question of the right to use trade-mark A.B.A. gave rise to the leading American case in this branch of law.[3] A prominent feature in these goods was and still is the permanence of the dye in the blue stripe, produced by the cold indigo vat. Still another illustration is the blue and white “shirting stripe” first made by Mr. Samuel Batchelder, at the Hamilton Mills, now so generally adopted for sailors’ shirts. The indigo dye enables the color to resist the roughest possible usage.
------------------------------------------------------------- [3] See the case stated at length in our article on Trade-marks, Bulletin, vol. i. p. 102. -------------------------------------------------------------
To recur to the application of indigo dyeing to wool and woollens. We have been unable, although we have written more than fifty letters of inquiry upon the subject, to learn of any peculiarity or improvements in the American processes of wool dyeing with indigo.[4] Our dyers are for the most part foreigners. For this reason, or because the art of indigo dyeing has long since reached perfection in the best establishments abroad, they rigidly pursue the old European methods. The best dyers regard the successful management of the warm fermenting vats for wool as the highest test of their art. We have already spoken of the complicity of the phenomena in fermentations. Practical dyers endow the fermenting vat with a sort of personality. “An indigo vat,” says one to us, “is more like a sick man than any thing in the world: you have to watch it as you would a sick patient, and give it physic or ferments to stir up the system and purify it.” [5] The diagnosis of a sick vat requires that sort of instinctive knowledge which experience gives to the practised physician. The impatience of our young Americans will not permit them to serve the long apprenticeship necessary to acquire the proper experience. The artisans not thoroughly trained will naturally prefer the dyes and processes introduced by modern science, which require but little skill in their application. It is a curious fact that the influence of the national government has been largely instrumental in preserving the old system of indigo dyeing. Thanks to the Quartermaster-General’s Bureau, or the man of science, General Meigs, who presides over it, indigo dyed cloths have been persistently insisted upon for the army. The late war gave a new impulse to indigo dyeing. A skilled dyer, whom we have consulted, was constantly employed in Connecticut, on a tour of professional inspection of a dozen or more different establishments making army goods. No doctor, he says, ever found in hospital practice more complications of disease than he found in the ailing vats. Among other difficulties there was a deficiency of imported woads, although the cultivation of excellent woad immediately sprung up in Connecticut. In the mean time carrot and rhubarb tops were used as substitutes for the fermenting material of the woad. Carrot-tops grown expressly for that purpose brought as high as twenty-five cents per pound. Since the war the requisitions for indigo dyed woollen goods have not relaxed, and the art is not likely to be lost.
------------------------------------------------------------- [4] A reply by Mr. D. R. Whitney, an extensive indigo importer, to a letter of inquiry, enables us to correct some errors in our former article, under the head of “commerce in indigo.” The value of export from India in 1862–63, stated in dollars, through a typographical error, should have been pounds sterling; thus, instead of $2,126,814, read £2,126,814. It is stated in our first article that the telegrams show a decline of price of indigo in the Indian trade of from 50 to 75 per cent; “per cent” should read “rupees,” which would make a decline of from 25 to 30 per cent. The reason for the decline, as stated by Mr. Whitney, is the unusually large crop of this year. The average crop of indigo in Bengal is about 100,000 maunds. The crop of this year is 135,000 maunds, about 30 to 35 per cent above the average.
According to Mr. Whitney, the consumption of Bengal indigo in the United States was 2,458 cases of 270 lbs. to a case on an average, in 1871; and in 1872, 1,802 cases. Guatemala indigo, 3,132 serroons in 1871, and 2,578 serroons in 1872.
[5] See notes on “sickness” of vats in Appendix. -------------------------------------------------------------
With the real difficulties which attend the process, it is hard for indigo dyeing to sustain itself in the face of cheap substitutes of easy application, such as the Nicholson blue. It is exceedingly difficult to piece dye with indigo and preserve a uniform hue upon the cloth. Hence indigo dyes are generally given in the wool. The wool absorbing the foreign material of the dye is more difficult to work in the operations of carding and spining. In other words, a finer and costlier wool is required. A great _desideratum_ therefore is a means of piece dyeing with indigo so as to preserve a perfect uniformity of hue throughout the piece. This, we are happy to say, has been recently successfully accomplished by one of the largest and most faithful of our cloth-making establishments. It would be premature, before the patents are secured for this invention, to explain the ingenious and expensive apparatus devised for this purpose, which constitutes in fact a battery of vats so arranged that the operation may be continuous. The experiments authorize the statement that bottom dyes of indigo, so desirable for a great variety of colors, can be applied with no other additional cost than that of the dyeing material. When this establishment, as it proposes, stamps upon the cards which designate goods, already so admirable in material and texture, “Warranted indigo dyed,” we shall regard it as an era in the American card-wool manufacture.
The old European woad vat process is that used in all our establishments. Mr. Henderson of the Washington Mills, whose experience as a practical dyer of wool is exceptionally large, informs us that he has found no work so instructive upon this process as Napier’s “Chemistry of Dyeing” (published by Henry Carey Baird, of Philadelphia, 1869). Napier’s description of the process is extracted from Dumas’s “Lectures on Dyeing.” The appreciation expressed by so competent a judge induces us to reprint Dumas’s description in an appendix to this article.
That we may give at least a general view of the whole subject, we will proceed to consider indigo in some relations not yet adverted to.
In Part I. of our notes we have treated only of the application of this substance in dyeing by means of reduction through the indigo vat. Indigo may be applied by means of reduction in the printing of fabrics, as well as in dyeing them. A true scientific arrangement would compel us next in order to consider this other application of indigo by means of reduction. But the more natural and practical order is to pursue the subject of dyeing, and to consider next the applications of the derivatives from indigo in dyeing proper.
SULPHURIC DERIVATIVES,—SAXON BLUE, &C.
The powerful action of sulphuric acid upon indigo, and the bright and lively blue color thereby produced, had been observed by chemists long ago; but no person appears to have applied this color upon cloth, until it was done about the year 1740, by Counsellor Barth, at Grossenhein, in Saxony. The vividness of the dye, and the facility with which it was applied, brought it into great vogue under the name of Saxon blue, from its origin. Its popularity in former times is evinced by the words of the old song, “The Blue Bells of Scotland:”—
“In what clothes, in what clothes is your Highland laddie clad? His bonnet’s of the Saxon blue, his waistcoat of the plaid.”[6]
------------------------------------------------------------- [6] First sung by Mrs. Jordan, about the year 1799. -------------------------------------------------------------
The Saxon blue consists simply of a solution of indigo, the Guatemala blue indigo being preferred, in sulphuric acid suitably diluted with water. The result of this reaction is not a single chemical substance, but two acids giving different tints, one called _sulpho-purpuric_ acid or _phenicine_, and the other _sulpho-indigotic_ acid; the first giving to wool a reddish-violet color, and the other a pure blue. A third compound has been indicated by Berzelius, the nature of which has not been determined. Whether one or the other of the two named acids, or the two combined, shall be produced by the reaction between the sulphuric acid and the indigo, depends upon the duration of the contact, the temperature of the mixture, and the nature and proportion of the acid used.
Persoz gives the following general receipt:—
“1 part by weight of indigo, finely rubbed. 1 „ „ „ „ Nordhaussen acid. 1 „ „ „ „ ordinary sulphuric acid. Leave for forty-eight hours, then heat until a drop turned into water will dissolve without producing a precipitate. Leave to cool, and dilute with water till the strength is brought to 18 Beaumé.”
