_The Zinc-lime Indigo Vat._ It will be necessary to explain these words--Indigo blue is insoluble and cannot be used for dyeing. If however it is "reduced" or changed to indigo white, it has, while it is in this form, an affinity for vegetable and animal fibre. These fibres will take it up from the solution and retain it. If they are then exposed to the air, the oxygen acts upon the indigo in the fibre and turns it back again to indigo blue. Various chemicals can be used to reduce indigo blue to indigo white. I propose to describe how the work is done with zinc dust and lime as reducing agents.

In course of time the word "vat" has been transferred from the dyeing vessels themselves to their contents; _i.e._, the indigo dye liquor.

By "vat," therefore, we understand not only the vessel used for dyeing indigo, but the solution of alkali salts of indigo white in water.

This definition distinguishes the _indigo vat_ completely from indigo extract, or any other improper purposes to which indigo may be put.

The zinc lime indigo vat is better than any other for dyeing cotton and linen. It is also very good for dyeing silk. It has many advantages over the hydrosulphite vat, as it is not nearly so much affected by changes of temperature and weather. It can be put to work after a six months' rest.

The disadvantage which it shares with the copperas vat, though in a less degree, is that there is a sediment which must not touch the stuff during the dyeing. This is avoided by hanging a net in the vat after the sediment has settled, or by dipping the skeins on rods.

It is essential that the indigo used should be of the best quality, and ground to so fine a powder that it will float on water. Coa.r.s.ely ground indigo will never reduce and can be found at the bottom of the vat unchanged. It should be so fine that no roughness is felt with the tongue. Buy the best quality indigo ready ground, and if possible mixed to a paste with water. A 20% paste, _i.e._ 20% of indigo and 80% of water, is a usual quant.i.ty. If indigo powder must be used it must be mixed to a paste very carefully, as it will, if properly ground, fly about like dust. The easiest method of mixing is to pour the required amount of boiling water into a jar (previously heated), then put in the indigo. Close the vessel tightly. The steam which rises will moisten the indigo so that it loses its tendency to fly about.

After 10 or 15 minutes it can easily be mixed with a stick. The zinc dust should be dry and not caked.

_The lime_ should be in hard lumps. It should be bought from a reliable chemist in a sealed container, and kept sealed till wanted.

If it is crumbling and cracking it has been exposed to damp air, and is partly slaked already, and therefore more or less useless.

As the indigo is more quickly reduced in a concentrated solution, a stock vat is first made and this is added to the dye vat as required.

The vessel for the stock vat should have a well-fitting lid. A stoneware jar with a bung will do very well. To make a stock vat sufficient to furnish a dye vat containing 15-20 gallons use:--

10 oz. Indigo 20% paste (or 2-1/2 oz. indigo pasted with 7-1/2 oz. of water), 1-1/2 oz. zinc dust, 4-5 oz. quick lime, 4-5 pints of water.

Mix the zinc dust to a paste with a little of the water, gradually add the indigo and the rest of the water. The heat of the water should be not less than 160F. as it will cool while the lime is being prepared. Slake the lime in a separate vessel by pouring about 5 oz.

of water over it. When it begins to hiss and break, add more water little by little. When all the lumps have cracked up stir till a thick even cream is made. Add this to the other ingredients in the stock vat. Stir well. The stock vat should have a temperature of 120-140F.

It should be stirred at intervals. The vessel should be stood in hot water to keep the temperature as near 120F. as possible. In about 5 hours the mixture has a pure yellow colour and is ready to add to the dye vat. (There is of course a blue-black sc.u.m of indigo on top.)

_Preparation of the dye vat._ The vessel used should be deep and upright so that an unnecessarily large surface is not exposed to the air, and a sufficient s.p.a.ce for dyeing is obtained above the sediment.

A galvanised dust bin, or a barrel (provided it is not of oak or any other wood which contains tannin), make good indigo vats. Put 16 gallons of water in the vat at a temperature of 65-70F. In order to counteract the effects of the atmospheric oxygen contained in the water of the vat, additions of zinc dust and lime are made some hours before the stock solution is added. A pinch of zinc dust and an ounce of lime, previously slaked, should be added and the vat stirred.

Stirring must always be done gently and smoothly, every effort being made not to take air into the vat. At the same time it must be stirred up from the bottom so that the sediment is mixed with the liquor above it. The best tool for this purpose is a broom stick, to one end of which a piece of wood is nailed, like a garden rake. When all is ready, carry the stock solution to the dye vat, and, to avoid splashing through the air, hold it in the water of the vat while gently pouring out half its contents. Stir up the vat and cover it until it shows a clear yellow colour under the surface of the sc.u.m.

This may not happen for 24 hours. A good way to test the colour of the vat is to push back the sc.u.m with the edge of a saucer or plate, then dip it halfway into the liquor. Against its white surface the colour of the liquor will be plainly seen. It should look like good light ale. If the liquor is greenish and sufficient time has elapsed, another pinch of zinc dust and a little more lime must be added as before, and the vat again stirred, allowed to settle and again tested.

A little difficulty may be found in getting the vat to start, but once it has worked well no difficulty will be found in starting it again.

It will work more easily as it gets older.

As indigo does not penetrate easily, every effort must be made to help it to do so. The stuff to be dyed must be thoroughly scoured so that no particle of grease, size, or any other impurity is present. Every effort must be made to prevent unreduced indigo from attaching itself to the cotton. Never begin to dye in a vat which is greenish. The unreduced indigo will attach itself to the stuff and be wasted. Your time will also be wasted in washing it off.

