The Dyeing of Woollen Fabrics Part 16

There is a further advantage about this vat, it is not necessary to prepare each individual vat separately, but a strong mother liquor or concentrated indigo solution may be prepared, and this only requires letting down with water to produce a vat of any required strength.

In the preparation of this vat, which was devised by Schutzenberger and Lalande, bisulphite of soda and zinc dust are used with either quick-lime or caustic soda. The bisulphite of soda is allowed to act on the zinc as will be detailed when an acid solution of sodium hydrosulphite NaHSO_{2}, more strictly hydrogen sodium hydrosulphite, is obtained. The acid solution of hydrosulphite has the property of rapidly reducing and dissolving indigo, and this solution may be used in dyeing. To prepare the hydrosulphite a vessel which is fitted (p. 142) with an agitator and can be closed is filled with zinc, either in the form of dust, foils, or granules. Then bisulphite of soda of 50 to 60 Tw. strength is poured over the zinc in sufficient quant.i.ty to cover it. All access of air should be avoided as much as possible, as it leads to oxidation. In the case of using zinc powder the action is often so rapid as to lead to heating, which also should be avoided.

The operation takes from an hour to two hours, when the liquor may be drawn off. It must be used immediately to dissolve the indigo; or otherwise, as it is a very unstable body, it is liable to decompose and become oxidised, when it loses its solvent properties. If more hydrosulphite is required, fresh bisulphite may be poured over the zinc which is left unused in the vessel; if no more is wanted the zinc which is left should be well rinsed in water and the vessel filled with water, so as to prevent any oxidation of the zinc, and so keep it ready for use when required. The liquor thus made will usually have a specific gravity of 62 Tw. The zinc which is used up in the preparation of the liquor is replaced by fresh zinc from time to time.

The liquor so obtained is, as stated above, rather unstable, and contains acid sodium hydrosulphite. By mixing with milk of lime, the acidity is neutralised, zinc oxide and calcium sulphite are thrown down, and a solution of neutral sodium hydrosulphite is obtained which is more stable and can be kept longer without decomposition. To prepare this, take 10 gallons of the acid liquor, as prepared in the manner described above, and mix it with 48 lb. of milk of lime, which is made from 2 lb. good quick-lime. Stir well together, allow all sediment to settle, or better, filter-press the ma.s.s. A liquor of 36 Tw. strength will usually be obtained. Do not let it stand too long before use, make it alkaline by adding a little lime.

To make the mother or stock indigo, the following method of (p. 143) procedure may be adopted. Indigo, say 10 lb., is ground into as fine a paste as possible with 13 lb. milk of lime, of such a strength that 1 gallon shall contain 30 oz. quick-lime. To this is then added so much of either the acid or the neutral sodium hydrosulphite as can be made from 90 lb. of bisulphite of soda, the mixture being kept at 150 F., until a comparatively clear, greenish yellow solution is obtained, this will contain about 1 lb. of indigo per gallon.

This mother liquor may be used in setting the vat as follows. The vat is filled with water which is heated to 120 F., about 200 gallons being used. To this is then added 1 gallon of either hydrosulphite or bisulphite of soda to destroy the free oxygen it contains, and prevent it from oxidising the indigo solution, which is next added. The quant.i.ty of the latter is solely regulated by the depth of shade it is desired to dye, and as soon as the requisite quant.i.ty has been added the dyeing may be proceeded with at once, and the first portion of goods put through will soon show the dyer whether too much or too little of the mother indigo has been added.

Continued use and the consequent agitation of the vat thereby generated causes it to become oxidised, and the vat acquires a greenish colour, and does not give fast colours. When this is noticed the use of the vat is stopped; it is heated to about 160 F., and a little lime and hydrosulphite added, when all the oxidised indigo in the vat will speedily be reduced, and the vat put into a workable condition again. By use this vat tends to become alkaline, and consequently will spoil the wool, making it harsh and brittle. This is remedied by adding a little hydrochloric acid.

#Holliday's Patent Indigo Vat.#--Messrs. Read Holliday & Sons have patented an improved method of making an indigo solution and the method of using it. They supply the indigo in the form of solution in two strengths, ordinary and concentrated. Both are used in the same way, only of the latter less, about one-fourth to one-third, is (p. 144) required than of the former. For those who would wish to buy their indigo ready prepared for use these are very convenient forms.

The best way of working the vat for wool is the following: 40 gallons of water heated to about 50 C., add 1/4 lb. of a mixture of 1-1/4 gallons bisulphite of soda, 52 Tw., and 1 lb. zinc dust, and, say, 1/2 gallon to 2 gallons, of the patent indigo solution, according to the depth of shade required. The boiled out wool is worked below the surface of the liquor for about three minutes, then taken out, and the excess of liquor squeezed back into the vat, the whole operation is repeated until the shade is arrived at. After dyeing, rinse in an acid bath of 1 to 2 Tw.

