Indigo - dyeing - hydrosulfite vat

This vat uses Sodium dithionite (hydrosulfite) and sodium hydroxide and is widely used at the present time
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The zinc hydrosulfite vat
The hydrosulphite-soda vat has found but little application in Great Britain, though it is used to a considerable extent on the Continent. When made up with synthetic indigo, it is quite free from sediment, and with high-class natural indigo, the insoluble matters are trifling in amount. On this account the hydrosulphite-soda vat is peculiarly adapted for use in dyeing machines. When ordinary vats are used, there is also the advantage of smaller vessels being needed, on account of no space being required for a bulky sediment. Naturally, with the employment of caustic soda for wool-dyeing, the greatest care must be taken to avoid an excess.
The Badische Anilin- und Soda-Fabrik recommend the following proportions for a strong vat :
Capacity of vat, 220 gallons.
Indigo pure 20 per cent
Caustic soda solution (42 Tw.)
Hydrosulphite solution (20 Tw.)
10 lbs.
3 pints.
74 gallons.
The mixture is heated to about 50 C, and allowed to stand for half to one hour, with occasional stirring. It is then poured into the dye-bath, which has been heated to 50 C, and the whole stirred. After standing for about half an hour dyeing may be commenced. For weaker vats a greater proportion of hydrosulphite is required.
Edmund Knecht, Christopher Rawson, Richard Loewenthal, Manual of dyeing : for the use of practical dyers, manufacturers, students, and all interested in the art of dyeing., 6th ed. (1920), London: Charles Griffin and company, limited; Vol. 1, p. 326


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