How To Use Hydrosulphite – Soda Vat For Wool
Hydrosulphite – Soda Vat For Wool Recipe
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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°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 cent.
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°F. and on no account to more than 140°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° and not above 140°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° to 140°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°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°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 waters.
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 White.
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.
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