The advantages of this new vat are that brighter shades are obtained and the darker shades with fewer dips, while the goods are dyed cleaner and the shades are more quickly obtained, and, we think, somewhat faster than by the other process.

There is also the advantage that no lime or other alkali is used with this new indigo vat. The wool should be boiled out before dipping, if the best results and even shades are desired.

#Potash-Indigo Vat.#--This is also a fermentation vat, and is set in the following manner: 5 lb. of madder and 4 lb. of bran are mixed with 50 gallons of water and heated for from three to four hours, until a temperature of from 180 to 212 F. is attained. Then 15 lb. of carbonate of potash are added and the liquor is allowed to cool down to about 120 F. Next 10 lb., more or less according to shade required, of finely ground indigo is added, and the whole is left for from forty-eight to sixty hours to ferment, being stirred up at intervals of twelve hours. This vat ferments in much the same way as the woad vat, and presents the same general appearances. It is not so liable to get out of order as the woad vat, and in consequence is (p. 145) much more easily managed. It does not, however, give such bright shades as either of the vats previously described, but it dyes a little quicker, and deeper shades can be produced. It is the best vat to use where indigo dyeing is carried on at irregular intervals, also for dyeing dark shades of navy blue and for giving an indigo bottom for dark blues, browns and greens. Such shades stand milling and alkalies very well.

#Soda-Indigo Vat.#--The soda-indigo vat is set in the following manner: 100 lb. bran is boiled with 200 gallons of water for three hours, then the liquor is allowed to cool from 100 to 120 F. Then 20 lb. of soda crystals, 5 lb. slaked lime, and 10 to 15 lb. ground indigo are added, the mixture being left for two or three days to ferment, and stirred up at intervals.

Sometimes a little more soda or a little lime is added, as may be judged from the appearance of the vat, these appearances being practically the same as those met with in the woad vat, which have already been described in detail.

The soda vat closely resembles the potash vat, but is cheaper to produce. It keeps its dyeing power longer, but is somewhat more liable to get out of order. It is like the potash vat, easier to manage than the woad vat, as with all the woad vats it is necessary after working them for a day to replenish them with a little indigo, soda, or potash, as the case may be, and a little bran.

Cleaner vats are obtained if treacle be subst.i.tuted for the bran, but the latter ferments better, and gives better results in working.

#Urine-Indigo Vat.#--This vat has almost, if not quite, gone out of use, being a rather unpleasant vat to work with, with few advantages over other vats. One advantage it possesses over the woad and potash vats is that it is the best for working on a small scale, but the modern zinc reduction vats run it very close in this respect. The vat is (p. 146) made as follows: To 50 gallons of stale urine 4 lb. of common salt are added, and the mixture heated to from 120 F. to 140 F. Then 1 lb.

madder and 1 lb. ground indigo are added, and the ma.s.s is well stirred. Then the mixture is allowed to stand until the indigo is completely reduced, when the vat is ready for dyeing.

#Indigo-Indophenol Vat.#--Messrs. Durand, Huguenin & Co. have introduced the use of Indophenol along with indigo in wool dyeing. Indophenol can be reduced in the same way as indigo, and fibres dipped in this reduced product on exposure to air turn blue in the same way as if dipped in an indigo vat.

By itself indophenol has not met with much favour from dyers for a variety of reasons, but it has been found that, mixed with indigo, it can be used in dyeing with some advantage on the score of cheapness.

The newly mixed vat is made in the following manner:--

In a convenient vessel 26 gallons of water, 15 lb. zinc dust, ground into a paste with 6 gallons of water, and 7 gallons bisulphite of soda of 55 Tw. strong are mixed. Then 8 pints caustic soda lye of 72 Tw., and 16 pints liquor ammonia are added, and the whole ma.s.s is well stirred up; 22 lb. good indigo of about 70 per cent. indigotine and 7-1/4 lb. Indophenol are thoroughly ground into a paste with 7 gallons of water and 2 pints caustic soda lye of 72 Tw. The paste is added to the previous mixture, and, after being well stirred in, sufficient water is added to make the total volume of liquor up to 100 gallons.

The ma.s.s is stirred up from time to time during a period of from thirty-six to forty-eight hours, by which time, as a rule, the indigo and Indophenol will have been completely reduced, and the vat have acquired a canary-yellow colour; if it has not, add a little more zinc dust and bisulphite of soda. It is ready for use when it has a good yellow colour.

