This vat uses zinc metal powder, calcium hydroxide and sodium bisulfite. Sodium dithionite is generated in situ
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The zinc sulfite vat
Sodium hydrosulphite was first recommended by Schützenberger and Lalande for reducing indigo, and is now very largely used in the dyeing of both wool and cotton. Hydrosulphite vats may be prepared either with lime or caustic soda. In either case a stock or standard vat should be prepared apart from the actual dyeing vat. In England the hydrosulphite-lime vat is used to a greater extent than the corresponding soda vat, and more particularly for the dyeing of wool, though it is also largely used for cotton yarn, &c. In the dyeing of cotton more lime and less hydrosulphite are required than for wool. The strong solution of reduced indigo may be conveniently prepared in a cask holding about 50 gallons in the following manner :- 20 lbs. of 60 per cent, indigo, thoroughly well ground to a smooth paste, are put into the cask with about 20 gallons of boiling water and mixed with milk of lime containing 24 lbs. of quicklime. In the meantime 7 gallons of bisulphite of soda (52° Tw.) have been slowly mixed with 7 lbs. of good zinc dust. In about a quarter of an hour the mixture will have become of a light greyish colour and will have lost its smell of sulphurous acid. It is then added to the mixture of indigo and lime, and the cask filled with hot water so as to bring the whole to a temperature of about 80° C. The mixture is well stirred and the cask covered to exclude air. It is allowed to stand, with occasional stirring, for about two hours, when the mixture will have acquired a deep yellow colour and is ready for use. In dyeing, the vat is filled with water and a small quantity of hydrosulphite solution or mixture of bisulphite and zinc is added (with a little milk of lime) to remove free oxygen. After half an hour or so the necessary amount of reduced indigo solution, according to shade required, is added, the vat well stirred, and allowed to stand for about an hour for the insoluble matters to settle. The dyeing may then be commenced.
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. 315
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