Hydrosulphite-soda Vat For Wool

2 ozs. powdered indigo.

7 fluid ozs. Caustic Soda solution (SG 1.2).

4 pints Sodium Hydrosulphite (SG 1.1).

The Stock Solution.--Take 2 ozs. of well pounded indigo, with enough

warm water (120 deg.F.) to make a paste, and grind in a pestle and

mortar for 10 minutes. Empty into a saucepan, capacity 1 gallon. Take

12 fluid ozs. of water adding gradually 3 ozs. of commercial caustic

soda 76 per
ent. This will give a solution of SG 1.2, which can be

tested with a hydrometer reading from 1000 to 2000, the 1000

representing SG 1 as for water.

Next take 5 pints water, add hydrosulphite slowly, stirring gently

until a reading of 1100 is shown (SG 1.1) on the hydrometer. If the

hydrosulphite be weighed beforehand and the stock of the same be kept

free from damp air, or great heat, for future vats the hydrometer can

be dispensed with; it is simply weighed out and added slowly to the

water. If added too quickly the hydrosulphite will cake, fall to the

bottom and be difficult to dissolve.

To the saucepan containing the indigo (100 per cent) add 7 fluid ozs.

of the caustic soda solution, then gradually add 3-1/2 pints of

hydrosulphite solution, stirring gently for 15 to 20 minutes. Heat

the saucepan to 120 deg.F. and on no account to more than 140

deg.F.--overheating will ruin the Stock Solution--let it stand for

half an hour, then test with a strip of glass. This should show a

perfectly clear golden yellow colour (turning blue in 45 secs.

approx.), free from spots. If dark spots show, this indicates

undissolved indigo, therefore gradually add hydrosulphite solution

(2-3 fluid ozs.). Wait 15 mins. and test with glass strip; if

incorrect continue this every 15 minutes until the glass indicates

clear yellow. If the Stock Solution is greenish white and turbid,

undissolved indigo white is present. Add then not more than a

teaspoonful at a time caustic soda solution until the Stock Solution

answers the glass test.

The Dye Vat should contain about 10 gallons of water heated to hand

hot, 120 deg. and not above 140 deg.F. Add 3 ozs. of hydrosulphite

solution stirring carefully, let it stand for 20 minutes; this renders

harmless any undissolved oxygen. Add a small cupful of the Stock

Solution, stir carefully without splashing. The vats should be greenish

yellow and should not feel slimy, an indication of too much caustic.

The vat is now ready to dye and is kept at 120 deg. to 140 deg.F.

Between dips add Stock Solution as required, if the vat goes blue and

turbid add 3 to 4 fluid ozs. of hydrosulphite and warm up to 140 deg.F.

and wait 30 minutes. As a last resort add caustic soda solution very

gradually. This should not be required if the Stock Solution is

properly prepared.

Start to dye with weak vats, 20 to 40 minute dips, and finish with

stronger vats. The more dips given to obtain a fixed shade, the faster

will be the yarn to washing and rubbing. The yarn must be oxidized by

exposure to the air for the same length of time as dipped. After the

final dip, pass the yarn through a 10 gallon bath of water to which is

added 3 ozs. of sulphuric acid, pure or hydrochloric. This neutralizes

the caustic used. Wash yarn at least twice in water.

Improvement of Defective Indigo Vat Dyes.

If, after washing until clear, the yarn should rub off badly, there is

but one remedy. Wash same in Fuller's earth, and if the shade is then

too pale, re-dye. If, through bad management of the vats, the yarn is

dull, pass the yarn through a hot bath (100% water, 1% acetic acid)

and wash in two waters. If yarn is streaky, take 10 gallons of water

at 120 deg.F., 1 oz. of hydrosulphite powder, 2 fluid ozs. liquid

ammonia fort. 880, and let yarn lie in same for 60 minutes. Wash in two


The following facts should be carefully noted:--

The Caustic Soda is the alkali which dissolves the Indigo White.

The Hydrosulphite reduces the Indigotine in the Indigo to Indigo


Indigo White is fixed on the yarn as Indigo White and on exposure to

the air becomes blue.

The yarn, on removal from the vat, should come out greenish yellow or

a greenish blue. The latter is for blue yarn and should not turn blue

too quickly (allow 60 seconds at least).

Rest the vats for 1 hour after 3 hours work. Never hurry the vats. It

is a good thing to have hydrosulphite slightly in excess as this

prevents premature oxidization; too much will strip off the indigo

white already deposited on the yarn.

Caustic Soda must always be used with the greatest caution or the yarn

will be tendered and ruined.

Finally, unless the yarn is completely scoured it is impossible to

obtain a clear colour, or a blue which will not rub off.

The figures given are for Indigo bearing 100% Indigotine, therefore in

using vegetable Indigo do not add all the Caustic or Hydrosulphite,

but depend on the glass test rather than on measurements.