Hellot says "when the vat, of whatsoever kind it be, is once prepared in a proper state, there is no difficulty in dyeing woollens or stuffs, as it is requisite only to soak them in clean warm water, to wring them, and then to immerse them in the vat, for a longer or shorter time, according as you would have the colour more or less deep. The stuff should be from time to time opened, that is to say, taken out and wrung over the vat and exposed to the air for a minute or two, till it becomes blue. For let your vat be what it will, the stuff will be green when taken out and will become blue when exposed to the air. In this manner it is very proper to let the colour change before you immerse your stuffs a second time, as you are thereby better enabled to judge whether they will require only one or several dips."—"The Art of Dyeing Wool," by Hellot.
The colour of the blue is brightened by passing the wool through boiling water after it comes out of the dye. Indigo is a substantive dye and consequently requires no mordant.
[13] 1). To Make Extract of Indigo.—
Put 2 lbs. of oil of vitriol into a glass bottle or jar, stir into it 8 oz. of powdered indigo, stirring briskly for ½ hour, then cover up and stir 4 or 5 times a day for a few days, then add a little powdered chalk to neutralise the acid. It should be added slowly, little by little, as the chalk makes the acid bubble up. Keep it closely corked.
2). [To Make Extract of Indigo].—
4 oz. sulphuric acid, ½ oz. finely ground Indigo. Mix like mustard, and leave to stand over-night. Prepare the wool by mordanting with 5 oz. alum to 1 lb. wool. Boil for ½ hour and dye without drying.
3). To Dye Wool with Indigo Extract
For 4 to 6 lbs. of wool. Stir 2 to 3 oz. of Indigo extract into the water of the dye bath. The amount is determined by the depth of shade required. When warm, enter the wool, and bring slowly to boiling point (about ½ hour) and continue boiling for another ½ hour. By keeping it below boiling point while dyeing, better colours are got, but it is apt to be uneven. Boiling levels the colour but makes the shade greener. This is corrected by adding to the dye bath a little logwood, 10 to 20 per cent. This should be boiled up separately, strained, and put in the bath before the wool is entered. Too much should be avoided however, as it dims the colour. It can be done in the same bath, but better results are got by separate baths. Instead of logwood a little madder is sometimes used; also Cudbear or Barwood.
4). To Dye Silk with Indigo Extract.
Dye at a temperature of 40 to 50°C. in a bath with a little sulphuric acid and the amount of indigo as is needed for the colour. Another method is to mordant the silk first with alum by steeping it for 12 hours in a solution of 25 per cent. and then, without washing, to dye with the Indigo Extract and about 10% of alum added to the dye bath. By this means compound colours can be made by the addition of cochineal, for purple, or old Fustic, Logwood, etc., for greys, browns and other colours.