Napier says that he has found the following method of preparing sulphate of indigo, in quantities for use, very satisfactory: “The indigo is reduced to an impalpable powder, and completely dried by placing it on a sand bath or flue for some hours at a temperature of about 150° F. For each pound of indigo six pounds of highly concentrated sulphuric acid are put into a large jar, or earthen pot, furnished with a cover. This is kept in as dry a place as possible, and the indigo is added gradually in small quantities. The vessel is kept closely covered, and care taken that the heat of the solution does not exceed 212°F. When the indigo is all added, the vessel is placed in such a situation that the heat may be kept up at about 150°F., and allowed to stand, stirring occasionally, for forty-eight hours. These precautions being attended to, we have uniformly found that any failure occurring was clearly traceable to the impurity of the indigo or weakness of the acid used.”
The processes for producing and separating the two acids derived from the combination of sulphur and indigo are minutely given by Berzelius, in vol. i. of his “Traité de Chemie,” who states this curious fact illustrative of the peculiar affinities of wool with certain dyeing substances. Wool or flannel thoroughly scoured, when immersed in the blue solution of indigo with sulphuric acid, acts as a base: it combines gradually with the acid blue, and becomes itself colored of a deep blue. When saturated with color, it is withdrawn. Fresh wool is introduced until the bath yields no more color. If sublimed or perfectly pure indigo is used, there remains in the bath nothing but free sulphuric acid. The wool thus plays the part of a base with which the blue acids combine. The dyed wool is afterwards washed and treated in feeble alkaline bath (ammonia), which redissolves the blue. This method of purifying the Saxon blue is still practised by French manufacturers.
The combination of indigo with sulphuric acid, sometimes improperly called sulphate of indigo, is known by the dyers here and in England under the name of _chemic_. The name of _chemic_ blue or green is also given the dyes formed from the indigo extract hereafter spoken of. It is largely used for making certain greens required in Scotch plaids.
The old Saxon blue or simple solution of indigo with sulphuric acid is now seldom prepared by the manufacturers themselves. It is now generally prepared for them, and furnished commercially under the name of _indigo extract_. The finer qualities used for fine dyeing and printing are known under the name of _carmines_ of indigo, _neutral extract_, _soluble indigo_, _ceruline_, &c.
The production of indigo carmines, which are simply alkaline sulphindigotates or sulpho-purpurates, is founded upon their insolubility in a liquid charged with a salt.
If, for example, we dissolve one part of indigo in four parts of fuming acid, and dilute the liquid with sixty or eighty times its weight of water, it will contain, besides the _sulphindigotic_ acid, an excess of sulphuric acid. By adding one part of crystals of soda so as to neutralize the bath, there will be formed not only sulphindigotate of soda, but sulphate of soda: as the former is insoluble in the saline liquid, the presence of the sulphate of soda causes the precipitation of the sulphindigotate in deep blue floccules. These are collected on woollen filters and washed to remove the sulphate of soda and a green coloring material, probably a modified chlorophyl, which the paste often contains, and which has the singular property of fixing itself on silk, but not on wool.
The carmines are divided according to their richness in indigo into simple carmine (4.96 per cent of indigo, water 89, saline materials 57), double carmine (10.2 per cent indigo, water 85, salts 4–8), triple carmine (12.4 per cent indigo water, 73.7, salts 13.9). A species of solid carmine known as Boiley blue or purple is in high repute in France.
The carmines may be tested by dyeing a specimen of wool in an acidulated bath to which cream of tartar has been added. The presence of the green matter, so objectionable to silk-dyers who make much use of these carmines, is detected by rubbing a small quantity of the carmine on a piece of glazed paper, which, when the color dries, gives a color varying from blue to a rich copper color: if any green coloring matter is left, it shows itself by a green aureola around the blue color. The method of applying the carmines in dyeing wool and silk,—for they are not adapted to cotton fabrics,—as given by M. de Kæppelin, is as follows:—
The operation is conducted in small wooden vats, provided with openings for manipulation, and pipes for inducting steam to heat the baths to the proper temperature. It consists of two parts, that of mordanting and dyeing. The former is thus conducted.
For each kilogram of tissue which has been previously scoured and bleached, there are provided 200 grammes of cream of tartar and 250 grammes of alum. These are dissolved in the bath of water of the vat, the temperature is raised to boiling heat, and the tissue is immersed in the bath f of an hour while it is worked over through the opening for manipulation. The pieces are then taken from the bath, to which is added a solution of the carmine in water containing a quantity of coloring matter proportionate to the intensity of the blue sought for. The solution ought to be prepared with care and passed through a silk sieve, so that the small insoluble grains which might have been left through bad fabrication may be left on the sieve. After the pieces have been manipulated in the colored bath, so as to exhaust the color and obtain the required blue, they should be rapidly washed in running water and dryed in the shade. Silk stuffs are dyed in the same way; but the alum should be previously applied cold by means of a saturated solution of alum, in which the stuffs should be immersed for an hour.
COLORS NOT FAST.
In regard to all the combinations of indigo with sulphuric acid, including the carmines, it must be observed that their application does not constitute true indigo dyeing: the colors are not fast. It is not pure indigotine which is fastened on the tissues as in the vat dyeing, but another compound of indigo with the sulphur. Berzelius observes that “the color of soluble indigo is fully as alterable and fugacious as that of the colors extracted by the decoction of vegetable materials. By a long exposure to the sun the indigo blue is destroyed: it becomes green during evaporation, and changes its nature.” The carmines as well as the sulphur acids are easily decolorized by reducing agents, such as hydrogen and sulphuretted hydrogen, although they gradually assume their original color when exposed to the atmosphere. We are informed by some of the older dealers that imported cloths and merino stuffs known as “Saxony” were formerly largely sold in our shops, but that, notwithstanding their attractiveness to purchasers, they were objectionable on account of the instability of their color.
APPLICATION OF INDIGO IN PRINTING STUFFS.
Our notes would be incomplete without some reference to the uses of indigo in printing fabrics. In pursuing this branch, we are embarrassed on the one hand by the consideration that the subject is too technical for the general reader, and on the other by the consciousness that it would be presumption in us to attempt to instruct those skilled in the art. It may not, however, be without benefit in producing a higher appreciation of science for the general reader to observe how science comes in play, even in the printing of a single color; while to the skilled reader our notes may possibly be of value as a vehicle for conveying some receipts taken from works not easily accessible.
PRINTING STUFFS OF WOOL AND SILK, AND STUFFS WITH COTTON WARPS.
This branch of our subject is directly allied to the one last considered, the application of the compounds of sulphur and indigo; for indigo is applied to printing wool and silk principally in the form of indigo carmines. These applications are less numerous than they were formerly, since they have been replaced by Prussian blue, and more recently by the aniline blues, which are now generally used. When the carmines are used, it is for making sky blues, and they enter into the composition of some greens and browns. The salts of alumina and vegetable acids are used to fix the indigo carmine upon tissues of wool and silk. Some receipts recommended by M. de Kæppelin, himself a practical printer, are given in a note.[7]
------------------------------------------------------------- [7] BLUE NO. 1. Indigo carmine 400 grammes. Alum 100 „ Oxalic acid 150 „ Boiling water 1¼ litre Gum water prepared in proportion of 1 kilogram to the litre 1¼ litre
GREEN NO. 1. Gum water as above 12 litres. Cuba lac 12 „ Alum 1 kilogram, 500 grammes. Oxalic acid 2 „ Indigo carmine 4 „
BOUILLON FOR THE GREENS AND BLUES. Boiling water 12 litres. Alum 600 grammes. Oxalic acid 750 „ Gum water 12 „
SKY BLUE FOR WOOLLEN STUFF WITH COTTON WARP.
