In the process just mentioned, we may observe, that the quantity of alum and of the weld plant used will be found very considerable: from one twelfth to a fourth of alum, and, according to the French method, four or five times more weld than the quantity of the wool.
When a process of dyeing has been scientifically conducted, the wool will take so much of the alum that the bath will hardly taste of it; and afterwards take the colour of the dye bath out of it; so that the remaining liquor put into a glass will be nearly like water.
The action of alum and tartar upon wool.
From the experiments of Dr. Ure, (Notes to Berthollet, vol. ii. p. 323.) it appears that alum has the property of increasing the solubility of cream of tartar; that as, in using alum and tartar, the wool is impregnated with alum and a large quantity of tartaric acid, these two salts should never be employed together, except when the colour is susceptible of being heightened and rendered brighter by acids, as is the case with cochineal, madder, and kermes. On the contrary, alum should never be employed for wools intended to be dyed with woad, or Brazil wood, the colour of which is easily destroyed or altered by acids.
To conclude these preliminary observations, wool has a strong and powerful affinity for all dyeing materials; and, therefore, the processes for dyeing wool are, in general, by no means so complicated as those for dyeing cotton, silk, &c.; although some colours, even to these, are readily, and without a complication of processes, imparted.
A pastil, or woad vat for BLUE.
Take, upon as small a scale as can conveniently be tried, a copper vessel, which will contain about twelve gallons, two thirds full of soft water, and one ounce of madder. Fix this small copper in a larger copper of water, so that the heat may be applied to keep the liquor in the smaller copper at a proper temperature; it will be then, in fact, a water bath.
Having kindled the fire in the afternoon, put in a good handful of bran and five pounds of woad; at five o'clock in the evening let it be well stirred and covered over, the liquor being about blood-warm; let the same heat be continued as nearly as possible, at least so as not to be lower than summer heat by the thermometer, nor higher than fever heat by the same instrument. The vat must again be well stirred at seven, at nine, at twelve at night, at two in the morning, and at four.
Hellot, describing this process, observes, that "the woad then working, some air bubbles began to rise pretty large, but few in number, and of a very faint colour; it had then two ounces of lime added, and was stirred; this was four o'clock in the morning; at five a pattern was put in, and at six it was taken out and the vat stirred. This pattern had received some colour. At seven o'clock another pattern was put in, and at eight it was stirred again. The second pattern was tolerably bright. An ounce of prepared indigo, (see p. 75.) was then added; at nine o'clock another pattern was put in; at ten it was stirred again, taking the pattern out, and putting in an ounce of lime because it began to smell sweetish; at eleven another pattern; at twelve at noon it was stirred again. This process was continued till five o'clock in the evening; then were added three ounces of prepared indigo; at six another pattern was tried, and at seven it was stirred again; the last pattern came out of a very good green, and became a bright blue. One ounce of lime was added to sustain it till nine o'clock the next morning; patterns were put in from time to time: the last was very beautiful. The vat was then filled up with water and a small quantity of bran and stirred; after which patterns were tried every hour till five o'clock in the evening, when, being in a proper state, it was immediately worked. Some lime was then added to preserve it; it was stirred and left to another opportunity to reheat."