This forms what may be called a "mother," or stock vat, from which (p. 147) the dyeing vat is made in the following manner: Take a sufficient quant.i.ty of water to make the dyeing vat, add some hydrosulphite of soda (see below) to destroy any oxidising action the vat liquor may have, then add sufficient of the stock vat to give the required shade, this point is one which must be determined by experience. The vat is now quite ready for use, and the wool is entered and treated in the usual manner.

After dyeing each lot of wool it is advisable to add some of the stock vat to replace the indigo abstracted by the goods. When a number of dyeings have been done, it is possible that the vat may become charged with oxidised indigo and lose its clean, yellow colour. It may be restored to its former conditions by adding some hydrosulphite of soda. Of course, after considerable use this, like all other indigo vats, becomes too highly charged with sediment, etc., to give excellent results, in which case the only thing that can be done is to throw the old vat away and start a new one.

The hydrosulphite of soda referred to above is made in the following way: 4-1/2 lb. zinc dust are ground into a paste with 5-1/2 gallons of water and then mixed with 4 gallons bisulphite of soda at 55 Tw., stirring well so as to keep the temperature down. Then add 3 pints caustic soda lye of 72 Tw., and 3-1/2 pints liquor ammonia. Finally, add sufficient water to make 13 gallons. After standing for two or three days the preparation is ready for use. It should be alkaline in property; if not, add a little ammonia to make it so. This vat gives very good bright shades, from sky blue to dark navy, which are equally as fast as pure indigo shades.

Sometimes woollen goods dyed with indigo rub badly. The causes of this defect vary from time to time, and in many instances are often obscure in their origin. All goods intended for indigo dyeing, and more especially when shades fast to rubbing are desired, should be (p. 148) thoroughly cleansed, and before pa.s.sing into the indigo vat should be thoroughly freed from any soap which may have been used in the boiling out. Then, after dyeing, they ought to be well rinsed in water and pa.s.sed through a sour made with sulphuric acid (2 lb. in 10 gallons), and then washed again. Vats highly charged with sedimentary matter, or with zinc or lime, are often the cause of loose shades. The remedy is obvious, _viz_., the discarding of such vats and the preparation of new ones, in fact old vats are perhaps more fruitful sources of loose shades than any other cause. Soft water suits indigo dyeing better than hard water, and is to be preferred.

It is not advisable to attempt to get full or deep shades of indigo at one dip, for such would necessitate the use of strong baths. Dyeings produced in this way are liable to rub badly, because the indigo lies mostly on the surface, to which it is more or less mechanically attached. Light shades of indigo are fast to rubbing, and by repeated dippings in a light vat or a medium shade vat deep shades of fair fastness to rubbing can be got.

As repeatedly stated, no indigo vat can be worked continuously with good results; the continual agitation induced by the pa.s.sage of the yarns or cloths into the liquor brings the liquor into contact with the air, and oxidation sets in, resulting in the indigo being thrown out of the liquor in its original form. When this happens the vat loses its original clear yellow or yellowish-brown colour and becomes greenish, a sure sign that the vat is getting out of condition to give good results. The remedy has been pointed out in dealing with each kind of vat, and consists essentially in adding to the vat more of the active reducing agent and allowing the vat to rest a while.

The dye-vats may be either round tubs or square wooden tanks; for yarn in hanks, when cloths or warps are being dyed, these may be fitted (p. 149) with winces and guide rollers so as to draw materials through the liquor.

The hawking machine shown in figure 22 is also very good for indigo cloth dyeing, and is largely used for this purpose.

[Ill.u.s.tration: Fig. 23.--Indigo Dye-vat.]

Figure 23 also shows an excellent machine for indigo dyeing on cloth.

In this the vat has a frame carrying guide rollers, round which the cloth pa.s.ses, so that it travels several times through the vat liquor in its pa.s.sage from one end of the vat to the other, the amount of liquor in the vat being so arranged that the cloth is entirely immersed the whole time. After going through the liquor the cloth pa.s.ses between a pair of squeezing rollers, in order to have any surplus liquor taken out, then it traverses the s.p.a.ce between sets of guide rollers arranged over the vat, during which time the indigo becomes oxidised and the blue develops, while finally it is (p. 150) plaited down on a table. The ill.u.s.tration clearly shows the working of the machine.

#Dyeing Wool with Indigo Extract.#--Sulphonated indigo, prepared by dissolving indigo in sulphuric acid, is sold under the name of "indigo extract," or "indigo carmine," in two forms--paste (containing, perhaps, 25 to 30 per cent. actual colour) and powder. Both forms are freely soluble in water, although some makes are more so than others.