First solution.—Boiling water 4 litres. Cyanuret of iron and potash 800 grammes. Second solution.—Boiling water 2 „ Tartaric acid 300 „
Third solution.—Cold water 3 „ Sulphuric acid 300 „
Pour in the first solution, then the second and third, agitating the color with a spatula after each new addition.
The following mixture is afterwards applied to the stuff:—
Gum water 12 litres. Water 6 „ Blue No. 1 for wool 3 „ -------------------------------------------------------------
In printing tissues of wool with cotton warp, the carmines are not used alone. They are combined in certain proportions with cyanites of iron and potash, to obtain upon the cotton a blue color of equal intensity with that produced by the carmines upon wool. It is also necessary to previously mordant the fabrics by means of a solution of oxide of tin or caustic soda which is precipitated on the fibres by passing through a bath of water, to which sulphuric acid has been added.
APPLICATIONS OF INDIGO IN PRINTING COTTON FABRICS.
Before entering upon methods used in large establishments, it may not be without interest to observe the processes still used in Java for printing calicoes, which the natives prefer to any imported from Europe. In Java there are no factories, and the women in each family make and dye or print all the cotton cloths required for their own consumption. They apply by means of a brush or pencil, which they use with great skill, to the cotton tissue which they wish to cover a thin coating of wax mixed with a little resin, the wax being applied to all the parts where the design, which has been first traced upon the cloth, requires that the fabric should remain uncolored. They then immerse the stuff several times in an indigo vat until they have obtained the desired tint. The stuff is afterwards washed and dried for a new application of the wax, carefully applied with a pencil as before. The cloth is then immersed in a bath of a different color, made with madder or catechu, but always of some dye which is perfectly stable; and the operation is repeated according to the number of colors desired. By these successive applications of wax and immersions into different vats, they succeed in producing very complicated and harmonious colors, while no European goods compare with them in stability of dye.
In the European, and our own manufacture, the blue bottoms upon vegetable fibres, made by immersion in the indigo vat, are combined with white impressions, or others variously colored, by two distinct methods. Sometimes there is printed upon the cloth before dyeing in the indigo vat a preparation called a reserve or resist, which prevents the indigotine from being deposited in the places where it is applied. Sometimes, on the contrary, the indigo, which has been uniformly fixed upon the fabric, is destroyed in certain places marked out by printing upon them certain chemical agents, called _discharges_.
The _reserves_ are mechanical, resisting the penetration of the dye, such as wax and pipe clay, or chemical. The last, through these acid or oxidizing properties, cause the precipitation of the indigotine before it has touched the fibre or penetrated into its pores. Such are the salts of copper and bi-chlorate of mercury. Other bodies perform the part both of mechanical and chemical reserves. The salts of zinc or alumina, for instance, which are frequently used, produce at the same time a deposit of indigo white and a gelatinous covering of hydrated oxide of zinc or aluminium. The composition of a good reserve is declared to be principally a question of good proportions of the constituent parts, varying with the strength of the vat and the intensity of the blue which is desired to be reserved. The first condition is that it hardens immediately after immersion in the vat: if it softens, on the contrary, it will cause the running of the color. In other words, the acidity of the impression should be proportionate to the strength and alkaline character of the vat. The white reserve, that most generally used, is composed of pipe clay, gum, verdigris, and sulphate of copper. The styles of work produced by dipping with reserves are generally of a cheap and low class. The system is clumsy and expensive, and is only tolerated because of the want of a method of directly applying indigo, which will yield the deepest shades.
Certain styles, formerly in great vogue, called _Lapis_, and forming one of the richest branches of the cotton-printing industry, are founded upon the use of reserves; and in these styles, by very simple means which we shall not attempt to describe, different colors produced from madder, catechu, &c., are produced upon the fabric so perfectly surrounded by blue that the eye cannot detect the slightest want of continuity. This fabrication has the greatest perfection in Russia. The imitation cashmere fabrics of cotton imported from that country, formerly much in fashion for dressing-gowns, are specimens of this fabrication. The great stability of the colors is a remarkable feature of these goods.
The system of resists or reserves possesses the inconveniences of not producing impressions of great firmness, and of requiring very strong vats. When the strength of the vat is partially exhausted, they may be thrown aside. These inconveniences are obviated by the system of discharges (_enlevages_). In this system the cloths are vat dyed of a uniform blue. The strength of the vat is of less importance, and it can be used until the indigo is quite exhausted. The means of destroying the indigo which has been fixed upon the fibre are founded on the use of active oxidizing agents, which transform the insoluble indigotine into soluble isatine. The agent generally used is chromic acid. As this acid cannot be incorporated with the thickening to be printed, as the thickening would produce oxide of chrome, the cloth is passed through a strong solution of chromate of potash, and dried in the shade. The required pattern is then printed on the cloth with a mixture whose principal elements are acids which are susceptible of setting free the chromic acid on the tissue, which then acts upon the indigo producing a white pattern. The acid generally employed for freeing the chromic acid is oxalic acid, thickened with British gum, dextrine, or starch, with the addition of pipe clay. To prevent running, nitric, sulphuric, or tartaric acid are sometimes used.[8]
------------------------------------------------------------- [8] Schutzenberger gives the following receipts:—
PREPARATION FOR DISCHARGE.
Water 2 litres. Yellow chromate 500 grammes.
WHITE DISCHARGE ON BLUE BOTTOM.
Tartaric acid 3 kilograms. Oxalic acid 250 grammes. Burnt starch 4 kilograms. Nitric acid 500 grammes. Water 4 litres.
De Kæppelin gives the following:—
WHITE DISCHARGE ON BLUE.
Water 2 litres. Starch 1 kilogram, 800 grammes. Oxalic acid 500 grammes. Tartaric acid 250 grammes. Sulphuric acid 375 grammes.
The pieces, having been dyed blue, are then placed in a solution of bichromate of potash in water, which is prepared in the ratio of 50 to 60 grammes to the litre, according to the intensity of the blue. The pieces thus prepared must be dried away from direct solar light or too much heat. In fact, under the action of these agents, the bichromate would be decomposed and the tissue altered. The pieces are often rolled up to prevent this effect. After the pieces are printed, they are passed into a vessel containing water and holding chalk in suspension in sufficient quantity to give it a milky aspect. The temperature of the bath is raised to 60° R. The excess of acid of the color applied is saturated by the chalk, and the excess of bichromate of potash with which the tissue is impregnated is dissolved in the bath. The pieces are afterwards washed and passed through slightly soapy water. -------------------------------------------------------------
By the method of discharges the white designs upon blue are brought out with a distinctness which it is impossible to obtain by resists, while the most delicate work of the graver can be exactly reproduced upon the tissue.
APPLICATION OF INDIGOTINE BY PRINTING.
The first step in the art of printing indigotine upon calicoes was the application of what is called pencil blue. Instead of immersing the fabrics in an indigo vat, the indigo white formed in a very strong indigo vat was thickened and applied locally to certain places on the cloth. The preparation was painted upon the cloth by means of pencils made of willow sticks, the ends of which were broomed up into a kind of brush. The style was hence called pencil blue. The methods now used to apply white indigo locally are of two kinds. The china blue process, and the solid blue process, sometimes called fast or precipitated blue. The china blue process derives its name from the resemblance of its color to the blue on the old china ware. It has great depth of tint, and permanency. It is scarcely used now, except for certain articles requiring great depth of color, such as certain furniture goods, and by the Germans and Swiss for the manufacture of calicoes for exportation to India.