The vat should be thoroughly stirred and allowed to settle each day before dyeing begins. When the sediment has settled, the froth should be carefully skimmed and kept to return to the vat when the day's dyeing is finished.

If a net is to be used it should be thoroughly wetted (if everything goes into the vat wet it will take less air with it). The net can be kept down by tying a few stones in a bag or an iron weight to the centre of it. If the hanks are to be dipped on a rod this may be of iron, or of wood suitably weighted. The hanks should not be less than 8 inches below the surface of the liquor and about 1 ft. above the bottom of the vat. The hanks should be turned after each dip, as, if the same end goes to the bottom each time it will be darker. A pulley over the vat to draw out the rod or net is convenient. The dyeings can then be allowed to drain a few seconds. Then wring each hank, shaking it out to get the air into it. After a sufficient airing, dip again.

Many short dips with airing between will produce faster colours. Dip 1 minute, wring and air 2 minutes. Dip 2 minutes, wring and air 4 minutes. Dip 5 minutes, and so on.

As linen and cotton look so very much darker when wet than when dry, a bit should be dried to judge if the colour is right.

Indigo can be dyed from the palest sky blue to black. The very palest shade of sky blue is never very fast. The virtue which indigo alone seems to possess is that, though it may become lighter with continual use, it also becomes a clearer and more lovely blue. This is especially so on cotton and linen, for which it is a superb dye. The varying shades of indigo of butchers' coats, sailors' collars, and French porters' blouses always give us pleasure.

CHAPTER XI

THE DYEING OF SILK

Silk is covered with a natural gum which has to be removed before the dyeing process can begin. This is done by boiling for one hour or more in a bath containing soap, 2 to 8 ozs. to the pound of silk according to the amount of gum on the silk. It is then well washed, and is ready for mordanting.

The mordants mostly used are _Alum_, for most of the bright colours.

_Tin_, for brightening some colours, and as a separate mordant for others. _Iron_, for black dyeing. _Chrome_, for certain browns such as catechu.

The princ.i.p.al Alum mordant is Acetate of Alumine, prepared as follows: Let 3 lbs. Alum and 3 ozs. chalk be dissolved in 1 gallon of warm water in an earthenware pan, add the chalk slowly to the Alum. Add 2 lbs. white acetate of lead, stir occasionally during 24 to 36 hours.

Let it remain 12 hours at rest. Decant and preserve the clear liquor, being careful not to stir up the sediment. Pour 2 gallons of water on the sediment, and stir occasionally for 12 hours. Let it rest 12 hours. Decant the clear and add to the first lot. Bottle for use. It keeps about three weeks. Of the mordant 2 parts are diluted with 1 of water, and the silk is well worked in this for 10 minutes, after being wetted down. Steep for 12 hours, wring out and dry. Wet down again and return to the Alum liquor, work for 10 minutes, steep 12 hours, dry.

When thoroughly dry, wash well in several changes of water before dyeing. For less bright colours one mordanting may be sufficient.

The mordant is used for successive batches of silk until exhausted; the fresher the mordant, the better for brighter colours. Silk should be dyed as soon after it is dried as is convenient.

Another Alum mordant. Dissolve 25 per cent of Alum in hot water and add 6 per cent carbonate of soda crystals. Fill up a jar with water and steep silk in it over-night. It must be washed before dyeing.

RECIPES FOR DYEING

(1) INDIGO VAT FOR BLUE

Silk is dyed in a similar manner as described for wool, but requires stronger vats and longer dips to obtain the same depth of colour. See page 33.

(2) INDIGO EXTRACT FOR BLUE

Dye at a temperature of 40 to 50C. with as much Indigo Extract dissolved in the bath as is required for the desired depth of shade.

If the silk has been first mordanted with alum, compound colours can be obtained by the addition of a red or yellow dye to the bath.

(3) CRIMSON

Mordant with Alum or Aluminium Acetate and dye with 40 to 50 per cent Cochineal. A teaspoonful of Tin, dissolved in cold water, may be added to brighten. Boil well. It is advisable to wash in soap after using tin as it prevents the latter making the yarn brittle.

(4) MADDER RED

Mordant with Alum or Aluminium Acetate. Dye with 80 to 100 per cent Madder and a handful of bran per pound of silk. Bring slowly to the boil in 1 hour, boil a few minutes. It should be brightened by boiling a short time in soap, with a little tin.

(5) YELLOW

Mordant with Alum or Aluminium Acetate. Various Dyes may be used.

_Weld_: Dye with 150 per cent. _Flavin_: Dye with 1 oz. to the pound, with a teaspoonful Tin. _Fustic_: Dye with 50 per cent, or more.

_Quercitron_: Dye with 10 to 20 per cent. A little chalk may be added towards the end.

The shades may be varied by the addition of small quant.i.ties of madder or cochineal. Orange may be obtained by the use of Madder, 2 to 4 ozs.

per pound, with Flavin or Fustic.

(6) GREEN

Greens may be obtained by dyeing with any of the yellow dyes and blueing in the Indigo Vat or with Indigo Extract. If the colour is thin, it should be dyed a deeper blue in the vat and then re-dyed with yellow. A strong clear yellow is needed for a good green.

(7) PURPLE