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
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.