We do not venture to condense the descriptions at our hand of the processes for applying the china blue and the solid blue, and translate those furnished by chemists of high authority. After the method indicated by Darwin in his recent works, we present them in smaller type, with the perhaps unnecessary suggestion that they may be passed over by the general reader.
_China blue_.—The theory of this printing blue, says Schutzenberger, is very simple. The indigo, reduced to an impalpable powder and thickened, is printed by a plate or roller. After drying, the tissue seems dyed blue, more or less deep, according to the proportion of coloring material used; but it is only a blue of application, which can be removed with the thickening, by the slightest washing. The object is now to reduce and redissolve the indigotine in place to enable it to penetrate the fibre at the end of a consecutive oxidization, and without producing a running of the color or altering the purity and distinctness of the contours of the design. I owe to M. Ed. Schwartz some valuable hints upon the fabrication of this style, which is also described with much care and details in the treatise on printing by M. Persoz.
The reduction of the indigo is obtained by alternate passages of the printed tissue into vats containing,—the first, quicklime slacked; the second, sulphate of iron; the third, soda. The operation is terminated by a passage through a bath of sulphuric acid, which removes the oxide of iron and precipitates the indigo white by hastening its oxidation.
The success depends upon the composition of the color printed, and above all upon the strength of the vats of immersion and the duration of the treatment.
The operator uses six vats,—for instance, two lime vats, provided each with 12 kilograms of lime; a copperas vat at 70 Beaumé; a caustic soda vat marking 140 Beaumé; a sulphuric acid vat with 500 grammes of acid (_par mesure d’eau_); and finally a vat of pure water.
The receipts for printing are:—
1. THE BLUE PREPARATION.
Ground indigo 4 kilograms. Acetate of iron 10 litres. Sulphate of iron 1 kilogram. Water 10 litres. Gum Senegal 6 kilograms.
Pass through a sieve; leave some time at rest, and stir whenever used. Caraccas indigo is preferred because it can be broken into a finer powder and gives a finer paste.
2. COLORS FOR ROLLER PRINTING NOS. 1, 2, 3, 4.
The blue preparation above 1, 1, 3, 4. Acetate of iron containing 700 grammes of gum per litre 2, 1½, ½, ½. Gum water at 600 grammes per litre 16, 2½, ½, ½.
These proportions can be varied according to the tint desired.
The piece is treated a quarter of an hour in the first lime vat by giving it a light movement from above to below; it is left a quarter of an hour in repose in the sulphate of lime vat; a quarter of an hour in the second lime vat; a quarter of an hour in the copperas vat; five minutes in the caustic soda; half an hour in the sulphuric acid, and then thoroughly rinsed.
To each lime vat there is given 2 kilograms of lime per piece of cloth. To the vitriol vat there is added 50 kilograms of sulphate of iron for each dozen pieces. The soda vat is renewed after 5 pieces by the addition of 12 kilograms of salt of soda, which has first been made caustic. The acid vat receives 25 kilograms of acid after 5 pieces, and ought to be renewed whenever it becomes saline. The other vats must be cleared out whenever the deposit becomes too great for success.
M. Ed. Schwartz recommends as important conditions, (1) the perfect causticity of the tissue, and an average strength of 140 Beaumé; (2) the neutrality of the sulphate of lime vat. For this end old iron should be boiled in it.
After leaving the sulphuric acid vat the pieces are rinsed in the water vat, then in river water, and afterwards should be soaked in a sulphuric acid bath at 40 Beaumé, for the purpose of dissolving the last traces of the peroxide of iron adhering to the fibre. The fabric is then washed in water and finally passed through a soapy water at 40° R.
_Solid or precipitated blue, Schutzenberger’s receipt_.—The process consists in printing indigo white precipitated in a vat, in a thick paste to dissolve it on the tissue by a passage through an alkaline bath (lime or soda), and of reprecipitating it by oxidizing it as soon as it has entered the fibre.
It is then the china blue process, minus the reduction which is made before printing, and consequently minus the sulphate of iron vat.
Indigo white is too alterable to be printed with success, so it is generally precipitated in combination with a stannic hydrate (hydrate of a salt of tin), which gives it body and preserves it from a too rapid oxidation.
The stannic indigotate in paste, or as it is generally called precipitate of indigo, is prepared by turning into the clear portion of a strong copperas vat an acid solution of protochlorate of tin, and filtering it upon woollen filters,—as much as possible away from the air. It would be better to prepare a strong tin vat by heating a mixture of indigo, caustic soda, and protochlorate of tin, and to precipitate by chlorohydric acid.[9]
------------------------------------------------------------- [9] Mr. T. P. Shepard gives in his valuable “Receipts for Calico Printing,” published in 1872, the following:—
NO. 52. INDIGO PRECIPITATE FOR FAST BLUE AND GREEN.
10 pounds quicklime, slacked with 6½ gallons water; then 2 pounds ground indigo finely rubbed in water are stirred in; then add 6 pounds copperas dissolved in 5 gallons of water; then add 5 gallons hot water and 15 gallons cold water.
Stir well from time to time, until the liquid has assumed a yellow color and deep blue veins or streaks appear on its surface. When this moment arrives, draw off the clear liquor, and precipitate every ten quarts of it with
½ pound tin crystals, dissolved in ½ pound muriatic acid.
To the remainder of the mixture of lime and indigo, 15 gallons of water may be added, and the whole stirred; and when settled, the indigo may be precipitated from the clear liquor as before. This operation may be repeated a second time before all the indigo is exhausted.
The indigo precipitate is to be collected on a muslin filter, and well squeezed out. -------------------------------------------------------------
The deposit is made into a paste with gum water; a salt of tin is often added to prevent oxidation. It is important to prevent the transformation of the indigo white into indigotine before printing. This indigotine would not fix itself on the fabric. Moreover, after printing, it is necessary to hasten the dissolution of the indigo white to enable it to penetrate the fibre. It is sufficient for this end to pass it through milk of lime. The stannic combination is immediately destroyed; the colorable matter unites itself with the lime, and the color passes into a pale gray with apple green. The indigo white becomes momentarily soluble; but the presence of the excess of lime and the thickening, as well as the attractive affinity of the thickening, prevent any running.
The piece on issuing from the lime water is placed in running water, when reoxidation commences, which this time fixes the color. The piece is finally passed through a sulphuric acid bath to absorb the lime, and washed.
By adding to the color a salt whose base precipitates in the milk of lime and oxidizes in the running water, and replacing the simple acid bath by an acid bath with yellow prussite, the intensity of the blue is increased through the formation of Prussian blue.
Although we have seen beautiful effects from the application of the solid blue of indigo on prints at our Pacific Mills, the colors produced by Prussian blue and aniline are so much more brilliant and easy of application that the use of indigo in printing goods for ordinary consumption is likely to decline rather than increase. It will be otherwise if we should ever manufacture for the East India markets. Here is a field still open for our manufacturers. Mr. Watson, in his beautiful work on “The Costumes of the People of India,” remarks that “British manufacturers have hitherto failed to appreciate Oriental tastes and habits, and hence supply but an insignificant part of the clothing of the two hundred million persons that form the population of what is commonly spoken of as India.” The great defect, he observes, is the want of stability of color in the cotton fabrics introduced,—this stability being an imperative demand in the Oriental markets.
