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.
WOAD
Woad is derived from a plant, Isatis tinctoria, growing in the North of France and in England. It was the only blue dye in the West before Indigo was introduced from India. Since then woad has been little used except as a fermenting agent for the Indigo vat. It dyes woollen cloth a greenish colour which changes to a deep blue in the air. It is said to be inferior in colour to indigo but the colour is much more permanent. The leaves when cut are reduced to a paste, kept in heaps for about fifteen days to ferment, and then are formed into balls which are dried in the sun; these have a rather agreeable smell and are of a violet colour. These balls are subjected to a further fermentation of nine weeks before being used by the dyer. When woad is now used it is always in combination with indigo, to improve the colour. Even by itself, however, it yields a good and very permanent blue.