The Zinc-lime Indigo Vat
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. Coarsely
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 quantity. 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 160 deg.F. 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 120-140
deg.F. It should be stirred at intervals. The vessel should be stood
in hot water to keep the temperature as near 120 deg.F. 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 scum of indigo on
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 space 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 65-70 deg.F. 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 scum.
This may not happen for 24 hours. A good way to test the colour of the
vat is to push back the scum 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.
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