The applications of indigo to cotton fabric are altogether secondary, in our mind, to its relations to the woollen manufacture. If we have felt called upon to say a word in behalf of the most ancient and best ally which the fibre of wool has ever had, it is because the vividness of color of the new products of coal, and the fascination which the application of the recent discoveries of science always possesses, is threatening the eclipse of the more ancient sober and solid dyes. Let the new colors have their place as auxiliaries, not as substitutes for the ancient dyes. Let them serve to give a bloom[10] to goods, but let the foundation be the good old dyes which the experience of ages has proved to be the most unalterable by light and air. The recent wonderful discovery of alizarine, or artificial madder, in coal tar products, has led practical men to expect too much from science. The opinion is quite prevalent among manufacturers that artificial indigotine has already been obtained from the same source. And some manufacturers are sanguine that the difficulties of indigo dyeing will thus be resolved. It is not improbable—for what is impossible to modern chemistry?—that this result will yet be partially obtained. But we have looked over all the recent foreign chemical reviews, and personally consulted some of our best chemists, and we can find no authority for the prevailing opinion that artificial indigotine has been produced. If the production of artificial indigotine should be realized, the only benefit would be the possible cheapening of the material. The difficulties of the indigo vat would still remain; for we cannot too often repeat, that in the very difficulties of the process, or in the insolubility of blue indigotine by ordinary agents, consists the excellence of the dye.
------------------------------------------------------------- [10] _Guernsey Blue_.—The darkest of the Nicholson Fast Blues. On a bottom of barkwood, camwood, madder, or inferior _indigo_, produces an indigo blue which will stand all the acid tests the same as colors made from indigo.
_Serge Blue_.—It will be found very serviceable to give bloom to goods dyed with indigo, and by itself shows a very good indigo test with nitric acid.—_Instructions for Working the Atlas Works Aniline Dyes_. -------------------------------------------------------------
APPENDIX.
DYEING WOOL IN THE WARM INDIGO VAT.
[_Extract from Dumas’s Lectures on Dyeing_.]
The value attached by practical wool-dyers to the following induces us to publish it without condensation:—
Indigo Blue.—We give a solid dye of indigo blue to wool by plunging it into an alkaline solution of indigo white, and then exposing it to contact with the air. The solution of indigo white is prepared in a vessel usually from eight to nine feet in depth, and six to seven feet in diameter. This size is very convenient for the requisite manipulations, and presents a large volume of water, which, when once heated, is capable of preserving a high temperature for a long time. This vessel should be made of wood or copper. It always bears the name of vat. These vats are covered with a wooden lid, divided into two or three equal segments. Over this lid are spread some thick blankets. Without this precaution the bath would come in contact with the atmospheric air, a portion of the indigo would absorb oxygen and become precipitated. There would also be a great waste of heat.
A most necessary operation, and one which has to be frequently repeated, consists in stirring up the deposit of vegetable and coloring matter which is formed in the vat, and intimately mixing it in the bath. For this purpose we employ a utensil called a _rake_, which is formed of a strong square piece of wood, set on a long handle. The workman takes hold of this with both hands, and, dipping the flat surface into the deposit at the bottom of the vessel, he quickly draws it up until it nearly reaches the surface, when, giving it a gentle shake, he discharges the matter again through the liquor of the bath. This manœuvre is repeated until the whole of the deposit seems to be removed from the bottom of the vessel. Before the tissue is dipped into the dye-bath, it should be soaked in a copper full of tepid water; it is then to be hung up and beaten with sticks. In this state it is plunged into the vat; it thus introduces less air into the bath, while it is more uniformly penetrated by the indigo solution. The cloth is now kept at a depth of from two to three feet below the surface of the liquid, by means of an open bag or piece of network fixed in the interior of an iron ring, which is suspended by cords, and fixed to the outside of the vat by means of two small iron hooks; the bag is thus drawn backwards and forwards without permitting it to come in contact with the air. When this operation has been continued for a sufficient length of time, the cloth is wrung and hung up to dry.
Flock wool is also, for the purpose of dyeing, enclosed in a fine net, which prevents the least particle from escaping, and which is fixed in the bath in the same way as in the foregoing case.
The many inconveniences attending the use of wooden baths, which necessitate the pouring of the liquor into a copper for the purpose of giving it the necessary degree of heat, have led to the general employment of copper vessels. These are fixed in brickwork, which extends half way up their surface, whilst a stove is so constructed at this elevation that the flame shall play around their upper part. By this means the bath is heated and kept at a favorable temperature without the liquor being obliged to be removed.
The potash vats are usually formed of conical-shaped coppers, surrounded by a suitable furnace. These may be constructed with less depth, inasmuch as there is less precipitation induced in the liquor. By using steam for heating the vats, we might dispense with the employment of copper vessels, and so return to those of wood.
The vats employed for dyeing wool are known under the names of the pastel vat, the woad vat, the potash vat the tartar-lee vat, and the German vat.
Pastel Vat.[11]—The first care of the dyer in preparing the vat should be to furnish the bath with matters capable of combining with the oxygen, whether directly or indirectly, and of giving hydrogen to the indigo. We must, however, be careful to employ those substances only which are incapable of imparting to the bath a color which might prove injurious to the indigo. These advantages are found in the pastel, the woad, and madder. This latter substance furnishes a violet tint when brought into contact with an alkali, and by the addition of indigo it yields a still deeper shade.
---------------------------------------------------------------------- [11] The distinction between pastel and woad is not very clear. Schutzenberger says: “Pastel, woad, and satis tinctoria is a _plant_ of the family of the crucifera. It would seem, however, that the term _pastel_ as used by the old French dyers is applied to the leaves of the woad which have been fermented, formed into paste, and afterwards into balls, and which contain much blue coloring matter. And the term _woad_ as distinguished from _pastel_ is applied to the unfermented plant.” ----------------------------------------------------------------------
In preparing the Indian vat, we ordinarily employ one pound of fine madder to two pounds of indigo. The madder is here especially useful, by reason of the avidity of some of its principles for oxygen.
The pastel vat, when prepared on a large scale, ordinarily contains from 18 to 22 lbs. of indigo; 11 lbs. of madder would suffice for this proportion, but we must also bear in mind the large quantity of water which we have to charge with oxidizable matters. I have invariably seen the best results from employing 22 lbs. to a vat of this size. Bran is apt to excite the lactic fermentation in the bath, and should therefore not be employed in too large a quantity; 7 to 9 lbs. will be found amply sufficient.
Weld, which is often used, is rich in oxidizable principles; it turns sour, and passes into the putrid fermentation with facility. Some dyers use it very freely; but ordinarily we employ in this bath an equal quantity of it to that of the bran. Sometimes weld is not added at all.
In most dye-houses the pastel is pounded before introducing it into the vat. Some practical men, however, maintain that this operation is injurious, and that it interferes with its durability. This is an opinion which deserves attention. The effect of the pastel, when reduced to a coarse powder, is more uniform; but this state of division must render its alterations more rapid. When the bath has undergone the necessary ebullition, the pastel should be introduced into the vat, the liquor decanted, and at the same time 7 or 8 lbs. of lime added, so as to form an alkaline lye which shall hold the indigo in solution. Having well stirred the vat, it should be set aside for four hours, so that the little pellets shall have time to become thoroughly soaked, both inside and out, and thus be prepared for fermentation. Some think coverings are to be spread over the vat, so as to preserve it from contact with the atmosphere. After this lapse of time, it is to be again stirred. The bath at this moment presents no decided character; it has the peculiar odor of the vegetables which it holds in digestion; its color is of a yellowish-brown.
