This vat just uses zinc metal powder and calcium hydroxide
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The zinc vat
The ingredients are often added direct to the dye-vat, and the reduction allowed to take place at the ordinary temperature; but it is far preferable to prepare the solution in a more concentrated form, at a higher temperature. At about 50° C. reduction is complete in four to five hours, and there is much less loss of indigo than takes place with the slow reduction at the ordinary temperature. As mentioned under the preparation of a copperas vat, a cask with a wellfitting lid forms a suitable vessel for preparing the standard solution of reduced indigo. For a vat of 400 gallons capacity the standard solution might be prepared as follows :-16 lbs. of 60 per cent. indigo, well ground with water into a smooth paste, are mixed with 8 lbs. of zinc powder, made into a thin paste, with 8 gallons of water. To this mixture milk of lime (containing 20 lbs. of quicklime), at a temperature of 50° C., is added, and the whole made up to about 30 gallons, with water at 50° C. The mixture is well stirred, covered, and allowed to stand, with occasional stirring, for about four hours, when, if in good order, it will have acquired a fine yellow colour, and be ready for use.
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. 317
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