Hellot describes two vats, in which the indigo is dissolved by means of urine. Madder is added to them, and in the one vinegar is put, in the other alum and tar tar, of each an equal weight to the indigo. The quantity of urine ought to be very considerable. It is probable the indigo is dissolved in them by the ammonia form ed in the urine. These vats are not so good as those before described ; less work can be performed with them, so that they are adapted only to small dye houses.
The colour dyed by a solution of indigo in sulphuric acid is called Saxon blue, from having been discovered at Gros senhayn, in Saxony, by Counsellor Barth. M. Poerner, who has paid great attention to this preparation, directs four parts of sulphuric acid to be poured on one of in digo, in fine powder: the mixture is to be stirred for some time. After having stood twenty-four hours, one part of good dry potash in fine powder is added ; the whole is to be again well stirred, and having stood twenty-four hours longer, more or less water is added gradu'ally.
To dye Saxon blue, the cloth is pre pared with alum and tartar ; a greater or less proportion of indigo is put into the bath, according as the shade required is, deep or light ; for deep shades the stuff must be passed several times through the bath ; light shades may he dyed after the deep ones, but they have more lustre when dyed in a fresh bath.
Of dyeing Silk blue.
Silk is dyed blue by the indigo vat be fore described. In general, a larger pro portion of indigo is put in than is there directed, but nearly the same quantities of bran and madder. Macquer says, that half a pound of madder for every pound of potash makes the vat greener, and the colour more fixed. When the vat is come to about two pounds of potash, three or four ounces of madd4r should be added ; it should then be raked, and in four hours it will be lit for dyeing. Its heat should be just what the hand can hear without pain. The silk is prepared for this bath by being boiled with thirty pounds of soap for every hundred pounds of silk, and being afterwards well cleans ed from it by two or more beetlings in a stream of water. As the silk is very lia ble to take the colour unevenly, it is ne cessary to dye it in small portions ; the workman dips each hank separately, and when he has turned it once or twice in the bath, he wrings it strongly over it, and airs it, to turn the green colour to a blue ; when the green is thoroughly changed, he throws it into some clear water, after which he wrings it several times with the pin. Care must be
that the silk dyed blue dry speedily. In the winter, and in damp' weather, it should be dried in a chamber heated by a stove, where it should be hung on a frame kept' in constant motion. When the bath grows weak, a pound of potash, an ounce of madder, and an handful of bran well washed, are added. Indigo is also put in when it appears to be wanted. Some dyers use vats grown weak to dye light shades, but fresh vats give a more beau tiful and permanent colour.
As indigo alone cannot give a deep blue, the silk must be prepared by re ceiving a ground, or other colour, pre; vious to dyeing. For the Turkey blue, a very strong archil bath is first given ; and for the French royal blue, a weaker one of the same kind. Cochineal is used also for the ground of another fine deep blue, which is more permanent. Verdigrise and logwood are also used for a preparatory colour, but produce a blue that is not per manent. It might be rendered more last ing, by making the shade lighter than that intended in this bath, afterwards dip ping it in the archil bath, and lastly in the blue vat. .
When raw silk is to be dyed blue, that which is ofa white colour should be cho sen. It should be thoroughly soaked in water, and afterwards put into the vat in separate hanks, in the same manner as the scoured silk. In general, raw silk takes the dye more readily ; wherefore, when it can be done, the scoured silk is put into the bath before it. If raw silk requires archil, or the other grounds mentioned, it should be treated as direct ed for silk in general.
The solution of indigo in sulphuric acid is also used for silk ; the colour called English blue is produced by it. To give silk this colour, it is first dyed a light blue, and then dipped in hot water, wash ed in a stream, and afterwards left in a bath made With the sulphate of indigo, to which a little tin has been added, till the proper shade is obtained, or the bath ex hausted.