Ordinarily, at the end of twenty-four hours, sometimes even after fifteen or sixteen, the fermentative process is well marked. The odor becomes ammoniacal, at the same time that it retains the peculiar smell of the pastel. The bath, hitherto of a brown color, now assumes a decided yellowish-red tint. A blue froth, which results from the newly liberated indigo of the pastel, floats on the liquor as a thick scum, being composed of small blue bubbles, which are closely agglomerated together. A brilliant pellicle covers the bath, and beneath we may perceive some blue or almost black veins, owing to the indigo of the pastel which rises towards the surface. If the liquor be now agitated with a switch, the small quantity of indigo which is evolved floats to the top of the bath. On exposing a few drops of this mixture to the air, the golden-yellow color quickly disappears, and is replaced by the blue tint of the indigo. This phenomenon is due to the absorption of the oxygen of the air by the indigogen from the pastel: in this state we might even dye wool with it without any further addition of indigo; but the colors which it furnishes are devoid of brilliancy and vivacity of tone, at the same time that the bath becomes quickly exhausted.
The signs above described announce, in a most indubitable manner, that fermentation is established, and that the vat has now the power of furnishing to the indigo the hydrogen which is required to render it soluble,—that contained in the pastel having been already taken up; this, then, is the proper moment for adding the indigo, which should be previously ground in a mill.
We stated above that the liquor of the vat should be previously charged with a certain quantity of lime; we also find in it ammonia generated by the pastel; but a part of these alkalies become saturated by the carbonic acid gas along with the proper acids of the madder and of the weld, as well as by the lactic acid produced by the bran during fermentation. The ordinary guide of the dyer is the odor, which, according to circumstances; becomes more or less ammoniacal. The vat is said to be either soft or harsh; if soft, a little more lime should be added to it. The fresh vat is always soft; it exhales a feeble ammoniacal odor accompanied with the peculiar smell of the pastel; we must, therefore, add lime to it along with the indigo; we usually employ from five to six pounds, and, after having stirred the vat, it is to be covered over. The indigo, being incapable of solution except by its combination with hydrogen, gives no sign of being dissolved until it has remained a certain time in the bath. We may remark that the hard indigoes, as those of Java, require at least eight or nine hours, whilst those of Bengal do not need more than six hours, for their solution. We should examine the vat again three hours after adding the indigo. We ordinarily remark that the odor is by this time weakened; we must now add a further quantity of lime, sometimes less, but generally about equal in amount to the first portion; it is then to be covered over again, and set aside for three hours.
After this lapse of time, the bath will be found covered with an abundant froth and a very marked copper-colored pellicle; the veins which float upon its surface are larger and more marked than they were previously; the liquor becomes of a deep yellowish-red color. On dipping the rake into the bath, and allowing the liquid to run off at the edge, its color, if viewed against the light, is of a strongly marked emerald-green, which gradually disappears, in proportion as the indigo absorbs oxygen, and leaves in its place a mere drop rendered opaque by the blue color of the indigo. The odor of the vat at this instant is strongly ammoniacal; we also find in it the peculiar scent of the pastel. When we discover a marked character of this kind in the newly formed vat, we may without fear plunge in the stuff intended to be dyed; but the tints given during the first working of the vat are never so brilliant as those subsequently formed; this is owing to the yellow-coloring matters of the pastel, which, aided by the heat, become fixed on the wool at the same time as the indigo, and thus give to it a greenish tint. This accident is common both with the pastel and the woad vats; it is, however, less marked in the latter.
When the stuff or cloth has been immersed for an hour in the vat it should be withdrawn; it would, in fact, be useless to leave it there for a longer time, inasmuch as it could absorb no more of the coloring principle. It is therefore to be taken from the bath and hung up to dry, when the indigo, by attracting oxygen, will become insoluble and acquire a blue color. Then we may replunge the stuff in the vat, and the shade will immediately assume a deeper tint, owing to renewed absorption of indigo by the wool. By repeating these operations, we succeed in giving very deep shades. We must not, however, imagine that the cloth seizes only on that portion of indigo contained in the liquor required to soak it. Far from such being the case, experience shows that, during its stay in the bath, it appropriates to itself, within certain limits, a gradually increasing quantity of indigo. We have here, then, an action of affinity, or perhaps a consequence of porosity on the part of the wool itself.
Woad Vat.—These vats are extensively employed at Louviers, and in the manufactories of the north of France. The bath is prepared in the same manner as in the foregoing case; the finely cut root is introduced into the copper along with 2 lbs. of pounded indigo, 9 lbs. of madder, and 15½ lbs. of slaked lime. The liquor is, after the necessary ebullition, poured upon the woad. This substance contains but a very small quantity of coloring principle; we must, therefore, add some indigo when preparing the vat, so as to indicate the precise instant when the mixture arrives at the point of fermentation so necessary for imparting hydrogen to the coloring principle, and for rendering it soluble. We must also use a larger quantity of lime, since the woad contains no ammonia resulting from previous decomposition, such as we find to be the case with the pastel of the south. When the vat is in a suitable state of fermentation, a rusty color becomes manifest, in addition to the signs already described in speaking of the pastel vat; besides the ammoniacal odor, the bath always retains the peculiar smell of the woad. The pounded indigo is now added, and we proceed, in the manner already detailed, to reduce it to a state of solution fit for dyeing.
The vats prepared by means of pastel have greater durability than those made with the woad; but it is thought that the colors given by the latter are more brilliant than those obtained from the former dye.
Modified Pastel Vat.—This vat is about 7 feet in depth, and 6½ feet in diameter. It is made of copper, and heated by steam. The lid is composed of three segments, each of which is formed of two planks, about an inch thick, and strongly secured together by bolts.
The beating is performed in the usual way, with sticks before the first dipping, after having moistened the cloth in tepid water. This operation is not subsequently repeated.
This vat is prepared with 13 lbs. of indigo, 17½ lbs. of madder, 4½ lbs. of bran, 9 lbs. of lime, and 4½ lbs. of potash. Having filled the vat, we heat it to about 200° Fah., and, as soon as the water is tepid, introduce 441 lbs. of pastel. The liquor becomes of a yellowish-brown color; small bubbles appear upon its surface, ordinarily at the end of four hours if the vat be heated by steam, but not until after eight or twelve hours where heat is applied by the common fire; in the latter case the mixture should be stirred every three hours. When the liquor displays the signs of fermentation, we add the above-mentioned ingredients, and cover the vat over; it is then to be set aside, stirring it every three hours, or oftener if the fermentative action be very rapid. Each time that it is stirred we are to add from 2 to 4 lbs. of lime; if fermentation proceed quickly we even use more, but in the contrary case less. After about eighteen hours, we plunge into the vat three pieces of common cloth, measuring from twenty to twenty-five ells in length each; when they have received six or seven turns, they are to be taken out again. The object of this is to remove the excess of lime from the bath. The vat is then set aside for three hours, when it is to be stirred, and 13 lbs. of indigo, with 2 lbs. of madder, added to it. We now again apply heat to the mixture.
If the vat contains a superabundance of lime, it will be unnecessary to add more; otherwise we throw in a further quantity. During the night it should be covered with a cloth, and a workman left to watch it. It is usually stirred once before the morning; but if it be deficient in lime, it will require this manipulation to be more frequently repeated, and also fresh lime added to it. On the following day the stirring should be continued every three hours, and so on for the next thirty hours, taking care to heat the vat from time to time. On the morning of the fourth day the dyeing may be commenced.
The temperature should be maintained at a pretty uniform point; if it be too hot, the blue takes a red reflection, by reason of the madder contained in the liquid. A vat thus prepared will last three months; we may even work it for double that period, but after the third month it appears to lose some of its indigo.