This quality of solubility is dependent upon the proportion of sulphuric acid which may have been used in the preparation of the extract. When this is small, what is termed indigo monosulphonic acid only is formed, which is but slightly soluble in water, and gives red shades. If a larger proportion of acid be used, then the indigo disulphonic acid is formed, which is fairly easily soluble in water, and gives bluer shades than the former.

As all forms of indigo extract are regular articles of commerce, details for their preparation will not be given here. It will suffice to say that indigo is heated with strong sulphuric acid until test samples show that the indigo has been completely dissolved, and it is then diluted with water and filtered. Sometimes it is sold in this condition under the term "chemic," but if this be used in dyeing wool it gives rather unsatisfactory results. When "sour extract" is required, the liquor filtered out is next treated with salt until all the colour has been precipitated out, when it is filtered off, drained, pressed and sold. Should "neutral" or "sweet" extract be required, then the acid liquor is neutralised with soda, and the product is salted out as before, drained and pressed to a suitable consistence. It is then sold as "indigo extract," or dried, at 150 F., to a powder, which is known as "indigo carmine".

All forms of indigo extract are dyed on wool from baths of (p. 151) Glauber's salt and sulphuric acid, and therefore they can be cla.s.sed with the acid-dyeing coal-tar colours. Indigo extract is notable for its level dyeing and penetrative properties, but it is not fast to light or milling.

Messrs. Read Holliday & Sons have a powder form of indigo extract which will be found very useful and to give better shades than the usual run of paste extract, while it only takes about one-fifth the quant.i.ty to give a similar shade. Working at the boil should be avoided with indigo extract, as tending to make the shades greenish in tone; from 170 to 180 F. will usually be found hot enough to dye good shades.

Indigo extract is not much used by itself in dyeing blues on wool, but it is extensively employed along with other dye-stuffs to produce an immense variety of shades--drabs, greens, fawns, greys, lilacs, etc., of which some examples will be given later on.

_Indigo Blue_.--Prepare a bath with 10 lb. indigo extract, 5 lb.

sulphuric acid, and 10 lb. Glauber's salt. Work just under the boil to shade.

_Sky Blue_.--The dye-bath contains 1 lb. indigo extract, 2 lb.

sulphuric acid, and 10 lb. Glauber's salt. Work at about 160 F. to shade.

#Dyeing Wool Blue with Logwood.#--This method of dyeing blue on wool has lost much of its importance since the introduction of the artificial dyes, but it is still employed when a blue fast to milling is wanted.

Logwood gives dark navy blue shades. The process is as follows: The wool is first mordanted by boiling for one and a half hours in a bath of 3 lb. bichromate of potash and 2-1/2 lb. of tartar. The operation must be so carried out that the non-oxidising green chrome mordant is developed on the fibre, and therefore the boiling must be thorough. In place of tartar, argols and oxalic acid are frequently used, while lactic acid or lignorosine might be employed. The dyeing is done (p. 152) in a bath of 20 to 25 lb. logwood, or 5 to 8 lb. logwood extract; the bath is started cold, heated slowly to the boil, and kept at that heat for one to one and a half hours. Between the mordanting and dyeing the wool should be well rinsed.

DYEING BLUE WITH COAL-TAR DYES.

The blue dyes derived from coal tar are very numerous, direct, basic, acid and mordant blues being known. The direct and basic dyes are very little used, but the acid and mordant dyes are extensively employed, as is indicated in the following recipes.

#Dyeing with Direct Dyes.# _Pale Blue_.--Prepare a dye-bath with 1/2 lb.

Sulphon Cyanine and 10 lb. Glauber's salt. Enter the goods, and work at the boil for one hour, then lift, wash and dry.

_Black Blue_.--Prepare a dye-bath with 3 lb. Sulphon Cyanine, 5 lb.

Glauber's salt, and 5 lb. acetate of ammonia; work at the boil for one hour. Sulphon cyanine works well with other dye-stuffs, and gives shades which are fast to milling.

#Dyeing with Acid Dyes.# _Bright Blue_.--Prepare a bath with 2 lb. borax and 1 lb. Alkali Blue B. Enter the wool at about 170 F., then heat to the boil, and work for half an hour; then lift, rinse lightly, and pa.s.s into a weak sour bath, with sulphuric acid to raise to the colour.

Soda may be used in place of borax, but the latter salt maintains the softness of the wool fibre better.

By using various brands of Alkali Blue (3 R to 7 B), various shades of blue from a reddish with the 3 R to a pure blue with the 6 B and 7 B brands may be dyed. The Alkali Blues are fairly fast to light.