We maintain the power of the vat by introducing every night 2¼ lbs. of madder. Some indigo is also added twice or three times a week. These additions are made in the evening. After the former, the vat is left at rest for forty-two hours; with the latter only for twenty-four, at the same time observing the precautions already indicated. At the end of three months, or sooner when we wish to stop the working of the vat, we exhaust the indigo; for this purpose we continue to charge it every night for the space of a month with madder, and dip into it white cloths, or more particularly woollen tissues, which become more or less loaded with the indigo. We must continue this plan until these matters take up no further color. The dippings are to be performed twice a day at first, but once only towards the termination. Many dyers make use of this bath for preparing a new vat, but it is better to throw this away and make it up afresh with common water.
Indian Vat.—These vats are of more simple and of more ready construction than the pastel or woad vats. We are to boil in water a quantity of madder and of bran, proportioned to the weight of indigo which we wish to employ. After two hours’ ebullition, we turn into this bath some tartar-lees, which are also to be boiled for an hour and a half or two hours, so as to charge the bath with whatever soluble matter they may contain; after this ebullition the bath should be allowed to cool, and the indigo, which has been previously ground, is then to be introduced. Supposing that we wish to employ 21 lbs. of indigo, the following would be the proportions used in preparing this vat: 41 lbs. tartar-lees, 13 lbs. of madder, and 5 lbs. of bran. These vats are usually mounted in coppers of a conical shape; a small fire should be kept up around them, so as to maintain a moderate and uniform heat. The indigo will usually be found dissolved at the end of twenty-four hours, often even after twelve or fifteen hours. The liquor has a reddish color in the new vats, and a green tint in those which are in a working state. The frothy surface, as well as the brilliant-colored pellicle, becomes manifested in this as in all other preparations of a like kind.
This species of vat has to be renewed much more frequently than the woad and pastel vats, from the indigo being more difficult to dissolve after a certain lapse of time. A moderate heat should be maintained in all these vats.
Potash Vat.—This species of vat is extensively employed at Elbœuf for the dyeing of wool in the flock. It presents in all respects a perfect analogy with the Indian vat; in fact, the action of the tartar-lee in the latter preparation depends entirely on the carbonate of potash which it contains. The ingredients used in the preparation of the potash vat are bran, madder, and the subcarbonate of potash of commerce.
We obtain the deep shades in this species of vat with greater celerity than in all others, a fact which undoubtedly depends on the greater power which potash has of dissolving indigo than is possessed by lime. Experience proves that the potash vat has the advantage in point of celerity of nearly a third; but this is balanced by the inconvenience resulting from the darker shade, which we must attribute to the large quantity of coloring matter of the madder dissolved by the alkaline lee, and which becomes fixed on the stuff with the indigo.
To render this vat in its most favorable state, the indigo should be made to undergo a commencement of hydrogenation before turning it into the mixture; for this purpose we prepare in a small copper a bath analogous to that in the vat, to which the pounded indigo is added. This bath is maintained for twenty-four hours at a moderate heat, taking care to stir it from time to time. The indigo assumes a yellowish color, becomes dissolved, and in this state is turned into the vat; we thus avoid many delays and losses in its preparation, and indeed it would be desirable if a similar plan were adopted with all these compounds.
German Vat.—This vat is of nearly similar dimensions to that used for the woad, being three times the size of the potash vat. Its diameter is about 6½ feet, and its depth 8½ feet. Having filled the copper with water, we are to heat it to 200° Fah.; we then add 20 pailsful of bran, 22 lbs. of carbonate of soda, 11 lbs. of indigo, and 54 pounds of lime, thoroughly slaked, in powder. The mixture is to be well stirred, and then set aside for two hours; the workman should continually watch the progress of the fermentation, moderating it more or less by means of lime or carbonate of soda, so as to render the vat in a working state at the end of twelve, fifteen, or, at the most, eighteen hours. The odor is the only criterion by which the workman is enabled to judge of the good state of the vat, he must therefore possess considerable tact and experience.
In the process of dipping we introduce 84 lbs., 106 lbs., or even 130 lbs. of wool, in a net bag, similar to that used in the woad vat, taking care that the bag is not allowed to rest against the sides of the copper. When the wool has sufficiently imbibed the color, we remove the bag containing it, and allow it to drain for a short time over the vessel. We operate in this way on two or three quantities in succession; we then remove the vat, and set it aside for two hours; we must be careful, from time to time, to replace the indigo absorbed by the wool, as also to add fresh quantities of bran, lime, and crystallized carbonate of soda, so as constantly to maintain the fermentation at a suitable point.
The German vat differs, then, from the potash vat by the fact that the potash is replaced by crystallized carbonate of soda and caustic lime, which latter substance also gives to the carbonate of soda a caustic character. It presents a remarkable saving as compared to the potash vat; hence the frequency of its employment; but it requires great care, and is more difficult to manage. It also offers considerable economy of labor; one man is amply sufficient for each vat.
The army cloth is usually dyed by means of the pastel vat, which gives the most advantageous results. We here make use of vats about 8½ feet in depth, and 5 feet in diameter, into which we introduce from 361 lbs. to 405 lbs of pastel or of woad, after previous maceration. The vat is to be filled with boiling water, and we then add to the bath 22 lbs. of madder, 17½ lbs. of weld, and 13 lbs. of bran. The mixture is to maintained in a state of ebullition for about half an hour; we next add a few pailsful of cold water, taking care, however, not to lower the temperature beyond 130° Fah.; during the whole of this time a workman, provided with a rake, keeps incessantly stirring the materials of the bath. The vat is then accurately closed by means of a wooden lid, and surrounded by blankets, so as to keep up the heat. It is now put aside for six hours; after this time it is again stirred by means of a rake, for the space of half an hour; and this operation should be repeated every three hours until the surface of the bath becomes marked with blue veins; we then add from six to eight pounds of slaked lime.
The color of the vat now borders on a blackish-blue. We immediately add the indigo in a quantity proportioned to the shade which we wish to obtain. The pastel in the foregoing mixture may last for several months; but we must renew the indigo in proportion as it becomes exhausted, at the same time adding both bran and madder. In general we employ—
11 to 13 lbs. of good indigo for 100 lbs. of fine wool. 9 to 11 lbs. of good indigo for 100 lbs. of common wool. 9 to 11 lbs. of good indigo for 131 yards of cloth dyed in the piece.
Management of the Vats.—A good condition of the vat is recognized by the following characters: The tint of the bath is of a fine golden-yellow, and its surface is covered with a bluish froth and a copper-colored pellicle. On dipping the rake into the bath, there escapes bubbles of air, which should burst very slowly; when they vanish quickly, it becomes an indication that we must add more lime. The paste which is found at the bottom of the vat, green at the moment of its being drawn up, should become brown in .the air; if, however, it remains green, this is a further sign that more lime is required. Lastly, the vat should exhale the odor of indigo. We usually complete the assurance of the vat being in a good state by plunging into it, after two hours’ respite, a skein of wool, which, on being withdrawn after the lapse of half an hour, should present a green color, but change directly to blue. We then once more mix the materials of the vat, and two hours after it may be considered ready for dyeing.
These vats, like those already described, are provided with a large wooden ring, the interior of which is armed with a kind of network, for the purpose of preventing the objects which are intended to be dyed coming in contact with the materials at the bottom of the vat; we, moreover, take the precaution of enclosing the wool or cloth in bags. These tissues, when plunged into the bath, should remain there for a longer or shorter time, according to the shade which we wish to obtain; one dipping, however, will never suffice for this object; usually we leave in the stuff for half an hour only; it is then to be taken from the bath, wrung, and exposed to the air. This operation is repeated until we have succeeded in procuring the desired shade; we ordinarily suffer three hours to elapse between each dipping. The heat of the vat should never be allowed to fall below 130° Fah. After each operation the bath must be well stirred, and fresh lime added; generally speaking, a pound a day will suffice. We re-establish the indigo about every second day. When once this vat is well mounted, and we are careful to examine its working, we may dye from two to four batches a day with it.
When the stuffs have acquired the desired shade, they are first to be washed in common water, and then in a very weak solution of hydrochloric acid (about one part in a thousand); after this they are again rinsed in pure water.
The Indian vat is much more easily managed than the foregoing; it presents less danger of failure, from the fact that it is quickly exhausted, and also from the fermentative process, which is so difficult to govern in the pastel vat; this vat not having time to change in character. It is prepared by first introducing an equal quantity of madder and of bran, and a triple quantity of potash; this is to be gradually heated until it reaches a temperature of 167° Fah., and we then add to it the indigo, thoroughly agitating the matters for half an hour. The vat is maintained at a temperature of 86° to 100° Fah., by keeping it closely covered, and at the same time the mixture is to be stirred occasionally at intervals of twelve hours. It should by this time present a beautiful green shade, the liquor being surmounted by a copper-colored pellicle and a purplish froth. We may now commence the dyeing, following the same course as with the pastel vat; but the stirrings being here repeated much more frequently than with the other mixture, we can dye a larger quantity of wool within a given time. When the vat ceases to give a brilliant blue, we must altogether renew it; if it be merely weakened, we add to it a small quantity of freshly prepared liquor containing a few pounds of potash, and a little less bran and madder. In giving the dark and the clear sky blues, we must be careful to employ a quantity of indigo proportioned to the color which we wish to obtain, or, better still, we may use the previously exhausted vat for the dark blue.
When exposed to the influence of the putrid fermentation, indigo is decomposed and loses its color. If rendered soluble, it obeys the impulse communicated to the azotized matters with which it is brought into contact, although, if macerated in pure water at the ordinary temperature, it is itself decomposed with great difficulty.
The pastel and the woad are very prone to the putrid fermentation, by reason of the large quantity of azotized matters which they contain, as do all the cruciferæ; they require therefore considerable care in their employment.
When a vat is mounted, if the fermentation be allowed to continue unchecked, after the appearance of the blue froth and the other signs already indicated, the liquor will acquire a yellow color similar to that of beer; the froth will become white; it will give out a stale smell and lose its ammoniacal odor; after a few days it will turn whitish, and exhale a smell at first similar to that of putrefied animal substances; then it will acquire the odor of rotten eggs, and set free sulphuretted hydrogen. The lime in the pastel and the woad vats, and the tartar-lee and potash in the other mixtures, are used for the purpose of preventing these accidents.
Besides the oxygenated compound, which is formed by the combination of oxygen with the extractive matters of the plants held in digestion, there is a production of carbonic acid which saturates the alkaline lee, and forms a carbonate of lime in the pastel vat. We find this attached to the sides of the vat in such quantity that the inside of these vessels becomes incrusted with it to a considerable depth. It is this product which dyers call the tartar of the vat; it effervesces with acids, and gives on analysis carbonic acid, lime, and a few particles of indigo. In the potash vat the solubility of the carbonate of potash prevents its deposition; but it is very probable that we have even here a formation of some carbonated products, perhaps in part formed at the expense of the carbonic acid of the air.
The soluble extractive principle being the only matter which remains in solution in the bath with the indigo, the lime, &c., we have formed deposits which, varying both in their volume and in the greater or less facility with which they are precipitated during the various periods of fermentation, lead to a more or less considerable waste of time. If we plunge a piece of woollen tissue into a vat which has been recently stirred, it will acquire a dark color, and will be found covered with brown stains which are with difficulty removed. When the woad or paste vat has been stirred, it need be left two or three hours only before plunging in the stuff, at least during the early months of its working, inasmuch as the pastel, being but slightly divided and attenuated, is readily precipitated; but when, by reason of its extreme division, in consequence of repeated operations, it is thrown down with less facility, the dipping should not be performed oftener than three times in the day.
The Indian vat requires less time than the others; we may even dye with it an hour after stirring the mixture. The potash, being soluble, forms no precipitate; while the ligneous fibre of the madder and the pellicles of the bran become deposited with great facility. We can also dip with these vats much oftener than with those made by pastel or woad.
SICKNESS OF THE WARM VAT FOR DYEING WOOL, AND ITS REMEDIES.
We have to thank that excellent practical magazine, “The American Chemist,” for the following notes on the sicknesses of the warm vat, by F. W. Kugler, translated from Reimann’s Färberzeintung:—
In the wool indigo vat, among the principal “sicknesses” is the blackening of the vat, or “sharpening.” This arises from the presence of too much lime. When “sharpened,” the liquor, instead of having a waxy yellow color with a dense blue film on its surface, has no film; while the liquor is a dark blackish-green, and on being stirred shows a gray or white scum on its surface, while it emits at the same time a pungent odor. If the vat is only slightly affected, it is sufficient to add some bran and madder and to let it stand over night. If it has not quite recovered by morning, it may be necessary to heat it up, agitate it, and let it stand for a couple of hours, after which perhaps the addition of a little lime will be necessary.
If the vat is much sharpened, it is recommended to sink in it a bag of bran, and leave it over night, when the fermentation will have restored the vat in a considerable degree; but it will be necessary to add lime cautiously and by degrees, to bring it to a proper state for working.
The theory of the souring of the vat is given. Butyric fermentation takes place under certain circumstances, butyric acid being formed; and hydrogen is set free, which reduces the indigo. The addition of lime makes the vat too strongly alkaline, and sets ammonia free, which gives the pungent odor of the soured (_verschäften_) vat. Simultaneously the lime with the white indigo forms a difficultly soluble compound, which settles, and thus interferes with the working of the vat. The excess of lime must be removed, which is accomplished by introducing bran, which causes a lactic fermentation; and the lactic acid neutralizes the excess of lime, and destroys the lime compound with indigo which had been formed. The lime may be neutralized by the use of mineral acids, but there is danger in that case of precipitating the indigo.
A second “sickness” is “becoming too sweet.” The symptoms are,—the blue veins and surface film disappear on stirring, the foam gives a rustling sound, the bath assumes a reddish-yellow color, blue goods placed in the bath lose their color, and the vat has an unpleasant odor.
The vat when “too sweet” needs to be brought to the regular temperature, and lime to be added cautiously until the vat is brought to its normal state. It is safer to add an excess of lime and “sour” the vat, and then bring it back according to the directions under that head, than to add too little, as less indigo is lost. To use up all the dye and to dye a light blue, as little lime should be present as is consistent with the workings of the vat.
The cause of the “falling away” of the vat is a too active fermentation, which produces considerable lactic acid, from which butyric acid forms, setting free hydrogen, thereby making white indigo, which, if the action is allowed to continue, changes to a compound from which the indigo cannot be recovered. If lime is added, the lactic and butyric acids unite with it and precipitate it, while the excess precipitates the white indigo, which is slowly recovered, as fermentation progresses, which forms lactic acid, which, taking the place of the white indigo, sets it free. Besides the sicknesses, there are various results of mismanagement, of which the first is overwarming, which causes the bath to turn brown, which is the beginning of the souring.
When the bath begins to sour from overheating, some logwood should be added and then bran, and the vat left to itself over night. The reason of it is that the temperature is too high for the desired fermentation to operate. The vat sometimes suddenly turns green, and even when indigo and the other necessary ingredients are added it remains of this color. This is called the “breaking up of the vat.” The reason is that the temperature is too low; to remedy it, it is necessary to add logwood and bran, warm it up, and stir, when it should stand for some